MIAMI UNIVERSITY The Graduate School Certificate for Approving the Dissertation

We hereby approve the Dissertation of Matthew E. Brueseke

Candidate for the Degree: Doctor of Philosophy

______William K. Hart, Director

______Alan R. Wallace, Reader

______Elisabeth Widom, Reader

______Brian S. Currie

______Michael R. Brudzinski

______Michael J. Pechan, Graduate School Representative

ABSTRACT

MID-MIOCENE MAGMATIC SYSTEM DEVELOPMENT IN THE NORTHWESTERN UNITED STATES

By Matthew E. Brueseke

This dissertation investigates the spatial, temporal, geochemical, and petrologic development and evolution of mid-Miocene volcanic systems in the southeastern Oregon Plateau region of Oregon and Nevada. This integrated field and laboratory investigation conclusively demonstrates that flood basalt volcanism occurred on the Oregon Plateau over at least a 2 m.y. duration, and provides the first comprehensive view into the development of a mid-Miocene Oregon Plateau volcanic field and its relationship with regional flood basalt volcanism.

The first portion of this study focuses on the geochemical and chronostratigraphic characteristics of flood basalt lava flows in the vicinity of Steens Mountain, Oregon. New

40Ar/39Ar ages and recalculated literature ages from the Steens Basalt type section illustrate that

multiple magmatic centers were present locally, and that Oregon Plateau flood basalt activity

was coeval with the main phase of Columbia River Basalt Group volcanism.

The remainder of this study focuses on the Santa Rosa-Calico volcanic field (SC) of

northern Nevada in order to better define and understand the link between mid-Miocene Oregon

Plateau mafic and silicic volcanism. In the SC, mafic through silicic eruptive loci and shallow

intrusive bodies are exposed along broadly north-south trending alignments, coincident with

regional lithospheric structures. At least sixteen physically and compositionally distinct units are

exposed in the SC representing approximately 2 m.y. of magma production. Local mafic

volcanism was dominated by the eruption of Steens Basalt magmas. SC silicic magmas were produced by basaltic magma induced crustal melting of granitoid upper crust and erupted from diverse vent types and locations. At least four distinct intermediate (andesite-dacite) magmatic systems also are documented. Physical, chemical, and isotopic data indicate that open-system petrogenetic processes played a substantial role in the generation of these magmas and also influenced the chemical characteristics of SC mafic and silicic units. The complex array of physical and chemical characteristics and processes displayed and documented by SC units provide an exceptional example of how compositionally diverse volcanic fields develop.

Moreover, these complexities establish an important link between regional mid-Miocene mafic magma production and the generation of silicic-dominated volcanic fields on the Oregon Plateau.

MID-MIOCENE MAGMATIC SYSTEM DEVELOPMENT

IN THE NORTHWESTERN UNITED STATES

A DISSERTATION

Submitted to the Faculty of Miami University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Geology

by

Matthew E. Brueseke Miami University Oxford, Ohio 2006

Dissertation Director: William K. Hart, Ph.D.

TABLE OF CONTENTS

Chapter 1: Introduction 1 References 4

Chapter 2: Distribution and Geochronology of 6 Oregon Plateau (U.S.A.) Flood Basalt Volcanism: The Steens Basalt Revisited

Abstract 7 Body Text 8 References 56 Appendix A: Appendix A: Argon Data Repository 63 Appendix B: Whole-Rock Major and Trace Element Data 71 Appendix C: Sample Locations and Descriptions 85

Chapter 3: Diverse mid-Miocene Silicic Volcanism Associated 100 with the Yellowstone-Newberry Thermal Anomaly

Abstract 101 Body Text 102 References 124 Appendix 1: 40Ar/39Ar Analyses of Single Crystal 157 Sanidines From Volcanic Rocks From the Santa Rosa-Calico Volcanic Field

Chapter 4: Geology and Petrology of the Mid-Miocene 172 Santa Rosa-Calico Volcanic Field, Northern Nevada

Body Text 172 References 321 Appendix 1: Analytical Methods 335 Appendix 2: Sample Locations and Descriptions 344 Appendix 3: Major, Trace, and Isotope Data 379 Appendix 4: 40Ar/39Ar Geochronology 412

Chapter 5: Summary and Suggestions for Future Research 425

ii LIST OF TABLES

Chapter 2: Distribution and Geochronology of 6 Oregon Plateau (U.S.A.) Flood Basalt Volcanism: The Steens Basalt Revisited

1 - Summary of 40Ar/39Ar Methods 29

2 - Summary of 40Ar/39Ar Age Results 30

3 - Summary of New and Published Oregon Plateau 31 Flood Basalt Radiometric Ages

4 - Major and Trace Element Geochemical Data 32 for Dated Oregon Plateau Lava Flows

Chapter 3: Diverse mid-Miocene Silicic Volcanism Associated 100 with the Yellowstone-Newberry Thermal Anomaly

1 - Summary of Sanidine 40Ar/39Ar Results for 155 Santa Rosa-Calico Volcanic Field Silicic Units

2 - Representative Major and Trace Element Geochemical 156 Data for Santa Rosa-Calico Volcanic Field Silicic Units

Chapter 4: Geology and Petrology of the Mid-Miocene 172 Santa Rosa-Calico Volcanic Field, Northern Nevada

1 - Summary of Assigned 40Ar/39Ar Ages 312

2 - Summary of SC Eruptive Units 313

3 - Representative Major and Trace Element Geochemical 314 Data for Santa Rosa-Calico Units Flood Basalt Radiometric Ages

4 - Results of Two Component Mixing Calculations for 320 Two SC Samples

iii LIST OF FIGURES

Chapter 2: Distribution and Geochronology of 6 Oregon Plateau (U.S.A.) Flood Basalt Volcanism: The Steens Basalt Revisited

1 - Cenozoic Volcanic and Tectonic Features of the Northwest USA 33

2 - 40Ar/39Ar ages and K/Ca Spectra for Newly Dated Lava Flows 35 From Stratigraphic Sections Peripheral to Steens Mountain

3 - 40Ar/39Ar age and K/Ca Spectra and Isotope Correlation Diagrams 38 For Dated Lava Flows From Steens Basalt at Steens Mountain

4 - 40Ar/39Ar age and K/Ca Spectra Diagrams for Low Resolution 40 Laser Heating Analyses for Steens Basalt at Steens Mountain Sample CH82-22G

5 - 40Ar/39Ar age and K/Ca Spectra Diagrams for Low Resolution 42 Laser Heating Analyses for Steens Basalt at Steens Mountain Sample CH82-22B

6 - Petrographic and Stratigraphic Relationships of Sampled Sections 44

7 - Total Alkali vs. Silica Diagram and Plot of Wt. % TiO2 vs. P2O5 46

8 - Chemostratigraphic Variation Diagrams From Sampled 48 Stratigraphic Sections, Discussed in this Study

9 - Chemostratigraphic Variation of the Steens Basalt at 50 Steens Mountain

10 - Select Major and Trace Element Variations of Oregon Plateau 52 Flood Basalt Lava Flows

11 - Stratigraphic Variation of Oregon and Columbia Plateau 54 Flood Basalt Units

Chapter 3: Diverse mid-Miocene Silicic Volcanism Associated 100 with the Yellowstone-Newberry Thermal Anomaly

1 - Cenozoic Volcanic and Tectonic Features of the Northwest USA 131

2 - Satellite Imagery and Generalized Geologic Map of the 133 Santa Rosa-Calico Volcanic Field

iv 3 - Generalized Stratigraphy of the SC 135

4 - Photographs of SC Silicic Units and Features 137

5 - Mineralogical Characteristics of SC Silicic Units 139

6 - Textural Characteristics of Select SC Silicic Units 141

7 - Coyote Mountain Ash-Flow Tuff Stratigraphy 143

8 - Cold Springs Tuff Exposures From the Central SC 145

9 - Total Alkali vs. Silica Diagram and Plot of A/NK vs. A/CNK 147

10 - Major and Trace Element Variations for Western SC Silicic Units 149

11 - Major and Trace Element Variations for Eastern SC Silicic Units 151

12 - Steens Basalt Normalized Multi-Element Diagrams of 153 SC Silicic Units

Chapter 4: Geology and Petrology of the Mid-Miocene 172 Santa Rosa-Calico Volcanic Field, Northern Nevada

1 - Cenozoic Volcanic and Tectonic Features of the Northwest USA 233

2 - Satellite Image and Map of the Santa Rosa-Calico Volcanic Field 235

3 - Generalized Stratigraphy of the SC 237

4 - Photographs of SC Units and Geologic Features 239

5 - Total Alkali vs. Silica Diagram and Plot of Wt. % K2O vs. SiO2 243

6 - Satellite Imagery and Generalized Geologic Map of the 245 Santa Rosa-Calico Volcanic Field

7 - Photomicrographs of Common SC Disequilibrium Textures 248

8 - Photomicrographs of Metamorphic Xenolith in a Tad1 Lava Flow 250

9 - Idealized Ash Flow (Ignimbrite) Flow Unit 252

10 - Coyote Mountain Ash-Flow Tuff Stratigraphy 254

v 11 - Cold Springs Tuff Composite Stratigraphic Section 256

12 - Photographs of SC Tuffaceous Units 258

13 - Plots of ppm Sr vs. Wt.% Silica, Tholeiitic vs. Calc-Alkaline 260 Discrimination Diagram, and A/NK vs. A/CNK

14 - Harker Diagrams Illustrating Major and Trace Element 262 Characteristics of SC Mafic to Intermediate Units

15 - MORB Normalized Trace Element Variation Diagram 265 for SC Mafic and Intermediate Units

16 - Chondrite Normalized REE Diagrams for All SC Units 267

17 - MORB Normalized Comparison of Regionally and 269 Locally Erupted Steens Basalt

18 - Harker Diagrams Illustrating Major and Trace Element 271 Characteristics of SC Mafic to Intermediate Units

19 - Upper Continental Crust Normalized Trace Element 274 Variation Diagram for SC Silicic Units

20 - Initial Sr and Nd Isotopic Characteristics of SC Units 276

21 - Initial Sr and Nd Isotopic Characteristics of SC Units vs. 278 Wt. % Silica

22 - Variations in Pb Isotopic Compositions of SC Units 280

23 - Combined Sr, Nd, and Pb Isotope Characteristics of SC Units 282

24 - Sr and Nd Isotopic Characteristics of Locally Exposed 284 Mesozoic and Younger Units

25 - Tba Normalized Trace Element Variation Diagram for SC Units 286

26 - Ppm Zr, wt. % K2O/MgO, and ppm Ba variations of SC Mafic 288 and Intermediate Units

87 86 27 - Wt. % SiO2 vs. K/P and Initial Sr/ Sr for SC Mafic and 290 Intermediate Units

28 - Binary Mixing Model Results 292

vi 29 - Comparison Between SC Silicic Units and Regional Upper 294 Crustal Melts

30 - Ppm Zr, wt. % K2O/MgO, and ppm Ba variations of SC Silicic 296 Units, Kg Bodies, and Calculated Melt Compositions

31 - Ppm Zr, wt. % K2O/MgO, and ppm Ba variations of All SC 299 Units, Kg Bodies, and Calculated Melt Compositions

32 - Detailed Stratigraphic Sections 300

33 - Generalized SC Geology and Geographic Features 304

34 - Geologic Development of the SC Through Time 306

35 - Chemical Variation of SC Products Through Time 308

36 - Subsurface Magmatic Processes Through Time 310

vii ACKNOWLEDGEMENTS

Writing this section is a long time coming and I could go on for quite a while, however, I’ll try to be as short and to the point as possible. First, I’d like to thank Bill Hart for taking a chance on a very interested, but not very focused undergraduate student way back in the Spring/Summer of 1996. Bill, if you had not given me the chance to work out in Hagerman that summer, right now I’d very likely be either working as an environmental consultant or working on modern carbonate deposition. Your willingness to help me, teach me, and advise me (in more than just geological research) has taught me how to be a scientist and more importantly, a mentor. I will always value the time we have spent together and thanks for being a great friend. Thank you to everyone who has served on my committees over the years: Brian Currie, Liz Widom, Mick Pechan, John Hughes, and Mike Brudzinski (from Miami) and also Alan Wallace for agreeing to make the trip out to Ohio at the end of February! Brian: while I did not always listen to your advice, I really have appreciated it over the years and also I value your friendship and what you have taught me while working/teaching in the field. Thanks to John Morton, who in addition to being a very fun person to talk to about life in general, spent hours of time helping me in the geochemistry labs at Miami. I’d like to thank Stan Mertzman (Franklin and Marshall College) for turning around vial after vial of samples in a timely manner and also Matt Heizler, for providing detailed and thorough geochronology throughout this project. I’d like to thank Cathy Edwards and Teresa Kolb for all of their help over the years as well. Thanks also to Dwight Baldwin for always giving encouragement to an undergraduate student whose priorities did not revolve around studying and Chris Haley for helping to spur that undergraduate into changing those priorities. Chris I’d like to also thank you for your advice and friendship over the years. I want to also thank my co-workers (especially John Stimac) and my Mineralogy/Petrology students at Eastern Illinois University who encouraged me during the final stages of finishing my dissertation and made this past year a great learning experience. Lauren Gilbert, Amy Maloy, and Jake Knight all contributed to this project through undergraduate research experiences and helped me see how fun and challenging it can be to mentor students. Additionally, you all livened up the field work! I’d like to thank all of my fellow graduate students. During my years at Miami, I made lots of great friendships with lots of different people, but I’ll only mention a few names here. I want to thank Ninad Bondre and

viii Steve Pasquale for your friendship and fun discussions about sports, politics, and geology. Andy Cyr, thanks for always being there with something to say (be that geology or primates). Charles Bruce Minturn III, thanks for all of your help while in the field and those nights hanging out on the porch at the Hagerman cabin. Kurt Shoemaker, thanks for all of the cheap beer and long nights of rambling on and on about mantle plumes, basalts, the merits and downsides to cheap beer, Nevada, and life in general. Someday I will finish all of those ribs and there will always be a Little Kings waiting for you in my fridge. Finally, I’d like to thank Darin Snyder. Darin, thanks for being the best officemate I could imagine and also for being a great friend. I’d also like to thank my Mom and Dad for letting me find my own path and always being there for me throughout this entire process. I can imagine what you were thinking when I was bringing home those “great” grades my first two years of college and I’m glad you kept hounding me to get my act together. Thanks for always giving me encouragement, listening to me, and also always fostering my interest in geology. Pretty amazing what a plastic dinosaur and some crinoid-rich gravel will end up doing! Finally, I’d like to thank my wife Emily. Em, your encouragement and patience have been major forces in helping me get through this past year. Thanks for always believing in me.

ix CHAPTER 1:

INTRODUCTION

Bimodal basalt-rhyolite magmatic systems are common in tectonic settings where lithospheric extension and/or the emplacement of large volumes of mantle derived magmas has occurred. Continuous emplacement of mafic magmas into the crust can lead to the production of silicic magmas through crystal fractionation and anatexis. Typically, a combination of processes including crustal melting, fractional crystallization, crustal assimilation, and magma mixing/hybridization have been called upon to explain the compositional characteristics of these silicic eruptive products (Walter et al. 1987; Tegtmeyer and Farmer, 1990; Hildreth et al. 1991; Wiebe, 1994; Gunnarsson et al. 1998). Intermediate composition (andesite-dacite) magmas are minor components in these dominantly bimodal systems and their presence usually suggests further evidence for open system processes including magma mixing and mingling (e.g., McMillan and Dungan, 1986; Honjo and Leeman, 1987; Walter et al., 1987; McCurry et al., 1999; Streck and Grunder, 1999; Johnson and Grunder, 2000). In the dominantly bimodal Cenozoic volcanic provinces of the northwestern United States (e.g. Snake River Plain-Yellowstone, Oregon Plateau, and northern Basin and Range) intermediate composition magmas appear to be confined to locations where extensional tectonism accompanied magma system development. Excellent examples of this include the 10.4 Ma Duck Butte eruptive center in southeastern Oregon (Johnson and Grunder, 2000); numerous small centers along the mid-Miocene northern Nevada rift (Wallace and John, 1998); and Magic Reservoir and Cedar Butte on/along the Snake River Plain (Honjo and Leeman, 1987; McCurry et al. 1999). This relationship is also apparent within mid-Miocene Oregon Plateau volcanic fields (e.g. McDermitt, Lake Owyhee; Santa Rosa-Calico), where andesite-dacite vents and lava flows are present within locally exposed packages (e.g. Rytuba and McKee, 1984; Cummings et al. 2000; Camp et al. 2003; this study). In addition to this association with intermediate magma generation, these mid-Miocene volcanic fields are also unique relative to their younger eastern Snake River Plain counterparts because of the combination of three other features: [1] their location in an area with transitional lithosphere (e.g., continental and oceanic; Leeman et al., 1992), [2] their diverse eruptive styles (e.g. domes, calderas, and fissural vents), and [3] their

1 intimate temporal and spatial relationship to major flood basalt eruptions and extensional structures. To further examine these characteristics and the association of mid-Miocene northwestern United States flood basalt volcanism with the development of silicic-dominated Oregon Plateau volcanic fields and coeval lithospheric extension, the following four questions are addressed in three separate chapters and form the basis for this study: 1) What is the complete temporal and chemical record of mid-Miocene Oregon Plateau flood basalt volcanism? 2) How do the eruptive history, eruptive styles, chronology, vent distribution geochemical diversity, and petrologic history of the Santa Rosa-Calico volcanic field relate to local and regional fault patterns and timing? 3) What is controlling the compositional diversity (basalt through high-Si rhyolite) within the Santa Rosa-Calico volcanic field? 4) Is mid-Miocene Oregon Plateau silicic eruptive activity intimately linked to increases in mafic magma input and did it cease at the same time that voluminous, regional flood basalt volcanism ceased? Chapter 2 addresses the temporal and chemical record of Oregon Plateau flood basalt volcanism through a stratigraphic examination of Steens Basalt lava flows that are exposed in close proximity to Steens Mountain. Field and geochemical data, in conjunction with nine new 40Ar/39Ar ages from these sections, define a 1.05 ± 0.46 Ma duration of local volcanism and indicate the presence of multiple eruptive loci. Additionally, new 40Ar/39Ar ages and recalculated literature ages from the Steens Mountain type section illustrate that Steens Basalt lava flows exposed at Steens Mountain are younger than typically quoted. Combined, these data indicate that Oregon Plateau flood basalt volcanism (Steens Basalt) was coeval with the main phase of Columbia River Basalt Group volcanism and regional silicic-dominated volcanic field development. To better define and understand the link between mid-Miocene Oregon Plateau mafic and silicic volcanism, a detailed study of the Santa Rosa-Calico volcanic field (SC) of northern Nevada was performed, of which the results are presented in Chapters 3 and 4. Chapter 3 illustrates the physical and chemical diversity of SC silicic eruptive products and shallow intrusive bodies. It also presents new 40Ar/39Ar ages that delineate an ~1 Ma duration of local silicic activity, coeval with local mafic magma input and extensional tectonism. Physically, the silicic products exposed in the SC are composed of both lava and ash flows that

2 erupted primarily from domes and fissures. Unlike other mid-Miocene Oregon Plateau silicic volcanic fields, SC silicic products are subalkaline and not dominated by caldera-forming volcanism. Only one locally derived pyroclastic unit, the Cold Springs tuff, is interpreted to be caldera-derived. Field, chemical, and chronologic data also illustrate that silicic volcanism in the SC varied drastically, both temporally and geographically. To better understand these physical, chemical, and temporal differences, and ultimately unravel how SC silicic magmas were generated, the spatial, stratgraphic, and petrologic context of these silicic units must be addressed in the broader context of the entire package of SC eruptive products. This context and the link between regional flood basalt volcanism and SC silicic magma generation is discussed in Chapter 4. Chapter 4 addresses the previously mentioned ideas with the ultimate goal of better understanding the relationship between mid-Miocene Oregon Plateau mafic and silicic volcanism. Field, chronologic, geochemical, and isotopic data were used in an effort to unravel the volcanic history of the SC, define the petrogenetic processes that influenced the generation/modification of SC magmas, and demonstrate the relationship between local and regional mid-Miocene volcanic and tectonic processes. In the SC, mafic through silicic eruptive loci and shallow intrusive bodies are exposed along broadly north-south trending alignments, coincident with regional lithospheric structures. At least sixteen physically and compositionally distinct mafic through silicic units are exposed in the SC and represent of ~2 Ma of magma production. Local mafic volcanism was dominated by the eruption of basaltic to basaltic- andesitic magmas that are chemically identical to regional Steens Basalt. SC silicic magmas were produced by crustal melting of locally exposed granitoid upper crust. Also present during SC activity were at least four distinct intermediate (andesite-dacite) magmatic systems. Physical, chemical, and isotopic data indicate that open-system petrogentic processes (e.g. magma mixing and assimilation-fractional crystallization) played a significant role in the generation of these intermediate magmas and also influenced the chemical characteristics of SC mafic and silicic units. The complex array of physical characteristics and processes that are displayed by and also affected SC units provide an exceptional example of how diverse volcanic fields develop and also record the interaction between regional mid-Miocene mafic magma production and the generation of silicic-dominated volcanic fields on the Oregon Plateau.

3 References Camp, V.E., Ross, M.E., and Hanson, W.E., 2003, Genesis of flood basalts and Basin and Range volcanic rocks from Steens Mountain to the Malheur River Gorge, Oregon: Geological Society of America Bulletin, v.115, p. 105-128.

Cummings, M.L., Evans, J.G., Ferns, M.L., and Lees, K.R, 2000, Stratigraphic and structural evolution of the middle Miocene syn-volcanic Oregon-Idaho graben: Geological Society of America Bulletin, v. 112, p. 668-682. Gunnarsson, B., Marsh, B.D., and Taylor, H.P., 1998, Generation of Icelandic rhyolites: silicic lavas from the Torfajokull central volcano: Journal of Volcanology and Geothermal Research, v. 83, p. 1-45. Hildreth, W., Halliday, A.N., and Christiansen, R.L., 1991, Isotopic and chemical evidence concerning the genesis and contamination of basaltic and rhyolitic magma beneath the Yellowstone Plateau volcanic field: Journal of Petrology, v. 32, p. 63-138. Honjo, N., and Leeman, W.P, 1987, Origin of hybrid ferrolatite lavas from Magic Reservoir eruptive center, Snake River plain, Idaho: Contributions to Mineralogy and Petrology, v. 96, p. 163-177.

Johnson, J.A., and Grunder, A.L., Grunder, 2000, The making of intermediate composition magma in a bimodal suite: Duck Butte eruptive center, Oregon, USA: Journal of Volcanology and Geothermal Research, v. 95, p. 175-195. Leeman, W.P., Oldow, J.S., and Hart, W.K., 1992, Lithosphere-scale thrusting in the western U.S. Cordillera as constrained by Sr and Nd isotopic transitions in Neogene volcanic rocks: Geology v. 20, p. 63-66. McCurry, M., Hackett, W.R., and Hayden, K., 1999, Cedar Butte and cogenetic Quaternary rhyolite domes of the eastern Snake River Plain, in, Hughes, S.S. and Thackray, G.D., eds., Guidebook to the geology of eastern Idaho: Idaho Museum of Natural History, p. 169-179. McMillan, N.J. and Dungan, M.A., 1986, Magma mixing as a petrogenetic process in the development of the Taos Plateau volcanic field, New Mexico: Journal of Geophysical Research, v. 91, p. 6029-6045. Rytuba, J.J., and McKee, E.H., 1984, Peralkaline ash flow tuffs and calderas of the McDermitt volcanic field, southeast Oregon and north central Nevada: Journal of Geophysical Research, v. 89, p. 8616-8628. 4 Streck, M.J. and Grunder, A.L., 1999, Enrichment of basalt and mixing of dacite in the rootzone of a large rhyolite chamber: inclusions and pumices from the Rattlesnake Tuff, Oregon: Contributions to Mineralogy and Petrology v. 136, p. 193-212. Tegtmeyer, K.J. and Farmer, G.L., 1990, Nd isotopic gradients in upper crustal magma chambers: evidence for in situ magma-wall rock interaction: Geology, v. 18, p. 5-9. Wallace, A.R., and John, D.A., 1998, New studies of Tertiary volcanic rocks and mineral deposits, Northern Nevada Rift, in, Tosdal, R.M., ed., Contributions to the gold metallogeny of northern Nevada: United States Geological Survey Open File Report 98-338, p. 264-278. Walter, R.C., Hart, W.K., and Westgate, J.A., 1987, Petrogenesis of a basalt-rhyolite tephra from the west-central Afar, Ethiopia: Contributions to Mineralogy and Petrology, v. 95, p. 462-480. Wiebe, R.A., 1994. Silicic magma chambers as traps for basaltic magmas: the Cadillac Mountain Intrusive Complex, Mount Desert Island, Maine: Journal of Geology, v. 102, p.423-437.

5 Distribution and Geochronology of Oregon Plateau (U.S.A.) Flood Basalt Volcanism: The Steens Basalt Revisited

Matthew E. Brueseke a, Matthew T. Heizler b, William K. Hart a*, and Stanley A. Mertzman c

a Department of Geology, Miami University, 114 Shideler Hall, Oxford, OH 45056

b New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801

c Department of Earth and Environment, Franklin and Marshall College, P.O. Box 3003 Lancaster, PA 17604

In review, Journal of Volcanology and Geothermal Research

3/13/06 version

* Corresponding author. Phone: 1-(513)-529-3216; Fax: 1-(513)-529-1542. E-mail address: [email protected] (W. K. Hart)

6 Abstract The timing and petrogenesis of mid-Miocene flood basalt volcanism in the northwest United States has been extensively addressed, yet the chemical characteristics and temporal details of the Steens Basalt, exposed on the Oregon Plateau, are poorly defined. Steens Basalt volcanism has generally been accepted to have occurred at ~16.6 Ma, coeval and/or just prior to the onset of Columbia River Basalt Group volcanism to the north. New major and trace element analyses and nine 40Ar/39Ar ages ranging from 15.51 ± 0.28 to 16.58 ± 0.18 Ma were obtained on Oregon Plateau flood basalt lava flows from stratigraphic sections in close proximity to Steens Mountain. Additionally, new 40Ar/39Ar ages were obtained on the uppermost and thirty-first lava flow down from the top of the ~1 km section of Steens Basalt exposed at Steens Mountain and yield eruption ages of 16.38 ± 0.20 and 16.36 ± 0.10 Ma, respectively. These new ages, along with recalculated literature ages indicate that the best age of the upper portion of the Steens Basalt type section is 16.35 ± 0.07 (2σ) Ma. Furthermore, the field relations between these Steens Basalt sections suggest that multiple eruptive centers were present in the vicinity of Steens Mountain. The chemical and chronologic data illustrate that flood basalts with the “Steens Basalt” chemical signature erupted across the southern Oregon Plateau over a much greater timespan than what is typically quoted for the Steens Mountain type section. These data suggest that the main volume of Steens Basalt volcanism erupted over at least an ~1 Ma duration from eruptive loci in the vicinity of Steens Mountain, while likely much less voluminous flood basalt volcanism appears to have occurred for >2 Ma from loci exposed across the Oregon Plateau. These new geochemical and geochronological constraints verify a common temporal link between the duration of Steens, Malheur Gorge-region, and Columbia River Basalt Group volcanism. This direct temporal link requires that petrogenetic and tectonic models of mid- Miocene northwestern U.S. flood basalt volcanism recognize that the northern (Columbia Plateau) and southern (Oregon Plateau) portions of this flood basalt province were erupting simultaneously and both remained active for the duration of regional flood basalt volcanism.

Keywords: Steens Mountain, Oregon Plateau, Miocene, flood basalts, 40Ar/39Ar, Yellowstone

7 Introduction and Regional Geology Renewed interest has recently highlighted some of the issues associated with the onset and duration of mid-Miocene flood basalt volcanism in the northwest United States (Hooper et al., 2002; Camp et al., 2003; Camp and Ross, 2004). The newly suggested stratigraphic relationships between eastern Oregon Plateau flood basalts have helped tentatively correlate lava flows erupted in the vicinity of Steens Mountain with tholeiitic volcanism further north. However, this link and its relationship with lava flows erupted throughout the entire areal extent of the Oregon Plateau are still somewhat poorly constrained. Here, we [1] present new geochemical data and 40Ar/39Ar ages obtained on mafic lava flows from the southern Oregon Plateau and [2] reconsider the areal extent, duration, and regional implications of flood basalt volcanism on the Oregon Plateau. These new data document at least an ~1 Ma duration of voluminous Steens Basalt flood basalt volcanism. Furthermore, major and trace element geochemistry and petrography link regionally exposed lava flows with the entire package of Steens Basalt exposed at Steens Mountain. Steens Basalt lava flows are generally relatively thin, characterized by a lack of interflow sediments, aphyric to coarsely plagioclase phyric textures, and mid-Miocene eruptive ages. These lava flows were originally studied by Fuller (1931) and later defined by Baksi et al. (1967) and Gunn and Watkins (1970) based on lava flows exposed at Steens Mountain. The chemical similarities we present and discuss validate earlier observations by Hart and Mertzman (1982), Carlson and Hart (1983; 1987), Hart and Carlson (1985), and Hart et al. (1989) who suggested that exposed flood basalts in this region span a greater temporal range than what is represented by the Steens Basalt type section. While presenting these observations, Carlson and Hart (1983; 1987) divided regional ~17 to ~14 Ma flood basalt lava flows into two types: [1] mafic lava flows that are chemically and isotopically similar to the Steens Basalt at Steens Mountain, which they considered Steens Basalt and [2] mafic lava flows with similar chemical and temporal characteristics to the Steens Basalt at Steens Mountain but different isotopic characteristics. This second group of isotopically dissimilar lava flows was termed “Steens-Type” Basalt and was grouped with a package of regionally extensive, younger (~12 ± 1 Ma) basaltic lava flows, which are petrographically and chemically similar, but isotopically unlike the older flood basalts. As part of this study, we clarify these semantic issues and demonstrate that the regional ~16 ± 1 Ma flood basalt lava flows originally termed “Steens- Type” Basalt by Carlson and Hart (1983) are compositionally correlative to Steens Basalt lava

8 flows and should be considered Steens Basalt sensu-stricto, even though they erupted over a much greater timespan and areal distribution than the lava flows exposed at Steens Mountain. These earlier observations and the data from this study demonstrate that flood basalt volcanism on the Oregon Plateau was temporally equivalent with the entire duration of major Columbia River Basalt Group (CRBG) volcanism. Additionally, these new data support the alternative view that Oregon Plateau flood basalt volcanism in this region did not cease through time as it propagated to the north as suggested by other workers (e.g. Geist and Richards, 1993; Camp, 1995; Hooper et al., 2002; Camp and Ross, 2004). As a result, tectonomagmatic models that are invoked to explain regional flood basalt volcanism and incorporate a northward propagating or focusing mantle upwelling must be reevaluated, and models that consider region-wide, ~17 to ~14 Ma flood basalt volcanism must be reconsidered. Large volumes of tholeiitic basalts and ferroandesites started erupting at ~17 Ma from loci juxtaposed against the western margin of the Wyoming Craton following a cessation in regional calc-alkaline volcanism (~19 to 24 Ma; Carlson and Hart, 1987; John, 2001). CRBG and Oregon Plateau tholeiitic basalt and basaltic andesite lava flows are the manifestation of the Yellowstone/Newberry melting anomaly and/or regional tectonic interaction between the underlying mantle and reactivated lithospheric structures (Christiansen and McKee, 1978; Carlson and Hart, 1987; Zoback et al., 1994; Dickinson, 1997; Humphreys et al., 2000; John and Wallace, 2000; Christiansen et al., 2002; Glen and Ponce, 2002; Wagner et al., 2000; Brueseke et al., 2003; Jordan et al., 2004). The stratigraphic details and temporal relationships of Oregon Plateau flood basalt volcanism are less well understood, even though the stratigraphic and chronologic details of CRBG flood basalt volcanism are fairly well constrained (Tolan et al., 1989; Baksi and Farrar, 1990). Additionally, unlike the eruptive products of the CRBG, it is often asserted that the Steens Basalt erupted in one brief magmatic event at ~16.6 Ma (Swisher et al., 1990; Hooper, 1997; Johnson et al., 1998; Hooper et al., 2002; Camp et al., 2003; Camp and Ross, 2004), contemporaneous and/or preceding the earliest Imnaha Basalt of the CRBG. The thickest exposures of Steens Basalt tend to be found in the vicinity of Steens Mountain, however, flood basalt eruptive loci (dikes) and eruptive products are present across the southern Oregon Plateau (Larson et al., 1971; Hart and Carlson, 1985; Mankinen et al., 1987; Brueseke and Hart 2004) (Figs. 1a and b) {POSITION FIGURE 1 AFTER}. Some workers have interpreted these data

9 to indicate that Steens Basalt volcanism was initially widespread across the Oregon Plateau, and gradually shifted to one large eruptive center situated at Steens Mountain (Mankinen, et al., 1987). Accompanying this spatial shift, one large magmatic event then occurred centered at Steens Mountain, forming the upper part of the ~75 to 100 flow, ~900 to 1000 m thick section of Steens Basalt, with outflow found as far west as Abert Rim, Oregon and as far south as northern Nevada (Hart and Mertzman, 1982; Carlson and Hart, 1983; Mankinen et al., 1987). Earlier paleomagnetic results on the Steens Basalt type section documented a N/R (normal polarity over reversed) paleomagnetic reversal within the upper portion of the flood basalt section and this reversal, the presence of thin lava flows, and the lack of thick interflow sedimentary horizons, were taken to indicate that the entire sequence of lava flows was erupted over a very short period of time (<10,000 yrs.; Baksi et al., 1967; Gunn and Watkins, 1970; Grommé et al., 1985; Mankinen et al., 1985; Mankinen et al., 1987). Prior 40Ar/39Ar geochronology yield indistinguishable ~16.6 Ma chronologic results for the uppermost and 31st (down from the top) flows of the Steens Basalt type section (Swisher et al., 1990), and additional 40Ar/39Ar ages from below these stratigraphic levels yield similar ages (Baksi and Farrar, 1990; Baksi et al., 1991). Although the data of Carlson and Hart (1983) and Hart and Carlson (1985) left open the possibility that the type Steens Basalt section might not be representative of the entire Steens Basalt magmatic event, subsequent workers have typically quoted the ~16.6 Ma age as solely representative of the entire duration of Steens Basalt volcanism. In this study we demonstrate that Oregon Plateau flood basalt eruptive activity was more complex than is commonly recognized and that the temporal characteristics of Oregon Plateau flood basalt volcanism mirrors other mid-Miocene Pacific Northwest flood basalt volcanism. Additionally, we normalize published Steens Mountain ages (Swisher et al., 1990; Baksi and Farrar, 1990; Baksi et al., 1991) to a common 40Ar/39Ar fluence monitor age (i.e. standard) to obtain an internally consistent eruption age for at least the upper 450 m of the Steens Mountain type section.

40Ar/39Ar Methodology and Age Assignments 40Ar/39Ar geochronology was conducted on eleven groundmass concentrates and one plagioclase separate at the New Mexico Geochronology Research Laboratory (NMGRL). Nine of the samples are from Oregon Plateau flood basalt lava flows collected from five stratigraphic

10 sections within the study area, whereas two samples are from Steens Mountain. All samples were analyzed by the furnace incremental heating age spectrum method using between nine and eleven heating steps (Table 1; Appendix A; Argon data repository). The basalts from Steens Mountain were analyzed in order to better directly compare age results from the entire study area and to determine the intercomparability of data from the NMGRL with previously published data. In addition to standard age spectrum analysis, the basalts from Steens Mountain were also analyzed by a 2-step age spectrum method that reproduces the analytical protocol used by Swisher et al. (1990). A summary of the analytical methods is presented in Table 1 and complete argon isotopic results are compiled in Appendix A. All new age data cited in the text are quoted at 2σ unless specifically noted otherwise. Age and K/Ca spectrum diagrams along with isotopic correlation plots are given in Figures 2 and 3 for the furnace step-heated samples {POSITION FIGURES 2 & 3 AFTER}. These samples yield age spectra with variable levels of complexity and methods for age assignment require some discussion. A variety of criteria for age assignment for step-heating data have circulated throughout the 40Ar/39Ar literature (cf. McDougall and Harrison, 1999). This variety is very evident when working with basaltic groundmass concentrate age spectra and adherence to strict criteria can be both beneficial and detrimental to the geochronological study. The benefits of using a pre-determined set of criteria is that it is straightforward to decide which samples yield reliable eruption ages based on the objective criteria. The disadvantage to this is that the pre-determined criteria may not always be applicable to all data sets in the study and there is no a priori way to know if the set criteria represent a geologically correct statistical model. Also, taking a fairly complex spectrum that happens to yield 3 consecutive steps that contain 50% of the total 39Ar overlapping at 2σ and concluding it has a well-behaved plateau (i.e. Fleck et al., 1977) can alleviate the responsibility to look more closely at the rest of the spectrum that is disturbed. It is possible that this disturbance is also influencing the integrity of the “plateau.” Perhaps all would agree that some sort of statistical test is required to justify an age assignment, but there is considerable disagreement on how to deal with data that violate (at some level) the predetermined statistical model. Isochron data have also been used to evaluate groundmass 40Ar/39Ar data (Singer and Pringle, 1996; Baksi, 1999; Heizler et al., 1999). Singer and Pringle (1996) suggest that isochron data of the chosen plateau steps should yield a 40 36 40 36 Ar/ Ari value within error of the atmospheric Ar/ Ar value of 295.5. Baksi (1999) suggests

11 the same; however he also requires the MSWD (mean square weighted deviate) value of the weighted regression to be less than 2.5. We view both of these criterion as useful approaches towards looking at the age spectrum data, but also recognize that if data chosen for the plateau age are statistically sound (i.e., normally distributed), the isochron data should yield the required atmospheric 40Ar/36Ar initial value (cf. Heizler et al., 1999). Also, choosing a cut-off value of 2.5 to distinguish between an isochron or errorchron might be misleading as the MSWD value needs to be evaluated based on the number of data points (Mahon, 1996). For this study we are fortunate that many of the samples produce age spectra that yield at least 6 consecutive heating steps containing no less than 70% of the total 39Ar released and have MSWD values that fall within the 95% confidence window for n-1 degrees of freedom (Figs. 2 and 3; Table 2). By any reasonable criterion, these groundmass samples can be considered as well behaved, despite having some discordance for early and late heating steps. Three samples (MB97-24; MB97-40; MB97-2) are more complex, but are still interpreted to yield useful data for determining eruption ages. MB97-2 reveals a significant age gradient that presumably records argon loss (Fig. 2b). This spectrum begins at about 5 Ma and climbs to about 16 Ma with the final 5 steps defining a normal distribution of ages with a weighted mean of 15.17 ± 0.36 Ma and an MSWD of 1.90. However, these five steps only contain about 34% of the total 39Ar released and we are not confident that the final heating steps have not also been disturbed by argon loss and therefore suggest that the assigned age represents a minimum age of eruption. Samples MB97-24 and MB97-40 yield very similar age spectra with climbing initial steps that reach a maximum before decreasing for the higher temperature steps (Figs. 2c, h). In both cases, combining steps D-I yield MSWD values of approximately 4.4 and fall outside the 95% confidence window. This suggests that these steps do not record a normal distribution of apparent ages and warrant some caution when considering these dates as representing eruption ages. In the case of MB97-24, steps D-I have a weighted mean age of 16.06 ± 0.36 Ma with step H being the cause of the high MSWD (Fig. 2c; Appendix A; Argon data repository). It is possible that step H is simply a random statistical outlier that could be tested by additional analyses, or the discordance could be related to geological affects (argon loss) or irradiation- induced artifacts (39Ar recoil). One alternative way to assign an age to this sample is to combine steps D-G to obtain a weighted mean age of 16.15 ± 0.30 Ma and MSWD of 2.72. This method uses greater than 50% of the 39Ar released and produces an MSWD that falls within the 95%

12 confidence window for 3 degrees of freedom. Additionally, steps E-G yield a weighted mean of 16.20 ± 0.20 Ma and an MSWD of 1.09. These data are clearly a population, but only comprise 45.8% of the total 39Ar. Because the 50% rule for the plateau segment is not up held, workers such as Baksi (1999) would define this as a marginal plateau. Similar combinations of steps can be used to evaluate an age for MB97-40. For instance, the final 4 heating steps that contain about 48% of the total 39Ar released yield an age of 15.21 ± 0.20 Ma and an MSWD of 1.04. We choose to combine steps D-I that yield an age of 15.51 ± 0.28 Ma in the recognition that this discordance for this part of the spectrum could be caused by 39Ar recoil redistribution. The main point of this discussion of age assignment is that there is no accepted (and perhaps there should not be) method to calculate eruption ages from somewhat disturbed spectra. It is however, important to evaluate different methods to see if these different methods might lead to age determinations that influence a geological interpretation. In the above three examples (MB97-24; MB97-40; MB97-2) the main observation is that the spectra seem to indicate argon loss and perhaps any age determined from these samples should be considered as minimum values for basalt eruption. The remaining six Oregon Plateau samples are less disturbed. Two of the samples (MB97-65 groundmass and MB97-32 plagioclase) are flat over 100% of the 39Ar released and presumably record accurate eruption ages (Fig. 2e, f). The final samples generally yield initial age gradients indicative of minor argon loss that are followed by significant plateau segments (Fig. 2a, c, d, g). The initial disturbance of the spectra is probably related to argon loss; however we suggest that the plateau segments are recording accurate eruption ages. The age spectrum for MB97-55 is slightly different compared to the other Oregon Plateau basalts in that it has some relatively old steps before decreasing to a well-defined flat segment (16.58 ± 0.18 Ma; MSWD = 1.62) for the remaining four heating steps (Fig. 2j). This pattern could result from 39Ar recoil and/or excess argon; however despite this complexity we suggest that the assigned age records an accurate eruption age. The isotope correlation data from the Oregon Plateau basalts are consistent with the ages provided by the age spectra. In no case is the isochron age significantly different from the assigned plateau ages; however there are a couple of cases where initial trapped 40Ar/36Ar components differ slightly from atmosphere (e.g. Fig. 2cc, hh, ii, jj). In most of these cases the MSWD values are slightly elevated and perhaps are recording regression of heating steps with

13 non-isochronous ages (i.e., age gradients) or are recording anomalous behavior related to 39Ar recoil. The two samples from Steens Mountain (CH82-22B and CH82-22G) are overall highly radiogenic and yield precise individual apparent ages (Fig. 3a, c; Appendix A; Argon data repository). Their spectra are somewhat different compared to the samples discussed above as they exhibit a slight age decrease across the spectrum with the final 10 to 20% of the 39Ar released yielding steps with a more dramatic age drop. Especially in the case of CH82-22G, the spectrum looks like a classic 39Ar recoil pattern with 39Ar being displaced from relatively high K locations into low K locations that degas at high temperature (Fig. 3c). Both spectra record plateau segments with apparent ages of about 16.56 Ma, however due to recoil redistribution we argue that the total gas ages of about 16.36 ± 0.10 Ma for CH82-22B and 16.38 ± 0.20 Ma for CH82-22G are more accurate eruptions ages. Due to data clustering, the isochron data for these samples do not significantly enhance age assignment (Fig. 3b, d). At the 2-sigma confidence level the isochron ages recorded by the plateau steps are not distinguishable from the plateau ages, but may not be meaningful because of probable recoil issues. There are large errors on the initial trapped 40Ar/36Ar components due to the low number of heating steps and due to the high and fairly constant radiogenic yields of individual steps used to define the linear regressions. For these reasons we do not apply great significance to the trapped component values, but recognize that the apparent value of 210 ± 60 (2σ) for CH82-22B is analytically less than the atmospheric value of 295.5, further suggesting possible recoil problems. Eight and nine aliquots of CH82-22B and CH82-22G, respectively were step-heated in two or three increments (Figs. 4, 5) {POSITION FIGURES 4 & 5 AFTER}. These are the identical samples analyzed by Swisher et al. (1990) and we attempted to reproduce their results. Swisher et al. (1990) combined the “B” steps of the two-step spectra to obtain a precise weighted mean age for each sample. This method was utilized to remove the relatively non-radiogenic, disturbed low temperature gas prior to sample fusion and thus attainment of a more accurate age compared to simple total fusion analyses. Our analyses are very similar to those of Swisher et al. (1990) as the first heating step is relatively non radiogenic followed by a fusion step that is typically about 90% radiogenic (Figs. 4, 5). The B-steps for CH82-22B yielded a weighted mean age of 16.48 ± 0.06 Ma (MSWD = 3.05). Eight of the nine B-steps from CH82-22G yield a well-defined (MSWD=1.22) weighted mean age of 16.46 ± 0.06 Ma. Run number 55006-22 for

14 CH82-22G yields an anomalously old apparent age of 17.27 ± 0.26 Ma and also has a low radiogenic yield of only 43.9% (Appendix A; Argon data repository). Perhaps this aliquot contains a phenocryst with excess argon or has an incompletely degassed xenocryst that is resulting in an inaccurate result. The 16.46 Ma B-step fall between the plateau ages (~16.56 Ma) and the total gas ages (~16.36 Ma) obtained from the higher resolution spectra (Fig. 3) and are consistent with the low-resolution homogenization of the overall age variations observed in the higher resolution age spectrum runs.

Eruption ages and normalization of published results For all but sample MB97-2 of the Oregon Plateau lava flows, the assigned plateau or preferred ages are interpreted to record accurate eruption ages. The high degree of argon loss evident in MB97-2 (Fig. 2b) requires a cautious interpretation and the assigned age of 15.17 ± 0.36 Ma perhaps represents a minimum age for eruption. In all cases the eruption ages are consistent with stratigraphic relationships (see next section) attesting to the robustness of the data set. Regional comparisons for the basalt geochronology reported by other workers are hampered somewhat by difficultly in normalizing all data to a common standard. Below we present data from the area that is compiled from a variety of publications (Table 3). This compilation shows that at least 5 different fluence monitors were used with only some having well determined intercalibration values (e.g. Renne et al., 1998). We have attempted to normalize all data to Fish Canyon (FC) tuff sanidine at 28.02 Ma such that discussion of the ages can be done from a consistent platform. Recall that we have also analyzed two samples from Steens Mountain so that our data is more readily compared to previous work by Swisher et al. (1990), Baksi and Farrar (1990), and Baksi et al. (1991). All of these studies suggest that a significant number of the Steens Mountain lavas were erupted over a short interval and/or that the N/R paleomagnetic transition at Steens Mountain was established within the resolution of the ages below and above the transition. The relative duration (<0.1 Ma) of the reversal is agreed upon by all datasets, however the reported age for it varies by about 0.5 Ma. Swisher et al. (1990) dated the top flow and the 31st down from the top (both stratigraphically higher than the N/R reversal) and obtained ages of 16.583 ± 0.048 Ma and 16.589 ± 0.022 Ma, respectively. Baksi and Farrar (1990) report plateau ages of 16.06 ± 0.10 Ma and 16.08 ± 0.12 Ma for

15 transitional basalts at Steens Mountain. Baksi et al. (1991) dated 11 samples (2 above, 2 below and 7 within the transition zone, all within the upper 450 m of the ~900 to 1000 m section; Mankinen, Pers. Comm.) and found no resolvable age difference and report an age for all samples at 16.2 ± 0.1 Ma. Each of these studies used a different fluence monitor that is likely contributing to some of the apparent discrepancies (Table 3). Our Steens Mountain samples (CH82-22G, CH82-22B) are identical to those analyzed by Swisher et al. (1990) with sample preparation also being identical (Table 1). Our preferred eruption ages are 16.38 ± 0.20 Ma for CH82-22G (top flow) and 16.36 ± 0.10 Ma for CH82-22B (31st down from top). Like previous studies, we cannot resolve an age difference between individual flows, but these ages are younger than those reported by Swisher et al. (1990). Considering that they used a basalt flux monitor standard that was internally calibrated to FC sanidine at 27.84 Ma (Swisher, Pers. Comm.) their ca. 16.59 Ma reported ages become 16.70 Ma and are about 0.35 Ma older than our preferred eruption age. The discrepancy is only about 0.25 Ma if we compare our “B-Step” weighted mean age to their result, but their data are still systematically older than ours. Because the Swisher et al. (1990) data only appear in an abstract and were monitored with a basalt flux monitor with uncertain intercalibration with Fish Canyon sanidine we suggest that the Swisher et al. (1990) age be abandoned as a robust absolute age for the Steens Basalt exposed at Steens Mountain. Their highly precise results are still very useful for documenting the relative duration of eruptions at Steens Mountain. The other pertinent data from Steens Mountain come from Baksi and Farrar (1990) and Baksi et al. (1991). The Baksi and Farrar (1990) data of 16.06 ± 0.10 Ma and 16.08 ± 0.12 Ma combine to yield a weighted mean of 16.07 ± 0.08 Ma. This age is relative to SB-3 biotite at 162.9 Ma which is intercalibrated with FC sanidine at 27.57 Ma (Lanphere and Baadsgaard, 2001). Therefore the Baksi and Farrar (1990) age of 16.07 ± 0.08 Ma relative to FC at 28.02 Ma becomes 16.33 ± 0.08 Ma and is essentially identical to our preferred eruption age. The 16.2 ± 0.1 Ma result of Baksi et al. (1991) was reported relative to Hbgr3 hornblende with an assigned age of 1071 Ma. Personal communication with Chris Hall at the University of Michigan reports that Hbgr3 intercalibrates well with Mmhb-1 hornblende at 520.4 Ma, the latter which has been intercalibrated by Deino and Potts (1990) with FC sanidine at 27.84 Ma. Thus, the 16.2 ± 0.1 Ma result of Baksi et al. (1991) becomes 16.30 ± 0.10 Ma relative to FC at 28.02 Ma. Therefore

16 our new result for Steens Mountain eruptions are very compatible with both results of Baksi and Farrar (1990) and Baksi et al. (1991) all of which indicate an eruption age of about 16.35 Ma. We combine our results from Steens Mountain, with the recalculated values of Baksi and Farrar (1990) and Baksi et al. (1991) to obtain a weighted mean age of 16.35 ± 0.07 Ma (2σ) and suggest that this age is the best estimate for the upper 450 m of the Steens Mountain type section. We recognize that we have not attempted to propagate the errors associated with the intercalibration of the fluence monitors. However, as reported by Renne et al. (1998) at least some standards can be intercalibrated at a precision of about 0.1% and thus the typical measurement errors for the ages reported here of about 0.5 to 1% seem to outweigh errors associated with recalculation of ages to a common standard value.

Stratigraphic Results Flood basalt lava flows crop out throughout the southern Oregon Plateau, but are best exposed in the vicinity of Steens Mountain (Figs. 1a and b). Steens Mountain and its southern extension the Pueblo Mountains, delineate an ~130 km north-northeast trending Cenozoic fault scarp. In contrast to CRBG lava flows, Steens Basalt and “Steens-Type” lava flows are thinner (~1 to 12 m versus ~50+ m thick) and typically show little to no columnar joint development. Steens Basalt lava flows are also commonly compound, consisting internally of flow lobes with rubbly and brecciated flow surfaces. These lava flows vary petrographically from aphyric flows and flow packages to extremely plagioclase-phyric (dominated by ~1 to 4 cm plagioclase crystals) flow packages and also include sparsely plagioclase phyric textured lava flows (hereafter termed intermediate-plagioclase), independent of stratigraphic position or geographic location. While interflow sediments are sparse, they do exist within packages of lava flows investigated in this study. In some cases the interflow horizons have been interpreted as regionally exposed tuffs by previous workers and have been used to mark the upper stratigraphic boundary of the Steens Basalt and separate it from petrographically similar basaltic lava flows (Sherrod et al., 1989; Johnson, 1995). While useful in field mapping, these boundaries are not supported by petrographic or chemical constraints from the lava flows themselves (Brueseke and Hart, 2002). In order to assess the chronological implications of these observations, five new stratigraphic sections (~100 to ~350 m thick exposures) in close proximity to Steens Mountain

17 (Figs. 1b, 6) were investigated in detail {POSITION FIGURE 6 AFTER}. Section locations were chosen to maximize the number of lava flows exposed in continuous stratigraphic context, thereby allowing flow-by-flow description and sampling. In the field, no attempt was made to discern discrete flow packages. However, based on up-section petrographic and chemical variations it is apparent that many of the “individual flows” that were sampled are from discrete flow packages of similar material, that were likely emplaced as inflated pahoehoe lobes. Geographically, two of the sections lie on or within the main Steens Mountain fault scarp ~50 km north of the type section and two additional sections lie less than 15 km across the Alvord Valley from the summit of Steens Mountain.

Folly Farm East The Folly Farm East section lies in the northwest quarter of section 4 of the Johnny Creek NW quadrangle and extends into the Folly Farm quadrangle (T30S, R37E). At this location, ~271 m of section is exposed and 30 individual lava flows were sampled (Fig. 6). While all three Steens Basalt petrographic types are present (aphyric, intermediate-plagioclase, plagioclase-phyric), this section is dominated by intermediate-plagioclase bearing lava flows. Additionally, it appears that most of these lava flows were emplaced as lobes, capped by ~3 to 12 m of aphyric lava flows. The intermediate-plagioclase and aphyric package is repeated three times and may represent the eruption of a stratified magmatic system, similar to what is present throughout the Steens Basalt type section (Gunn and Watkins, 1970; Stewart, 1992). Approximately 58 m of plagioclase-phyric lava flows occur near the top of the section and comprise the thickest flow unit exposed at this location. In addition to the aphyric lava flows interspersed throughout the section, two zones of brecciated and blocky lava flows are exposed near its base. The basal lava flow exposed at this location yields a 40Ar/39Ar age of 16.31 ± 0.20 (Fig. 6; Table 2). Additionally, unpublished paleomagnetic data from this section indicate that the entire package of lava flows erupted during a normal geomagnetic interval (Smith, 1987).

Folly Farm West The Folly Farm West section lies in sections 14 and 23 of the Lambing Canyon quadrangle (T29S, R36E). Here, 22 lava flows are present and each was sampled (Fig. 6). Along with the lava flows/flow units, three tuffaceous/sedimentary horizons are present within

18 this package. Conformably overlying the uppermost exposed basalt flow are four ~3 to 6 m thick aphyric, tholeiitic andesite lava flows. The second andesite flow down section yields a minimum 40Ar/39Ar age of 15.17 ± 0.36 Ma. The remaining 300 m of the ~345 m section is composed of aphyric, intermediate-plagioclase, and plagioclase-phyric textured mafic lava flows that can be grouped into five flow units based on petrography and chemical variations. The basal lava flow from the lowermost unit yields a 40Ar/39Ar age of 16.06 ± 0.36 Ma. Approximatly 27 and 67 m above the lowest exposed lava flow lie two thin tuffaceous horizons, both less than 1 m thick. Additionally, a third, thinner tuffaceous horizon crops out ~80 m from the top of the flood basalt section within a package of intermediate-plagioclase textured lava flows. The lowermost exposed tuffaceous horizon may be correlative with the ~16.1 Ma Tuff of Oregon Canyon, sourced in the nearby McDermitt volcanic field (Rytuba and McKee, 1984).

Route 78 North The Route 78 North section lies in the northwest quarter of section 30 of the Folly Farm quadrangle (T28S, R37E). Fourteen lava flows are present and all were sampled in ~105 m of section exposed in a fault block within the Brothers fault zone (Fig. 6). This section is dominated by plagioclase-phyric lava flows in its lower half. Plagioclase separates extracted from a lava flow exposed approximately in the middle of the section yield a 40Ar/39Ar age of 16.03 ± 0.30 Ma. Five intermediate-plagioclase to aphyric textured lava flows are exposed in the upper ~50 m of section. The uppermost exposed lava flow of this group and the section yields a 40Ar/39Ar age of 16.24 ± 0.50 Ma, constraining the age of this section to <16.3 Ma.

Miranda Flat This section lies directly across Miranda Flat from the main Steens Mountain fault scarp and is in the western half of section 4 of the Miranda Flat quadrangle (T33S, R35E). Twenty- four lava flows were sampled in succession from 104 m of section (Fig. 6). Similar to the upper portion of the Mickey Butte section (see below), lava flows exposed here are much thinner (~5 m average) than at other sampled sections. Exposed lava flows range from aphyric to plagioclase- phyric in texture and the upper half of the section is composed of two ~30 m packages that grade up-section from plagioclase-phyric, to intermediate-plagioclase, to aphyric lava flows. At this

19 location, the uppermost lava flow yields a 40Ar/39Ar age of 16.40 ± 0.18 Ma, while the second lava flow from the base of the section yields a 40Ar/39Ar age of 16.33 ± 0.20 Ma.

Mickey Butte This section lies along the western flank of Mickey Basin, a northern extension of the Alvord Valley in the southeast quarter of section one of the Mickey Springs quadrangle (R35E, T33S). From the valley floor to the summit of Mickey Butte is ~608 m of section, of which, ~365 m was investigated (Fig. 6). Within this section 19 of 46 lava flows were sampled. Like the other sections, petrographic type was variable, though intermediate-plagioclase lava flows dominate the upper portion of the sampled section. When variations do occur, they appear to represent packages of petrographically similar flows, but do not exhibit the apparent cyclicty of petrographic types present at Folly Farm East or Miranda Flat. Individual lava flows range from ~1 to ~24 m in thickness and the upper portion of this section is dominated by thin (~ <5 m average) sheet-like lava flows that are interpreted as near-vent facies. Two features set Mickey Butte apart from the other sections: 1) the presence of local dikes and 2) a distinct difference in eruptive age between the upper and lower portions of the section. Three dikes are exposed at the Mickey Butte section and their average trend is N13ºE. The best exposed dike cuts through ~250 m of section and appears to feed lava flows in the upper, unsampled, portion of the section. The other dikes cut through the basal portion of the section and are all petrographically similar to Steens Basalt lava flows. Three lava flows from this section were selected for 40Ar/39Ar analyses. Two of these lava flows, exposed near the bottom and upper portion of the package yield similar ages of 16.58 ± 0.18 Ma and 16.48 ± 0.30 Ma, respectively. The lava flow directly underlying the uppermost sampled flow yields a 40Ar/39Ar age of 15.51 ± 0.28 Ma. Hook (1981) documented a normal over reverse paleomagnetic polarity change at this location and its approximate location within the Mickey Butte section is shown in Figure 6. Based on the new 40Ar/39Ar ages, this polarity change is an older reversal than what is present within the upper portion of the Steens Basalt at Steens Mountain. All of these data combined appear to indicate the presence of at least a local magmatic hiatus near the top of the sampled section and indicate that the Mickey Butte region was characterized by local eruptive activity.

20 Steens Mountain In comparison to the newly sampled sections, the Steens Basalt type section at Steens Mountain, is much thicker (~900 to 1000 m) and exposes at least 75 to 100 lava flows (Gunn and Watkins, 1970; Johnson et al., 1998). Exposed lava flows are petrographically similar to flows sampled in this study and range from aphyric to plagioclase-phyric varieties (Gunn and Watkins, 1970). These lava flows also vary petrographically throughout the section and at the type section, meter scale flow packages of extremely plagioclase-phyric lava flows are often interbedded with aphyric varieties (Gunn and Watkins, 1970; Mankinen et al., 1985). The age results of these Steens Mountain basalts is discussed above and it is here suggested that the upper ~450 m of the section falls within a tight age range of 16.35 ± 0.07 Ma. The fact that only the top ~450 m of the section is well dated leaves open the possibility that older lava flows are preserved lower in the section, possibly akin to those exposed at Mickey Butte. In summary, the presence of dikes and the petrographic differences that reveal the intra- section unconformity at Mickey Butte, help demonstrate that flood basalt volcanism in this region was complex. Furthermore, the distal facies of regional silicic pyroclastic units and thin interflow sedimentary zones found at Folly Farm West separate dissimilar geochemical and petrographic flow packages. These data combined with the new 40Ar/39Ar ages demonstrate that periods of volcanic inactivity were present over at least an ~1 Ma duration, as local volcanism stopped and then resumed at different eruptive loci. We feel that the presence of multiple eruptive centers and irregular paleotopography created by pre- and syn-volcanic extension has resulted in extensive interfingering of these lava flows. The geologic data from these sampled stratigraphic sections convincingly reveals that flood basalt volcanism in the vicinity of Steens Mountain was much more complex and occurred over a longer period of time than typically quoted for the Steens Basalt type section.

Chemostratigraphy of Oregon Plateau Flood Basalts The chemical diversity of Oregon Plateau flood basalts from locations peripheral to Steens Mountain has received little attention, even though the geochemical diversity of Steens Basalt lava flows has been well documented at their Steens Mountain type section (Gunn and Watkins, 1970; Carlson and Hart, 1983; 1987; Johnson et al., 1998). Oregon Plateau and Steens Basalt lava flows are dominantly basaltic, but also may be more chemically evolved and range in

21 composition from ~48 to ~56 wt. % SiO2 (Fig. 7a; Table 4; Appendix B; Chemistry)

{POSITION FIGURE 7 AFTER}. Camp et al. (2003) used lower TiO2/P2O5 ratios and other chemical criteria (e.g. lower average Sr and Ba values) to differentiate Steens Basalt lava flows from a group of chemically similar mid-Miocene tholeiitic lava flows they called Venator Ranch basalts. Figure 7b illustrates the TiO2/P2O5 variation of Oregon Plateau lava flows from this study and Steens Basalt lava flows from Steens Mountain. It is apparent that while most of the

lava flows sampled in this study have TiO2/P2O5 values similar to the Steens Basalt lava flows from Steens Mountain, a few have lower values more similar to Venator Ranch lava flows. However, these lava flows have much higher Sr (623 to 730 ppm) and Ba (753 to 1023 ppm) values than Venator Ranch lava flows. Because these “Venator Ranch” like lava flows are only found at the Miranda Flat section, they may reflect the presence of a localized magmatic system. At a regional level, major and trace element concentrations of Steens Basalt are consistent with their derivation via fractional crystallization of more primitive magmas, coupled with open system processes (e.g. crustal assimilation; Carlson and Hart, 1987; Hart et al., 1989). Hart et al. (1989) studied ~17 to ~16 Ma flood basalts exposed in the Pueblo Mountains (the southern continuation of Steens Mountain) and demonstrated that the same isotopic and geochemical variations found at the Steens Basalt type section are found in an ~1 km section of flood basalt lava flows to the south and can be attributed to assimilation-fractional crystallization (AFC) processes. These identical variations (increasing incompatible element abundance and more radiogenic 87Sr/86Sr isotopic ratios that correspond to increased stratigraphic level) demonstrate that Steens Basalt crops out to the south, at least to the Nevada border. Similar up- section chemical variations led Binger (1997), Johnson et al. (1998), and Camp et al. (2003) to divide the Steens Basalt at Steens Mountain into “upper” and “lower” Steens Basalt. While up section chemical and isotopic variations are virtually identical between the Steens Basalt at Steens Mountain and the Steens lava flows in the Pueblo Mountains, the presence of abundant dikes in close association with the Pueblo flood basalts suggest that many of the exposed lava flows in this region were erupted locally (Hart et al., 1989). Recent paleomagnetic work (Jarboe et al., 2003) reveals a N/R polarity reversal in the Pueblo Mountain section, supporting the chemical and age data of Hart et al. (1989) that links this package of Steens Basalt to the lava flows exposed at Steens Mountain.

22 In the following discussion we present data that verifies the chemical correlation between Steens Basalt and ~16 ± 1 Ma flood basalts from this study. For clarity, these regional ~16 ± 1 Ma flood basalts will be termed Oregon Plateau flood basalts throughout the remainder of this section. Additionally, for geochemical comparison, we have also divided Steens Basalt lava flows from the type section into the “upper” and “lower” Steens Basalt of previous workers. Without discussing each section in detail, Figures 8 and 9 illustrate the broad chemical variation of Oregon Plateau flood basalts and the Steens Basalt exposed at Steens Mountain. It is also clear that within-section chemical variability of Oregon Plateau flood basalts is common at all locations (Fig. 8) {POSITION FIGURE 8 AFTER}. In addition, while less evolved compositions are present within the lower ~400 m of lava flows exposed at Steens Mountain (Fig. 9; e.g. higher Ni, lower Zr and other incompatible trace element concentrations, lower

average SiO2), chemically similar lavas flows are occasionally exposed in the upper portion of the section {POSITION FIGURE 9 AFTER}. Additionally, similar, less evolved lava flows are exposed at all of the newly sampled locations (Fig. 8). For example, at the Folly Farm East location, lava flows with “lower Steens” chemical signatures are found at different intervals throughout the <16.35 Ma section. These “lower Steens” lava flows are found in each sampled section, regardless of age or stratigraphic position. Conversely, lava flows with “upper Steens” chemical signatures are also found in each of the newly sampled sections. Similar within section variations of interbedded “lower” and “upper” Steens lava flows are also noted by Binger (1997) and Camp et al. (2003), at the northern edge of the Oregon Plateau. These chemostratigraphic variations (Fig. 8) demonstrate that the postulated chemical disparity between “upper” and “lower” Steens chemical types is not a function of age or stratigraphic position. Between section correlations of individual lava flows from this study and Steens Basalt from Steens Mountain based on petrographic and bulk chemical characteristics was attempted through a number of statistical means, but in all cases, proved unsuccessful (Brueseke and Hart, 1999). We believe that this is not due to the inherent geochemical heterogeneity within individual lava flows, but rather is a direct result of the volcanic systems that were active in this region. For example at the Mickey Butte location, the substantial age difference near the top of the section corresponds with significant major and trace element chemical differences (Fig. 8). These variations at Mickey Butte and the other investigated locations, likely reflect the presence of multiple, localized eruptive systems, active during the entire period of Oregon Plateau flood basalt volcanism.

23 Throughout this duration, these vents erupted lava flows and flow units with chemical and petrographic variations similar to those exposed and erupted at Steens Mountain, over a much broader timescale. Essentially, at any given time during the duration of this flood basalt event, any chemical or lithologic type of Steens Basalt was being erupted from numerous, scattered eruptive loci. The chemical similarities between Steens Basalt exposed at Steens Mountain and other Oregon Plateau flood basalts are best illustrated in Figure 10 {POSITION FIGURE 10 AFTER}. It is apparent from these diagrams that unlike flood basalt lava flows from the Malheur Gorge-region to the north (Binger, 1997; Hooper et al., 2002; Camp et al., 2003), those exposed in the vicinity of Steens Mountain are less distinguishable chemically. That is, while the individual Malheur Gorge-region units define discrete chemical groups and, in some cases, chemically overlap with Steens Basalt, Oregon Plateau flood basalts do not define “tight” chemical groups based on age or stratigraphic position. On the contrary, Oregon Plateau flood basalts define a larger, more “diffuse” chemical group. These fundamental similarities between

Oregon Plateau flood basalts are most evident on diagrams of SiO2 vs. Zr and Rb. While some “lower” Steens lava flows from Steens Mountain are less evolved and Oregon Plateau flood basalts tend to be more similar to “upper” Steens lava flows, (Figs. 8, 9, and 10), Oregon Plateau flood basalts and the Steens Basalt type section define the same chemical group. Unlike observations for the Columbia Plateau and the Malheur Gorge-region (e.g. Hooper, 2000; Camp et al., 2003), Figure 10 demonstrates that using chemical characteristics to differentiate among flood basalt lava flows from the Oregon Plateau is difficult. This approach does not work for identifying “sub-groups” of the Steens Basalt. As a result, the entire duration of Oregon Plateau flood basalt volcanism must be treated in a chronostratigraphic, time-transgressive context while being compared with the contemporaneous CRBG and Malheur Gorge-region basalts. This supports earlier observations by Hart and Carlson (1985) that regional, mid-Miocene tholeiitic mafic lava flows (their ~17 to ~14 Ma “Steens-Type” Basalts) from throughout the Oregon Plateau are chemically equivalent to lava flows exposed at Steens Mountain and should be considered as Steens Basalt.

24

Regional implications and significance To reiterate, the nine new 40Ar/39Ar ages obtained on Steens Basalt lava flows range from 15.51 ± 0.28 Ma to 16.58 ± 0.18 Ma and the two lava flows originally dated by Swisher et al. (1990) from the type Steens section yield new, essentially identical, ages of 16.38 ± 0.20 Ma (uppermost) and 16.36 ± 0.10 Ma (31st down). Conservatively, these data define a 1.05 ± 0.46 Ma duration of local (to Steens Mountain) Steens flood basalt volcanism. As evident in Table 3, the time period over which much of the tholeiitic Malheur Gorge-region basalt was produced in east-central Oregon (Hooper et al., 2002) overlaps with Steens Basalt volcanism. Other age data that are included in this table are from older and possibly less precise K-Ar age determinations, as well as more recent 40Ar/39Ar data from chemically similar mafic lava flows present in the Owyhee Mountains and Santa Rosa-Calico volcanic field of northern Nevada (Shoemaker and Hart, 2004; Brueseke et al., 2003). Lees (1994) completed a study linking the flood basalts of the Malheur Gorge-region with the Imnaha and Grande Ronde basalts of the CRBG and further connected the plagioclase- phyric and aphyric “lithotypes” found within the Malheur Gorge-region basalts to the dominantly plagioclase-phyric Imnaha and dominantly aphyric Grande Ronde Basalts. Following Lees (1994), Binger (1997) documented the geochemical and petrographic similarities between the Imnaha and Grand Ronde basalts, the Malheur Gorge-region basalts, and the “lower” Steens Basalt exposed at Steens Mountain. Binger (1997) also noted that while the “upper” Steens Basalt is chemically evolved like the Birch Creek/Hunter Creek Basalts (Malheur Gorge-region) and the Grande Ronde Basalt, it is in fact, dissimilar chemically. These observations were further discussed by Hooper et al. (2002). Furthermore, Camp et al. (2003) suggested that the areal/volumetric extent of “upper” Steens Basalt decreases to the north of Steens Mountain, while lava flows with the “lower” Steens Basalt chemical affinity continue northward and physically link with the lower Pole Creek lava flows of the Malheur Gorge-region. 40Ar/39Ar ages from these Malheur Gorge-region lava flows (Table 3; Hooper et al., 2002) are temporally indistinguishable and could be interpreted to suggest that in the Malheur Gorge-region at any given time during the flood basalt event, lava flows of any “lithotype” (plagioclase-phyric to aphyric) or chemotype were being erupted. Lees (1994) suggested that these differing “lithotypes” might be the eruptive products of different stages of magma chamber development

25 in multiple eruptive chambers throughout the Malheur Gorge-region over an ~4 Ma duration (~18.4 to ~14.5 Ma; Lees, 1994). This observation is very similar to what we observe in the vicinity of Steens Mountain, where multiple lithotypes of Steens Basalt crop out in stratigraphic sections independent of eruptive age. New 40Ar/39Ar ages obtained on mafic lava flows and shallow intrusive bodies that are chemically identical to Steens Basalt from the southern Oregon Plateau/northern Nevada rift transition (i.e. Santa Rosa-Calico volcanic field) document the occurrence of local Oregon Plateau mafic (i.e. Steens Basalt) volcanism from 16.73 ± 0.04 to 14.32 ± 0.40 Ma (standardized to 28.02 Ma Fish Canyon tuff), contemporaneous with and/or just preceding flood basalt volcanism on the Columbia Plateau (Table 3, Brueseke and Hart, 2003; Brueseke et al., 2003). Based on these observations and previously documented eruptions of similar aged material, it is evident that tholeiitic volcanism was occurring throughout the Oregon Plateau during the ~17 to ~14 Ma time period (Hart and Mertzman, 1982; Hart and Carlson, 1985; Carlson and Hart, 1987; Lees, 1994; Hooper et al., 2002). The chemical and geochronologic data presented in this study document that Oregon Plateau tholeiitic flood basalt volcanism (i.e. Steens Basalt) is more temporally extensive than often recognized. These data and recent regional studies suggest that the main episode of Steens Basalt volcanism occurred for an ~1.05 ± 0.46 Ma duration and that across the Oregon Plateau, less voluminous Steens volcanism occurred locally for a greater duration (~2.4 Ma). As a result, we suggest that Oregon Plateau flood basalt volcanism (the Steens Basalt) is the lateral and temporal equivalent of the entire Malheur Gorge suite, as well as the CRBG (Fig. 11) {POSITION FIGURE 11 AFTER}. Furthermore, the time-transgressive nature of Steens volcanism indicates that the N/R magnetic reversal present within the upper portion of the Steens Mountain section is not the only reversal present in regionally exposed packages of Steens Basalt lava flows and may not be correlative to the earliest paleomagnetic

transition (N0/R0) used by workers to correlate packages of lower CRBG lava flows (Fig. 11). In summary, the same large-scale mantle processes controlling the generation and eruption of the CRBG on the Columbia River Plateau were similarly affecting the Oregon Plateau during the mid-Miocene, not just at ~17 Ma, when Oregon Plateau flood basalt volcanism may have initiated. However, the data presented in this study do not support the hypothesis that Oregon Plateau flood basalt volcanism became rapidly younger to the north-northwest, the consequence of a migrating melt anomaly related to a spreading mantle plume head (Camp and Ross, 2004).

26 Discussion and Conclusions New field observations, chemical analyses, and 40Ar/39Ar ages, combined with prior geochronology help to better document the duration of Oregon Plateau flood basalt volcanism. Coeval with the entire duration of CRBG volcanism, Steens Basalt volcanism was intimately linked to both regional and local tectonomagmatic processes. Regionally, the Oregon Plateau has experienced greater extension in the last 20 Ma than the Columbia Plateau and likely experienced more widespread, but focused (at specific locations like the northern Nevada rift) extensional tectonism during the period of flood basalt volcanism (Carlson and Hart, 1987; Wells and Heller, 1988; Hooper and Conrey, 1989; Zoback et al., 1994; Cummings et al., 2000). This extension also controlled the availability of upwelling Steens Basalt magma and led to the widespread areal distribution of Steens Basalt eruptive loci across the southeastern Oregon Plateau. During the mid-Miocene flood basalt event, the reactivation of regional lithospheric structural heterogeneities throughout the Oregon Plateau likely helped focus the upwelling mafic magma at locations that were actively extending and also helped localize the distribution of compositionally diverse (basalt through rhyolite) volcanism present at some of these locations (John and Wallace, 2000; Brueseke and Hart, 2001; Brueseke and Hart, 2004). While the upper portion of the type section at Steens Mountain only exposes ~16.35 Ma Steens Basalt lava flows, this flood basalt episode was much more widespread temporally and geographically. The widespread locations of the other eruptive loci besides Steens Mountain (Fig. 1b), lack of correlation between Steens Basalt sections on a flow-by-flow basis, and presence of multiple flow units within Steens Basalt stratigraphic sections, reveals that during the span of Steens Basalt volcanism, numerous eruptive centers were active in the region. Throughout the southern Oregon Plateau, eruptive loci of Steens Basalt are found in the Owyhee Mountains (ID/OR), at Catlow Peak (OR), at Orevada View (OR), and near the Jordan Craters (OR) volcanic field (Mankinen et al., 1987; Hart, unpublished data). In addition, ~17 to ~14 Ma mafic lava flows and dikes are found further south within the northern Nevada rift and its transition with the Oregon Plateau (Zoback et al., 1994; Brueseke and Hart, 2001; John and Wallace, 2000; Brueseke and Hart, 2003), to the west in CA (e.g. the ~16 Ma Lovejoy Basalt; Wagner et al., 2000), and ~16.5 Ma mafic lava flows are present to the east in the vicinity of the Jarbidge Mountains (e.g. the Seventy-Six Basalt; Hart and Carlson, 1985; Rahl et al., 2002). Furthermore, the intimate link between the timing of mid-Miocene Oregon Plateau silicic and

27 intermediate volcanic field development (e.g. McDermitt, Lake Owyhee, Northwest Nevada, and Santa Rosa-Calico) and Steens Basalt volcanism implies a petrogenetic connection between basalt production and the development of regional polygenetic volcanic fields and caldera complexes. The availability of upwelling mafic magma, coupled with ongoing extension affected the duration and location of the coeval intermediate through silicic volcanism in these systems. While the most volumetric tholeiitic volcanism in the region may have been present to the north, contemporaneous flood basalt volcanism was occurring on the southern Oregon Plateau during the entire timespan that CRBG lava flows were erupted. The field observations, chemical data, and 40Ar/39Ar geochronology presented in this study agree with this prior observation and accordingly, provide a direct temporal link between eruptions of the Oregon Plateau wide Steens Basalt and other regionally extensive flood basalt volcanism. This direct temporal link requires that petrogenetic and tectonic models of mid-Miocene northwestern U.S. flood basalt volcanism recognize that the northern (Columbia Plateau) and southern (Oregon Plateau) portions of this flood basalt province were erupting simultaneously, and both remained active for the duration of regional flood basalt volcanism.

Acknowledgements We thank John Morton (Miami University) for his assistance with DCP-AES elemental analyses and Claire McKee for her assistance in the field. We also thank Ed Mankinen (U.S.G.S.) for his help with answering detailed questions about the magnetostratigraphic record present within the Steens Basalt at Steens Mountain and the precise stratigraphic locations of previously dated lava flows from the Steens section. National Science Foundation Grant EAR-0106144 (Hart), a Miami University Faculty Research Grant (Hart), and a Geological Society of America Student Research Grant (Brueseke) have provided the necessary financial support for this research.

28 TABLE 1. SUMMARY OF 40AR/39AR METHODS.

Sample preparation and irradiation: Groundmass concentrates obtained by hand-picking 1-2 mm rock fragments free of visible phenocrysts. Samples rinsed in dilute HCl followed by a thorough ultrasonic wash in distilled water. CH82 samples were picked free of coarse plagioclase, reacted with dilute HCl and HF and followed by boiling in distilled water for 1 hour. Plagioclase separate hand-picked from crushed sample, washed in dilute HCl and ultrasonically cleaned in distilled water. Samples were loaded into machined Al discs and irradiated in 3 separate packages in the D-3 position, Nuclear Science Center, College Station, TX. NM-89 and NM-110 were 7 hour irradiations whereas NM-179 was 14 hours. Neutron flux monitor Fish Canyon Tuff sanidine (FC-2). Assigned age = 28.02 Ma (Renne et al., 1998).

Instrumentation: Mass Analyzer Products 215-50 mass spectrometer on line with automated all-metal extraction system. Most samples were step-heated in a double vacuum Mo resistance furnace.

CH82 samples also step-heated using a defocused Synrad 50 W C0 2 laser.

Flux monitor single crystal sanidines fused by a 50 watt Synrad CO2 laser. Furnace analysis: Reactive gases removed during a 7 minute heating with a SAES GP-50 getter operated at ~450°C. Additional cleanup (6 minutes) following heating with 2 SAES GP-50 getters, 1 operated at ~450°C and 1 at 20°C. Gas also exposed to a W filament operated at ~2000°C. Laser step-heating analysis: Samples heated for 0.5-1 minute. Reactive gases removed during heating followed by a 5 minute reaction with 2 SAES GP-50 getters. 1 operated at ~450°C and 1 at 20°C. Gas also exposed to a W filament operated at ~2000°C and a cold finger operated at -140°C.

Analytical parameters: System sensitivities: NM-89 furnace=2.3; NM-110 furnace=3.0; NM-179 furnace=2.88; NM-179 laser=1.65. All values times 10 -16 moles/pA. Total system blank and background: Furnace NM-89 = 130, 1.4, 0.1, 0.9, 0.6 x 10 -17 moles for masses 40, 39, 38, 37, 36, respectively. Total system blank and background: Furnace NM-110 = 1400, 2.6, 1.3, 0.8, 5.6 x 10 -17 moles for masses 40, 39, 38, 37, 36, respectively. Total system blank and background: Furnace NM-179 = 100, 0.7, 0.4, 0.4, 0.5 x 10 -17 moles for masses 40, 39, 38, 37, 36, respectively. Total system blank and background: Laser NM-179 = 52, 1.5, 0.1, 0.1, 0.3 x 10-17 moles for masses 40, 39, 38, 37, 36, respectively.

J-factors determined to a precision of ± 0.1% (1s) by CO2 laser-fusion of 4-6 single crystals from each of 6-10 radial positions around the irradiation tray.

Correction factors for interfering nuclear reactions were determined using K-glass and CaF 2 and are as follows: 40 39 36 37 39 37 NM-89&110: ( Ar/ Ar)K = 0.0±0.0004; ( Ar/ Ar)Ca = 0.000280±0.000005; and ( Ar/ Ar)Ca = 0.00070±0.00002. 40 39 36 37 39 37 NM-179: ( Ar/ Ar)K = 0.0±0.0004; ( Ar/ Ar)Ca = 0.0002878±0.000003; and ( Ar/ Ar)Ca = 0.0006765±0.000005. TABLE 2. SUMMARY OF 40AR/39AR AGE RESULTS.

Sample Section Age Total Steps MSWD Age ± 2σ 39 Location Analysis Steps Used % Ar (Ma) (Ma) MB97-2 FFW plateau 9 5 34.3 1.90 15.17 0.36 MB97-40 MB plateau 9 6 76.3 4.431 15.51 0.28 MB97-32 R78N plateau 11 11 100.0 0.34 16.03 0.30 MB97-24 FFW plateau 9 6 77.0 4.371 16.06 0.36 MB97-25 R78N plateau 9 6 71.0 0.76 16.24 0.50 86-29 FFE plateau 9 6 88.8 1.34 16.31 0.20 MB97-87 MF plateau 9 7 92.9 0.87 16.33 0.20 CH82-22B Steens Mt TGA 9 9 100.0 NA 16.36 0.10 CH82-22G Steens Mt TGA 9 9 100.0 NA 16.38 0.20 MB97-65 MF plateau 9 8 98.1 0.90 16.40 0.18 MB97-47 MB plateau 9 6 84.4 1.67 16.48 0.30 MB97-55 MB plateau 9 4 51.5 1.62 16.58 0.18 Section location abbreviations are as follows: FFE, Folly Farm East; FFW, Folly Farm West; R78N, Route 78 North; MF, Miranda Flat; MB, Mickey Butte; NA, Not Applicable. Samples selected for geochronology were analyzed at the New Mexico Geochronology Research Laboratory via the incremental heating age spectrum method (see Data repository for detailed information). Groundmass concentrates were used for each analysis with the exception of the coarsely plagioclase-phyric sample, MB97-32, where plagioclase was analyzed. All of the new plateau ages are interpreted to represent eruptive ages. The ages listed are calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory standard at 28.02 Ma and errors include 0.2% (2σ) error in J. TGA is total gas age and represents the preferred eruption age of the Steens Mt. samples. 1MSWD outside of 95% confidence window for n-1 degree of freedom.

30 TABLE 3. SUMMARY OF NEW AND PUBLISHED OREGON PLATEAU FLOOD BASALT RADIOMETERIC AGES.

Location Unit Range or weighted mean age TypeFluence Monitor Ages relative to (Ma ± 2σ) (K-Ar or 40Ar/39Ar)and Age FC at 28.02 Ma (±2σ) Regional OP 1 Oregon Plateau basalt 17 ± 1 to 14 ± 1 K-Ar Pueblo Mts. 2 Oregon Plateau basalt 17.0 ± 0.3 to 16.0 ± 0.4 K-Ar Steens Mountain 3 Steens Basalt 16.58 ± 0.10 to 16.59 ± 0.04 40Ar/39ArBCR-2 basalt, 15.6 Ma11 16.70 ± 0.0410 Steens Mountain 4 Steens Basalt 16.20 ± 0.20 40Ar/39Ar Hbgr3 hornblende, 1071 Ma 16.30 ± 0.20 Steens Mountain 5 Steens Basalt 16.06 ± 0.20 to 16.08 ± 0.24 40Ar/39ArSB-3 biot, 162.9 Ma 16.33 ± 0.1610 Malheur Gorge 6 Hunter Creek Basalt 15.8 ± 0.6 40Ar/39Ar GA1550 biot, 98.8 Ma 15.8 ± 0.6 Malheur Gorge 6 Birch Creek Basalt 15.9 ± 0.2 to 14.4 ± 0.8 40Ar/39Ar GA1550 biot, 98.8 Ma 15.9 ± 0.2 to 14.4 ± 0.8 Malheur Gorge 6 Upper Pole Creek 16.8 ± 0.6 to 15.2 ± 5.6 40Ar/39Ar GA1550 biot, 98.8 Ma 16.8 ± 0.6 to 15.2 ± 5.6 Malheur Gorge 6 Lower Pole Creek 17.0 ± 1.6 to 15.8 ± 5.6 40Ar/39Ar GA1550 biot, 98.8 Ma 17.0 ± 1.6 to 15.8 ± 5.6 Owyhee Mts. 7 Oregon Plateau basalt 16.10 ± 0.17 40Ar/39Ar FC san, 27.84 Ma 16.20 ± 0.17 8 40 39 SC volcanic field Oregon Plateau basalt 16.45 ± 0.04 to 14.05 ± 0.40 Ar/ Ar FC san, 27.55 Ma 16.73 ± 0.04 to 14.32 ± 0.40 9 40 39 10 Steens Mountain Steens Basalt 16.38 ± 0.20 to 16.36 ± 0.10 Ar/ ArFC san, 28.02 Ma 16.37 ± 0.09 9 40 39 Steens Mt. vicinity Oregon Plateau basalt 16.58 ± 0.18 to 15.51 ± 0.28 Ar/ Ar FC san, 28.02 Ma 16.58 ± 0.18 to 15.51 ± 0.28 1Hart and Carlson, 1985; 2Hart et al., 1989; 3Swisher et al., 1990; 4Baksi, et al., 1991; 5 Baksi and Farrer (1990); 6Hooper et al., 2002; 7Shoemaker and Hart, 2004; 8Brueseke et al., 2003 (SC, Santa Rosa-Calico);9 This study; 10Weighted means were calculated from age ranges and then normalized to 28.02 Ma;11 Information of flux monitor provided by C. Swisher. TABLE 4. MAJOR AND TRACE ELEMENT GEOCHEMICAL DATA FOR DATED OREGON PLATEAU LAVA FLOWS.

Sample MB97-2 MB97-24 MB97-25 MB97-32 MB97-40 MB97-47 MB97-55 MB97-65 MB97-87 86-29 CH82-22B CH82-22G

SiO2 59.46 51.22 49.39 50.21 53.36 50.73 51.17 47.98 47.38 50.39 47.30 47.30

TiO2 2.03 2.55 1.93 2.32 2.56 2.58 2.28 2.09 2.06 2.14 2.20 2.38

Al2O3 13.45 13.18 15.16 17.32 14.40 14.97 15.17 15.92 16.30 15.68 16.40 15.70

Fe2O3 8.86 14.78 13.18 11.83 12.11 13.65 12.52 13.49 13.43 13.15 13.90 15.10 MnO 0.14 0.22 0.19 0.16 0.19 0.20 0.15 0.18 0.18 0.20 0.19 0.19 MgO 2.13 4.16 6.09 4.00 3.46 4.33 5.15 6.24 6.34 4.88 5.09 5.73 CaO 5.51 7.64 9.11 9.18 6.78 7.16 8.76 8.53 8.77 8.35 8.79 8.25

Na2O 3.28 3.34 2.88 3.25 3.42 3.56 3.16 3.16 3.25 3.62 3.00 3.14

K2O 2.80 1.52 0.59 0.95 2.32 1.90 1.17 0.92 0.89 1.27 1.29 0.99

P2O5 0.39 0.36 0.31 0.35 0.65 0.64 0.56 0.41 0.32 0.44 0.37 0.34 LOI 1.08 -0.06 0.21 0.28 1.05 1.26 1.11 0.42 0.40 1.50 0.31 0.47 TOTAL 99.13 98.91 99.04 99.85 100.30 100.98 101.20 99.34 99.32 101.62 98.84 99.59 Ba 1226 561 419 397 742 314 525 374 345 620 ------Ce 64 38 30 36 62 38 46 27 31 52 ------Co 40 41 45 36 28 50 40 49 52 38 ------Cr 25 31 36 33 11 43 3 44 23 --- 60 70 Cu 17 244 25 92 162 90 41 212 177 62 ------Ga 20.6 26.1 25.1 27.4 25.0 24.6 24.2 24.2 24.0 23.5 ------La 30 20 16 17 33 14 25 14 12 27 ------Nb 15.5 11.6 14.9 13.4 17.6 14.3 15.9 10.7 10.4 14.2 ------Ni 12 22 27 53 12 118 48 129 136 66 ------Pb 15956115554------Rb 78 35 12 22 53 8.2 18 15 13 29 13 14 Sc 25.6 33.5 27.3 25.6 24.9 29.3 23.9 27.7 27.7 ------Sr 333 397 463 498 461 479 511 490 532 514 553 476 Th 8.2 4.6 3.4 4.4 7.8 2.6 5.4 4.3 3.8 5.1 ------U 2.8 1.5 1.6 1.2 1.6 0.6 1.1 0.2 1.3 0.4 ------V 330 406 254 265 275 334 299 350 352 212 ------Y 44 37 25 30 43 30 32 29 29 36 ------Zn 131 118 117 103 132 127 134 114 111 109 ------Zr 282 194 135 166 270 171 197 151 144 213 140 100 Note: Major element concentrations are reported as weight percent oxides and expressed as raw data; trace element concentrations are reported in ppm. Major elements were analyzed by the techniques outlined in Katoh et al. (1999) at Miami University by DCP-AES (Direct Current Argon Plasm Atomic Emission Spectrometry), except for CH82-22B and G. Trace elements were analyzed by XRF (X-ray fluorescence) at Franklin and Marshall College by techniques outlined in Mertzman (2000) except for CH82-22B and G. Major and trace element compositions for CH82-22B and G were determined by XRAL Ltd. by XRF and originally reported in Carlson and Hart (1987). Figure 1a & b. (a) Map of the northwestern United States depicting select Cenozoic volcanic and tectonic features. Shaded region is the approximate extent of mid-Miocene flood basalt volcanism (after Hart and Carlson 1985; Camp and Ross 2004). Also shown are major flood basalt dike swarms/eruptive loci (black lines), Malheur Gorge-region (MG), Oregon-Idaho graben (OIG) and Northern Nevada rift (NNR) and associated magnetic anomalies corresponding to zones of lithospheric extension/mafic magma emplacement (black dashed lines; Cummings et al., 2000; Glen and Ponce 2002), commonly depicted volcanic fields of the Yellowstone-Snake River plain province (dashed circles); BJ, Bruneau-Jarbidge (~12.5 to <11 Ma); TF, Twin Falls (~10 to 8.6 Ma); PC, Picabo (~10 Ma); HS, Heise (~6.7 to 4.3 Ma); and YS, Yellowstone (<2.5 Ma), and age isochrons (dashed lines, ages in Ma) of Oregon High Lava Plains silicic volcanism (N, Newberry Volcano; after Jordan et al., 2004). Also illustrated is the initial 87Sr/86Sr 0.706 and 0.704 isopleths (after Armstrong et al. 1977; Kistler and Peterman 1978; Leeman et al. 1992; Crafford and Grauch 2002), which are commonly interpreted to demarcate the western edge of the Precambrian North American craton (the 0.706 isopleth) and a zone of transitional lithosphere between the older craton and Mesozoic accreted terranes to the west (between the 0.706 and 0.704 isopleth). (b) Shaded digital elevation model of the southeastern Oregon Plateau depicting documented flood basalt eruptive loci, study area (dashed box) and sampled section locations, and regional geographic and tectonic features. White circles indicate southeastern Oregon Plateau flood basalt eruptive loci/shallow intrusive bodies. Notice widespread location of Steens Basalt eruptive loci and lack of loci identified/exposed on the Owyhee Plateau portion of the Oregon Plateau. SM, Steens Mountain; OIG, Oregon-Idaho graben; OM, Owyhee mountains; WSRP, western Snake River Plain; OWP, Owyhee Plateau; BR-T, Bull Run-Tuscarora Mountains; NNR, eastern northern Nevada rift; SC, Santa Rosa- Calico volcanic field; MD, McDermitt volcanic field.

33 o WA ID a 117 b OIG WSRP 3 Sr isotope 2 OM 0.706 line Sr isotope 1 o 0.704 line 46 SM 4 OR ID 5 CRB Dikes Study 1 Folly Farm East MT YS area 2 Folly Farm West HS N 2 6 OWP ? 10 MG 3 Route 78 North PC ? OIG 4 TF Miranda Flat o Steens Dikes 42 ? ? BJ MD NV o ? 5 Mickey Butte 42 OR CA ? ? SC Steens ? ? Mountain ? ? NV UT WY OWP ? NNR ? BR-T 50 km NNR

o o o o o 123 120 117 114 111

Brueseke et al. Figure 1

34 Figure 2a-j. 40Ar/39Ar age and K/Ca spectra for dated lava flows from stratigraphic sections peripheral to Steens Mountain. Plateau age errors are (1σ). Isotope correlation diagrams report intercept results determined by York (1969) linear regression results and errors are 1σ.

35 a) 0.004 86-29 gm 1 86-29 gm aa) A FFE K/Ca 0.003 25 0.01 Ar

40 I

16.31±0.10 Ma (MSWD = 1.34) Ar/ 0.002 H 36 Steps D-I B 15 D E F G H I Age= 16.30±0.12 Ma G C C 0.001 D 40 36 ± B Ar/ Ari= 296 2 TGA = 15.83±0.11 Ma F

Apparent Age (Ma) n= 6, MSWD= 1.6 5 0

b) 0.004 1 MB-97-2 gm bb) MB-97-2 gm A Ar FFW K/Ca 0.003 25 0.01 40

15.17±0.18 Ma (MSWD = 1.9) Ar/ 0.002 I H 36 B Steps E-I G C 15 Age= 15.4±0.3 Ma F E F I D D GH 0.001 40 36 ± E C Ar/ Ari= 288 8 A B TGA = 11.58±0.17 Ma n= 5, MSWD= 2.3 Apparent Age (Ma) 5 0 0.004 MB-97-24 gm c) 1 cc) A MB-97-24 gm

FFW K/Ca

Ar 0.003 25 0.01 40 16.06±0.18 Ma (MSWD = 4.37) I H Ar/ 0.002

36 B Steps D-I C 15 E F Age= 16.29±0.11 Ma D G H I D C 0.001 G 40Ar/36Ar = 287±3 E B i F TGA = 14.64±0.16 Ma n= 6, MSWD= 3.1 Apparent Age (Ma) 5 0 0.004 MB-97-25 gm d) 1 MB-97-25 gm dd) R78N K/Ca Ar 0.003 A 25 0.01 H 40 16.24±0.25 Ma (MSWD = 0.76) I Ar/ G 0.002 B 36 Steps D-I 15 D E F Age= 16.3±0.3 Ma C A I 0.001 C H 40 36 ± FD B G Ar/ Ari= 291 6 E TGA = 14.7±0.4 Ma n= 6, MSWD= 0.8 Apparent Age (Ma) 5 0 0.004 MB-97-32 plag e) 1 MB-97-32 plag ee) R78N A K/Ca

Ar 0.003 25 0.01 16.03±0.15 Ma (MSWD = 0.34) 40 Ar/ 0.002 36 All Steps 15 J K Age= 16.2±0.4 Ma D I 0.001 C E 40 36 ± Ar/ Ari= 280 50 F TGA = 16.24±0.29 Ma EB B G n= 11, MSWD= 0.38 FG Apparent Age (Ma) J 5 0 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.1 Cumulative % 39Ar Released 39Ar/40Ar Brueseke et al. Figure 2a-e 36 0.004 f) ff) MB-97-65 gm 1 MB-97-65 gm K/Ca MF 0.003 A 25 0.01 Ar 40

16.40±0.09 Ma (MSWD = 0.83) Ar/ 0.002 B I 36 All Steps H 15 Age= 16.42±0.09 Ma C D E F G H B C I 0.001 40Ar/36Ar = 293.8±1.4 i D G TGA = 16.24±0.18 Ma n= 9, MSWD= 0.74 F Apparent Age (Ma) E 5 0 0.004 MB-97-87 gm g) 1 MB-97-87 gm gg)

K/Ca A MF Ar 0.003 25 0.01 40 B ±

16.33 0.10 Ma (MSWD = 0.87) Ar/ 0.002 36 I H Steps C-I C 15 D E F G H Age= 16.49±0.14 Ma C I 0.001 B 40 36 ± D G Ar/ Ari= 288 4 TGA = 16.23±0.47 Ma E F Apparent Age (Ma) n= 7, MSWD= 0.46 5 0 0.004 MB-97-40 gm h) 1 MB-97-40 gm hh)

MB K/Ca A

Ar 0.003 25 0.01 40 B ± 15.51 0.14 Ma (MSWD = 4.43) Ar/ 0.002 I HC 36 Steps D-I G 15 D FE D E F Age= 16.1±0.2 Ma C G H I 0.001 40 36 ± Ar/ Ari= 282 5 B TGA = 14.62±0.20 Ma

Apparent Age (Ma) n= 6, MSWD= 3.9 5 0 0.004 97-47 gm i) 97-47gm ii) 1 A K/Ca

MB Ar 0.003 I

25 0.01 40 16.48±0.15 Ma (MSWD = 1.67) Ar/ B 0.002 36 Steps C-I 15 C D E F G H Age= 16.65±0.13 Ma C I 0.001 D 40 36 ± H B Ar/ Ari= 283 3 E TGA = 15.98±0.15 Ma F G Apparent Age (Ma) n= 7, MSWD= 1.3 5 0 0.004 j) jj) MB-97-55 gm 1 MB-97-55 gm

K/Ca A MB

Ar 0.003 25 0.01 A 16.58±0.09 Ma (MSWD = 1.62) 40 B Ar/ 0.002 C 36 Steps D-I I 15 D E C F G H Age= 16.68±0.08 Ma H B I 0.001 40Ar/36Ar = 320±8 D TGA = 18.90±0.58 Ma i G Apparent Age (Ma) n= 6, MSWD= 8.5 E F 5 0 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.1 39 Cumulative% Ar Released 39Ar/40Ar 37 Brueseke et al. Figure 2f-j Figure 3a-d. 40Ar/39Ar age and K/Ca spectra and isotope correlation diagrams for dated lava flows from Steens Basalt at Steens Mountain. Errors are 1σ and isochron regressions use the method of York (1969). Sample CH82-22G is from the top flow whereas CH82-22B is from the 31st flow down from the top.

38 0.004 (A) 55005-01, CH82-22B (B) 55005-01, CH82-22B 1 K/Ca 0.003 25 0.01 ± I

16.55 0.05 Ma (MSWD = 1.91) Ar

20 40 0.002 Ar/

15 B C D E F 36 G H ± 0.001 Isochron age = 16.72 0.07 Ma H Apparent age (Ma) 10 40Ar/36Ari = 210±30 I Steps B-F G ± F TGA=16.36 0.05 Ma MSWD = 1.8, n = 5 B CDE 5 0 0.004 (C) 55006-01, CH82-22G 1 I (D) 55006-01, CH82-22G

K/Ca A 0.003 25 0.01

16.59 ± 0.05 Ma (MSWD = 0.15) Ar 20 40 0.002 A

Ar/ H

15 B C D E F 36 G G 0.001 ± H Isochron age = 16.64 0.15 Ma 10 40Ar/36Ar = 290±30 Apparent age (Ma) i B ± Steps C-F TGA=16.38 0.10 Ma I MSWD= 0.19, n=4 F C D E 5 0 0 20 40 60 80 100 0 0.05 0.10 0.15 0.20 Cumulative % 39Ar Released 39Ar/40Ar

Brueseke et al. Figure 3a-d Figure 4. 40Ar/39Ar age and K/Ca spectra diagrams for low resolution laser heating analyses for Steens Basalt at Steens Mountain. Errors are 1σ. Sample CH82-22G is from the top flow.

40 Ar* 80

0 (b) 55005-22, CH82-22B (c) 55005-23, CH82-22B 4

% 40 (a) 55005-21, CH82-22B

0 1

0.1 K/Ca

18 0.01

16 B A B B A

Apparent age (Ma) 14 A TGA=16.48 ± 0.07 Ma TGA=16.17 ± 0.07 Ma TGA=16.47 ± 0.06 Ma

80 Ar* (d) 55005-24, CH82-22B (e) 55005-25, CH82-22B (f) 55005-26, CH82-22B 0 4 40 %

0 1

0.1 K/Ca

18 0.01

16 B B B

A A A

Apparent age (Ma) 14 TGA=16.37 ± 0.07 Ma TGA=16.46 ± 0.07 Ma TGA=16.53 ± 0.06 Ma

0 40 80 80 Cumulative % 39Ar Released

Ar* (g) 55005-27, CH82-22B (h) 55005-28, CH82-22B 0 4 40 % 0 1

0.1 K/Ca

18 0.01

16 B B A A

Apparent age (Ma) 14 TGA=16.45 ± 0.06 Ma TGA=16.127 ± 0.08 Ma

0 40 80 0 40 80 Cumulative % 39Ar Released Cumulative % 39Ar Released

Brueseke et al. Figure 4a-h 41 Figure 5. 40Ar/39Ar age and K/Ca spectra diagrams for low resolution laser heating analyses for Steens Basalt at Steens Mountain. Errors are 1σ. Sample CH82-22B is from the 31st flow down from the top.

42 (c) 55006-22, CH82-22G

Ar* 80

0 (a) 55006-21, CH82-22G (b) 55006-20, CH82-22G 4

% 40

0 1

0.1 K/Ca

18 0.01

B 16 B B

AA A A

Apparent age (Ma) 14 TGA= 16.25 ± 0.07 Ma A TGA= 16.28 ± 0.07 Ma TGA= 17.22 ± 0.13 Ma

(d) 55006-23, CH82-22G 80 (e) 55006-24, CH82-22G (f) 55006-25, CH82-22G Ar* 0 4 40 % 0 1

0.1 K/Ca

18 0.01

16 B B B

14 TGA= 16.41 ± 0.09 Ma Apparent age (Ma) ± TGA= 16.43 0.08 Ma TGA= 16.35 ± 0.07 Ma A A A

80 (g) 55006-26, CH82-22G (h) 55006-27, CH82-22G (i) 55006-28, CH82-22G Ar* 0

4 40 % 0 1

0.1 K/Ca

18 0.01

16 B B B

14 Apparent age (Ma) TGA= 16.41 ± 0.08 Ma TGA= 16.39 ± 0.06 Ma TGA= 16.33 ± 0.08 Ma A A A

0 40 80 0 40 80 0 40 80 Cumulative % 39Ar Released Cumulative % 39Ar Released Cumulative % 39Ar Released

Brueseke et al. Figure 5a-i 43 Figure 6. Petrographic and stratigraphic relationships of sampled sections. For details of each stratigraphic section, see discussion under stratigraphic results section. 40Ar/39Ar ages obtained at each section are illustrated and each dashed horizontal line either indicates a dated lava flow or other horizon of geologic interest (T, tuffaceous/sedimentary horizon). The entire section at Folly Farm East consists of normal polarity lava flows and the reverse to normal polarity transition at Mickey Butte is shown at its approximate stratigraphic level. The scale near the Folly Farm West section is applicable to all sections.

44 Folly Farm West 345m, 22 flows total Folly Farm East 15.17± 0.36 Ma 271m, 30 flows

T 70 m

16.31± 0.20 Ma T 16.06± 0.36 Ma 16.1 Ma Tuff of Oregon Canyon? Route 78 North 105m, 14 flows Mickey Butte 16.24± 0.50 Ma 365 m, 46 flows 15.51± 0.28 Ma 16.03± 0.30 Ma

16.48± 0.30 Ma Normal Miranda Flat 104m, 24 flows Reversed 16.40± 0.18 Ma

16.58± 0.18 Ma 16.33± 0.20 Ma

Covered interval Aphyric Plagioclase-phyric

Younger andesite Intermediate-plagioclase Blocky a’a-like

Brueseke et al. Figure 6

45 Figure 7a & b. (a) Total alkali vs. silica diagram (LeBas et al., 1986) illustrating similarities between Steens Basalt lava flows from Steens Mountain and Oregon Plateau flood basalts presented in this study. B= basalt; BA= basaltic andesite; A= andesite; TB= trachybasalt; BTA=

basaltic trachyandesite; TA= trachyandesite. (b) Plot of TiO2 vs. P2O5 (wt. %) illustrating compositional variation of Oregon Plateau flood basalts from this study and Steens Basalt lava flows from Steens Mountain (Johnson et al., 1998). Also depicted are fields for the Malheur Gorge-region tholeiitic flood basalts (Camp et al., 2003) and Venator Ranch basalts (Camp et al., 2003). Notice that most of the Oregon Plateau flood basalts from this study fall along the array defined by the Steens Basalt from Steens Mountain. Eight samples from this study plot above the Venator Ranch field, defining a local subgroup exposed at the Miranda Flat section.

46 8 Oregon Plateau Flood Steens Basalt BTA TA Basalts from this study from Steens Mountain 6 “Venator Ranch Malheur Gorge TB Basalts” Basalts

O+K O

22 4 A 1.4 BA 1.2 2 B

Wt.%Na a 0 1.0

45 50 55 60 O 0.8 25 Wt. % SiO2 0.6

Wt.%P 0.4 0.2 b 0.0 1.0 1.5 2.0 2.5 3.0 3.5

Wt. % TiO2

Brueseke et al. Figure 7a & b

47 Figure 8. Chemostratigraphic variation diagrams from sampled stratigraphic sections, discussed in this study. All five sections are depicted to scale and each point represents the stratigraphic position of sampled lava flows. Numbers below the section name are the thickness (m) and the

number of sampled lava flows out of the total lava flows exposed. SiO2 is expressed as weight percent oxide; Zr and Ni are expressed as ppm. The scale near the Folly Farm East section is applicable to all sections.

48 2) Folly Farm West 300m, 18/18 1) Folly Farm East 271m, 30/30

70 m

16.06 Ma± 0.36

16.31 0.20± Ma 47 49 51 53 55 57 100 200 300 0 40 80 120 160

SiO2 Zr Ni 48 50 52 54 56 100 150 200 250 0 40 80 120 160

SiO2 Zr Ni 3) Route 78 North 5) Mickey Butte 105m, 14/14 365m, 19/46 16.24 0.50± Ma 15.51 0.28± Ma

16.03 0.30± Ma

16.48 0.30± Ma 49 51 53 55 57 50 100 150 200 0 40 80 120 160

SiO2 Zr Ni

16.58 0.18± Ma 4) Miranda Flat 104m, 24/24 16.40 0.18± Ma

48 50 52 54 56 100 200 300 0 40 80 120 160

SiO2 Zr Ni 16.33 0.20± Ma

47 49 51 53 55 57 100 200 300 0 40 80 120 160

SiO2 Zr Ni

Brueseke et al. Figure 8

49 Figure 9. Chemostratigraphic variation of the Steens Basalt at Steens Mountain (data from Johnson et al. (1998) hung at approximate stratigraphic position to depict within section chemical variation and is shown to illustrate the broad chemical variation within the section). 40Ar/39Ar ages from Baksi and Farrar (1990), Baksi et al. (1991), and Swisher et al. (1990) are also shown at their approximate stratigraphic positions and normalized to a 28.02 Ma age for the Fish Canyon tuff standard. Also, the approximate stratigraphic “zone” that the N/R polarity change present at this location is found within is depicted in gray (based on the detailed stratigraphy of Mankinen et al., 1985). “Upper” and “lower” Steens Basalt division of Binger (1997), Johnson et al. (1998) and Camp et al. (2003) is also depicted.

50 Steens Mountain Steens Basalt type section ~900 to 1000m 16.38 ± 0.20 Ma (This study) 16.70 ± 0.05 Ma (Swisher et al., 1990)

16.36 ± 0.10 Ma (This study) 16.70 ± 0.02 Ma (Swisher et al., 1990) Zone of N/R paleomagnetic 16.33 ± 0.16 Ma transition (Baksi and Co-workers, 1990; 1991)

“Upper Steens” “Lower Steens” 304 m

47 49 51 53 55 57 100 200 300 0 200 400

SiO2 Zr Ni

Brueseke et al. Figure 9

51 Figure 10. Major and trace element plots depicting chemical variations of Oregon Plateau flood

basalt lava flows. SiO2 and TiO2 are expressed as weight percent oxide; Rb, Ba, Zr, and Ni are expressed as ppm. Lava flows from each section in this study are plotted, in addition to lava flows from Steens Mountain (divided into “upper” and “lower” Steens, minus ten samples with Ni >200 ppm; data from Johnson et al., 1998) and from other ~17 to ~14 Ma southeastern Oregon Plateau locations (Owyhee Mountains and Santa Rosa-Calico volcanic field, data from Mellott, 1987; Shoemaker and Hart, 2004; Brueseke and Hart, unpublished data).

52 60

50

40

30 Rb

20

10

1200

1000

800

600 Ba

400

200

300

250

200 Zr

150 Folly Farm East

100 Folly Farm West

Route 78 North

200 Miranda Flat

Mickey Butte

150 “Upper” Steens

“Lower” Steens

100 Ni SE Oregon Plateau

50

SiO2 TiO2 0 47 49 51 53 55 57 12 3 4

Brueseke et al. Figure 10 53 Figure 11. Stratigraphic variation of Oregon and Columbia Plateau flood basalt units. Diagram

based on Figure 4 of Camp and Ross (2004). R0, N0, etc. are the paleomagnetic intervals present within the lower Columbia River Basalt Group (Imnaha and Grande Ronde Basalts) while the N and R represent the reversal present in the upper portion of the Steens Basalt exposed at Steens Mountain. Not depicted is the relationship of flood basalt units within the complex transition zone between the Oregon and Columbia Plateaus (Malheur Gorge region and further west). Black zones depict periods of no volcanism while white zones depict periods of flood basalt activity. In the context of this study, the Steens Basalt exposed at Steens Mountain (thin white line) represents just a small portion of the regional Steens chronostratigraphic record.

54 Main phase of Pacific Northwest U.S.A. flood-basalt volcanism

Oregon Proximal to Steens Columbia Plateau Steens Mt. Mountain Plateau ~15.0 Ma ~15.0 Ma

N2

~15.4 Ma ? R2 Grande Ronde N ~15.8 Ma Steens 1 Basalt Steens ? Basalt R Basalt 1 ~16.2 Ma N Imnaha ? N0 R Basalt R0 ~16.6 Ma ~16.6 Ma

Brueseke et al. Figure 11

55 References Cited

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58 Hooper, P.R., Binger, G.B., and Lees, K.R., 2002. Age of the Steens and Columbia River flood basalts and their relationship to extension-related calc-alkalic volcanism in eastern Oregon. Geol. Soc. Am. Bull. 114, 43-50. Humphreys, E.D., Dueker, K.G., Schutt, D.L., and Smith, R.B., 2000. Beneath Yellowstone: Evaluating plume and nonpplume models using telesiesmic images of the upper mantle. GSA Today 10, 1-6. Jarboe, N.A., Coe, R.S., Glen, J.M., and Renne, P.R., 2003. A study of mid-Miocene Yellowstone hotspot volcanics and the search for the Steens Mountain reversal. Eos Trans AGU 84, 46. John D.A., 2001. Miocene and early Pliocene epithermal gold-silver deposits in the northern Great Basin, western United States: characteristics, distribution, and relationship to magmatism. Econ. Geol. 96, 1827-1853. John, D.A., and Wallace, A.R., 2000. Epithermal gold-silver deposits related to the northern Nevada Rift: Geology and Ore Deposits 2000. The Great Basin and Beyond Symposium: Geol. Soc. Nev. Proc., pp. 155-175. Johnson, J.A., 1995. Geologic evolution of the Duck Creek Butte eruptive center, High Lava Plains, southeastern Oregon. M.S. Thesis, Corvallis, Oregon, Oregon State University, 168 p. Johnson, J.A., C.J. Hawkesworth, C.J., Hooper, P.R., and Binger, G.B., 1998. Major- and trace- element analyses of Steens Basalt, Southeastern Oregon. U.S. Geol. Surv. Open File Rep. 98- 482, 30 p. Jordan, B.T., Grunder, A.L., Duncan, R.A., and Deino, A.L., 2004. Geochronology of age- progressive volcanism of the Oregon High Lava Plains: Implications for the plume interpretation of Yellowstone. J. Geophys. Res. 109, B10202 DOI:10.1029/2003JB002776 Katoh, S., Danhara, T., Hart, W.K., and WoldeGabriel, G., 1999. Use of sodium polytungstate solution in the purification of volcanic glass shards for bulk chemical analysis. Nat. Hum. Act. 4, 45-54. Kistler, R.W., and Peterman, Z.E., 1978. Reconstruction of crustal California on the basis of initial strontium isotopic compositions of Mesozoic granitic rocks. U. S. Geol. Surv. Prof. Pap. 107, pp. 1-17. Lanphere, M.A., and Baadsgaard, H., 2001. Precise K-Ar, 40Ar/39Ar, Rb-Sr, and U/Pb mineral ages from the 27.5 Ma Fish Canyon tuff reference standard. Chem. Geol. 175, 653-671.

59 Larson, E.E., Watson, D.E., and Jennings, W., 1971. Regional comparison of a Miocene geomagnetic transition in Oregon and Nevada. Earth Planet. Sci. Lett. 11, 391-400. LeBas, M.J., Le Maitre, R.W., Streckeisen, A., and Zanettin, B., 1986. A chemical classification of volcanic rocks based on the total alkali-silica diagram. J. Pet. 27, 745-750. Leeman, W.P., Oldow, J.S., and Hart, W.K., 1992. Lithosphere-scale thrusting in the western U.S. Cordillera as constrained by Sr and Nd isotopic transitions in Neogene volcanic rocks. Geology 20, 63-66. Lees, K.R., 1994. Magmatic and tectonic changes through time in the Neogene volcanic rocks of the Vale area, Oregon, North Western USA. Ph.D. thesis, Milton Keynes, United Kingdom, The Open University, 284 p. Mahon, K.I., 1996. The New “York” regression: Application of an improved statistical method to geochemistry. Int. Geol Rev. 38, 293-303. Mankinen, E.A., Prévot, M., Grommé, C.S., and Coe, R.S., 1985. The Steens Mountain (Oregon) geomagnetic polarity transition 1. Directional history, duration of episodes, and rock magmatism. J. Geophys. Res. 90, 10,393-10,416. Mankinen, E.A., Larson, E.E., Grommé, C.S., Prévot, M., and Coe, R.S., 1987. The Steens Mountain (Oregon) geomagnetic polarity transition 3. Its regional significance. J.Geophys. Res. 92, 8057-8076. McDougall, I., and Harrison, T.M., 1999. Geochronology and Thermochronology by the 40Ar/39Ar method. Oxford University Press, 269 p. Mellott, M.G., 1987. Geochemical, petrologic, and isotopic investigation of andesites and related volcanic rocks in the Santa Rosa Range and Bloody Run Hills, Nevada; tectonic implications. M.S. Thesis, Oxford, Ohio, Miami University, 164 p. Mertzman, S.A., 2000. K-Ar results from the southern Oregon - northern California Cascade Range. Ore. Geol. 62, 99-122. Rahl, J.M., Foland, K.A., and McGrew, A.J., 2002. Transition from contraction to extension in the northeastern Basin and Range; new evidence from the Copper Mountains, Nevada. J. Geol. 110, 179-194. Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., and DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chem. Geol. 145, 117-152.

60 Rytuba, J.J. and McKee, E.H., 1984. Peralkaline ash flow tuffs and calderas of the McDermitt volcanic field, southeast Oregon and north central Nevada. J. Geophys. Res. 89, 8616-8628. Sherrod, D.R., Minor, S.A., and Vercoutere, T.L., 1989. Geologic map of the Sheepshead Mountains, Harney and Malheur counties, Oregon. United States Geol. Surv. Misc. Field Stud. Map MF- 2079, 1 sheet. Shoemaker, K.A., and Hart, W.K., 2004. Temporal controls on basalt genesis and evolution on the Owyhee plateau, Idaho and Oregon. In B. Bonnichsen, C.M. White, and M. McCurry (Eds.), Tectonic and magmatic evolution of the Snake River Plain volcanic province. Idaho Geol. Surv. Bull. 30, 16 p. Singer, B.S., and Pringle, M.S., 1996. Age and duration of the Matuyama-Brunhes geomagnetic polarity reversal from 40Ar/39Ar incremental heating analyses of lavas. Earth Planet. Sci. Lett. 139, 47-61. Smith, J.A., 1987. Volcanism and Paleomagnetism of the Sheepshead Mountains, Oregon. Undergraduate Thesis, Lancaster, Pennsylvania, Franklin and Marshall College, 23 p. Steiger, R.H., and Jager, E., 1977. Subcommisson on geochronology: Convention on the use of decay constants in geo-and cosmochronology. Earth Planet. Sci. Lett. 36, 359-362. Stewart, M.A., 1992. Petrogenesis of high alumina flood basalts, Steens Mountains, Oregon. M.S. Thesis, Bloomington, Indiana, Indiana University, 65 p. Swisher, C.C., Ach, J.A., and Hart, W.K., 1990. Laser fusion 40Ar/39Ar dating of the type Steens Mountain Basalt, southeastern Oregon and the age of the Steens geomagnetic polarity transition. Eos Trans AGU 71, 1296. Taylor, J.R., 1982. An introduction to error analysis: The study of uncertainties in physical measurements. Univ. Sci. Books, Mill Valley, California. Tolan, T.L., Reidel, S.P., Beeson, M.H., Anderson, J.L., Fecht, K.R., and Swanson, D.A., 1989. Revisions to the estimates of the areal extent and volume of the Columbia River Basalt Group. In S.P. Reidel and P.R. Hooper (Eds.), Volcanism and tectonism in the Columbia River flood-basalt province. Geol. Soc. Am. Spec. Pap. 239, Denver, Colorado, pp. 367-378. Wagner, D.L., Deino, A.L., Renne, P.R., and Saucedo, G.J., 2000. Age and origin of the Lovejoy Basalt of Northern California. Geol. Sci. Am. Abs. w/Prog. 32, 159. Wells, R.E., and Heller, P.L. 1988. The relative contribution of accretion, shear, and extension to Cenozoic tectonic rotation in the Pacific Northwest. Geol. Soc. Am. Bull. 100, 325-338.

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62 Appendix A: Argon data repository

Distribution and Geochronology of Oregon Plateau (U.S.A.) Flood Basalt Volcanism: The Steens Basalt Revisited Brueseke et al. Submitted to J. Volcanology and Geothermal Research

Argon isotopic analyses were preformed at the New Mexico Geochronology Research laboratory (NMGRL), NM Tech, Socorro New Mexico. Analytical methods are provided in Table 1 in the manuscript. Complete details of the operation of the NMGRL can be found at: http://geoinfo.nmt.edu/publications/openfile/argon/home.html.

Details of the age calculations are provided in the footnotes of Data Repository Tables 1 and 2.

References cited:

Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., and DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chemical Geology, 145, 117-152.

Steiger, R.H., and Jäger, E., 1977. Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planet. Sci. Lett., 36, 359-362.

Taylor, J.R., 1982. An Introduction to Error Analysis: The Study of Uncertainties in Physical Measurements,. Univ. Sci. Books, Mill Valley, Calif., 270 p.

63 Brueseke et al. Data Repository Table 1. Argon isotopic data for Oregan Plateau basalts.

ID Temp 40 39 37 39 36 39 39 K/Ca 40 39 Age Ar/ Ar Ar/ Ar Ar/ Ar ArK Ar* Ar ±1σ (°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

86-29, groundmass FFE, B2:110, 25.47 mg, J=0.0007868, D=1.00468±0.00093, NM-110, Lab#=50462-01 # A 625 80.47 0.5604 258.3 2.582 0.91 5.2 1.7 5.9 1.7 # B 700 16.00 0.8389 24.94 7.90 0.61 54.4 7.0 12.32 0.47 # C 750 14.81 1.087 15.85 6.22 0.47 69.0 11.2 14.45 0.57 D 800 14.41 1.824 10.91 17.30 0.28 78.7 22.7 16.04 0.21 E 875 13.80 2.446 7.937 21.40 0.21 84.5 37.1 16.50 0.17 F 975 13.77 2.769 8.139 20.89 0.18 84.2 51.0 16.42 0.18 G 1075 17.39 2.198 21.65 14.47 0.23 64.2 60.7 15.81 0.28 H 1250 30.27 2.667 64.05 52.0 0.19 38.2 95.5 16.37 0.16 I 1700 36.18 17.64 88.27 6.77 0.029 31.9 100.0 16.53 0.65 Integrated age ± 1σ n=9 149.6 K2O=2.89 % 15.83 0.11 Plateau ± 1σ steps D-I n=6 MSWD=1.34 132.8 0.156 88.8 16.31 0.10

MB-97-2, groundmass FFW, D4:89, 15.39 mg, J=0.0007966, D=1.0017099±0.00078, NM-89, Lab#=9166-01 # A 625 174.2 0.1610 580.4 8.57 3.2 1.6 12.3 3.9 1.0 # B 700 14.09 0.2230 25.90 15.10 2.3 45.8 34.1 9.25 0.13 # C 750 15.06 0.3400 23.83 12.22 1.5 53.4 51.7 11.53 0.15 # D 800 15.39 0.6014 19.28 9.77 0.85 63.3 65.7 13.95 0.17 E 875 15.43 1.086 16.82 8.22 0.47 68.4 77.6 15.11 0.18 F 975 17.08 1.722 21.30 5.94 0.30 64.0 86.1 15.65 0.25 G 1075 19.03 2.561 31.28 2.439 0.20 52.6 89.6 14.34 0.56 H 1250 21.77 8.323 41.71 3.86 0.061 46.5 95.2 14.59 0.47 I 1650 25.71 9.556 54.26 3.36 0.053 40.7 100.0 15.08 0.46 Integrated age ± 1σ n=9 69.5 K2O=2.19 % 11.66 0.17 Plateau ± 1σ steps E-I n=5 MSWD=1.90 23.8 0.135 34.3 15.17 0.18

MB-97-24, groundmass FFW, D2:89, 17.28mg, J=0.0007977, D=1.0017099±0.00078, NM-89, Lab#=9164-01 # A 625 336.0 0.8230 1129.6 2.566 0.62 0.7 4.9 3.3 2.1 # B 700 16.20 0.7593 29.70 4.74 0.67 46.2 13.9 10.75 0.31 # C 750 16.48 1.115 24.03 4.78 0.46 57.5 23.0 13.59 0.30 D 800 14.96 1.446 14.35 5.09 0.35 72.4 32.6 15.55 0.27 E 875 13.73 1.466 8.810 9.51 0.35 81.9 50.7 16.13 0.15 F 975 13.92 1.147 8.681 9.70 0.44 82.3 69.1 16.41 0.15 G 1075 16.56 1.074 18.39 4.86 0.47 67.7 78.4 16.08 0.29 H 1250 29.46 9.774 67.29 5.57 0.052 35.2 88.9 14.98 0.31 I 1650 32.60 9.452 75.67 5.82 0.054 33.8 100.0 15.90 0.32 Integrated age ± 1σ n=9 52.6 K2O=1.48 % 14.64 0.16 Plateau ± 1σ steps D-I n=6 MSWD=4.37 40.6 0.140 77.0 16.06 0.18

64 Brueseke et al. Data Repository Table 1. Argon isotopic data for Oregan Plateau basalts.

ID Temp 40 39 37 39 36 39 39 K/Ca 40 39 Age Ar/ Ar Ar/ Ar Ar/ Ar ArK Ar* Ar ±1σ (°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

MB-97-25, groundmass R78N, D5:89, 16.58mg, J=0.0007974, D=1.0017099±0.00078, NM-89, Lab#=9167-01 # A 625 463.7 2.333 1559.6 1.245 0.22 0.7 7.4 4.4 3.4 # B 700 19.64 3.276 37.15 1.780 0.16 45.5 18.0 12.84 0.76 # C 750 15.76 5.132 20.69 1.853 0.099 63.9 29.0 14.48 0.74 D 800 13.55 7.041 9.525 3.031 0.072 83.5 47.0 16.28 0.44 E 875 13.09 6.723 7.973 3.30 0.076 86.2 66.5 16.24 0.41 F 975 14.52 6.561 11.93 2.411 0.078 79.5 80.9 16.60 0.55 G 1075 23.16 6.636 45.41 0.887 0.077 44.4 86.1 14.82 1.52 H 1250 41.84 28.39 115.0 1.581 0.018 24.4 95.5 14.93 1.03 I 1650 47.78 47.11 133.6 0.757 0.011 25.5 100.0 18.06 1.98 Integrated age ± 1σ n=9 16.84 K2O=0.49 % 14.76 0.37 Plateau ± 1σ steps D-I n=6 MSWD=0.76 11.97 0.042 71.0 16.24 0.25

MB-97-32, plagioclase R78N, E14:89, 22.64mg, J=0.0008080, D=1.0017099±0.00078, NM-89, Lab#=9182-01 A 700 46.63 22.41 134.3 0.135 0.023 18.9 0.9 13.0 10.0 B 850 12.66 20.02 12.52 0.416 0.025 83.9 3.5 15.6 3.2 C 950 12.06 19.59 7.056 0.631 0.026 96.2 7.5 17.1 2.1 D 1050 12.03 19.40 8.926 1.005 0.026 91.4 13.9 16.2 1.3 E 1150 12.74 18.34 11.68 0.778 0.028 84.8 18.8 15.9 1.7 F 1200 13.30 18.14 10.25 0.375 0.028 88.5 21.2 17.3 3.6 G 1250 12.66 18.82 8.843 0.324 0.027 91.7 23.2 17.1 4.1 H 1300 14.66 17.94 19.20 0.253 0.028 71.4 24.8 15.4 5.3 I 1350 13.00 18.45 8.029 0.599 0.028 93.5 28.6 17.9 2.2 J 1400 12.55 18.46 8.234 1.828 0.028 92.8 40.2 17.12 0.76 K 1650 12.33 18.97 10.30 9.42 0.027 88.0 100.0 15.97 0.16 Integrated age ± 1σ n=11 15.77 K2O=0.33 % 16.24 0.29 Plateau ± 1σ steps A-K n=11 MSWD=0.34 15.77 0.027 100.0 16.03 0.16

MB-97-65, groundmass MF, D3:89, 15.95mg, J=0.0007968, D=1.0017099±0.00078, NM-89, Lab#=9165-01 # A 625 910.2 3.670 3051.5 0.737 0.14 1.0 1.9 12.6 6.5 B 700 28.54 3.234 60.61 1.662 0.16 38.2 6.1 15.64 0.84 C 750 17.65 3.035 22.63 2.731 0.17 63.5 13.1 16.08 0.51 D 800 14.28 3.232 10.16 4.00 0.16 80.8 23.3 16.55 0.34 E 875 12.71 2.658 4.572 7.70 0.19 91.1 42.9 16.61 0.18 F 975 12.27 2.105 3.445 11.48 0.24 93.1 72.2 16.38 0.13 G 1075 12.73 1.728 4.750 3.46 0.30 90.1 81.1 16.44 0.38 H 1250 19.42 8.986 29.80 2.867 0.057 58.5 88.4 16.35 0.51 I 1650 26.78 12.21 57.28 4.55 0.042 40.6 100.0 15.69 0.38 Integrated age ± 1σ n=9 39.2 K2O=1.19 % 16.24 0.18 Plateau ± 1σ steps B-I n=8 MSWD=0.90 38.4 0.124 98.1 16.396 0.090

65 Brueseke et al. Data Repository Table 1. Argon isotopic data for Oregan Plateau basalts.

ID Temp 40 39 37 39 36 39 39 K/Ca 40 39 Age Ar/ Ar Ar/ Ar Ar/ Ar ArK Ar* Ar ±1σ (°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

MB-97-87, groundmass MF, A4:89, 15.66 mg, J=0.0008023, D=1.0017099±0.00078, NM-89, Lab#=9149-01 # A 625 5082.7 4.637 17150.3 0.561 0.11 0.3 1.4 22 29 # B 700 40.18 3.271 103.8 2.210 0.16 24.3 7.1 14.14 0.75 C 750 21.43 3.303 35.56 2.309 0.15 52.2 13.1 16.17 0.63 D 800 16.16 3.361 16.53 4.03 0.15 71.5 23.5 16.68 0.37 E 875 13.14 2.747 6.497 7.60 0.19 87.1 43.1 16.52 0.19 F 975 12.78 2.319 5.669 8.42 0.22 88.4 64.8 16.30 0.17 G 1075 14.27 1.827 10.18 2.967 0.28 80.0 72.5 16.46 0.48 H 1250 20.95 5.925 35.33 4.97 0.086 52.5 85.3 15.92 0.34 I 1650 27.95 12.91 61.26 5.70 0.040 39.1 100.0 15.87 0.35 Integrated age ± 1σ n=9 38.8 K2O=1.19 % 16.23 0.47 Plateau ± 1σ steps C-I n=7 MSWD=0.87 36.0 0.108 92.9 16.33 0.10

MB-97-40, groundmass MB, D1:89, 17.28mg, J=0.0007984, D=1.0017099±0.00078, NM-89, Lab#=9163-01 # A 625 615.6 3.020 2061.4 3.84 0.17 1.1 4.1 9.7 3.5 # B 700 26.16 0.4423 64.07 9.02 1.2 27.8 13.7 10.43 0.23 # C 750 20.89 0.7294 37.22 9.33 0.70 47.6 23.7 14.28 0.21 D 800 19.58 0.6373 29.26 10.20 0.80 56.1 34.5 15.76 0.17 E 875 18.16 0.9165 24.36 16.46 0.56 60.8 52.1 15.84 0.12 F 975 19.02 1.601 28.69 14.66 0.32 56.1 67.7 15.33 0.14 G 1075 19.17 1.599 30.00 6.41 0.32 54.4 74.6 14.98 0.25 H 1250 23.89 4.725 46.55 18.96 0.11 44.1 94.8 15.15 0.15 I 1650 26.80 6.530 55.64 4.89 0.078 40.7 100.0 15.70 0.38 Integrated age ± 1σ n=9 93.8 K2O=2.63 % 14.62 0.20 Plateau ± 1σ steps D-I n=6 MSWD=4.43 71.6 0.206 76.3 15.51 0.14

97-47, groundmass MB, B3:110, 26.73 mg, J=0.0007864, D=1.00468±0.00093, NM-110, Lab#=50463-01 # A 625 264.3 3.391 871.8 1.602 0.15 2.6 1.6 9.9 3.0 # B 700 23.88 2.946 49.53 3.68 0.17 39.7 5.3 13.4 1.0 C 750 16.19 2.608 18.89 4.55 0.20 66.8 9.9 15.31 0.83 D 800 14.49 2.643 11.55 10.49 0.19 78.0 20.6 15.98 0.36 E 875 14.42 2.799 11.06 13.96 0.18 78.9 34.7 16.10 0.29 F 975 13.72 2.651 7.019 21.73 0.19 86.5 56.7 16.78 0.19 G 1075 12.68 2.477 3.807 16.38 0.21 92.7 73.2 16.63 0.25 H 1250 16.78 5.033 18.93 16.29 0.10 69.2 89.7 16.44 0.28 # I 1700 46.23 23.41 128.9 10.17 0.022 21.8 100.0 14.48 0.51 Integrated age ± 1σ n=9 98.9 K2O=1.82 % 15.98 0.15 Plateau ± 1σ steps C-H n=6 MSWD=1.67 83.4 0.164 84.4 16.48 0.15

66 Brueseke et al. Data Repository Table 1. Argon isotopic data for Oregan Plateau basalts.

ID Temp 40 39 37 39 36 39 39 K/Ca 40 39 Age Ar/ Ar Ar/ Ar Ar/ Ar ArK Ar* Ar ±1σ (°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

MB-97-55, groundmass MB, D6:89, 16.58mg, J=0.0007983, D=1.0017099±0.00078, NM-89, Lab#=9168-01 # A 625 9755 4.421 32522 0.344 0.12 1.5 1.1 199 46 # B 700 46.57 3.573 120.2 1.504 0.14 24.4 5.8 16.33 0.64 # C 750 24.44 3.652 43.19 1.84 0.14 49.0 11.6 17.22 0.44 # D 800 16.39 2.386 14.67 4.92 0.21 74.8 27.1 17.59 0.17 # E 875 12.71 1.942 3.334 6.83 0.26 93.5 48.5 17.07 0.11 F 975 12.16 1.475 2.302 9.37 0.35 95.4 78.0 16.65 0.08 G 1075 12.54 1.041 4.297 3.38 0.49 90.6 88.6 16.29 0.21 H 1250 15.85 8.271 18.00 2.70 0.062 70.8 97.1 16.18 0.27 I 1650 22.01 19.29 40.79 0.917 0.026 52.5 100.0 16.78 0.79 Integrated age ± 1σ n=9 31.8 K2O=0.93 % 18.90 0.58 Plateau ± 1σ steps F-I n=4 MSWD=1.62 16.4 0.145 51.5 16.578 0.091

Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions. Ages calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory standard at 28.02 Ma (Renne et al., 1998). Errors quoted for individual analyses include analytical error only, without interfering reaction or J uncertainties. Integrated age calculated by quadratically combining isotopic measurements of all steps. Integrated age error calculated by quadratically combining errors of isotopic measurements of all steps. Plateau age is inverse-variance-weighted mean of selected steps. Plateau age error is inverse-variance-weighted mean error (Taylor, 1982) times squareroot MSWD where MSWD>1. Plateau and integrated ages incorporate uncertainties in interfering reaction corrections and J factors (0.1%). Decay constants and isotopic abundances after Steiger and Jager (1977). # symbol preceding sample ID denotes analyses excluded from plateau age calculations. 39 K20 estimated from Ar signal, sample weight and J-factor. D = 1 AMU discrimination in favor of light isotopes Correction factors: 39 37 ( Ar/ Ar)Ca = 0.00070 ± 0.00002 36 37 ( Ar/ Ar)Ca = 0.000280 ± 0.000005 38 39 ( Ar/ Ar)K = 0.0129 40 39 ( Ar/ Ar)K = 0.0000 ± 0.0004

67 Brueseke et al. Data repository Table 2. Argon isotopic results for Steens Mountain samples.

ID Power/Temp 40 39 37 39 36 39 39 K/Ca 40 39 Age Ar/ Ar Ar/ Ar Ar/ Ar ArK Ar* Ar ±1σ (W/°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

CH82-22B, groundmass concentrate, 5.9 8mg, J=0.0016460, D=1.0035±0.0005, NM-179AA, Lab#=55005-21 A 0.25 10.89 2.670 18.77 5.99 0.19 51.2 27.9 16.50 0.17 B 15 6.289 3.721 3.549 15.4 0.14 88.4 100.0 16.467 0.066 Integrated age ± 1σ n=2 21.4 K2O=0.84 % 16.476 0.071

CH82-22B, groundmass concentrate, 5.81 mg, J=0.0016460, D=1.0035±0.0005, NM-179AA, Lab#=55005-22 A 0.25 234.0 1.900 803.6 0.115 0.27 -1.4 0.6 -10 7 B 15 6.282 3.564 3.647 20.1 0.14 87.7 100.0 16.322 0.048 Integrated age ± 1σ n=2 20.2 K2O=0.82 % 16.174 0.066

CH82-22B, groundmass concentrate, 4.37 mg, J=0.0016460, D=1.0035±0.0005, NM-179AA, Lab#=55005-23 A 0.20 10.60 2.411 18.30 3.03 0.21 50.9 17.8 15.99 0.18 B 15 6.164 3.463 2.922 14.0 0.15 90.8 100.0 16.579 0.051 Integrated age ± 1σ n=2 17.0 K2O=0.91 % 16.474 0.057

CH82-22B, groundmass concentrate, 4.52 mg, J=0.0016460, D=1.0035±0.0005, NM-179AA, Lab#=55005-24 A 0.20 15.02 2.529 33.47 2.19 0.20 35.6 13.7 15.82 0.34 B 15 5.959 3.835 2.478 13.9 0.13 93.2 100.0 16.456 0.050 Integrated age ± 1σ n=2 16.1 K2O=0.83 % 16.370 0.067

CH82-22B, groundmass concentrate, 4.77 mg, J=0.0016460, D=1.0035±0.0005, NM-179AA, Lab#=55005-25 A 0.20 12.49 3.003 24.94 3.15 0.17 43.0 19.8 15.92 0.25 B 15 6.096 4.319 2.925 12.7 0.12 91.8 100.0 16.598 0.055 Integrated age ± 1σ n=2 15.9 K2O=0.78 % 16.464 0.070

CH82-22B, groundmass concentrate, 5.12 mg, J=0.0016460, D=1.0035±0.0005, NM-179AA, Lab#=55005-26 A 0.20 65.61 1.648 200.5 0.279 0.31 9.9 1.5 19.3 2.5 B 15 5.910 3.282 2.105 19.0 0.16 94.2 100.0 16.493 0.038 Integrated age ± 1σ n=2 19.3 K2O=0.88 % 16.533 0.055

CH82-22B, groundmass concentrate, 5.10 mg, J=0.0016460, D=1.0035±0.0005, NM-179AA, Lab#=55005-27 A 0.20 15.80 2.284 35.62 2.17 0.22 34.6 11.2 16.20 0.37 B 15 5.921 3.500 2.216 17.2 0.15 94.0 100.0 16.484 0.038 Integrated age ± 1σ n=2 19.3 K2O=0.89 % 16.453 0.057

CH82-22B, groundmass concentrate, 4.01 mg, J=0.0016460, D=1.0035±0.0005, NM-179AA, Lab#=55005-28 A 0.20 13.52 2.554 28.74 2.00 0.20 38.8 14.2 15.55 0.39 B 15 5.790 3.899 1.998 12.1 0.13 95.5 100.0 16.393 0.066 Integrated age ± 1σ n=2 14.1 K2O=0.82 % 16.273 0.082

68 Brueseke et al. Data repository Table 2. Argon isotopic results for Steens Mountain samples.

ID Power/Temp 40 39 37 39 36 39 39 K/Ca 40 39 Age Ar/ Ar Ar/ Ar Ar/ Ar ArK Ar* Ar ±1σ (W/°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

CH82-22B, groundmass concentrate, 31.21 mg, J=0.0016460, D=1.004±0.001, NM-179AA, Lab#=55005-01 A 625 4.758 1.334 11.67 0.027 0.38 29.9 0.0 4 38 B 700 5.877 2.378 1.415 9.04 0.21 96.3 9.3 16.76 0.13 C 750 5.757 3.554 1.620 16.0 0.14 96.9 25.7 16.536 0.092 D 800 5.751 2.161 1.109 21.8 0.24 97.5 48.0 16.600 0.061 E 875 5.762 1.253 0.9118 19.5 0.41 97.2 67.9 16.565 0.072 F 975 5.915 1.009 1.629 15.7 0.51 93.3 84.0 16.327 0.089 # G 1075 5.892 3.064 3.061 7.93 0.17 89.1 92.1 15.549 0.148 # H 1250 6.672 22.75 11.78 7.10 0.022 76.8 99.4 15.394 0.261 # I 1700 21.48 22.30 63.86 0.585 0.023 21.0 100.0 13.537 2.072 Integrated age ± 1σ n=9 97.8 K2O=0.74 % 16.359 0.050 Plateau ± 1σ steps A-F n=6 MSWD=1.91 82.2 0.25 84.0 16.548 0.053

CH82-22G, groundmass concentrate, 4.72 mg, J=0.0016444, D=1.0035±0.0005, NM-179AA, Lab#=55006-20 A 0.25 46.48 2.197 145.4 0.533 0.23 7.9 2.2 10.9 1.4 AA 0.25 5.711 2.021 2.059 1.26 0.25 92.4 7.5 15.60 0.29 B 15 6.096 3.942 2.867 22.0 0.13 91.6 100.0 16.535 0.052 Integrated age ± 1σ n=3 23.8 K2O=1.18 % 16.283 0.071

CH82-22G, groundmass concentrate, 4.36 mg, J=0.0016444, D=1.0035±0.0005, NM-179AA, Lab#=55006-21 A 0.20 13.57 2.219 28.96 2.63 0.23 38.3 17.4 15.40 0.29 B 15 6.250 4.025 3.540 12.5 0.13 88.7 100.0 16.423 0.051 Integrated age ± 1σ n=2 15.1 K2O=0.82 % 16.247 0.069

CH82-22G, groundmass concentrate, 4.43 mg, J=0.0016444, D=1.0035±0.0005, NM-179AA, Lab#=55006-22 A 0.20 49.63 2.086 150.4 0.594 0.24 10.8 3.7 15.9 1.6 B 15 13.28 3.747 26.27 15.5 0.14 43.9 100.0 17.27 0.11 Integrated age ± 1σ n=2 16.1 K2O=0.86 % 17.22 0.13

CH82-22G, groundmass concentrate, 4.53 mg, J=0.0016444, D=1.0035±0.0005, NM-179AA, Lab#=55006-23 A 0.20 65.87 2.212 210.7 0.347 0.23 5.8 2.1 11.2 2.6 B 15 6.034 3.741 2.594 16.0 0.14 92.6 100.0 16.538 0.061 Integrated age ± 1σ n=2 16.3 K2O=0.85 % 16.426 0.083

CH82-22G, groundmass concentrate, 4.04 mg, J=0.0016444, D=1.0035±0.0005, NM-179AA, Lab#=55006-24 A 0.20 145.1 2.494 470.0 0.145 0.20 4.4 1.1 19.0 6.2 B 15 6.275 3.728 3.584 13.7 0.14 88.2 100.0 16.380 0.052 Integrated age ± 1σ n=2 13.8 K2O=0.80 % 16.408 0.086

CH82-22G, groundmass concentrate, 4.06 mg, J=0.0016444, D=1.0035±0.0005, NM-179AA, Lab#=55006-25 A 0.20 21.62 2.210 57.53 0.98 0.23 22.2 6.7 14.22 0.71 B 15 6.295 3.817 3.545 13.7 0.13 88.5 100.0 16.498 0.054 Integrated age ± 1σ n=2 14.7 K2O=0.85 % 16.346 0.072

CH82-22G, groundmass concentrate, 5.82 mg, J=0.0016444, D=1.0035±0.0005, NM-179AA, Lab#=55006-26 A 0.20 69.17 2.483 216.0 0.430 0.21 8.0 2.3 16.4 2.7 B 15 6.081 3.959 2.967 18.5 0.13 91.1 100.0 16.408 0.044 Integrated age ± 1σ n=2 18.9 K2O=0.76 % 16.409 0.078

69 Brueseke et al. Data repository Table 2. Argon isotopic results for Steens Mountain samples.

ID Power/Temp 40 39 37 39 36 39 39 K/Ca 40 39 Age Ar/ Ar Ar/ Ar Ar/ Ar ArK Ar* Ar ±1σ (W/°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma) CH82-22G, groundmass concentrate, 4.96 mg, J=0.0016444, D=1.0035±0.0005, NM-179AA, Lab#=55006-27 A 0.20 77.03 2.003 247.6 0.307 0.25 5.3 1.7 12.0 2.3 B 15 5.994 3.849 2.578 17.6 0.13 92.8 100.0 16.462 0.047 Integrated age ± 1σ n=2 17.9 K2O=0.85 % 16.386 0.064

CH82-22G, groundmass concentrate, 3.96 mg, J=0.0016444, D=1.0035±0.0005, NM-179AA, Lab#=55006-28 A 0.20 36.88 2.181 110.7 0.563 0.23 11.8 4.1 12.9 1.3 B 15 6.101 3.870 2.934 13.2 0.13 91.2 100.0 16.473 0.061 Integrated age ± 1σ n=2 13.8 K2O=0.82 % 16.329 0.081

CH82-22G, groundmass concentrate, 30.91 mg, J=0.0016444, D=1.004±0.001, NM-179AA, Lab#=55006-01 # A 625 250.2 1.865 817.2 1.12 0.27 3.5 1.3 26.1 4.8 # B 700 6.086 1.647 2.697 12.1 0.31 89.2 15.6 16.05 0.11 C 750 6.057 1.803 2.034 12.3 0.28 92.6 30.1 16.58 0.10 D 800 5.984 1.930 1.839 20.6 0.26 93.7 54.4 16.571 0.075 E 875 6.022 1.880 1.883 18.3 0.27 93.4 76.1 16.632 0.076 F 975 6.377 3.090 3.534 12.8 0.17 87.7 91.2 16.56 0.11 # G 1075 7.434 5.256 9.096 3.02 0.097 69.9 94.7 15.40 0.37 # H 1250 8.861 38.81 26.80 4.40 0.013 47.9 99.9 12.88 0.38 # I 1700 202.4 18.15 713.3 0.044 0.028 -3.4 100.0 -21 26 Integrated age ± 1σ n=9 84.5 K2O=0.64 % 16.38 0.10 Plateau ± 1σ steps C-F n=4 MSWD=0.15 63.9 0.24 75.6 16.591 0.046

Notes Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions. Ages calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory standard at 28.02 Ma (Renne et al., 1998). Errors quoted for individual analyses include analytical error only, without interfering reaction or J uncertainties. Integrated age calculated by quadratically combining isotopic measurements of all steps. Integrated age error calculated by quadratically combining errors of isotopic measurements of all steps. Plateau age is inverse-variance-weighted mean of selected steps. Plateau age error is inverse-variance-weighted mean error (Taylor, 1982) times squareroot MSWD where MSWD>1. Plateau and integrated ages incorporate uncertainties in interfering reaction corrections and J factors (0.1%). Decay constants and isotopic abundances after Steiger and Jager (1977). # symbol preceding sample ID denotes analyses excluded from plateau age calculations. 39 K20 estimated from Ar signal, sample weight and J-factor. D = 1 AMU discrimination in favor of light isotopes Correction factors: 39 37 ( Ar/ Ar)Ca = 0.0006765 ± 0.000005 36 37 ( Ar/ Ar)Ca = 0.0002878 ± 0.000003 38 39 ( Ar/ Ar)K = 0.0131 40 39 ( Ar/ Ar)K = 0 ± 0.0004

70

Appendix B: Whole-rock major and trace element data

Distribution and Geochronology of Oregon Plateau (U.S.A.) Flood Basalt Volcanism: The Steens Basalt Revisited Brueseke et al. Submitted to J. Volcanology and Geothermal Research

Major element concentrations are reported as weight percent oxides and expressed as raw data; trace element concentrations are reported in ppm. Major elements were analyzed by the techniques outlined in Katoh et al. (1999) at Miami University by DCP-AES (Direct Current Argon Plasma Atomic Emission Spectrometry). Trace elements were analyzed by XRF (X-ray fluorescence) at Franklin and Marshall College by techniques outlined in Mertzman (2000). Position is individual lava flow position at the stratigraphic section where the flow was sampled (1 = uppermost exposed flow, 2 = 2nd flow down-section, 3 = 3rd flow down-section, etc.).

References cited:

Katoh, S., Danhara, T., Hart, W.K., and WoldeGabriel, G., 1999. Use of sodium polytungstate solution in the purification of volcanic glass shards for bulk chemical analysis. Nat. Hum. Act. 4, 45-54. Mertzman, S.A., 2000. K-Ar results from the southern Oregon - northern California Cascade Range. Ore. Geol. 62, 99-122.

Folly Farm East

Sample 86-1 86-2 86-3 86-4 86-4* 86-6 86-6* 86-7* 86-8* Location FFE FFE FFE FFE FFE FFE FFE FFE FFE Position 123 4 567 8 9

SiO2 52.27 48.99 49.83 49.76 51.88 49.79 50.19 48.86 51.70

TiO2 1.96 2.44 2.61 1.56 1.93 1.77 2.09 2.32 1.87

Al2O3 14.82 15.45 15.65 20.72 18.96 19.33 17.24 17.58 14.09

Fe2O3 13.24 13.08 13.33 9.34 10.83 9.99 11.58 12.19 13.35 MnO 0.18 0.18 0.18 0.12 0.14 0.13 0.16 0.15 0.20 MgO 4.62 5.02 4.13 3.59 3.81 3.53 4.65 4.59 5.04 CaO 8.02 10.09 9.35 10.31 9.61 9.77 9.31 8.89 8.93

Na2O 3.25 3.33 3.39 3.47 3.58 3.50 3.32 3.44 3.37

K2O 1.19 0.72 0.96 0.63 0.83 0.81 1.00 1.00 1.06

P2O5 0.29 0.32 0.39 0.21 0.26 0.27 0.49 0.38 0.30 L.O.I. 0.98 1.07 1.07 0.70 0.78 0.74 0.96 0.97 0.93 Total 100.82 100.69 100.89 100.41 102.61 99.63 100.99 100.37 100.84

Ni 17 55 42 59 49 49 66 68 36 Cr ------Sc ------V 247 287 258 155 158 171 188 151 326 Ba 621 333 473 282 392 325 417 493 529 Rb 33 16 25 12 20 18 24 21 25 Sr 418 426 417 536 522 525 477 508 390 Zr 182 181 218 121 151 139 169 204 149 Y 30 38 40 23 27 25 31 34 37 Nb 13.1 11.5 14.1 8.0 9.9 9.3 10.6 14.2 7.8 Ga 22.6 24.2 25.9 23.1 25.0 23.2 24.9 24.9 21.2 Cu 20 137 116 61 127 111 92 69 100 Co 42 35 32 29 31 29 34 37 38 Zn 114 101 112 73 73 77 90 103 101 Pb ------U 1.3 1.0

72 Folly Farm East

Sample 86-9 86-10 86-11 86-12 86-13 86-14 86-15 86-16 86-17 Location FFE FFE FFE FFE FFE FFE FFE FFE FFE Position 10 11 12 13 14 15 16 17 18

SiO2 51.11 49.85 49.05 49.09 49.22 50.97 51.35 51.52 48.82

TiO2 2.41 2.22 2.09 2.09 2.18 2.53 1.90 2.08 1.99

Al2O3 13.70 15.32 15.78 15.97 15.93 13.39 15.44 15.14 15.76

Fe2O3 13.49 12.91 12.78 12.87 12.69 14.95 12.99 13.25 14.19 MnO 0.19 0.18 0.17 0.17 0.15 0.22 0.18 0.17 0.19 MgO 5.07 5.19 6.29 5.87 4.48 4.05 5.13 4.00 5.90 CaO 8.92 9.91 9.79 9.47 9.30 7.65 7.83 7.34 8.76

Na2O 3.05 3.24 3.27 3.24 3.22 3.49 3.72 3.84 3.47

K2O 1.14 0.72 0.65 0.80 0.74 1.75 1.30 1.41 0.80

P2O5 0.36 0.29 0.33 0.37 0.36 0.40 0.32 0.38 0.27 L.O.I. 1.11 0.81 0.96 1.01 0.58 0.86 0.99 0.71 1.03 Total 100.55 100.64 101.16 100.95 98.85 100.26 101.15 99.84 101.18

Ni 21 42 75 75 79 22 81 74 90 Cr ------Sc ------V 276 253 246 215 236 372 235 305 235 Ba 460 369 327 377 363 642 571 685 434 Rb 25 16 13 17 11 38 27 30 11 Sr 405 450 485 496 503 391 472 478 503 Zr 221 175 170 189 187 203 159 172 138 Y 38 32 31 33 35 42 31 31 30 Nb 15.6 12.8 13.4 14.6 14.6 10.5 7.7 9.3 7.4 Ga 22.7 23.1 22.5 23.7 23.1 23.9 22.3 22.2 23.5 Cu 35 41 38 41 56 221 133 195 116 Co 33 35 44 38 36 37 43 41 45 Zn 124 104 106 107 113 119 95 111 90 Pb ------U 0.7 0.1 1.3 0.4

73 Folly Farm East

Sample 86-18 86-19 86-20 86-21 86-22 86-23 86-24 86-25 86-26 Location FFE FFE FFE FFE FFE FFE FFE FFE FFE Position 19 20 21 22 23 24 25 26 27

SiO2 50.87 48.80 54.25 48.13 49.02 48.89 49.80 48.54 49.05

TiO2 2.52 1.38 2.05 1.84 2.11 2.19 2.57 2.08 2.40

Al2O3 14.19 15.50 14.08 16.68 16.27 15.18 14.74 15.43 15.83

Fe2O3 14.18 11.58 12.08 13.83 13.54 13.38 14.12 13.07 13.41 MnO 0.19 0.17 0.18 0.20 0.20 0.18 0.19 0.18 0.18 MgO 4.20 7.48 3.24 5.19 5.08 6.12 5.19 6.49 5.24 CaO 7.69 10.90 6.61 9.09 8.70 9.67 8.57 9.72 9.36

Na2O 3.64 2.94 3.73 3.33 3.64 3.28 3.38 3.23 3.60

K2O 1.48 0.34 2.05 0.73 1.03 0.68 1.18 0.57 0.83

P2O5 0.38 0.16 0.44 0.26 0.50 0.35 0.46 0.30 0.39 L.O.I. 0.87 0.78 0.46 0.72 0.92 0.65 0.98 0.79 1.07 Total 100.21 100.03 99.17 100.00 101.01 100.57 101.18 100.40 101.36

Ni 48 129 29 129 63 58 77 153 78 Cr ------Sc ------V 316 248 268 278 224 258 237 225 221 Ba 692 183 864 354 513 349 529 264 414 Rb 30 6.7 50 11 16 12 26 8.1 15 Sr 455 311 450 515 539 473 464 479 510 Zr 185 101 210 118 150 171 243 149 190 Y 36 26 34 29 34 30 39 29 35 Nb 9.8 5.7 9.9 7.3 8.5 12.1 17.9 11.5 14.1 Ga 23.8 17.5 21.3 22.1 23.4 21.8 23.3 22.1 24.3 Cu 265 93 151 99 129 67 81 57 47 Co 35 46 34 50 38 42 41 47 38 Zn 105 88 109 95 94 115 125 105 101 Pb ------U 1.2

74 Folly Farm East & West

Sample 86-27 86-28 86-29 MB97-5 MB97-6 MB97-7 MB97-8 MB97-10 MB97-11 Location FFE FFE FFE FFW FFW FFW FFW FFW FFW Position 28 29 30 1 2 3 4 5 6

SiO2 48.04 49.96 50.39 50.03 50.88 50.42 49.71 51.74 52.15

TiO2 1.97 2.19 2.14 2.36 1.94 2.26 2.26 2.54 1.87

Al2O3 15.66 15.43 15.68 16.53 18.83 14.79 15.56 13.31 15.43

Fe2O3 13.23 13.26 13.15 11.89 10.55 13.03 13.16 15.02 12.49 MnO 0.18 0.24 0.20 0.16 0.14 0.18 0.18 0.22 0.18 MgO 6.38 4.81 4.88 4.28 3.89 5.60 5.88 4.18 4.72 CaO 9.89 8.22 8.35 9.61 9.61 9.69 9.59 7.69 7.26

Na2O 3.41 3.53 3.62 3.11 3.48 3.04 3.04 3.44 3.57

K2O 0.57 1.31 1.27 0.80 0.83 0.69 0.66 1.52 1.42

P2O5 0.27 0.44 0.44 0.36 0.18 0.21 0.36 0.30 0.34 L.O.I. 0.70 0.93 1.22 0.42 -0.03 0.15 -0.18 0.09 0.53 Total 100.30 100.32 101.34 99.55 100.30 100.06 100.22 100.05 99.96

Ni 110 67 66 43 56 42 66 21 80 Cr ------47 57 62 48 16 23 Sc ------26.6 22.4 29.0 25.9 33.2 22.8 V 248 227 212 273 209 287 277 417 280 Ba 353 678 620 396 373 322 327 566 562 Rb 7.0 32 29 20 17 14 14 34 30 Sr 505 514 514 431 519 439 488 383 485 Zr 135 220 213 178 144 171 180 195 164 Y 28 37 36 34 27 31 32 39 30 Nb 10.1 15.0 14.2 13.0 10.9 13.4 15.5 10.8 9.3 Ga 21.5 23.6 23.5 22.6 22.8 20.9 21.5 21.7 20.1 Cu 64 59 62 71 76 42 41 196 146 Co 45 38 38 34 32 41 41 42 42 Zn 103 115 109 106 91 124 123 131 110 Pb ------6 6 5 6 10 8 U 1.0 2.3 0.4 1.1 1.1 0.1 0.9 1.4 0.4 Th 0.8 3.7 5.1 1.7 2.2 1.2 0.3 2.0 2.0 La 15 26 26 16 16 16 21 21 16 Ce 35 49 52 32 33 35 38 41 35

75 Folly Farm West

Sample MB97-12 MB97-13 MB97-14 MB97-15 MB97-16 MB97-17 MB97-18 MB97-20 MB97-21 Location FFW FFW FFW FFW FFW FFW FFW FFW FFW Position 7 8 9 101112131415

SiO2 52.82 51.84 50.73 48.19 46.60 45.71 47.13 49.75 49.70

TiO2 2.34 2.62 2.57 1.86 1.98 1.86 1.87 2.21 2.21

Al2O3 13.85 13.89 13.82 16.23 15.60 15.71 15.99 15.26 14.87

Fe2O3 13.84 13.73 13.32 13.12 13.72 12.89 13.31 12.82 12.83 MnO 0.20 0.19 0.19 0.18 0.19 0.20 0.17 0.18 0.17 MgO 4.05 3.94 4.59 6.08 6.62 6.32 6.22 5.54 5.40 CaO 6.97 6.99 8.40 8.78 8.62 8.59 8.77 9.70 9.27

Na2O 3.54 3.54 3.07 3.39 3.12 3.05 3.19 3.12 3.00

K2O 1.78 1.51 1.44 0.77 0.79 0.77 0.75 0.66 0.77

P2O5 0.42 0.45 0.62 0.35 0.37 0.33 0.46 0.32 0.12 L.O.I. 0.39 0.34 0.64 0.43 0.81 1.05 0.82 0.41 0.42 Total 100.20 99.04 99.39 99.38 98.42 96.48 98.68 99.97 98.76

Ni 35 39 28 101 126 132 139 43 42 Cr 6 1025333038456767 Sc 26.8 29.1 26.1 23.6 27.7 26.7 26.1 30.6 28.6 V 327 382 312 282 321 306 300 293 303 Ba 712 639 609 343 334 348 327 298 310 Rb 36 32 34 13 14 12 14 9 16 Sr 441 435 452 496 464 472 494 441 430 Zr 200 188 253 117 127 125 123 173 171 Y 37 34 39 27 28 27 27 31 30 Nb 11.1 10.6 18.2 7.3 8.0 7.6 7.5 13.5 13.6 Ga 21.0 21.3 22.5 20.6 19.7 19.5 20.1 20.2 20.5 Cu 189 197 61 156 204 162 182 46 43 Co 38 40 36 47 52 50 52 41 41 Zn 121 120 142 102 110 102 102 123 117 Pb 10109 566555 U 0.4 0.3 0.6 0.4 0.4 0.7 0.3 0.3 0.1 Th 3.2 1.4 3.4 1.6 0.8 1.0 1.4 1.4 1.6 La 23 19 30 12 16 14 9 16 18 Ce 44 38 54 23 24 27 28 38 35

76 Folly Farm West & Route 78 North

Sample MB97-22 MB97-23 MB97-24 MB97-25 MB97-26 MB97-27 MB97-28 MB97-29 MB97-30 Location FFW FFW FFW RTE 78N RTE 78N RTE 78N RTE 78N RTE 78N RTE 78N Position 161718123456

SiO2 50.29 48.84 51.22 49.39 49.65 49.40 55.35 52.48 49.95

TiO2 2.31 2.30 2.55 1.93 2.07 2.08 1.05 1.84 1.96

Al2O3 15.35 15.99 13.18 15.16 15.53 15.79 16.06 14.28 18.57

Fe2O3 13.31 13.48 14.78 13.18 13.20 12.47 9.73 12.33 10.67 MnO 0.17 0.18 0.22 0.19 0.19 0.18 0.15 0.19 0.15 MgO 5.39 5.63 4.16 6.09 5.47 5.64 4.66 5.42 3.99 CaO 9.38 9.57 7.64 9.11 9.22 10.04 6.78 8.87 9.66

Na2O 3.05 3.02 3.34 2.88 2.96 2.81 3.22 3.05 3.29

K2O 0.75 0.71 1.52 0.59 0.66 0.60 1.87 1.25 0.77

P2O5 0.29 0.35 0.36 0.31 0.28 0.25 0.28 0.23 0.24 L.O.I. 0.13 1.06 -0.06 0.21 0.36 0.16 1.64 0.36 0.73 Total 100.42 101.13 98.91 99.04 99.59 99.42 100.79 100.30 99.98

Ni 43 73 22 27 22 76 79 46 62 Cr 71 50 31 36 27 179 87 115 27 Sc 29.6 27.8 33.5 27.3 24.8 30.4 21.9 29.9 21.9 V 285 278 406 254 257 312 222 274 236 Ba 342 328 561 419 464 270 323 486 318 Rb 17 11 35 12 13 12 53 27 16 Sr 437 481 397 463 469 404 292 415 531 Zr 171 173 194 135 147 143 89 143 139 Y 30 30 37 25 27 31 28 32 26 Nb 13.7 14.7 11.6 14.9 16.0 11.5 9.6 9.3 12.0 Ga 20.8 20.6 26.1 25.1 25.2 25.3 19.8 23.4 25.8 Cu 40 41 244 25 23 166 122 114 88 Co 41 44 41 45 46 44 33 38 33 Zn 118 125 118 117 120 105 94 97 94 Pb 65 9 5541065 U 0.7 0.3 1.5 1.6 0.6 0.3 2.9 2.1 1.6 Th 1.4 1.9 4.6 3.4 3.5 3.2 4.0 3.9 3.8 La 17 19 20 16 17 16 8 15 15 Ce 37 35 38 30 30 33 13 29 27

77 Route 78 North & Mickey Butte

Sample MB97-31 MB97-32 MB97-33 MB97-34 MB97-35 MB97-36 MB97-37 MB97-38 MB97-39 Location RTE 78N RTE 78N RTE 78N RTE 78N RTE 78N RTE 78N RTE 78N RTE 78N MB Position 7 8 9 10 11 12 13 14 1

SiO2 49.96 50.21 50.11 51.04 50.41 50.74 49.70 49.59 52.65

TiO2 2.20 2.32 2.39 1.81 2.28 2.06 2.15 2.40 1.44

Al2O3 17.40 17.32 17.07 19.40 16.05 18.45 18.20 17.56 16.13

Fe2O3 11.60 11.83 12.26 9.96 12.46 10.82 11.48 12.21 10.62 MnO 0.16 0.16 0.17 0.14 0.17 0.14 0.15 0.16 0.15 MgO 4.23 4.00 4.20 3.96 5.24 4.20 4.74 4.61 5.66 CaO 9.36 9.18 9.01 9.85 8.57 9.27 9.39 8.75 7.72

Na2O 3.23 3.25 3.22 3.30 3.07 3.36 3.29 3.29 3.16

K2O 0.81 0.95 0.88 0.75 1.06 0.81 0.73 0.96 1.38

P2O5 0.24 0.35 0.22 0.26 0.33 0.35 0.35 0.42 0.26 L.O.I. 0.94 0.28 -0.03 0.12 0.47 0.59 0.67 0.25 0.51 Total 100.13 99.85 99.50 100.59 100.11 100.79 100.85 100.20 99.68

Ni 143 53 57 60 79 80 79 80 111 Cr 48 33 27 62 103 112 65 51 52 Sc 28.3 25.6 24.7 21.6 25.4 25.8 25.4 23.0 22.4 V 334 265 282 209 276 237 276 212 241 Ba 394 397 427 314 409 365 386 455 501 Rb 6.9 22 17 15 26 15 26 21 28 Sr 462 498 496 545 450 498 450 511 422 Zr 116 166 170 133 181 154 181 191 154 Y 26 30 30 24 31 27 31 31 25 Nb 12.9 13.4 13.6 11.2 14.6 13.2 13.0 16.4 10.1 Ga 26.6 27.4 27.6 26.4 25.5 25.2 26.8 26.7 23.0 Cu 117 92 78 62 115 98 105 120 124 Co 38 36 39 32 44 37 40 43 39 Zn 102 103 108 85 119 102 119 118 108 Pb 46557 5 7 6 8 U 0.3 1.2 2.0 1.2 1.6 1.7 1.6 1.0 1.1 Th 3.4 4.4 4.8 4.1 3.4 4.7 3.4 4.6 6.2 La 17 17 22 13 17 17 17 19 14 Ce 36 36 41 31 39 35 33 42 33

78 Mickey Butte

Sample MB97-40 MB97-42 MB97-43 MB97-44 MB97-45 MB97-46 MB97-47 MB97-48 MB97-49 Location MB MB MB MB MB MB MB MB MB Position 2 345678910

SiO2 53.36 47.76 48.96 47.60 48.42 47.77 50.73 48.18 47.99

TiO2 2.56 1.98 2.49 2.57 2.14 2.31 2.58 2.27 1.72

Al2O3 14.40 15.54 15.07 15.08 16.64 15.13 14.97 15.62 16.21

Fe2O3 12.11 12.79 12.85 14.49 12.88 14.02 13.65 13.90 13.02 MnO 0.19 0.16 0.16 0.19 0.17 0.20 0.20 0.19 0.18 MgO 3.46 6.62 4.91 5.11 4.73 5.99 4.33 5.81 5.54 CaO 6.78 9.23 6.85 8.56 8.26 9.21 7.16 9.04 9.26

Na2O 3.42 2.99 3.61 3.17 3.44 3.06 3.56 3.04 2.96

K2O 2.32 0.52 1.72 0.93 1.22 0.73 1.90 0.76 0.66

P2O5 0.65 0.35 0.66 0.50 0.44 0.44 0.64 0.39 0.28 L.O.I. 1.05 0.98 2.53 1.71 3.10 -0.05 1.26 0.86 1.99 Total 100.30 98.92 99.81 99.91 101.44 98.81 100.98 100.06 99.81

Ni 12 129 82 74 104 44 118 131 143 Cr 11 59 41 27 25 19 43 47 28 Sc 24.9 25.1 23.9 26.3 25.9 26.0 29.3 25.8 28.3 V 275 248 323 382 341 361 334 311 334 Ba 742 232 691 414 370 677 314 350 275 Rb 53 5.4 25 27 18 38 8.2 11 6.9 Sr 461 506 493 544 465 498 479 512 462 Zr 270 130 230 158 201 251 171 171 116 Y 43 26 37 29 34 41 30 32 26 Nb 17.6 12.3 15.2 11.1 16.6 16.5 14.3 16.2 9.2 Ga 25.0 24.3 25.1 24.6 26.5 25.7 24.6 24.8 22.8 Cu 162 68 141 204 109 217 90 176 229 Co 28 45 39 43 49 38 50 53 53 Zn 132 111 127 114 140 131 127 135 102 Pb 1148659554 U 1.6 1.7 0.6 1.3 0.9 1.3 0.6 1.8 0.3 Th 7.8 3.6 5.4 5.2 3.6 5.9 2.6 3.4 3.4 La 33 19 30 19 21 31 14 23 14 Ce 62 26 61 40 41 63 38 41 25

79 Mickey Butte

Sample MB97-50 MB97-51 MB97-52 MB97-53 MB97-55 MB97-56 MB97-57 MB97-58 MB97-59 Location MB MB MB MB MB MB MB MB MB Position 11 12 13 14 15 16 17 18 19

SiO2 47.41 48.92 49.27 48.27 51.17 50.13 49.06 48.43 48.49

TiO2 2.25 2.21 1.64 1.92 2.28 1.79 1.88 1.75 1.81

Al2O3 15.20 15.65 15.97 15.39 15.17 15.46 18.16 15.86 15.92

Fe2O3 13.76 12.40 11.24 12.36 12.52 11.47 10.56 12.05 11.80 MnO 0.18 0.14 0.15 0.17 0.15 0.16 0.15 0.19 0.19 MgO 5.77 6.00 7.64 6.27 5.15 6.71 4.04 7.59 7.05 CaO 8.90 8.86 9.89 9.89 8.76 9.71 9.33 9.77 9.77

Na2O 2.96 3.05 2.70 2.91 3.16 2.88 3.34 2.80 2.81

K2O 0.86 0.88 0.51 0.66 1.17 0.79 0.97 0.49 0.53

P2O5 0.43 0.43 0.27 0.30 0.56 0.37 0.33 0.29 0.30 L.O.I. 1.75 1.52 1.06 1.63 1.11 1.13 1.42 0.97 1.24 Total 99.47 100.06 100.34 99.77 101.20 100.60 99.24 100.19 99.91

Ni 164 126 168 124 48 85 68 106 105 Cr 157 83 234 100 3 86 46 75 71 Sc 29.2 24.6 28.4 27.3 23.9 24.2 25.1 25.6 27.3 V 400 277 262 291 299 262 285 262 278 Ba 359 369 225 277 525 347 332 203 220 Rb 10 16 6.5 9.4 18 17 17 6.6 6.6 Sr 480 480 446 505 511 475 601 479 477 Zr 160 184 115 137 197 141 140 118 123 Y 30 30 22 26 32 25 28 23 24 Nb 11.9 15.3 10.5 13.2 15.9 12.5 10.5 11.3 11.9 Ga 24.4 24.7 21.8 22.6 24.2 22.5 25.0 22.0 22.3 Cu 198 72 75 59 41 42 321 44 46 Co 53 45 47 48 40 44 33 48 47 Zn 123 119 102 123 134 120 79 116 114 Pb 564455534 U 0.9 1.1 1.2 1.6 1.1 1.1 1.0 1.0 1.5 Th 3.3 4.3 3.5 2.9 5.4 4.7 4.0 3.3 3.5 La 16 24 15 16 25 15 16 10 17 Ce 36 43 29 32 46 34 29 30 26

80 Miranda Flat

Sample MB97-65 MB97-66 MB97-67 MB97-68 MB97-69 MB97-70 MB97-71 MB97-72 MB97-73 Location MF MF MF MF MF MF MF MF MF Position 123456789

SiO2 47.98 47.99 48.06 48.10 51.79 50.10 47.45 47.62 48.03

TiO2 2.09 1.75 2.27 2.35 2.09 1.48 2.72 2.74 2.73

Al2O3 15.92 15.85 17.10 15.94 15.64 20.80 15.52 15.28 14.91

Fe2O3 13.49 13.48 12.99 13.94 12.14 8.93 15.02 15.17 15.06 MnO 0.18 0.19 0.18 0.19 0.18 0.13 0.21 0.21 0.21 MgO 6.24 6.92 4.42 5.73 3.60 3.73 5.81 5.09 5.78 CaO 8.53 8.81 8.75 8.35 6.24 10.10 8.64 8.61 8.39

Na2O 3.16 2.94 3.37 3.27 4.09 3.54 3.25 3.18 3.22

K2O 0.92 0.70 0.98 1.04 2.13 0.87 1.09 1.16 1.18

P2O5 0.41 0.33 0.52 0.51 0.72 0.20 0.44 0.31 0.47 L.O.I. 0.42 0.42 1.02 0.09 0.79 0.15 0.27 0.68 0.48 Total 99.34 99.38 99.66 99.51 99.41 100.03 100.42 100.05 100.46

Ni 129 144 95 117 45 69 110 106 110 Cr 44 49 23 61 17 25 28 35 33 Sc 27.7 29.0 27.6 25.5 22.5 20.6 27.1 29.1 28.9 V 350 335 338 329 307 248 389 395 379 Ba 374 269 373 429 825 331 441 445 431 Rb 15 13 15 14 35 11 17 20 22 Sr 490 405 526 509 730 687 471 459 446 Zr 151 118 167 180 195 114 205 208 205 Y 29 27 31 32 33 21 37 37 36 Nb 10.7 8.3 11.9 12.2 13.9 10.1 14.0 14.2 14.0 Ga 24.2 23.0 26.1 25.7 24.0 24.6 26.2 25.9 25.8 Cu 212 221 245 217 261 91 284 298 283 Co 49 57 45 50 37 30 50 52 51 Zn 114 103 104 116 112 80 126 124 125 Pb 555594566 U 0.2 0.9 1.2 1.2 2.1 1.5 1.2 1.4 0.7 Th 4.3 3.0 4.7 5.1 5.1 4.3 5.5 5.3 4.1 La 14 10 16 19 32 13 21 21 24 Ce 27 22 37 40 61 32 37 38 42

81 Miranda Flat

Sample MB97-74A MB97-75 MB97-76A MB97-77 MB97-78 MB97-79 MB97-80 MB97-81 MB97-82 MB97-83 Location MF MF MF MF MF MF MF MF MF MF Position 10 11 12 13 14 15 16 17 18 19

SiO2 50.35 49.69 50.44 53.42 53.48 52.70 54.90 51.12 49.70 50.04

TiO2 2.37 2.66 2.57 2.52 2.51 2.21 1.91 2.54 2.49 2.46

Al2O3 18.14 17.01 15.57 14.52 14.23 15.15 15.26 14.50 15.55 15.20

Fe2O3 11.96 12.93 13.41 11.93 11.89 11.24 9.81 12.50 13.24 13.18 MnO 0.17 0.18 0.19 0.20 0.21 0.19 0.18 0.20 0.20 0.19 MgO 3.63 3.44 4.48 3.36 3.44 3.64 2.92 3.89 5.01 4.82 CaO 9.13 8.80 8.13 7.01 7.00 6.69 5.72 7.20 7.44 7.26

Na2O 3.71 3.60 3.67 3.59 3.69 3.90 4.13 3.73 3.62 3.75

K2O 1.21 1.38 1.59 2.22 2.35 2.36 2.84 2.07 1.78 1.81

P2O5 0.49 0.55 0.69 0.66 0.85 0.98 1.27 0.69 0.62 0.62 L.O.I. 0.02 0.40 0.05 0.56 0.33 0.20 0.68 1.44 -0.03 -0.31 Total 101.18 100.64 100.79 99.99 99.98 99.26 99.62 99.88 99.62 99.02

Ni 46 47 55 14 13 24 7 24 80 79 Cr 8 11 1 22 23 15 13 26 31 33 Sc 23.9 26.2 27.3 27.6 27.7 23.6 17.6 27.7 26.4 24.5 V 329 349 353 286 269 248 172 345 340 318 Ba 485 526 589 784 779 858 1023 686 713 745 Rb 17 24 30 46 51 44 54 35 31 30 Sr 606 566 560 472 468 623 701 474 548 543 Zr 186 205 220 273 275 244 253 242 237 240 Y 32 35 37 43 43 39 38 40 39 39 Nb 13.2 14.1 14.9 17.4 17.3 15.4 15.7 16.3 15.8 16.0 Ga 27.9 28.4 27.3 25.9 25.7 23.6 23.3 24.8 25.8 26.6 Cu 200 302 217 250 170 137 47 147 167 185 Co 33 34 38 29 31 30 23 35 42 40 Zn 103 104 112 131 129 130 139 126 135 127 Pb 68 7 911911877 U 1.4 1.4 0.7 2.5 2.4 2.0 1.4 1.7 1.4 1.8 Th 4.8 5.4 5.6 7.4 8.1 7.2 7.6 6.8 5.4 5.7 La 21 25 26 34 35 33 35 29 29 30 Ce 39 44 53 68 67 61 75 61 56 59

82 Miranda Flat & Mickey Butte Dikes

Sample MB97-84 MB97-85 MB97-86 MB97-87 MB97-88 MB97-41 MB97-54 MB97-60 MB97-61 Location MF MF MF MF MF MB Dikes MB Dikes MB Dikes MB Dikes Position 20 21 22 23 24

SiO2 53.05 53.11 52.81 47.38 47.54 48.81 50.07 48.10 48.65

TiO2 1.95 2.06 2.07 2.06 2.34 1.70 2.47 2.34 2.28

Al2O3 15.73 15.73 16.00 16.30 15.58 19.98 15.68 16.42 15.69

Fe2O3 10.82 10.91 11.24 13.43 14.08 10.01 14.12 12.33 13.74 MnO 0.17 0.18 0.17 0.18 0.18 0.13 0.20 0.17 0.17 MgO 3.94 3.63 3.87 6.34 5.36 3.88 4.37 5.00 5.04 CaO 6.69 6.44 6.64 8.77 8.11 9.45 8.30 8.48 7.95

Na2O 3.88 4.00 4.08 3.25 3.40 3.45 3.43 3.33 3.37

K2O 2.20 2.31 2.08 0.89 1.14 0.82 1.37 1.13 1.31

P2O5 0.72 0.71 0.76 0.32 0.49 0.37 0.33 0.37 0.54 L.O.I. 0.32 0.38 -0.05 0.40 0.97 3.01 0.55 1.08 0.48 Total 99.47 99.46 99.67 99.32 99.19 101.61 100.89 98.75 99.22

Ni 44 30 40 136 130 62 40 72 73 Cr 16 16 17 23 35 24 19 29 18 Sc 20.4 19.7 20.8 27.7 27.3 21.2 26.7 26.1 25.6 V 251 239 262 352 372 268 419 365 377 Ba 784 847 802 345 430 336 542 436 471 Rb 40 43 35 13 18 13 28 18 28 Sr 661 678 675 532 512 648 514 535 520 Zr 212 220 211 144 178 108 188 174 175 Y 32 33 32 29 32 22 34 33 31 Nb 14.0 14.1 13.7 10.4 12.5 8.6 12.8 12.7 12.4 Ga 24.7 24.3 24.4 24.0 24.7 24.0 25.3 25.3 24.9 Cu 96 80 74 177 184 139 260 200 204 Co 33 29 33 52 47 35 42 40 47 Zn 121 121 119 111 122 87 121 103 115 Pb 9108464856 U 2.3 2.4 1.7 1.3 1.1 0.9 2.1 1.1 1.7 Th 5.4 7.1 7.1 3.8 4.0 3.4 5.3 4.4 4.7 La 30 29 27 12 18 12 22 20 20 Ce 59 64 56 31 36 26 45 45 40

83 Folly Farm West Andesites

Sample MB97-1 MB97-2 MB97-3A MB97-4 Location FFW And FFW And FFW And FFW And Position 12 3 4

SiO2 57.37 59.46 57.83 58.11

TiO2 1.95 2.03 1.96 1.98

Al2O3 13.05 13.45 13.24 13.09

Fe2O3 11.15 8.86 11.35 11.36 MnO 0.14 0.14 0.15 0.15 MgO 2.19 2.13 2.21 2.23 CaO 5.31 5.51 5.37 5.39

Na2O 3.33 3.28 3.22 3.22

K2O 2.47 2.80 2.67 2.55

P2O5 0.38 0.39 0.33 0.35 L.O.I. 0.77 1.08 0.84 0.84 Total 98.11 99.13 99.17 99.27

Ni 6128 8 Cr 16 25 20 19 Sc 25.2 25.6 24.2 24.3 V 287 330 272 294 Ba 964 1226 927 999 Rb 70 78 75 74 Sr 313 333 312 313 Zr 270 282 274 272 Y 39 44 40 39 Nb 15.2 15.5 15.3 15.5 Ga 19.6 20.6 20.3 20.1 Cu 18 17 17 18 Co 29 40 27 29 Zn 129 131 128 132 Pb 15 15 15 15 U 2.9 2.8 3.2 1.6 Th 6.8 8.2 7.8 7.6 La 26 30 33 28 Ce 58 64 56 59

84 APPENDIX C: SAMPLE LOCATIONS AND DESCRIPTIONS

Sample ID: MB97-1 Type: Younger Andesite 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Top of first exposure, blocky flow in places, oxidized base. Large vesicles aligned parallel with flow. Zeolite/Calcite in vesicles. Conchoidal fracture in places and banding. Micorphenocrysts of plagioclase and spotted texture in a dark gray matrix. ~12 to 15’ thick, sampled near base.

Sample ID: MB97-2 Type: Younger Andesite 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Massive, weathered texture similar to 97-1. Micorphenocrysts of plagioclase, green alteration in places. Again conchoidal fracture and spheroidal weathering to blocks within larger round spheroids. ~10’.

Sample ID: MB97-3A & B Type: Younger Andesite 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: MB97-3A: Highly oxidized flow top with large areas of vesicular rubble. Appears to be flow ropes along upper surfaces. Spheroidal weathering in massive central portion of flow above a highly sheared platy base. Brecciated below. From massive zone, massive dark gray matrix with microphenocrysts of plagioclase. Large vesicles when present. MB97-3B: From upper brecciated portion of lava flows. Dark gray-black with oxidized portions, breccia clast (vitrophyric).

Sample ID: MB97-4 Type: Younger Andesite 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Massive, dark gray lava flow like 97-1,2,3. Microphenocrysts of plagioclase in a dark gray matrix. Vesicles aligned parallel to flow, discontinuous exposure, ~15’ maximum in places.

Sample ID: MB97-5 Type: Steens Basalt- plagioclase-phyric 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Upper flow of Steens Basalt sequence. Coarsely plagioclase-phyric (1-3 cm phenocrysts) in a gray, diktytaxitic matrix. Vesicular top and bottom, with massive interior that has slight columnar jointing. Entire exposure ~50’ and may be a compound flow.

Sample ID: MB97-6 Type: Steens Basalt- plagioclase-phyric 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Coarsely plagioclase-phyric (1-2cm phenocrysts), more acicular plag than MB97-5. Diktytaxitic gray matrix. Again, slight columnar jointing within the flow, ~40’ thick.

85 Sample ID: MB97-7 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: ~50’ thick. Small phenocrysts of plagioclase within a light gray matrix. Mafic microphenocrysts (aug?) also present. Massive, non-vesiculated matrix, much less phyric than MB97-6 & 7.

Sample ID: MB97-8 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Thinner flows below this flow. Vesicular to almost diktytaxitic gray matrix. <1 cm plagioclase phenocrysts with an occasional larger crystal. Altered mafic minerals (pyroxene?) visible. Overall, this material is also relatively altered compared to overlying basalt flows.

Sample ID: MB97-9A Type: Tuffaceous horizon 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Bright orange baked zone between flows. ~1.5” thick with apparent welded fiamme inside; welded fall deposit. MB97-B; sampled plagioclase-phyric pseudo-column. May have been from slump block, sample not processes for chemistry.

Sample ID: MB97-10 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: ~15’ thick exposure, up to 25’. Fine grained massive and no vesicles. Microphenocrysts of plagioclase within a dark gray matrix. Possible olv and pyroxene, altered. Rubbly top that grades into massive lower jointed zone.

Sample ID: MB97-11 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: ~20’ thick with a rubbly base. Pseudo-columnar massive central portion of the flow; vesicular and rubbly top. Dark gray sugary texture with small vesicles present. Microphenocrysts of plagioclase with some larger laths. Olv and pyroxene also present.

Sample ID: MB97-12 Type: Steens Basalt- aphyric 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Highly undulatory outcrop and vesicular throughout; rubbly flow. Dark gray almost diktytaxitic matrix w/abundant microphenocrysts of plagioclase feldspar.

Sample ID: MB97-13 Type: Steens Basalt- aphyric 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Rubbly top and bottom with massive interior. Slight columnar jointing in interior portion of the flow. Sparse plagioclase phenocrysts in places and olivine microphenocrysts altered to iddingsite.

86 Sample ID: MB97-14 Type: Steens Basalt- aphyric 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Small flow (~10’), red/purple/gray matrix w/abundant microphenocrysts of plagioclase (up to 2mm). Matrix color due to oxidation because of thin thickness?

Sample ID: MB97-15 Type: Steens Basalt- aphyric 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: ~40-50’ thick, with rubbly base and top. Very fresh, HAOT-like material. Slight columnar jointing present and abundant vesicle trains. Light gray matrix with microphenocrysts of olivine and plagioclase (up to a couple mm). White alteration is present in small quantities. Rubble zone is ~15’ thick.

Sample ID: MB97-16 Type: Steens Basalt- aphyric 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Flow appears to be ~20’ thick. Vesicular top, massive center and rubbly basal portion. Seems to be finer near the base, where the sample was taken. Fine grained and massive with abundant microphenocrysts of plagioclase (scattered larger laths also present, up to ~1.5 cm). Slight presence of olivine, maybe some altered microphenocrysts.

Sample ID: MB97-17 Type: Steens Basalt- aphyric 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Small 10-15’ thick flow. Basal rubble zone/flow breccia and some horizontal jointing. Large up to boulder sized clasts in basal flow breccia. Fine to medium grained matrix with white alteration and a weathering rind. Microphenocrysts of olivine (iddingsite) and plagioclase. Again, dark gray color with small vesicles (vesicle-fill present). Might also be some pyroxenes (small dark crystals with no red-alteration).

Sample ID: MB97-18 Type: Steens Basalt- Blocky a’a like 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: From thick brecciated flow. Large boulder sized clasts of massive basalt within monolithologic breccia. Appears to be an a’a flow. Blocky zones within breccia appear to be draped by zones of more massive material. Sample taken from massive zone. Dark gray matrix with white alteration (spotty texture). Abundant microphenocrysts of plagioclase and olivine.

Sample ID: MB97-19 Type: Steens Basalt- plagioclase-phyric 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Small rubble-crop below MB97-18 (~9’ high). Slight columnar jointing in places, large vesicles throughout the outcrop. At base, thin exposure of white tuffaceous material that appears to be in place. Sample has large laths of plagioclase (up to ~2cm) within a light gray matrix. Altered olivine also present (almost metallic looking). Sample collected from base of rubble-crop, appears that this may be a slump block from further up-section. Chemically similar to MB97-5 and 6.

87 Sample ID: MB97-20 Type: Steens Basalt- aphyric 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Thin ~15’ thick flow with upper rubble zone and massive interior. Platy horizontal fractures abundant within this massive zone. Fine to medium grained gray matrix with small microphenocrysts (up to ~3mm) of plagioclase. Olivine phenocrysts (up to ~1mm) also present, but highly weathered (some nice euhedral crystals are present). Diktytaxitic texture present in matrix and pipe vesicles present throughout the flow.

Sample ID: MB97-21 Type: Steens Basalt- Blocky a’a like 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Thick upper rubbly zone, a’a like. In places, more massive material. Overall, this mixture of material is ~80’ thick. ~30-40’ down is a very distinct ~3’ thick orange, oxidized baked zone. Blocky weathering with numerous 1-2 mm clasts within matrix. Could be a soil or also could be a tuffaceous zone (Oregon Canyon tuff?). This appears to underlie the thick massive blocky material that MB97-20, 21, and 22 were sampled from. MB97-21 taken in upper portion of thick rubbly zone form a more massive portion of this zone. Small plagioclase crystals (~2mm) in a dark gray, fine-grained matrix. 1.5-2mm olivine crystals also present.

Sample ID: MB97-22 Type: Steens Basalt- Blocky a’a like 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Very similar to 20 and 21; Plagioclase phenocrysts (up to ~4mm) and abundant olivine (up to ~2mm) in a highly altered bluish matrix.

Sample ID: MB97-23 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: ~50’ thick rubbly to massive flow. Thick massive section with abundant vesicle trains. Material is similar but larger microphenocrysts of plagioclase in a highly altered dark gray-blue matrix (water alteration?) Olivine present as well, also highly altered and abundant vesicle fill present throughout the outcrop.

Sample ID: MB97-24 Type: Steens Basalt- aphyric 7.5’ topo quad: Lambing Canyon, OR Locality: Folly Farm West Description: Large (~10-15’ thick) rubbly zone separates MB97-23 and 24. Extremely fine-grained dark gray matrix. Abundant plagioclase microphenocrysts (<1 mm) and large and numerous vesicles throughout the massive zone of the flow. Blue alteration also present and no noticeable olivine.

Sample ID: MB97-25 Type: Steens Basalt- aphyric 7.5’ topo quad: Folly Farm, OR Locality: Route 78 north Description: Fine grained, light gray-blue matrix. Microphenocrysts of plagioclases with some larger crystals scattered throughout (up to 4 mm; <1-2%). Sample collected from ~10’ from rubbly top of hill (flow-top?), exposure ~25’ thick with platy top and massive interior. Quite vesicular.

88 Sample ID: MB97-26 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Folly Farm, OR Locality: Route 78 north Description: After highly oxidized flow break, a new ~20-25’ thick flow. Fresh, fine grained gray matrix. Abundant plagioclase microphenocrysts and phenocrysts (up to ~1 cm long, ~0.4 to 0.5 cm wide). Plagioclase phenocrysts are zoned and twinned, with dark rims. Again, more plagioclase than previous sample and larger crystals.

Sample ID: MB97-27 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Folly Farm, OR Locality: Route 78 north Description: Thin flow with vesicular top and rubbly bottom (~15’ total). Slight sense of columnar jointing also. Sampled from base of massive interior portion. Medium grained gray diktytaxitic matrix with large phenocrysts of plagioclase (up to 1.5 cm long), possible altered olivine also. Plagioclase not as abundant or large as plagioclase-phyric flows.

Sample ID: MB97-28 Type: Steens Basalt- aphyric 7.5’ topo quad: Folly Farm, OR Locality: Route 78 north Description: After covered interval, come to rubbly oxidized flow top with vesicular zone below rubble at the top of a massive zone that has platy-jointing. Flow is thin (~10’ thick) and highly vesiculated and weathered throughout. Dark to medium gray fine grained matrix. Microphenocrysts of plagioclase present which are much less abundant than MB97-27. No olivine visible also and alteration present within vesicles.

Sample ID: MB97-29 Type: Steens Basalt- aphyric 7.5’ topo quad: Folly Farm, OR Locality: Route 78 north Description: Sampled from pseudo-columnar region of thin flow. Highly weathered and full of vesicles. Open textured dark to medium gray matrix. Small microphenocrysts of plagioclase are abundant with possible clusters of olivine. Occasional larger (~3mm) plagioclase crystals are also present.

Sample ID: MB97-30 Type: Steens Basalt- plagioclase phyric 7.5’ topo quad: Folly Farm, OR Locality: Route 78 north Description: Down through covered interval and come to zone of plagioclase-phyric flows. First is ~15’ thick with vesiculated bottom. Plagioclase phenocrysts up to ~3 m long. Open textured gray matrix and abundant phenocrysts of olivine.

Sample ID: MB97-31 Type: Steens Basalt- plagioclase phyric 7.5’ topo quad: Folly Farm, OR Locality: Route 78 north Description: Plagioclase-phyric lava flow, similar to MB97-30. Slight columnar jointing starting to become present and abundant vesicle trains/pipes. Open textured, gray medium grained matrix plagioclase phenocrysts up to 3 cm long and 2 cm wide. Olivine microphenocrysts also present.

89 Sample ID: MB97-32 Type: Steens Basalt- plagioclase phyric 7.5’ topo quad: Folly Farm, OR Locality: Route 78 north Description: ~15’ thick with vesicular and oxidized basal margin. Again, plagioclase-phyric with abundant 2.5 to 3 cm long, 1 to 1.5 cm wide phenocrysts. Open textured, medium grained matrix with some olivine microphenocrysts.

Sample ID: MB97-33 Type: Steens Basalt- plagioclase phyric 7.5’ topo quad: Folly Farm, OR Locality: Route 78 north Description: ~15’ thick. Plagioclase phyric lava flow with slight columnar jointing. Plagioclase phenocrysts are less abundant than previous two samples, but still ~ 3 cm long. Trace olivine microphenocrysts and an open-textured, medium grained gray matrix.

Sample ID: MB97-34 Type: Steens Basalt- plagioclase phyric 7.5’ topo quad: Folly Farm, OR Locality: Route 78 north Description: Again, plagioclase-phyric flow in an open-textured gray matrix. Up to 3 cm long and 2 cm wide plagioclase phenocrysts. Olivine microphenocrysts also present and small vesicle trains throughout the outcrop. ~15’ thick.

Sample ID: MB97-35 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Folly Farm, OR Locality: Route 78 north Description: Very fresh material, ~15’ thick flow. Plagioclase-porphyritic, but not as abundant plagioclase as above flows. Plag is ~2 cm long and up to 0.5 cm wide. Glomeroporphyritic plagioclase and olivine as well. Massive fine to medium grained matrix, abundant vesicles.

Sample ID: MB97-36 Type: Steens Basalt- plagioclase porphyritic 7.5’ topo quad: Folly Farm, OR Locality: Route 78 north Description: Past talus slope. Another ~15’ thick plagioclase-phyric flow. At base, nice oxidized zone. Somewhat rubbly exposure due to weathering and slight hint of columnar jointing. Gray to red- gray matrix which is massive and fine grained. Within this are abundant plagioclase phenocrysts (same size as above samples) and 0.5 to 1 mm olivine microphenocrysts.

Sample ID: MB97-37 Type: Steens Basalt- plagioclase porphyritic 7.5’ topo quad: Folly Farm, OR Locality: Route 78 north Description: Horizontal vesicle trains present and again, a plagioclase phyric flow. ~20’ thick. Plagioclase phenocrysts present (up to ~2 cm) in a fine grained, slightly open-textured matrix. Olivine microphenocrysts are also present and are iddingsitized.

Sample ID: MB97-38 Type: Steens Basalt- plagioclase porphyritic 7.5’ topo quad: Folly Farm, OR Locality: Route 78 north Description: Highly weathered outcrop, likely due to interaction with standing water. Plagioclase- porphyritic, with similar sized crystals as above samples. Iron staining present throughout the outcrop. Gray, open-textured matrix. Possible microphenocrysts of olivine.

90 Sample ID: MB97-39 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: ~20-15’ thick highly vesiculated flow. Fine grained matrix and some scatted plagioclase phenocrysts (up to ~3 cm long) that are aligned parallel to the flow. Possible small olivine microphenocrysts and alteration.

Sample ID: MB97-40 Type: Steens Basalt- aphyric 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: Uppermost of a package of four small, fine grained flows. ~5-6’ thick with uppermost pahoehoe flow-top and oxidized zone. Sample is dark gray, fine grained with a few microphenocrysts of plagioclase and glomerophenocrysts of plagioclase and olivine.

Sample ID: MB97-41 Type: Steens Basalt- plagioclase phyric dike 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: Plagioclase-phyric dike that appears to be ~10’ thick. Plagioclase phenocrysts up to ~3 cm long. Plagioclase crystals are altered dark-blue/brown (secondary Fe-staining?). Possible olivine. Fine grained-dark gray to black matrix also present.

Sample ID: MB97-42 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: Fine to medium grained light/medium gray matrix. Abundant phenocrysts of olivine (~2 mm) and plagioclase phenocrysts up to ~ 1 cm long. Some glomeroporphyritic plagioclase and olivine.

Sample ID: MB97-43 Type: Steens Basalt- aphyric 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: Down through three flows similar to MB97-42. Plagioclase content seems to increase down- section also. ~15’ thick with a massive center. Small microphenocrysts of plagioclase in a dark gray, fine-grained matrix. Slight alteration.

Sample ID: MB97-44 Type: Steens Basalt- blocky 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: Down through thick (~80’) exposure of blocky material. Abundant, highly oxidized clasts which either indicate a very thick blocky flow or a locus of local eruptive activity. Given the close proximity of this material with the dike (MB97-41), this material could be a near vent facies. On the other hand, it could also be an overthickened brecciated zone related to a flow. Some draping of rubble over massive zones and it appears in places that basal shearing is present in the massive zones. Sample taken from massive zone; large phenocrysts of plagioclase in a fine-grained matrix are present, resembling the intermediate plagioclase variety. Olivine microphenocrysts also present.

91 Sample ID: MB97-45 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: Upper flow of 4-5 thin (~5 - 10’ thick) flows. Coarser textured than MB97-46; small laths of plagioclase in a gray sugary matrix. Abundant olivine microphenocrysts also present.

Sample ID: MB97-46 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: Lowermost sample in sequence of thin flows. ~15’ thick flow with small laths of plagioclase within a gray matrix. Microphenocrysts of olivine also present and some vesicles also. Blue alteration present as well.

Sample ID: MB97-47 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: Sampled massive section of relatively thin flow. Massive with occasional vesicles in a light gray, sugary textured matrix. Abundant microphenocrysts of olivine and plagioclase. Large phenocrysts of plagioclase also present in places (~ 8 mm). Glomeroporphyritic clots of olivine and plagioclase present as well.

Sample ID: MB97-48 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: More plagioclase-phyric thin flow. Flow is ~5 – 10’ thick and highly altered/weathered. Dark gray altered open textured matrix with abundant phenocrysts of plagioclase (up to 0.5 cm and equant). Plagioclase crystals are rounded in shape and not that tabular. Olivine microphenocrysts also present.

Sample ID: MB97-49 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: ~5-10’ undulatory flow. Fine to medium grained dark gray matrix. Slight vesicles present in hand sample, not many massive zones within flow. Small and thin plagioclase laths along with number of round, larger plagioclase phenocrysts (like MB97-48). Olivine is extremely abundant, almost the most olivine rich we have seen.

Sample ID: MB97-50 Type: Steens Basalt- aphyric 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: Sampled from upper flow of package of 5 thin aphyric flows. Thin (~5’) spheroidal weathering, fine grained material. Dark gray, massive matrix with plagioclase and olivine microphenocrysts. Flows below appear similar, but may have an occasional larger plagioclase phenocryst in places.

92 Sample ID: MB97-51 Type: Steens Basalt- plagioclase phyric 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: ~10 – 15’ thick plagioclase phyric lava flow. Within package of a number of similar flows. Abundant 1-2 cm plagioclase phenocrysts in a dark gray/black fine grained and massive matrix. Some olivine. Overall, outcrops are very rubbly and altered.

Sample ID: MB97-52 Type: Steens Basalt- plagioclase phyric 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: Through ~10’ thick plagioclase-phyric flow to another. Massive and large, with a platy top. ~40’ thick. Sampled from ~20’ up from base of flow. Dark massive matrix with abundant 1-2 cm plagioclase phenocrysts and some small olivine microphenocrysts.

Sample ID: MB97-53 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: Down through thin aphyric flow to a new lithology. ~20-30’ thick with a dark gray/black massive and sugary matrix. Plagioclase phenocrysts are ~0.3 – 0.5 cm long and olivine is also present (1-5 mm). Again, a highly weathered outcrop (greenish color).

Sample ID: MB97-54 Type: Steens Basalt- plagioclase phyric dike 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: From dike(s) near base of Mickey Butte section. Appears to be one dike that splits into four on little knob. Sampled dike trends N20E. 1-3 cm plagioclase phenocrysts in a fine, dark matrix. These dikes are cutting through covered intervals and plagioclase phyric lava flows.

Sample ID: MB97-55 Type: Steens Basalt- aphyric 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: ~25’ thick fine grained flow. Dark gray, fine grained matrix. Plagioclase and olivine microphenocrysts are present. Also appears to be noticeable pyroxenes. Below and above this flow are covered intervals with occasional fine grained dark gray think flows like MB97-55.

Sample ID: MB97-56 Type: Steens Basalt- aphyric 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: ~15’ thick flow of fine grained basalt. Dark, fine grained matrix with microphenocrysts of olivine. No noticeable traces of olivine or pyroxene, but resembles MB97-55. Also, same context; thin aphyric flows between covered intervals.

Sample ID: MB97-57 Type: Steens Basalt- plagioclase phyric 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: ~25’ below exposure of thin aphyric flow. Plagioclase phyric lava flow, similar to dikes. Large 1–3 cm plagioclase phenocrysts in a dark, fine grained matrix. No observed olivine. Very similar to other plagioclase phyric lava flows we have sampled throughout the sections.

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Sample ID: MB97-58 Type: Steens Basalt- aphyric 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: Brown/orange platy aphyric lava flows, similar to those exposed above MB97-57. Dark gray/greenish matrix in hand sampled, highly fractured and massive. No obvious phenocrysts of any type. Flow is ~30’ thick.

Sample ID: MB97-59 Type: Steens Basalt- aphyric 7.5’ topo quad: Mickey Springs, OR Locality: Mickey Butte Description: 20’ below MB97-58 and rubble zone. Again, similar aphyric material. Dark, fine grained matrix with no obvious plagioclase or olivine crystals. Overall, outcrop is platy and extremely weathered. Basal material at this location.

Sample ID: MB97-60 Type: Steens Basalt- plagioclase phyric dike 7.5’ topo quad: Mickey Springs, OR Locality: South of Mickey Butte (Sec 15, T33S, R35E, NE¼, NE¼) Description: Dike exposed south of Mickey Butte. Trends N5W. 1-6 cm plagioclase phenocrysts in a brown weathered matrix. Some phenocrysts are up to 4 cm wide. Plag phenocrysts are extremely flow aligned at this location. Dike is ~20’ thick and dipping to the SW at 75-80°. Dike cuts through fine grained material that weathers orange (similar to lower Mickey Butte flows).

Sample ID: MB97-61 Type: Steens Basalt- intermediate plagioclase dike 7.5’ topo quad: Miranda Flat, OR Locality: South of Mickey Butte (Sec 10, T33S, R35E, SE¼, SE¼) Description: Other dike exposed south of Mickey Butte. Dike trends N10E and splits up-slope from sampled location and continues as two dikes. Dike is platy and massive, unlike other dikes sampled from the vicinity of Mickey Butte. Fine grained and massive, easily fractured matrix. Plagioclase phenocrysts are present and are as big as ~3.5 cm. Olivine microphenocrysts are also present. Dike is ~15-20’ thick.

Sample ID: MB97-62 Type: Welded tuff 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Above Miranda Flat section on higher hill; Miranda Flat section is down-dropped along a west dipping normal fault from this high point. Encounter thin tuffaceous rubble. Buff to pink color, welded fiamme and abundant feldspar.

Sample ID: MB97-63 Type: Steens Basalt- aphyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Upper mafic lava flow below tuffaceous material. Fine grained, gray open-textured matrix. Phenocrysts of feldspars are up to 0.1 cm and microphenocrysts of olivine also appear present. Smaller plagioclase laths are present in the matrix.

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Sample ID: MB97-64 Type: Steens Basalt- aphyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Dark gray matrix with abundant vesicles. Some degree of open-texturedness. Plagioclase and olivine microphenocrysts are also present.

Sample ID: MB97-65 Type: Steens Basalt- aphyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Massive and very fine-grained dense matrix. Glomeroporphyritic olivine and plagioclase. Olivine and plagioclase microphenocrysts are also abundant. Slight columnar jointing also present and in some places this material is very platy.

Sample ID: MB97-66 Type: Steens Basalt- aphyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Massive and very fine grained lava flow. Abundant plagioclase and olivine microphenocrysts and glomeroporphyritic clusters of both minerals. Sugary, gray matrix. Similar to MB97-65 material. ~25’ thick.

Sample ID: MB97-67 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Thin, ~3-5’ thick flow. Slightly open-textured matrix. Abundant olivine microphenocrysts and plagioclase phenocrysts up to ~1.5 cm long.

Sample ID: MB97-68 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: ~10 thick, massive flow. Open textured, dark gray matrix. Plagioclase phenocrysts up to ~3cm long, olivine microphenocrysts and glomeroporphyritic plagioclase + olivine clots also present. Vesicle trains present within the outcrop and in the hand sample.

Sample ID: MB97-69 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: ~15’ thick lava flow. Abundant microphenocrysts of olivine within a light gray, fine grained massive altered matrix. No obvious plagioclase laths within the matrix. Possibly some small pyroxene microphenocrysts.

Sample ID: MB97-70 Type: Steens Basalt- plagioclase phyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: ~25’ thick plagioclase phyric lava flow. Columnar jointing present. Light gray open- textured holocrystalline matrix. Abundant olivine and plagioclase phenocrysts. Plagioclase up to ~4 cm long and ~3 cm wide. Glomeroporphyritic olivine and plagioclase clots also.

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Sample ID: MB97-71 Type: Steens Basalt- aphyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: At base of MB97-70; vesicular top with a massive interior. Flow is ~15’ thick. Sampled above basal rubble zone. Fine grained, open-textured dark gray matrix. Microphenocrysts of plagioclase and olivine also present. Massive outcrop with platy fractures and vertical jointing.

Sample ID: MB97-72 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: ~5’ thick flow with basal rubble zone. Dark to medium gray fine grained matrix with vesicles. Occasional phenocryst of plagioclase (~0.5 cm) and microphenocrysts of olivine. Alteration present within vesicles.

Sample ID: MB97-73 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: ~12’ thick highly vesicular lava flow. Massive center with pseudo-columnar jointing. Similar petrographically to MB97-72; dark gray fine grained matrix with alteration. Plagioclase phenocrysts up to 2 cm as well as olivine microphenocrysts.

Sample ID: MB97-74A Type: Steens Basalt- plagioclase phyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: ~10’ thick flow. Plagioclase phenocrysts up to ~4 cm long within a fine, open-textured matrix. Easily weathered and no noticeable olivine.

Sample ID: MB97-74B Type: Steens Basalt- plagioclase phyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: From within basal section of MB97-74 flow. Blocky chunks of fine, dark matrix material. Appears to be basal rubble zone breccia. Heterolithologic; some of these clasts are plagioclase phyric, some are olivine rich, some aphyric.

Sample ID: MB97-75 Type: Steens Basalt- plagioclase phyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: ~15’ thick, with slight columnar jointing. Light gray, slightly open-textured matrix. Plagioclase phenocrysts up to ~3 cm long, 2-3 cm wide. Large phenocrysts of olivine within matrix (up to 4 mm).

96 Sample ID: MB97-76A Type: Steens Basalt- plagioclase phyric zone 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: ~30’ thick flow, heterogeneous textures throughout. Texturally zoned from plagioclase phyric, underlain by olivine-rich material, underlain by again a plagioclase-rich zone and then a more aphyric basal part of the flow that is vesiculated. Likely density settling within slowly cooling and inflating flow (evidence of tubes and injection of new batch of magma?; lower ~10’ of 30’ total; upper 20’ is plagioclase phyric)? This sample is from the plagioclase phyric upper. Plagioclase phenocrysts are up to 3 cm and tabular, in a gray/salt and pepper matrix. 3 mm olivine phenocrysts also present and glomeroporphyritic olivine and plagioclase clots. Olivine similar in size to that found in MB97-76C.

Sample ID: MB97-76B Type: Steens Basalt- aphyric zone 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Dark, aphyric material at base of plagioclase phyric zone (liquid!). Dark and massive with some plag laths (sparse, large laths). Highly vesicular and very dense. Occasional olivine phenocrysts.

Sample ID: MB97-76C Type: Steens Basalt- olivine zone 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Extremely rich in olivine (> 50%). ~2-3’ above contact with plagioclase phyric material and zone below (within this strange flow). Olivine phenocrysts are up to 4 mm within a gray salt and pepper, fine to medium grained matrix. Plagioclase phenocrysts are present in places, but dominantly stratified olivine phenocrysts.

Sample ID: MB97-77 Type: Steens Basalt- aphyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: ~10’ thick, right below MB97-76 flow. Dark massive, fine grained matrix. Extremely dense material. Sparse vesicles also. Small laths of plagioclase (~1 mm) and no noticeable olivine. Slight open textured also in spots. This may have flowed into above flow or vice-versa (related)?

Sample ID: MB97-78 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Red, highly oxidized material. Silica rich? Appears to be ~15’ thick. Sampled near base where not oxidized. Fine grained dark to medium gray matrix. Occasional large 1 cm plagioclase lath and pyroxenes appear to be in the matrix (< 1 mm).

Sample ID: MB97-79 Type: Steens Basalt- aphyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: ~8’ thick flow, same weathered look as MB97-78. Fairly dense, fine grained gray matrix. No visible olivine, but some small mm sized plagioclase laths. Might be glomeroporphyritic clots of plagioclase and olivine, but hard to tell.

97

Sample ID: MB97-80 Type: Steens Basalt- aphyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Again, ~10’ thick flow, similar to MB97-78 and 79. Massive and vesicular in places; sparkly textured. Fine grained, dark matrix with plagioclase in the matrix. No noticeable olivine and possibly some pyroxene.

Sample ID: MB97-81 Type: Steens Basalt- intermediate plagioclase 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Down through thick rubble zone (10-15’). Platy base of flow. Dark gray, fine grained matrix with plagioclase phenocrysts up to 1.3 cm. Olivine microphenocrysts also present (2-3 mm). Smaller plagioclase laths are present too (1-3 mm). Bluish alteration present within vesicles.

Sample ID: MB97-82 Type: Steens Basalt-aphyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: ~20’ thick, blockier flow than those above. Massive, dense, fine grained dark matrix. Microphenocrysts of plagioclase (1-2 mm) present and no noticeable olivine. Thickness varies from place to place and within this flow are large boulder sized clasts and small, scoria fragments.

Sample ID: MB97-83 Type: Steens Basalt- aphyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Massive, horizontally fractured unit. Fine to medium grained light gray matrix. Small microphenocrysts of plagioclase are present within matrix. Maybe olivine, but hard to tell.

Sample ID: MB97-84 Type: Steens Basalt- aphyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Rubbly flow top, thin ~3’ thick flow. Massive center, then break for underlying thin flow. Fine grained dark matrix. Plagioclase phenocrysts are present (up to 1cm long, but mostly all small!). Possible olivine but real small and maybe some pyroxene as well.

Sample ID: MB97-85 Type: Steens Basalt- aphyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Thin, ~10’ thick flow. Dark gray, fine grained matrix. Plagioclase phenocrysts are present, up to 5 mm and sparse. Olivine microphenocrysts also present.

Sample ID: MB97-86 Type: Steens Basalt- aphyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: Through 2-3’ of rubble to third flow in this sequence of thinner flows (~15-20’ thick). Dark gray, sugary matrix with plagioclase laths up to 4 mm. Olivine microphenocrysts also present and more abundant than MB97-85. Glomeroporphyritic clots of olivine and plagioclase present too.

98 Sample ID: MB97-87 Type: Steens Basalt- plagioclase phyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: ~30’ and very massive. Bottom is platy. Fine grained light to medium gray matrix. Plagioclase phenocrysts up to 3 cm long are abundant. Olivine present also, up to 3 mm. Both minerals are present in glomeroporphyritic clots.

Sample ID: MB97-88 Type: Steens Basalt- plagioclase phyric 7.5’ topo quad: Miranda Flat, OR Locality: Miranda Flat Description: ~7’ of upper rubble zone to an ~15’ thick flow (total). Fine, massive, dark gray matrix. Plagioclase phenocrysts are abundant, but smaller than MB97-87 (1-3 cm). Olivine microphenocrysts also present, but no glomeroporphyritic clots.

99 Diverse mid-Miocene Silicic Volcanism Associated with the Yellowstone- Newberry Thermal Anomaly

Matthew E. Brueseke a1, William K. Hart b, and Matthew T. Heizler c

a Department of Geology/Geography, Eastern Illinois University, 600 Lincoln Avenue Charleston, IL 61920-3099

b Department of Geology, Miami University, 114 Shideler Hall, Oxford, OH 45056-2473

c New Mexico Bureau of Mines and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801

Accepted by Visiting Editor for publication in the Bulletin of Volcanology special volume “Petrogenesis and volcanology of anorogenic rhyolites in the northwest intermountain USA"

4/4/06 version

1 Corresponding author. Phone: 1-(217)-581-2827, Fax: 1-(217)-581-6613 E-mail addresses: [email protected] (M. Brueseke), [email protected] (W.K. Hart), [email protected] (M. Heizler) 100 Abstract The Santa Rosa-Calico volcanic field (SC) of northern Nevada is a complex, multi- vent mid-Miocene eruptive complex that formed in response to regional lithospheric extension and flood basalt volcanism. Santa Rosa-Calico volcanism initiated at ~16.7 Ma, concurrent with regional Steens-Columbia River flood basalt activity and is characterized by a complete compositional spectrum of basalt through high-silica rhyolite. To better understand the relationships between upwelling mafic magmatism, coeval extension, and magmatic system development on the Oregon Plateau we have conducted the first comprehensive study of Santa Rosa-Calico silicic volcanism. Detailed stratigraphic-based field sampling and mapping illustrate that silicic activity in this volcanic field was primarily focused along its eastern and western margins. At least five texturally distinct silicic units are found in the western Santa Rosa-Calico volcanic field, including abundant lava flows, eruptive loci, and shallow intrusive bodies. Similar physical features are found in the eastern portion of the volcanic field where four physically distinct units are present. The western and eastern Santa Rosa-Calico units are characterized by abundant macro- and microscopic disequilibrium textures, reflecting a complex petrogenetic history. Additionally, unlike other mid-Miocene Oregon Plateau volcanic fields (e.g. McDermitt), the Santa Rosa-Calico volcanic field is characterized by a paucity of caldera- forming volcanism. Only the Cold Springs tuff, which crops out across the central portion of the volcanic field, was caldera-derived. Major and trace element geochemical variations are present within and between eastern and western Santa Rosa-Calico silicic units and these chemical differences, coupled with the observed disequilibrium textures, illustrate the action of open- system petrogenetic processes and melt derivation from heterogeneous source materials. The processes and styles of Santa Rosa-Calico silicic magmatism are linked to three primary factors, local focusing of and thermal and material contributions from the regional flood basalt event, lithospheric extension within the northern portion of the Northern Nevada rift, and interaction of mid-Miocene silicic magmas with pre-Santa Rosa-Calico lithosphere. Similar processes and styles of mid-Miocene silicic volcanism likely occurred across the Oregon Plateau in regions characterized by both focused lithospheric extension and localized mafic magmatism. Keywords: Miocene, Oregon Plateau, Owyhee-Humboldt, Santa Rosa-Calico, silicic, Steens Basalt, Yellowstone

101 Introduction

Nowhere is volcanism associated with the Yellowstone-Newberry mantle upwelling more diverse than in the Oregon-Idaho-Nevada tri-state region, the southeastern Oregon Plateau. Continuous mafic volcanism from ~16.7 Ma to the present as well as the only silicic volcanism associated with the initial manifestation of the upwelling is found across this region. Recent studies dealing with mid-Miocene northwestern United States volcanism have focused on the Steens and Columbia River flood basalts, their relationship to younger regional Cenozoic volcanism, and its relationship to mid-Miocene mineralization and rift-development (Fig. 1a; Zoback et al. 1994; Wallace and John 1998; Cummings et al. 2000; John and Wallace 2000; John et al. 2000; John 2001; Camp et al. 2003; Wallace 2003; Camp and Ross 2004; Jordan et al. 2004). While these studies have focused attention on regional mafic volcanism and silicic volcanism peripheral to the loci of mid-Miocene activity (the Oregon Plateau), the details of mid-Miocene Oregon Plateau silicic activity remain poorly understood. The most comprehensive information comes from the McDermitt volcanic field, also often cited as the “ground zero” of the Yellowstone-Snake River plain volcanic system (Pierce and Morgan 1992). However, numerous other large and small, dominantly silicic mid-Miocene volcanic systems are present across the Oregon Plateau (Fig. 1b), including the Santa Rosa-Calico volcanic field (SC) of northern Nevada (Brueseke and Hart 2001). The SC lies at the junction of the Northern Nevada rift and Owyhee Plateau (Fig. 1b), an ideal location to further investigate the relationships between mid-Miocene flood basalt volcanism, magmatic system development, and tectonism. In this paper, we present a portion of the results of the first comprehensive field, chronostratigraphic, geochemical, and petrologic study of the SC, the portion focused on the silicic components (>64 wt. % SiO2). This contribution will (1) distinguish the SC from contemporaneous Oregon Plateau volcanic systems (e.g. McDermitt, Owyhee-Humboldt), (2) formally define the SC as a locus of mid-Miocene volcanism and potential source for regionally exposed tephra, (3) detail the spatial, temporal, physical, and bulk chemical characteristics of the silicic magmatic products, and (4) provide first-order constraints on the petrogenetic processes active within the SC. Isotopic studies now underway will allow for a more detailed petrogenetic treatment and will be presented elsewhere in the context of the entire suite of SC magmatic products.

102

The Santa Rosa-Calico Volcanic Field: Regional Overview and Geologic Setting At ~16.7 Ma, the Steens flood basalt started erupting across the Oregon Plateau. This unit is best known at its Steens Mountain type section where ~1 km of ~16.3 Ma lava flows crop out; however, other regionally exposed Steens basalt eruptive loci and flows are found across the Oregon Plateau (Hart and Carlson 1985; Carlson and Hart 1987; Camp et al. 2003; Brueseke et al. in review). The geologic, chemical, and chronologic evidence found in the Oregon Plateau flood basalt record suggest that Steens Basalt lava flows erupted for at least ~2 m.y., coeval with the development and inception of regional silicic activity (Brueseke and Hart 2000; 2002). Widespread flood basalt activity ceased at ~14 Ma, following which, small volume monogenetic basaltic eruptions have occurred to the present across the region (Hart 1985; Jordan et al. 2004; Shoemaker 2004). Silicic activity ceased in the Idaho-Oregon-Nevada border region at ~14 to 12 Ma and progressed toward the northeast and northwest, forming the Yellowstone and Newberry trends of caldera/dome development (Walker 1969; 1974; MacLeod et al. 1975; Pierce and Morgan 1992; Christiansen et al. 2002; Jordan et al 2004). Initial ~16.7 to 14 Ma Oregon Plateau silicic volcanism was diffuse across the entire region and consisted largely of voluminous ash flow eruptions from large, nested caldera complexes. The best known of these is the McDermitt volcanic field, however; the Northwest Nevada and Lake Owyhee volcanic fields were also active during this period (Noble et al. 1970; Greene and Plouff 1981; Rytuba and McKee 1984; Ach and Swisher 1990; Rytuba et al. 1991; Bussey 1995; Castor and Henry 2000). These mid- Miocene systems were characterized by multiple eruptions of extensive ash-flow sheets and subsequent rhyolite dome formation (Fig. 1b). Less well documented are the abundant rhyolite dome complexes that also formed during the mid-Miocene (Fig. 1b). Regionally, the eruptive products of these mid-Miocene systems are best represented in the tephra fall record and numerous studies have been performed to better characterize these deposits (Perkins et al. 1998; Perkins and Nash 2002).

General Geology The Santa Rosa-Calico volcanic field is located in north-central Humboldt County, Nevada, overlapping with the northern portion of the Santa Rosa mountain range, which forms its western

103 boundary (Fig. 2a). The eastern SC boundary is defined by the Calico Mountains and the southern SC boundary is defined by the basin bounding normal fault at the northern end of Paradise Valley; the northern boundary is less well defined physically (Fig. 2a, b). The northern Santa Rosa range and the Calico Mountains help delineate the central SC, an oval topographic depression (the Goosey Lake depression), which is divided into two distinct sub-basins. Most of the SC lies within the Humboldt-Toiyabe National Forest and topography in the SC is rugged. Pleistocene glaciation and syn- and post-SC faulting provide excellent exposures, and numerous unimproved roads allow reasonable access to most portions of the SC. Prior work includes mapping ± petrologic/geochemical studies of the metamorphic and granitoid core of the Santa Rosa Range (e.g., Compton 1960; Shieh and Taylor 1969; Stuck 1993), reconnaissance stratigraphic, chronologic, and petrologic/geochemical studies of volcanism along the margins of the Santa Rosa Range (e.g., LeMasurier 1965; 1968; Larson et al. 1971; Hart and Carlson 1985; Carlson and Hart 1987; Mellott 1987), mapping and geochemical studies of volcanic formations and volcanic-hosted epithermal mineral deposits in the Buckskin-National region of the northern SC (e.g., Winchell 1912; Lindgren 1915; Roberts 1940; Willden 1964; Vikre 1985a, b), and mapping and remote sensing studies of regional fault and vent patterns (e.g., King 1984; McCormack 1996). A package of Triassic metasedimentary rocks and a suite of Cretaceous granitoid plutonic bodies are exposed throughout both the main Santa Rosa range and its southern extension, the Bloody Run Hills (Compton 1960; Stuck 1993; Wyld et al. 2001; Wyld et al. 2003). Within the granitoid suite, two isotopically and chemically distinct intrusive events are recognized; the ~102 Ma Santa Rosa-Andorno group and the ~85 Ma Granite Peak-Sawtooth group (Stuck 1993). Santa Rosa-Andorno group granitoid is also exposed along the basin bounding fault at the southern SC margin, as well as along the eastern base of the Calico Mountains. The Santa Rosa range is being affected by Recent Basin and Range block faulting along its western and southern margins, causing exposed SC units to dip east and north toward the Goosey Lake Depression at ~10 - 15º. This style of deformation initiated at ~ 11 - 10 Ma (Colgan et al. 2004); however, the Santa Rosa range must have existed as a basement high prior to the mid-Miocene. Pre-SC (~35 - 20 Ma) basalt through dacite lava flows unconformably onlap granitoid and metasedimentary basement at high elevations near parts of the northern range crest and locally derived mid- Miocene lava flows onlap granitoid exposed at the base of the Calico Mountains (Figs. 2 and 3).

104 Such paleotopographic irregularities, together with syn-volcanic normal faulting accounts for the variable thickness of the SC assemblage which ranges from at least ~400 to >1000 meters (e.g., LeMasurier 1965; 1968; Vikre 1985a, b; Mellott 1987; Brueseke and Hart unpublished data).

Mid-Miocene geologic processes SC volcanism initiated at ~16.7 Ma and continued to at least ~14 Ma, spanning an ~2.7 m.y. duration of volcanism (Brueseke et al. 2003). Silicic activity occurred from ~16.6 to ~15.4 Ma (Table 1 & Appendix 1 [Electronic supplementary material]; Brueseke et al. 2003). Figure 3 illustrates the complex stratigraphic relationships among pre-SC and SC derived units. The most dominant SC mafic units are chemically identical to regionally exposed Steens Basalt, but were derived locally (Brueseke and Hart 2003). Stewart and Carlson (1976) grouped much of the mafic and intermediate SC units into one unit, however, at least four chemically and geographically distinct andesite-dacite packages are observed (Maloy et al. 2003; Brueseke and Hart 2004). Also, some of the intermediate units mapped by Stewart and Carlson (1976) are part of an older, early Miocene suite of calc-alkaline subduction related andesite/dacites correlative to the Steens Mountain volcanics that underlie Steens Basalt at Steens Mountain (Fuller 1931; Mellott 1987). Full details of the mafic to intermediate SC volcanism will be presented elsewhere. Although sedimentary units are poorly exposed in the study area, lacustrine and fluvial basin-fill strata are present in and proximal to the SC, primarily in two sedimentary depo-centers. The Hardscrabble basin along the southeastern SC margin exposes at least 150 m of interbedded sediment, tephra fall deposits, and ash-flow tuffs. In the Goosey Lake depression, lacustrine and fluvial volcanogenic strata are exposed in patches and reach a maximum thickness of ~20 m. Gilbert et al. (2003) used tephrostratigraphic correlation of tuffs from these central SC deposits to conclude that the Goosey Lake depression was actively subsiding by at least 15.8 m.y. ago. Numerous post-SC faults and fault zones are exposed throughout the study area. The most pronounced zone divides the Goosey Lake depression into two sub-basins and is roughly aligned with numerous silicic eruptive loci, likely reflecting the reactivation of earlier structures. For descriptive purposes throughout the remainder of this paper, we use this fault zone (Fig. 2b) to divide SC silicic volcanism into two broad regions, (1) the western including Eightmile Mountain and vicinity and (2) the eastern including Odell Mountain and Black Dome. The

105 Goosey Lake depression is referred to as the central SC. In addition to discrete silicic domes, abundant dikes, plugs, and shallow intrusive bodies are found throughout the volcanic field and local dikes broadly trend N-S, similar to other regionally exposed mid-Miocene shallow intrusive bodies (e.g. dikes in the Northern Nevada rift; Zoback et al. 1994). Some of these eruptive loci are well exposed and others are highly dissected due to post emplacement faulting and recent glaciation. As a result, in some cases we were unable to identify a specific vent for local units, but facies changes in these units suggested that a vent was nearby (Fig. 2b). This post-eruptive faulting has also helped obscure the areal extent of many SC units. Consequently, volumetric estimates and magma production rates are difficult to estimate without further geologic mapping.

Methods Employed Field sampling and mapping, major and trace element geochemical data, and chronologic data provide a comprehensive examination of the physical, chemical, and temporal diversity of SC silicic units. Samples were collected from SC exposures to provide detailed stratigraphic and spatial information and our sampling campaign followed earlier work by LeMasurier (1965, 1968), Vikre (1985a, b), and Mellot (1987) and W.K. Hart (Pers. Comm.). Approximately two hundred and seventy new samples were collected, of which two hundred and fifty were selected for whole rock geochemical analyses. Included among these samples are the SC silicic units discussed in this study. All samples analyzed for major and trace element concentrations were examined to ensure that the freshest material possible was prepared for analytical work and all obvious signs of post-eruptive alteration (e.g. weathering rinds) were removed during processing. However, no attempt was made to remove all potential syn-magmatic contaminants from each sample prior to geochemical analyses. Individual pumice clasts and bulk glass were only purified and analyzed separately from Cold Springs tuff deposits, however, these results are not discussed in this study. Major element concentrations were measured by Direct Current Argon Plasma Atomic Emission Spectrometry (DCP-AES) at Miami University following the procedures outlined in Katoh et al. (1999). Trace element concentrations were determined by X- Ray Fluorescence (XRF) at Franklin and Marshall College following the technique outlined in Mertzman (2000). 40Ar/39Ar radiometric dating was employed to constrain temporal relationships among SC silicic units. Fifteen sanidine crystals per sample were analyzed by the laser fusion method at the New Mexico Geochronology Research Laboratory (NMGRL). The

106 chronologic results and a summary of the methods are presented in Table 1, while the detailed information is presented in Appendix 1. The silicic units defined and discussed in this study exhibit substantial morphological, petrographic, and geochemical diversity. Most of these units display modal evidence for the incorporation of non-juvenile lithologies or/and complex fluctuations in magma chemistry during their differentiation. We generically refer to this evidence as “disequilibrium” features or textures, which commonly present themselves as sieved and resorbed feldspar crystals, mafic phase rich crystal clots, xenoliths of SC or pre-SC mafic to intermediate volcanic rocks, xenoliths of local granitoid basement, and xenocrysts of Mg-olivine.

Physical Diversity of Santa Rosa-Calico Silicic Units The following four points summarize the key physical observations of SC silicic units and provide the context for the more detailed discussion that follows. They also serve as a foundation from which the whole rock geochemistry must be interpreted. (1) Numerous eruptive loci are identified, are concentrated along the western and eastern margins of the volcanic field, and are primarily of two types, domes and fissures, (2) no SC-derived, large-volume, areally extensive eruptive units are identified, (3) disequilibrium features indicative of open-system magmatic evolution are ubiquitous and the specific nature of these features is linked to eruption location, and (4) the combined spatial, temporal, and petrographic features suggest that SC silicic activity did not result from catastrophic or periodic eruptions from one large magmatic system.

Western SC Silicic volcanism in the western SC is characterized by five physically and chemically distinct units: (1) Hinkey Summit-Coal Pit Peak lava flows (hypersthene rhyodacite of LeMasurier 1965; 1968); (2) Western margin lava flows; (3) Porphyritic rhyolite intrusive bodies and lava flows;

(4) Eightmile Mountain region lava flows (Tr3; Rhyolite of Buckskin Mountain of Vikre 1985b); and (5) Flow-banded rhyolite intrusive bodies and lava flows (laminated rhyolite of LeMasurier 1965; 1968). Because of the mid-Miocene epithermal mineralization that affected northern Nevada, we focused our efforts away from those areas where mineralization and alteration is prevalent (e.g. Buckskin-National, Buckskin Mountain, and Spring City areas). The salient field and petrographic characteristics of these units are described below.

107

Hinkey Summit-Coal Pit Peak lava flows The oldest western SC silicic units are 16.4 Ma rhyodacite-rhyolite lava flows exposed in the vicinity of Hinkey Summit that overlie the basal SC package of mafic and intermediate lava flows (Brueseke et al. 2003; Fig. 2b, Tsc and Fig. 3). In some locations, intermediate lava flows are also found interbedded with these Hinkey-Coal Pit Peak lava flows. These lava flows range from ~15 - 120 m in thickness and appear to be thickest along the southern margin of the SC (southeast of Hinkey Summit), suggesting a nearby source. These lava flows often form thick, crudely columnar jointed walls and weather to thin plates (Fig. 4a). Flow margin breccias are present at some exposures and upper portions of flows are often flow-banded and vitrophyric. Petrographically, they are characterized by abundant 1 - 3 mm sanidine crystals in an aphanitic light grey/pink-purple matrix (Fig. 5). In thin section, plagioclase and sanidine, quartz, clinopyroxene, orthopyroxene, and occasionally biotite are present. Feldspars are often resorbed and sieve-textured and occasional mafic xenoliths are present.

Western margin lava flows The western margin lava flows are best exposed along the crest of the northern Santa Rosa range between Granite Peak and Buckskin Mountain (Fig. 2b, Tsc). Based on their physical similarities, LeMasurier (1965; 1968) did not differentiate these lava flows from the Hinkey- Coal pit silicic units. Western margin silicic lava flows are characterized by platy weathering, cliff-forming outcrops and, in some exposures, by brecciated tops. Some exposures of these lava flows were mapped by Vikre (1985b) and grouped into a northward thickening (up to ~100 m),

extensive unit (Tr2; Crystal-lithic rhyolite tuff). South of Buckskin Mountain, the western margin flows vary in thickness from ~25 - 35 m. This region was extensively glaciated during the Pleistocene; thus, this unit may have been much more areally and volumetrically extensive than current exposures indicate. Field relationships coupled with the prior mapping of Vikre (1985b) suggest that this unit was sourced north of Buckskin Mountain. Petrographically, these lava flows are also similar to Hinkey-Coal pit lava flows; plagioclase and sanidine, sparse mafics (clinopyroxene, orthopyroxene, and occasionally biotite), and quartz are present in flows of this unit (Fig. 5). Resorbed, sieved, and complexly zoned plagioclase is also present.

108 Porphyritic rhyolite lava flows and intrusive bodies The porphyritic rhyolite is best exposed between Hinkey Summit and Buckskin Mountain as abundant shallow intrusive bodies and large flow-dome complexes (Fig. 2b, Tpr and Tsi). These magmas were emplaced/erupted at ~16.2 Ma (Table 1). This is the most widespread and abundant western SC silicic unit and the most spectacular exposure is a large hypabyssal body found southeast of Hinkey Summit. Porphyritic rhyolite dikes are concentrated in two north- south trending zones that cut older SC units between Hinkey Summit and an eroded porphyritic rhyolite coulée south of Buckskin Mountain. These dikes range from 6 - 12 m across and have well exposed vitrophyric margins. Porphyritic rhyolite lava flows are best exposed in coulées located along the southern and western SC margins. A neck and near-vent bedded pyroclastic deposits associated with the western coulée are located south of Buckskin Mountain, and provide the best evidence for local eruption of this unit (Fig. 4b). The eastern margin of this flow forms a prominent ~120 m cliff along the western margin of the Goosey Lake Depression and the areal extent of this unit is at least 11 km2. This areal estimate suggests that at least ~1.3 km3 of magma erupted from this location. Satellite imagery reveals prominent ogives on the surface of this flow, also suggesting an eastward flow direction. Sedimentary units and tuffaceous sediment exposed under the flow margin suggest the Goosey Lake depression already existed prior to the eruption of this unit. The southern flow-dome complex is also well exposed and areally extensive (at least 15 km2). Preliminary mapping and thickness estimates suggest that this lava flow represents at least ~1 - 1.5 km3 of erupted magma. It erupted along the southern margin of the Goosey Lake depression and field relationships indicate that it flowed into a topographic low (Fig. 4c). The domal vent of this coulée is visible on satellite imagery. Unlike other SC silicic units, porphyritic rhyolite intrusive bodies and lava flows are amphibole bearing (Fig. 5). Additionally, these units are much more crystal rich than any other SC silicic unit; amphibole and plagioclase phenocrysts are present up to 5 mm and biotite

phenocrysts are as large as 2 mm (Fig. 6a). This unit lacks clinopyroxene and high-SiO2 varieties lack orthopyroxene. As with other western SC units from the Hinkey Summit region, abundant disequilibrium textures are observed in the porphyritic rhyolite. Resorbed and complexly zoned feldspars are the most common; however, granitoid(?) clots and mafic xenoliths are also present in some samples (Fig 6c, d).

109 Eightmile Mountain lava flows Eightmile Mountain area lava flows crop out along the northern SC margin, in the vicinity of Eightmile Mountain; the thickest exposures (>190-m-thick) are found just east of Eightmile Mountain (Fig 2b, Tem). This unit was mapped by Vikre (1985b) as the Rhyolite of Buckskin 2 Mountain (Tr3). Satellite imagery and field mapping indicate that this unit extends over ~20 km and may represent 2 - 4 km3 of magma. Eightmile mountain lava flows overlie a locally derived package of andesite-dacite lava flows and this stratigraphic relationship suggests that these silicic lava flows are <16 Ma. The best defined vent for this unit is a northeast trending fissure located southeast of Eightmile Mountain. Abundant boulder to pebble sized, pervasively oxidized, frothy tack-welded bombs and highly weathered north-south trending dikes help define this eruptive locus (Fig. 6b). These features and a lack of any domal body or steeply dipping near- vent carapace deposits indicate that Eightmile Mountain lava flows erupted from fissures. Based on topography and stratigraphic relationships, we also believe that another less well exposed vent region is located to the east (Fig. 2b). The near vent facies and overall outcrop patterns associated with Eightmile lava flows are very similar to proximal and distal features exhibited by fissure-fed rhyolite lava flows along the Owyhee front of the western Snake River Plain graben (e.g. Reynolds Creek lava flow(s); Bonnichsen et al. 2004). Eightmile Mountain lava flows weather to plates, similar to Hinkey-Coal Pit and western margin silicic lava flows. Petrographically, these have an aphanitic, gray to purple/pink flow- banded matrix, often with abundant 1 - 3 mm sanidines. Partially resorbed plagioclase feldspar is the major phenocryst phase, with trace amounts of oxides, apatite, and zircon (Fig. 5). Resorbed, sieved, and complexly zoned plagioclase as well as plagioclase + clinopyroxene clots are observed in some Eightmile Mountain area silicic units and when present, are much less abundant compared to other SC silicic units. Highly fractured, euhedral to subhedral Mg-rich olivine xenocrysts are also occasionally present and likely reflect incorporation during magma ascent.

Flow-banded rhyolite lava flows and intrusive bodies Flow banded rhyolite intrusive bodies and lava flows are best exposed in the vicinity of Hinkey Summit, cross-cut older SC volcanic units, and appear to be time-transgressive (Fig. 2b, Tsc and Fig. 3). In some locations along the western margin of the Goosey Lake depression, these lava

110 flows directly overlie porphyritic rhyolite lava flows. Flow banded lava flows are best exposed along the north side of Forest Service road 084 near where it drops into the Goosey Lake Depression and as a large columnar jointed intrusive body located southwest of Hinkey Summit. These lava flows and intrusive bodies are characterized by ubiquitous 1 - 3 mm laminations that give an open-textured appearance and they also lack phenocrysts. Where exposed as lava flows, they are characterized by a basal vitrophyre and abundant 2 - 5 cm wide lithophysae. Additionally 1 - 15 cm rounded obsidian clasts and weathered fragments are present in drainages and on Holocene to Recent pediment surfaces across the SC. These obsidian fragments may be physically related to the flow banded-rhyolite bodies (i.e. an eroded obsidian dome). LeMasurier (1965; 1968) suggested that flow banded rhyolite lava flows may be as thick as 120 m; however, we interpret the bulk of what he mapped as flow banded rhyolite in these thick exposures as a fine-grained variety of the porphyritic rhyolite unit. In other locations around Hinkey Summit, ~2 - 4 m thick white discontinuous dikes are exposed and are all extremely flow-banded and crystal poor. Plagioclase, sanidine, quartz, and biotite are all found in this unit but are rarely observable in hand sample (all <1 mm). Trace amounts of oxide and apatite are also visible in thin section. No disequilibrium features are observed in this unit.

Eastern SC Silicic volcanism in the eastern SC initiated at ~16.6 Ma (Table 1; Fig. 3) and is characterized by three physically and chemically distinct units: (1) Calico Mountains area units; (2) Odell Mountain area units; and (3) Coyote Mountain-Zymns Butte-Black Dome area units. Unlike western SC silicic units, stratigraphic relationships and geochronology suggest that the eastern region silicic volcanism ended by 16.4 Ma. The salient field and petrographic characteristics of these units are described below.

Calico Mountains silicic units The most areally extensive and physically diverse package of eastern SC silicic units are exposed in the Calico Mountains. Calico area units include lava-like ash-flow tuffs, lava flows, and numerous silicic vent deposits (Fig. 2b, Tsc). In the thickest portion of the Calico Mountains pile, these units are interbedded with dacitic and andesitic lava flows and form an ~400 m thick package that unconformably overlies Santa Rosa-Andorno group granitoid. In the lower 200 m

111 of this package, silicic units that crop out are predominantly highly welded, lava-like ash-flow tuffs (Fig. 4d). Where exposed, their basal portions are vitrophyric and lack a brecciated zone. Additionally, some of these units exhibit extreme rheomorphism, lack vesicles, and are xenolith- rich, suggesting their emplacement as pyroclastic flows (Fig. 6e). Sparse 1 - 3 mm phenocrysts of highly resorbed and sieve-textured potassium feldspar and plagioclase are present and are also commonly shattered, again suggesting a pyroclastic origin. Clinopyroxene and orthopyroxene are also present while oxides, apatite, and zircon are found in trace amounts. Two distinct xenolith varieties are observed: plagioclase ± clinopyroxene ± orthopyroxene clots, and clasts that resemble local mafic and intermediate lava flows (Fig. 5). In the upper 200 m, an ~150-m-thick ash-flow tuffs underlies ~50 m of rhyolite lava flows that are exposed at Capitol Peak, the highest peak in the Calico range. The ~150-m-thick Capitol Peak ash-flow tuff is the thickest unit exposed in the northern Calico Mountains and is a distinct cliff-forming unit (Fig. 4e). A vitrophyric base, slight columnar jointing, rheomorphic folding, abundant 2 - 5 cm brown to green mafic xenoliths, and abundant 1 - 3 mm rounded potassium feldspar crystals helps to distinguish this unit from other local silicic units. Some of the mafic xenoliths bear 1 - 3 cm feldspars and physically resemble the plagioclase-phyric variety of Steens Basalt. Stratigraphically above this ash-flow tuff are at least two rhyolite lava flows. Texturally, these units are feldspar and pyroxene rich; potassium feldspar and orthopyroxene are the dominant phenocryst phases. Trace amounts of clinopyroxene, oxide, apatite, zircon, and sparse plagioclase are also present, as are resorbed feldspars. Vents for these units may be located further north in the northern Calico range (King 1984). Further south in the central and southern Calico Mountains, thicker exposures of rhyolite lava flows are present from just north of where Forest Service road 531 crosses the range (Mahogany Pass) to the north fork of the Little Humboldt River (Fig. 2b). Here, the ~50 - 100- m-thick Mahogany Pass rhyolite lava flow overlies basaltic andesite and andesite lava flows. An eroded rhyolite dome that is the source of this sanidine-rich porphyritic unit is present just to the north (Hill 7502 on the Capitol Peak 1:24,000 U.S.G.S. quadrangle). At this small topographic high, vertical fins of vitrophyre rim a broadly circular plug-like feature and are spatially associated with abundant 15 - 20 cm tan feldspar-rich pumice clasts. South of Mahogany Pass in the central and southern Calico range, the Mahogany Pass rhyolite overlies a texturally identical, but compositionally distinct rhyolite lava flow. We interpret this underlying flow to have been

112 erupted from an unidentified vent in the southern Calico range. Although highly dissected by post-emplacement faulting, the field relationships suggest that the central and southern Calico Mountains were characterized by a number of overlapping silicic coulées. Petrographically, the central and southern Calico silicic lava flows are similar and are characterized by phenocrysts of potassium and plagioclase feldspar, oxides, and traces of apatite, zircon, and orthopyroxene (Fig. 5). In these units, plagioclase feldspars are commonly partially resorbed/rounded and occasional granitoid(?) clots are present.

Odell Mountain area silicic units The Odell Mountain area silicic units are best exposed along the east fork of the Quinn River in the vicinity of Odell Mountain, where ~295 m of silicic material crops out Fig. 2b, Tom). Ash- flow tuffs, lava flows, and an eroded rhyolite dome/coulée are exposed in this region. Based on field relationships and geochronology, Odell area silicic volcanism occurred at ~16.6 Ma (Table 1). At Odell Mountain proper, interbedded lava flows, ash-flow tuffs, and air-fall tuffs are poorly exposed. The uppermost unit is an ~150-m-thick crudely columnar jointed and flow- banded feldspar and quartz-rich lava flow. A sequence of feldspar and orthopyroxene-bearing, highly welded ash-flow tuffs underlie this lava flow. Also abundant in these ash-flow tuffs are 1 - 3 cm mafic xenoliths, and shattered Mg-rich olivine xenocrysts are present in one of these flows. The lowest exposed stratigraphic units are quartz and potassium feldspar-rich rhyolite lava flows that crop out along the south fork of the Quinn River. In these lava flows, resorbed and sieved feldspars are present and at least some of these lava flows erupted from an eroded dome exposed south of Odell Mountain (Fig. 2b). Oxidized vertical dikes and frothy near vent carapace deposits help define this vent. Field relationships and satellite imagery illustrate that outflow from this source was directed to the north, southeast, and southwest where it was onlapped by Eightmile Mountain area lava flows. To the south, this unit is onlapped by a xenolith-rich Odell Mountain ash-flow tuff and a younger ash-flow tuff derived from the Goosey Lake depression (Cold Springs tuff; Knight et al. 2004).

Coyote Mountain-Zymns Butte area silicic units The Coyote Mountain-Zymns Butte-Black Dome area lies to the south of the Calico Mountains and at the southeastern end of the prominent central SC fault zone (Fig. 2b, Tct and Tsc). In the

113 vicinity of Coyote Mountain, silicic volcanism was occurring by at least ~16.4 Ma marked by the eruption of the Coyote Mountain tuff from Coyote Mountain. The other well exposed silicic vents in this region are domes; Zymns Butte and Black Dome (Fig. 4f). At and peripheral to Coyote Mountain, the Coyote Mountain ash-flow tuff forms a shallow (~10º) dipping ignimbrite plateau that extends away from the summit in all directions (Fig. 4g). Along the flanks of this vent, block and ash deposits (above ash-flow tuff) and air-fall tuffs (below ash-flow tuff) are locally well exposed, suggesting that Coyote Mountain erupted more than once. The Coyote Mountain ash-flow tuff is a highly welded, rheomorphic, and petrographically monotonous unit (Fig. 7). Andesite and dacite lava flows underlie the Coyote Mountain sequence. Stratigraphic relationships between Zymns Butte, Black Dome, and the Coyote Mountain ash-flow tuff are difficult to discern, but we suggest that these domes post-date Coyote Mountain activity. Unlike other SC silicic domes, Zymns Butte and Black Dome do not appear to be associated with substantial outflow, although at least one lava flow emanated from Black Dome (Fig. 4f). The Coyote-Zymns-Black area units are potassium feldspar-rich and contain abundant evidence of disequilibrium including xenolithic clots of plagioclase ± orthopyroxene ± clinopyroxene and abundant resorbed and sieved feldspars.

Goosey Lake depression ash-flow tuffs and air-fall tuffs The Goosey Lake depression preserves evidence of at least two physically distinct ash-flow tuffs. The most widespread is moderately welded and intermittently exposed across the Goosey Lake depression and along the western SC margin as a ~1 to 5-m-thick simple cooling unit, originally mapped by Vikre (1985b) as Twt (Crystal-lithic rhyolite tuff). This unit’s most distinguishing characteristic is the presence of 1 - 2 cm long fiamme in a welded, variably colored matrix (most commonly beige/yellow), and where exposed, its base is vitrophyric. Field relationships suggest that its source was north/northwest of the SC and texturally, this unit resembles distal outflow from the McDermitt volcanic field (Rytuba and McKee 1984). A very distinct ash-flow tuff, herein named the Cold Springs tuff, is generally observed underlying the above unit and is best exposed in the Goosey Lake Depression (Fig. 4h; Fig. 8). At Cold Springs Butte, composite exposures of the Cold Springs tuff are at least 50 m thick. Along the southeastern SC margin, Cold Springs tuff flow units are much thinner and likely represent distal facies (Fig. 4h). In the Goosey Lake Depression, the basal portion of the Cold Springs tuff is well exposed and overlies

114 lacustrine strata (Fig. 8a). The main body of the ash-flow tuff appears to be composed of at least one ~18 to 30-m-thick, crystal rich compound cooling unit (Fig. 8b). Sparse lithic fragments of welded ash-flow tuff and andesite-dacite lava flows are also found in main body deposits near Cold Springs Butte. While no eruptive loci can be directly linked with this ash-flow tuff, the thickest outcrops and only occurrence of lithic fragments are found near Cold Springs Butte, suggesting that this region may be proximal to source. A roughly oval ~2.5 x ~3.5 km region visible on aerial imagery is present in the vicinity of Cold Springs Butte and may reflect the presence of a small caldera, the only caldera system within the volcanic field.

Chemical Diversity of Santa Rosa-Calico silicic units Unlike other mid-Miocene Oregon Plateau volcanic fields, SC products display compositional variation from basalt through high-silica rhyolite and span this spectrum with no compositional gaps (Fig. 9a and Table 2). The entire SC suite mimics a subalkaline differentiation trend (Fig. 9a) and the welded ash-flow(s) exposed in the Goosey Lake depression is the only silicic material that possesses a peralkaline (comenditic) composition (Fig. 9b). This chemical difference supports our field interpretation that this unit may have been derived from the nearby McDermitt volcanic field or a yet identified source. All locally derived silicic units straddle the peraluminous-metaluminous divide, and those from the western SC tend to be more peraluminous while those from the eastern SC are only mildly peraluminous (Fig. 9b). Major and trace element data (Fig. 10 and 11) also illustrate that the Cold Springs tuff is chemically unlike other SC silicic units. This supports the field relationships that suggest it erupted from an isolated magmatic system, possibly from the only caldera-forming system within the SC. In order to focus on the main ~16.6 - 16.2 Ma package of SC-derived silicic activity, the younger (< 15.8 Ma) Cold Springs and McDermitt derived ash-flow tuffs are not further discussed in this paper.

Chemical Diversity of Western SC Silicic Volcanism Figure 10 illustrates selected major and trace element characteristics for the western SC silicic units. Within and between units, variations exist as evidenced by obvious data clusters (subgroups) and arrays. Most notable are the between unit sub-parallel data arrays that further highlight the physical, petrographic, and chronologic distinctions discussed previously. Hinkey

115 Summit-Coal Pit Peak lava flows define two distinct clusters between 67 - 74 wt. % SiO2. These clusters are particularly evident in the Zr versus silica plot. Viewed together, these subgroups define a single array in element-element space and lava flows from both subgroups stratigraphically overlie high-silica Hinkey-Coal Pit lava flows. Furthermore, the high Ba concentrations (up to 5500 ppm) of the low-silica subgroup and the overall trend of decreasing

Ba with increasing SiO2 for all Hinkey-Coal Pit lava flows are unique within the SC silicic suite. For example, physically similar Western Margin and Eightmile Mountain lava flows are not as enriched in Ba or Zr. The porphyritic rhyolite intrusive bodies and lava flows also span a wide range in silica content and the most evolved are compositionally similar to flow banded rhyolites. Two chemical subgroups are present within this unit based on silica content: a low silica group (~69 - 71 wt. %) representing the southern SC margin lava flow and a higher silica subgroup (>72 wt. %) that represents all other exposures. Within the higher silica subgroup substantial chemical

variation is present. This variation is best seen in plots of K2O, TiO2, Sr, Th, Nb versus silica.

The samples falling between ~74 and ~75 wt. % SiO2 and the single sample at ~72 wt. % SiO2 represent the large shallow intrusive body exposed southeast of Hinkey Summit. The remaining high-silica samples are from the dikes exposed between Hinkey Summit and Buckskin Mountain and the coulée along the western margin. Eightmile Mountain region lava flows and near-vent clastogenic deposits range from ~65 to ~74 wt. % SiO2. Scatter is observed in major element concentrations (and Sr) across this silica range, whereas most trace elements define near linear arrays. The overall major and trace element characteristics of these lava flows and near-vent pyroclastic material are most similar to western margin lava flows. All flow-banded rhyolite intrusive bodies and lava flows are high-silica (>76 wt. %) rhyolites. While minor variations are present, this unit defines a cluster in major element space. Within this unit, certain trace elements (e.g., Th and Nb) define two distinct subgroups, yet lava flows and intrusive bodies from these subgroups are texturally indistinguishable. It is interesting to note that obsidian collected from along the southern margin and near the center of the Goosey Lake depression is chemically linked to these subgroups, indicating that at least two physically distinct obsidian bodies were present in the western SC.

116 Chemical Diversity of Eastern SC Silicic Volcanism Eastern SC silicic volcanism is also characterized by a suite of geochemically diverse units (Fig. 11). However, the within unit variability is typically more pronounced than that observed in the western units, whereas the between unit variability is less pronounced. Calico Mountain area units are the most physically diverse in the eastern SC and their major and trace element

variations reflect this diversity. Calico units span a wide range in wt. % SiO2, and major and trace element characteristics define at least four subgroups separated by gaps in silica content. Our comprehensive sampling in this area indicates that these subgroups truly represent the available material. Moreover, the subgroups roughly correspond to stratigraphic position, with increasing silica up-section (e.g., Th vs. SiO2). The least evolved samples are the xenolith-rich ash-flow tuffs exposed below the Capitol Peak ash-flow tuff, and their chemical compositions likely reflect the presence of relatively high-MgO xenoliths. The subgroup of samples clustered between 71 and 73 wt. % SiO2 include the lava flows exposed at Mahogany Pass that erupted from a small dome in the central Calico range. At higher silica concentrations (~73 to 76 wt. %

SiO2), two subgroups are present: a high Th and Nb, low Sr subgroup similar in chemistry to Odell Mountain units and a cluster of low Th, Nb, and high Sr samples. The two samples that define the high Nb subgroup are the stratigraphically youngest material in the Calico range and their chemical affinity with Odell Mountain units suggests that they are petrogenetically related to Odell Mountain units. The high Sr subgroup includes a dike exposed north of Mahogany Pass and the thick lava flows in the southern Calico range.

Odell Mountain area units are restricted to compositions >73 wt. % SiO2 and are the most evolved eastern SC group. However, at least two distinct subgroups appear to be present. The lower silica subgroup encompasses a mafic-xenolith rich ash-flow tuff and a lava flow that is the lowest stratigraphic unit in the Odell region. The >75 wt. % SiO2 subgroup includes the only exposed Odell area rhyolite dome and the stratigraphically youngest lava flows exposed at Odell Mountain. Coyote Mountain-Zymns Butte and Black Dome area units exhibit the same range in major and trace element concentrations as those from the Calico Mountains. The least evolved Coyote Mountain area silicic units are dacitic to trachydacitic and underlie the Coyote Mountain ash-flow tuff. The ash-flow tuff itself appears to be chemically zoned, with the array from ~69

to ~75 wt. % SiO2 reflecting this up-section zonation. At ~74 to ~75 wt. % SiO2, two subgroups

117 of Coyote Mountain samples are present (high and low Nb) that represent the upper portion of the ash-flow tuff (high Nb) and under/overlying welded fall and block and ash deposits (low Nb). Additionally, Zymns Butte falls within the high-silica and high Nb group cluster. Two samples that were collected from Black Dome and its inferred outflow fall within the array defined by Coyote-Zymns units. In order to summarize and further interpret the geochemical characteristics discussed above, figure 12 illustrates multi-element plots for the western and eastern silicic units. All concentrations are normalized to an average Steens Basalt composition since this provides the best estimate for the mafic component involved in SC magmatism. The western SC porphyritic and flow banded rhyolites exhibit broadly similar trace element patterns characterized by pronounced depletions in Sr and Ba as compared to other western SC units (Fig. 12a), and pronounced depletion in V as compared to all SC silicic units. As previously noted, the porphyritic rhyolite exhibits considerable within-unit variability. Hinkey-Coal Pit, western margin, and Eightmile Mountain units exhibit similar overall patterns that are distinct from those exhibited by the flow banded and porphyritic rhyolites. However, figure 9a illustrates that the Hinkey-Coal Pit lava flows possess greater Sr, Ba and Zr diversity including pronounced Zr and Ba enrichment and Sr depletion relative to Eightmile Mountain and western margin lava flows. This supports the temporal and spatial relationships that preclude a direct petrogenetic link between Eightmile Mountain and Hinkey-Coal Pit lava flows. Eastern SC units are illustrated in figure 12b. The trace element patterns further substantiate the similarities between the uppermost lava flows in the Calico Mountains and Odell Mountain area units. The patterns defined by the main group of Calico lava flows are nearly identical to those from Coyote Mountain area units. The lower Calico ash-flow tuffs exhibit trace element characteristics unique among all SC silicic units. The transition metal abundances highlight this point, and reflect the modal abundance of mafic xenoliths in these units (e.g. greater xenolith content coupled with higher transition metal concentrations). Ignoring these highly contaminated lower Calico ash-flow tuffs, the overall trace element characteristics exhibited by the eastern units are more similar to each other than is the case in the western SC.

118 Implications of Chemical Heterogeneity Previous work restricted to the western SC attempted to constrain the petrogenetic processes responsible for the local silicic magmatism. For example, LeMasurier (1965; 1968) concluded that the bulk chemical variability was due to different degrees of fractional crystallization of a basaltic parental magma. However, Mellott (1987) found isotopic heterogeneities within and between western SC silicic units, with the Hinkey-Coal Pit and western margin lava flows yielding initial Sr isotopic compositions between 0.7046 and 0.7058. Larger ranges and more 87 86 radiogenic values ( Sr/ Sri = 0.7048 to 0.7076) characterize the porphyritic and flow banded rhyolite units. Mellott (1987) invoked one or more of the following processes to explain these data: (1) mixing between chemically and isotopically heterogeneous mantle and crustal melts, (2) partial melting of isotopically evolved pre-Miocene crust (e.g. local granitoid or metasedimentary rocks), and (3) open system fractional crystallization (AFC) of a more mafic parent. Later recognition by Stuck (1993) that the local Cretaceous granitoid basement is 87 86 comprised of at least two chemically and isotopically distinct components ( Sr/ [email protected] Ma = 0.7048 to 0.7058, Santa Rosa-Andorno group; 0.7061 to >0.7071, Granite Peak-Sawtooth group; 0.7095 to >0.7426, cross cutting aplite dikes) supports some of Mellott’s conclusions. Locally 87 86 erupted Steens Basalt lava flows and shallow intrusive bodies are characterized by Sr/ Sri values less than 0.7045 (Hart and Carlson, 1985; Carlson and Hart 1987; Mellot, 1987; Brueseke and Hart unpublished data). Preliminary isotopic data from the units discussed in this study 87 86 indicate an isotopic similarity to both granitoid groups ( Sr/ Sri > 0.7045; Brueseke and Hart, unpublished data) and this observation coupled with the earlier isotopic results of Mellot (1987) suggest that simple crystal fractionation of a more mafic parental magma is not solely responsible for the chemical diversity exhibited by SC silicic units. Compositionally similar silicic melts were documented by Petcovic and Grunder (2003) in a study of silicic melt generation in the Wallowa Mountains of Oregon. There, rhyodacitic and high-Si rhyolite melts were generated via the emplacement of Columbia River Basalt Group dikes into Cretaceous hornblende-biotite granodiorite to tonalite wallrock (Petcovic and Grunder, 2003). Additionally, crystal clots present in some SC silicic units are mineralogically similar (Fig. 7c, d) to the plagioclase feldspar, clinopyroxene, orthopyroxene, and oxide restite that formed as a consequence of melting granitoid upper crust (Petcovic and Grunder, 2003). We suggest that a

119 similar process is likely responsible for the generation of most SC silicic units, initiated and driven by upwelling Steens Basalt magmas. Differing degrees of partial melting of local granitoid basement may explain the high Ba concentrations found in the Hinkey Summit-Coal Pit Peak lava flows (Fig. 7) and post melt- generation fractional crystallization could potentially account for some of the trace element variations illustrated in Figure 9 (e.g. pronounced negative Sr, Ba, and V anomalies for the flow banded and porphyritic rhyolites and Odell and upper Calico lava flows). However, the abundant disequilibrium textures present within SC silicic units and the overall trace element diversity within and between units demonstrate that complex magmatic processes (e.g. heterogeneous parental magmas, crustal assimilation, magma mixing) have contributed to the observed chemical variations. This physical evidence also points to some level of location specific open system behavior. For example, xenolithic granitoid clots (anatectic restite or/and bulk granitoid) are most abundant in those western SC units with eruptive loci in the vicinity of Hinkey Summit (Fig. 2) where the massive granitoid core of the Santa Rosa Range looms only kilometers away. In contrast, in areas predominated by thick mafic to intermediate flow packages (e.g., Eightmile Mountain, Odell Mountain, Calico Mountains; Fig 2), the silicic units preserve a more variable lithic assemblage including Mg-olivine xenocrysts and basalt-andesite xenoliths. Further Sr, Nd, and Pb isotope work currently in progress should better define how the observed chemical and isotopic differences between and within units vary as a result of crustal melting and/or melting combined with other petrogenetic processes including interaction with local pre- and mid-Miocene crust and contemporaneously produced less-evolved magmas.

Discussion and Conclusions New field mapping, 40Ar-39Ar geochronology, and chemical analyses, coupled with prior reconnaissance studies combine to produce the first comprehensive picture of silicic volcanism in the Santa Rosa-Calico volcanic field. In the western SC, at least five physically and chemically distinct silicic units are present and crop out along a north-northwest trending zone that extends from Hinkey Summit to the National Mining district. Conservatively, one laterally heterogeneous, evolving magmatic system can be invoked for the western SC; however, we prefer an interpretation that calls on at least two physically isolated systems. The central SC is dominated by locally and regionally derived ash-flow tuffs, sedimentary basin deposits, and lava

120 flows that were deposited/emplaced into an actively subsiding structural basin. Included in this area is the only local evidence of explosive, caldera forming eruptions and outflow products (Cold Springs tuff). This unit possesses a unique geochemical fingerprint (Fig. 10 and 11), suggesting derivation from an isolated and younger magmatic system. The physical and chemical characteristics of lava flows and ash-flow tuffs from the eastern SC indicate that they were derived from separate loci along the eastern margin of the field. However, their overall chemical similarity and broadly coeval emplacement suggest a commonality in petrogenetic processes. We therefore suggest that eastern SC silicic activity owes its origin to the tapping of a single crustal magma chamber (or related sill-like bodies) along N-S trending faults during the initiation of SC volcanism. A similar style of volcanism was proposed to explain ignimbrites of the Sierra Madre Occidental that are spatially and temporally associated with coeval extensional tectonism (Aguirre-Díaz and Labarthe-Hernández, 2003). Aguirre-Díaz and Labarthe- Hernández (2003) also observed rhyolite domes directly overlying lag-breccia and ignimbrites aligned along major faults. This situation may be present in the Calico range where pyroclastic units (the lower ash-flow tuffs) are generally overlain by effusive products (the middle to upper rhyolite domes and flows). Disruption by faulting and/or mafic magma injection could have provided the trigger for localized eruption, leading to the presence of the discrete eastern SC eruptive loci. In this scenario, the minor chemical variations present may reflect heterogeneous magmatic processes affecting localized portions of the larger eastern system. The Eightmile Mountain lava flows of the western SC were also derived from fissural vents and resemble the areally extensive lava flows that erupted along the Owyhee Front at the initiation of western Snake River Plain graben formation (Bonnichesen 2004; Bonnichsen et al. 2004). Similar broad compositional variation and styles of volcanism are also present at the 10.4 Ma Duck Butte Eruptive Center (north of Steens Mountain). Here, rhyodacite vents are present parallel to basin and range faults and local eruptive units drape and were cut by faults active during volcanism (Johnson and Grunder 2000). The only locally derived silicic unit that appears to have had a caldera origin is the Cold Springs tuff, the youngest unit recognized in this study. Its apparent eruption from a caldera in the central SC may reflect a cessation in localized extension (e.g. a cessation in regional rifting) during the waning stages of SC silicic activity prior to Cold Springs tuff volcanism at ~15.4 to 15.5 Ma. Prior workers have suggested that the Goosey Lake depression formed due to

121 magmatic subsidence related to local, large-scale caldera forming volcanism and attributed the abundant eruptive loci that “ring” the Goosey Lake depression to post-caldera resurgence along ring faults (Ekren et al. 1984; Vikre 1985a, b; McCormack 1996). However, the data from this study do not support this interpretation for the main SC silicic eruptive products. The paucity of caldera-forming volcanism in the SC is interpreted to be a direct function of its location within the Northern Nevada rift, where active extension created an environment that frequently disrupted developing crustal magma bodies. The interplay between volcanism and focused extensional tectonism suggested for the SC may help to better understand the origin of Miocene silicic volcanism on the adjacent Owyhee Plateau (Fig. 1b). The silicic products exposed in this region are often attributed to caldera forming volcanism related to the “Owyhee-Humboldt eruptive center” (originally defined as “a basalt-filled structural basin”; Bonnichsen 1985). However, little is known about the chemical diversity and eruptive sources of these areally extensive units, except for those that erupted from the Juniper Mountain volcanic center, which are lava flows and not caldera-derived (Manley and McIntosh 2002). Across the Owyhee Plateau, numerous, small silicic eruptive centers and shallow intrusive bodies are present and must reflect multiple eruptive loci (Fig. 1b). Additionally, no field-based geologic evidence exists to suggest the presence of caldera-related volcanic systems (Coats 1968; Ekren et al. 1984; Manley and McIntosh 2002). The multiple and abundant loci, diverse eruptive styles, and crustal response to upwelling mafic magmas documented in the SC suggest that the southern Owyhee Plateau may have been characterized by similar features, rather than a large caldera system like those depicted across the eastern Snake River Plain. In summary, the various lines of evidence presented in this paper point to generation and evolution of Santa Rosa-Calico silicic magmas through a complex set of processes involving mantle- and crustal-derived melts and heterogeneous open system differentiation in physically isolated upper crustal magma bodies. At the root of these complexities are two fundamental aspects of the regional tectonomagmatic development; (1) active mid-Miocene extension, and (2) crustal modification and addition via the voluminous Steens-Columbia River magmatic event. Santa Rosa-Calico silicic volcanism was characterized by numerous eruptive loci distributed along roughly north-south trending alignments. These alignments are likely lithospheric, similar to the Northern Nevada rift and Oregon-Idaho graben, and their presence coupled with the field

122 relationships present in the central SC indicates that regional mid-Miocene rifting was occurring locally. Furthermore, new age data demonstrate that the silicic activity was coeval with regional Steens flood basalt volcanism. Because of its location in a zone of focused extensional tectonism and its spatial and temporal association with continuous upwelling mafic magma, the styles and chemical characteristics of Santa Rosa-Calico silicic volcanism illustrate the important interplay between tectonism and volcanism, and distinguish the Santa Rosa-Calico volcanic field from other mid-Miocene northwestern United States volcanic fields.

Acknowledgements We thank Stan Mertzman (Franklin & Marshall College) for XRF trace element analyses, John Morton (Miami U.) for his assistance with DCP analyses, Alan Wallace (USGS) for his continued discussions on the evolution of the Northern Nevada rift and southern SC margin, Wes LeMasurier for his detailed field map of the Hinkey Summit region, and Peter Vikre for his discussions about Buckskin Mountain. We thank Charles Bruce Minturn III, Lauren Gilbert, Amy Maloy, Jacob Knight, and Steve Pasquale for their valuable and often entertaining assistance in the field and for work on related sub-projects. We also thank Katharine Cashman and Craig White for providing constructive and thoughtful reviews and Mike McCurry for handling this manuscript. Primary financial support for this research was provided by the National Science Foundation (EAR-010S6144 to Hart) and a 2001 Geological Society of America Student Research Grant (Brueseke).

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126 Katoh S, Danhara T, Hart WK, WoldeGabriel G (1999) Use of sodium polytungstate solution in the purification of volcanic glass shards for bulk chemical analysis. Nat and Hum Act 4:45- 54 King JK (1984) Utility of Landsat imagery for recognition of Tertiary silicic and intermediate volcanic features in northern Nevada: unpublished M.S. thesis, Univ Wyoming, Laramie, pp 1-486 Kistler RW, Peterman ZE (1978) Reconstruction of crustal California on the basis of initial strontium isotopic compositions of Mesozoic granitic rocks. US Geol Surv Prof Pap 107, pp 1-17 Knight JE, Brueseke ME, Hart WK (2004) Physical, petrographic, and geochemical characterization of ash flow volcanism: the mid-Miocene Cold Springs Tuff of the Santa Rosa Calico Volcanic Field, Nevada: Geol Soc Am Abs w/Prog 36:77 Larson EE, Watson DE, Jennings W (1971) Regional comparison of a Miocene geomagnetic transition in Oregon and Nevada. Earth Planet Sci Lett 11:391-400 Leeman WP, Oldow JS, Hart WK (1992) Lithosphere-scale thrusting in the western U.S. Cordillera as constrained by Sr and Nd isotopic transitions in Neogene volcanic rocks. Geology 20:63-66 LeBas MJ, LeMaitre RW, Streckeisen A, Zanettin B (1986) A chemical classification of volcanic rocks based on the total alkali-silica diagram. J Pet 27:745-750 LeMasurier WE (1965) Volcanic geology of Santa Rosa Range, Humboldt County, Nevada. Ph.D. Thesis, Stanford Univ, Stanford, pp 1-126 LeMasurier WE (1968) Crystallization behavior of basalt magma, Santa Rosa Range, Nevada. Geol Soc Am Bull 79:949-972 Lindgren W (1915) Geology and mineral deposits of the National mining district, Nevada. US Geol Surv Bull B0601, pp 1-58 MacLeod NS, Walker GW, McKee, EH (1975) Geothermal significance of eastward increase in age of Upper Cenozoic rhyolitic domes in southeastern Oregon. US Geol Surv Open-File Rep 75-348:21 Maloy AK, Brueseke ME, Hart WK, Minturn CB III (2003) The generation of intermediate composition magmas in a bimodal setting; evidence from the Santa Rosa-Calico volcanic field, Nevada. Geol Soc Am Abs w/Prog 35:5

127 Manley CR, McIntosh WC (2002) The Juniper Mountain volcanic center, Owyhee County, southwestern Iadaho: Age relations and physical volcanology. In: Bonnichsen B, White CM, McCurry M (eds.), Tectonic and magmatic evolution of the Snake River plain volcanic province. Idaho Geol Surv Bull 30 pp 205-227 McCormack JK (1996) Large-scale arcuate structures concentric with the McDermitt caldera complex. In: Coyner AR, Fehey PL (eds.), Geology and ore deposits of the American Cordillera. Geol Soc Nevada Symp Proc pp 625-633 Mellott MG (1987) Geochemical, petrologic, and isotopic investigation of andesites and related volcanic rocks in the Santa Rosa Range and Bloody Run Hills, Nevada; tectonic implications. M.S. Thesis, Miami Univ, Oxford, pp 1-164 Mertzman SA (2000) K-Ar results from the southern Oregon - northern California Cascade Range. Oregon Geol 62:99-122 Noble DC, McKee, EH, Smith JG, Korringa, MK (1970) Stratigraphy and geochronology of Miocene volcanic rocks in northwestern Nevada: US Geol Surv Prof Paper D23-D32. Perkins ME, Nash BP (2002) Explosive silicic volcanism of the Yellowstone Hotspot; the ash fall tuff record. Geol Soc Am Bull 114:367-381 Perkins ME, Brown FB, Nash WP, McIntosh W, Williams SK (1998) Sequence, age, and source of silicic fallout tuffs in middle to late Miocene basins of the northern Basin and Range province. Geol So. Am. Bull 110:344-360 Petcovic, HL, Grunder AL (2003) Textural and thermal history of partial melting in tonalitic wallrock at the margin of a basalt dike, Wallowa Mountains, Oregon. J Pet 44:2287-2312 Pierce KL, Morgan LA (1992) The track of the Yellowstone hot spot: Volcanism, faulting, and uplift. In: Link PK, Kuntz, MA, Platt, LB (eds.) Regional Geology of Eastern Idaho and Western Wyoming. Geol Soc Am Mem 179:1-53 Roberts RJ (1940) Quicksilver deposit at Buckskin Peak, National mining district, Humboldt County, Nevada; a preliminary report. US Geol Surv Bull 0922-E:115-133 Rytuba JJ, McKee EH (1984) Peralkaline ash flow tuffs and calderas of the McDermitt volcanic field, southeast Oregon and north central Nevada. J Geophys Res 89:8616-8628 Rytuba JJ, Vander Meulen DB, Barlock VE (1991) Tectonic and stratigraphic controls on epithermal precious metal mineralization in the northern part of the Basin and Range,

128 Oregon, Idaho, and Nevada. In: Buffa RH, Coyner AR (eds.) Geology and ore deposits of the Great Basin fieldtrip guidebook compendium. Geol Soc Nevada pp 636-644 Shieh YN, Taylor HP (1969) Oxygen and hydrogen isotope studies of contact metamorphism in the Santa Rosa range, Nevada and other areas. Cont Min Pet 20:306-356 Shoemaker KA (2004) The tectonomagmatic evolution of the late Cenozoic Owyhee Plateau, northwestern United States. Ph.D. Dissertation, Miami Univ, Oxford, pp 1-288 Stewart JH, Carlson JE (1976) Cenozoic rocks of Nevada. Nevada Bur Mines Geol Map 52 Stuck RJ (1993) Petrology and geochemistry of a late Cretaceous granitoid suite, Santa Rosa mountain range, Humboldt County, Nevada. M.S thesis, Miami Univ, Oxford, pp 1-179 Vikre PG (1985a) Precious metal vein systems in the National District, Humboldt County, Nevada. Econ Geol 80:360-393 Vikre PG (1985b) Geologic map of the Buckskin Mountain quadrangle, Nevada: Nevada Bur Mines Geol Map 88 Walker GW (1969) Geology of the High Lava Plains province. In: Weissenborn AE (ed.) Mineral and Water Resources of Oregon, Oregon Dept of Geol and Min Ind Bull 64: 77-79 Walker GW (1974) Some implications of Late Cenozoic volcanism to geothermal potential in the High Lava Plains of south-central Oregon. Ore Bin 36:109-119 Wallace AR (2003) Geology of the Ivanhoe Hg-Au district, northern Nevada; influence of Miocene volcanism, lakes, and active faulting on epithermal mineralization. Econ Geol 98:409-424 Wallace AR, John DA (1998) New studies of Tertiary volcanic rocks and mineral deposits, Northern Nevada Rift. In: Tosdal RM (ed.) Contributions to the gold metallogeny of northern Nevada. US Geol Surv Open File Rep 98-338:264-278 Willden R (1964) Geology and mineral deposits of Humboldt County, Nevada. Nevada Bur Mines Geol Bull 0097-5281:1-158 Winchell AN (1912) Geology of the National mining district, Nevada. Mining and Scientific Press, New York, pp 655-659 Wyld SJ, Rogers JW, Wright JE (2001) Structural evolution within the Luning-Fencemaker fold- thrust belt, Nevada; progression from back-arc basin closure to intra-arc shortening. J Struct Geol 23:1971-1995

129 Wyld SJ, Copeland P, Rogers JW (2003) Metamorphic evolution of the Luning-Fencemaker fold-thrust belt, Nevada; illite crystallinity, metamorphic petrology, and 40Ar/39Ar geochronology. J Geol 111:17-38 Zoback ML, McKee EH, Blakely RJ, Thompson GA (1994) The Northern Nevada rift: Regional tectono-magmatic relations and middle Miocene stress direction. Geol Soc Am Bull, 106:371-382

130 Figure 1. (a) Map of the northwestern United States depicting select Cenozoic tectonomagmatic features. Shaded region is the approximate extent of mid-Miocene flood basalt volcanism (after Hart and Carlson 1985; Camp and Ross 2004). Also shown are major flood basalt dike swarms/eruptive loci (black lines), Oregon-Idaho graben and magnetic anomalies in Nevada corresponding to zones of lithospheric extension/mafic magma emplacement (black dotted- dashed lines; Cummings et al. 2000; Glen and Ponce 2002), major volcanic fields of the Yellowstone-Snake River plain province (dashed circles); BJ, Bruneau-Jarbidge (~12.5 - <11 Ma); TF, Twin Falls (~10 – 8.6 Ma); PC, Picabo (~10 Ma); HS, Heise (~6.7-4.3 Ma); and YS, Yellowstone (<2.5 Ma), and age isochrons (dashed black lines, ages in Ma) of Oregon High Lava Plains silicic volcanism (N, Newberry Volcano; after Jordan et al., 2004). The SC lies between the initial 87Sr/86Sr 0.706 and 0.704 isopleths (after Armstrong et al. 1977; Kistler and Peterman 1978; Leeman et al. 1992; Crafford and Grauch 2002). (b) Shaded relief map of the southern Oregon Plateau illustrating the locations of major mid-Miocene silicic volcanic systems. SC, Santa Rosa-Calico volcanic field; MD, McDermitt volcanic field; LO, Lake Owyhee volcanic field; NWNV, Northwest Nevada volcanic field (e.g. Virgin Valley, High Rock, Hog Ranch, and unnamed calderas); HVLM, Hawks Valley-Lone Mountain dome complex; SI, Silver City- Delamar dome complex; JM, Juniper Mountain volcanic center; CC, Circle Creek volcanic center; J, Jarbidge Rhyolite loci; SS, Snowstorm Mountains dome complex. Unnamed black circles are other rhyolite dome complexes/eruptive loci/shallow intrusive bodies. HLP, High Lava Plains; SM, Steens Mountain; OIG, Oregon-Idaho graben; WSRP, western Snake River Plain; OP, Owyhee Plateau; NNR, Northern Nevada rift and related lineaments. Mid-Miocene extensional features from figure 1a are also depicted.

131 WA ID a o 44 OIG b Sr isotope 0.706 line LO Sr isotope HLP o 46 0.704 line SI WSRP

CRB Dikes SM HVLM MT OP JM YS MD OR ID 2 6 HS N NV 10 SC CC J PC Steens TF NWNV Dikes BJ SS o 42 OR CA SC NV UT WY NNR o 40 o o 120 115

o o o o o 123 120 117 114 111

132 Brueseke et al., Figure 1 Figure 2. (a) Landsat 7 image of the Santa Rosa-Calico volcanic field. Geographic features within the SC are: CPP, Coal-Pit Peak; HS, Hinkey Summit; GLD, Goosey Lake depression; BM, Buckskin Mountain; EM, Eightmile Mountain; OM, Odell Mountain; CM, Calico Mountains; HB, Hardscrabble Basin. Features peripheral to the SC are: PV, Paradise Valley; MSR, main Santa Rosa range; QR, Quinn River Valley; OP, Owyhee Plateau. Dashed white box is the approximate region shown in figure 2b. (b) Generalized geologic map of the SC (after LeMasurier 1965; 1968; Vikre 1985b, Mellott 1987; King 1984; Brueseke and Hart this study). Units and symbols are depicted in legend. Geographic features not present on figure 2a are: GP, Granite Peak; ZB, Zymns Butte; BD, Black Dome. Major Au-Ag-Hg mining districts from north to south are: National, Buckskin-National, and Spring City. Map units are as follows: Trms, Triassic metasediments (Norian); Kg, Cretaceous granitoid; Tarc, undivided arc volcanics (late Eocene to early Miocene); Tom, Odell Mountain area units; Tct, Coyote Mountain area units; Tpr, porphyritic rhyolite; Tem, Eightmile Mountain lava area lava flows; Tsc, Other mid- Miocene volcanic units including Calico Mountain silicic units and Goosey Lake depression ash- flow tuffs; Tsi, silicic intrusive bodies (mainly porphyritic rhyolite); Tsed, undivided Miocene sediment including lacustrine and fluvial basin deposits and interlayered tephra fall deposits. Hardscrabble basin sedimentary sequence includes distal Cold Springs tuff exposures; Tsmt, Miocene Swisher Mountain tuff (likely derived from the mid-Miocene Juniper Mountain eruptive center); Qsed, undivided Holocene and Pliocene sediment.

133 Tsmt

b

Qsed

Tsmt ZB

Mountains

Tct

Calico

SC dike orientation

n=62

Tsc

CT

Qsed

Tsc

Lava flow direction

Tsed

OM

BD

GLD

Tom

Forest Service Road Marker

Tsc FS 531

State Road Marker

Silicic dike zone

Silicic vent region

Major mine

CPP

FS 531

792

Ì

Ì

Tem

GLD

uff

Tpr

Kg FS 084

Tsc

Tsed

Tpr

Tsc

EM

Tem

Qsed

Ì

Possible caldera for Cold Springs T

HS Tsi 792

Major normal fault

Improved Road

Silicic dome

Tsc

Tpr

BM

TsiÌ

Tsc

Tarc Tarc

Tsi

Kg

GP

5km FS 084

Tsc

Trms

N

Qsed

a

OP

SC

10 km

N

Rift

CM

Kg

Tct

Tpr

Tem Tom

Tarc

Nevada

Tsmt

Trms

Qsed

Northern

CT

Tsc

GLD

OM

Tsi

CPP

PV

GLD

EM

Tsed

HS

BM

Generalized Stratigraphy (see figure caption for unit descriptions)

QR

MSR

NV

OR

134 Brueseke et al., Figure 2 Figure 3. Generalized stratigraphy of the SC illustrating the complex stratigraphic relationships found across the volcanic field (not to scale). All SC and pre-SC ages, not including the Cretaceous granitoid ages, are based on new Ar-Ar determinations from this study and from Brueseke et al. 2003. New 40Ar-39Ar data are provided in Table 1 and Appendix 1 (Electronic Supplementary Material). Ages calculated relative to FC-2 Fish Canyon tuff sanidine interlaboratory standard at 28.02 Ma.

135 Western SC- Greater Hinkey Summit Region 16.0 Ma 14.3 Ma Eastern SC- Central Central SC- Goosey Calico Mountains Region Lake Depression 16.2 Ma

13.9 Ma 15.5 Ma 16.5 Ma 16.4 Ma

14.9 to 15.8 Ma vvvvvvv 16.7 Ma vvvvvvv 16.5 Ma 22.5Ma SC Derived vvvvvvvvvv 23.5Ma vvvvvvv >16 Ma 16.5 Ma 35.4Ma

Pre-SC

Fluvial-lacustrine strata ~19-24 Ma calc-alkaline vvv with interbedded tuffs andesite assemblage Rhyolite ~43-28 Ma basalt to Porphyritic rhyolite rhyolite assemblage Ash flow ~85 Ma Granite Peak - Sawtooth group granitoids Rhyodacite ~102 Ma Santa Rosa - Dacite Andorno group granitoids Andesite

Pre-Santa Rosa -Calico units

Santa Rosa-Calico Norian (Late Triassic) derived units Basalt & Metasediments Basaltic andesite

136 Brueseke et al., Figure 3 Figure 4. Photographs of SC silicic units and features. (a) Silicic lava flows and porphyritic dikes exposed at Hinkey Summit. (b) Porphyritic rhyolite neck and near-vent pyroclastic deposits exposed south of Forest Service Road 84 along the western SC margin. (c) Porphyritic rhyolite lava flow along the southern SC margin. (d) Recumbent fold in a lower Calico lava-like ash-flow tuff exposed in the Calico Mountains along the eastern SC margin. (e) Columnar jointing in the Capitol Peak ash-flow tuff and overlying silicic units at Capitol Peak, Calico Mountains. (f) Black Dome in the southeastern SC with inferred outflow dipping to the north. (g) Coyote Mountain surrounded by exposures of the Coyote Mountain ash-flow tuff, dipping away from the vent region. (h) Distal exposure of Cold Springs tuff flow units exposed in the Hardscrabble basin along the southeastern SC margin.

137 Hinkey-Coal Pit lava flows

Porphyritic shallow intrusives

a b

c d

Capitol Peak Black Dome

Capitol Peak ash flow e f

Coyote Cold Springs Mountain tuff flow units

g h 138 Brueseke et al., Figure 4 Figure 5. Mineralogical characteristics of SC silicic units. Solid lines indicate phase is present in all samples examined. Dashed lines indicate phase is present in some of the samples and the absence of a line indicates that the phase is not present. Mafic, mafic xenoliths that resemble local basalt and andesite; Diseq, other disequilibrium textures including resorbed and sieved textured feldspar, amphibole dehydration, and possible clots of local granitoid crust. The silicic units as discussed in the text are: CM, Calico Mountains area; CZ, Coyote Mountain-Zymns Butte-Black Dome area; OM, Odell Mountain area; EM, Eightmile Mountain region; WM, Western margin lava flows; HCP, Hinkey-Coal Pit lava flows; PR, Porphyritic rhyolite; FBR, Flow banded rhyolite. The observed silica range(s) for each of these units is shown below the x- axis.

139 Plag

K-spar

Qtz PR+FBR+HCP+WM+CM+OM+High-Si CZ

PR+FBR+HCP+WM+High-Si CZ Biot

PR only Amph

HCP+WM+CM+OM+CZ Cpx

PR+HCP+WM+CM+CZ Opx Fe-Ti Oxide

Apatite

PR+EM+WM+CM+OM+CZ Zircon

EM+lower CM ash flows+OM Mafic All but FBR Diseq 65 68.371.5 74.8 78

SiO2 (wt. %) CM CZ OM EM WM HCP PR FBR

140 Brueseke et al., Figure 5 Figure 6. Textural characteristics of select SC silicic units. (a) Porphyritic rhyolite, notice the dark biotite and amphibole crystals and large white feldspars. (b) Clastogenic spatter from Eightmile Mountain lava flow(s) vent region. (c) Clinopyroxene + orthopyroxene + plagioclase + oxide crystal clot from a porphyritic rhyolite vitrophyre (plane light). (d) Same image under cross-polars. (e) Complex textural relationships in a lower Calico Mountains lava-like ash-flow- tuff (plane light). (f) Welding textures in a highly welded variety of Cold Springs tuff. Notice the resorbed feldspar at the top of the image. (plane light).

141 a b

cpx

plag opx ox c 500µ m d500µ m

xenolith 500µ m

mingling and/or e weldingtextures f 500µ m

142 Brueseke et al., Figure 6 Figure 7. Sketch illustrating the Coyote Mountain ash-flow tuff. 1= Ground layer/surge, 2= main body of ash-flow tuff; 2a is basal ash layer and 2b is main body of flow.

143 Block and ash flow with upper lapilli tuff

Upper massive to banded zone of densely welded ash-flow tuff

2b Lower platy, flow folded and locally brecciated zone of densely welded ash-flow tuff

Basal moderately welded

2a glassy vitrophyre

Basal poorly welded vitrophyric lapilli ash

Ground

Layer (1)

Pumiceous lapilli tuff (fall) -darker, more welded upper Fall -lighter less welded basal

Precursor

Intermediate lava flows

Brueseke et al., Figure 7

144 Figure 8. Cold Springs tuff exposures from the central SC. (a) Contact of Cold Springs tuff and associated pyroclastic deposits with underlying lacustrine strata. (b) Poorly welded Cold Springs tuff main body deposit. White “blobs” in cliff face are pumices. (c) Lithophysael zone in an outcrop of highly welded Cold Springs tuff.

145 Welded Basal Ash

Ground Surge

Precursor Fall Deposits Lacustrine Strata a

b c

Brueseke et al., Figure 8 146 Figure 9. (a) Total alkalies vs. silica diagram (LeBas et al. 1986) illustrating compositional spectrum of sampled SC lava flows, ash-flow tuffs, eruptive loci, and shallow intrusive bodies. B, basalt; TB, trachybasalt; BA, basaltic andesite; BTA, basaltic trachyandesite; A, andesite; TA, trachyandesite; D, dacite; TD, trachydacite; RD, rhyodacite; R, rhyolite. (b) Plot of A/NK

(molecular Al2O3/Na2O+K2O) vs. A/CNK (molecular Al2O3/CaO+Na2O+K2O).

147 1.5 10 SC sample n=286 TD 1.4 8 TA BTA 1.3 6 R

O+K O 22 TB D 1.2 4 A/NK A RD BA 1.1

Wt. %2 Na B Metaluminous Peraluminous a 1.0 0 Peralkaline 45 50 55 60 65 70 75 Wt. % SiO b 2 0.9 Western SC unit Cold Springs Tuff 0.8 0.9 1.0 1.1 1.2 1.3 A/CNK Eastern SC unit Peralkaline ash flow

148 Brueseke et al., Figure 9 Figure 10. Major and trace element variations for western SC silicic units (major elements in wt. %; trace in ppm). Notice the chemical differences exhibited by each unit. Also shown is the field for the Cold Springs tuff (except Nb), illustrating its chemical dissimilarity to western SC silicic units. Cold Springs tuff Nb data is not included to better illustrate Nb differences among western SC silicic units. However, Cold Springs tuff Nb concentrations range from 33 - 65 ppm.

149 1.0 2 700

600 0.8 500

0.6 400 1 0.4 300 200 0.2 100 TiO2 MgO Zr 0.0 0 0

5000 0.3 6 4000

0.2 5 3000

2000 0.1 4 1000

PO25 KO2 Ba 0.0 3 0

40 300 20 30 200 20 15 100 10 Sr Th Nb 0 0 10 64 66 68 70 72 74 76 78 64 66 68 70 72 74 76 78 64 66 68 70 72 74 76 78

SiO2 SiO2 SiO2 West Porphyritic Rhyolite Flow-banded Rhyolite Western Margin Units Eightmile Mountain Units Hinkey Summit-Coal Pit Units

150 Brueseke et al., Figure 10 Figure 11. Major and trace element variations for eastern SC silicic units (major elements in wt.%; trace in ppm). Chemically, these units are more similar to each other than western SC units and are chemically distinct from the Cold Springs tuff (same fields as figure 7). Like in figure 10, Cold Springs tuff Nb data is not included to better illustrate Nb differences among eastern SC silicic units.

151 1.0 2 700

600 0.8 500

0.6 400 1 0.4 300 200 0.2 100 TiO2 MgO Zr 0.0 0 0

5000 0.3 6 4000

0.2 5 3000

2000 0.1 4 1000

PO25 KO2 Ba 0.0 3 0

40 40 300

30 30 200 20 20

100 10 10 Sr Th Nb 0 0 0 64 66 68 70 72 74 76 78 64 66 68 70 72 74 76 78 64 66 68 70 72 74 76 78

SiO2 SiO2 SiO2 East Calico Mountains area units Odell Mountain area units Coyote-Zymns Butte area units Black Dome and outflow

152 Brueseke et al., Figure 11 Figure 12. Multi-element diagrams illustrating the chemical variation of units from the (a) Western SC and (b) Eastern SC.

153 100 Flow banded rhyolite Porphyritic rhyolite 10

1

.1

.01

100 Eightmile Mountain Hinkey-Coal Pit 10 and Western SC lava flows

Rock/Steens Basalt 1

.1

.01 (a) Western SC .001 Sr RbBa Th LaNb Zr Y ZnGa Sc V Ni Cr CoCu Sr RbBa Th LaNb Zr Y ZnGa Sc V Ni Cr CoCu 100 Odell Mountain and Lower Calico ash flows 10 upper Calico lava flows

1

.1

.01

100 Coyote Mountain, Calico lava flows 10 Zymns Butte, (w/out upper) and Black Dome

Rock/Steens Basalt 1

.1

.01 (b) Eastern SC .001 Sr RbBa Th LaNb Zr Y ZnGa Sc V Ni Cr CoCu Sr RbBa Th LaNb Zr Y ZnGa Sc V Ni Cr CoCu

154 Brueseke et al., Figure 12 Table 1 Summary of sanidine 40Ar/39Ar results for Santa Rosa-Calico volcanic field silicic units

Sample Unit Location L# MSWD n K/Ca ±1s Age ±1s MB02-44 Tpr Southern margin 54404 1.39 12 41.4 5.9 16.16 0.02 MB02-19 Tpr Western coulée 54400 1.01 15 57.3 4.8 16.18 0.02 MB00-18 Tpr Southern coulée 55041 1.24 14 33.1 6.5 16.23 0.02 MB03-10B Tom Klondike Canyon 55043 1.53 15 28.9 3.1 16.43 0.03 MB00-33 Tom Odell Mountain 54403 2.87 15 33.6 3.9 16.55 0.02 MB03-45 Tct Zymns Butte 55047 0.65 15 23.9 3 16.4 0.04 MB01-54A Tct Coyote Mt. 54405 3.96 13 17.9 3.8 16.44 0.03 MB01-76 Tcm Southern Calico Mts. 54407 1.92 15 12.3 1.9 16.47 0.02 MB01-27 Tcm Capitol Peak 54401 0.98 15 29 4.2 16.5 0.02 MB00-32B Tcm Mahogany Pass 54402 1.53 13 11.9 3.1 16.51 0.02 MB03-36A Cold Springs tuff Hardscrabble Basin 55046 1.71 15 8.7 1.7 15.4 0.04 MB02-55 Cold Springs tuff Cold Springs Butte 54406 2.38 15 9 2.1 15.46 0.02 MB03-26F Cold Springs tuff Holloway Meadows 55044 0.38 15 3.6 2.3 15.75 0.08 L# = Lab number K/Ca error is 1σ standard deviation of grains used for weighted mean age. n = number of grains included in weighted mean calc

Sample preparation and irradiation: Minerals separates provided by Matt Brueseke. Separates were loaded into machined Al discs and irradiated for either 7 (NM-172) or 15 hours (NM-181) in D-3 position, Nuclear Science Center, College Station, TX. Neutron flux monitor Fish Canyon Tuff sanidine (FC-2). Assigned age = 28.02 Ma (Renne et al., 1998).

Instrumentation: Mass Analyzer Products 215-50 mass spectrometer on line with automated all-metal extraction system.

Single crystal sanidines fused by a 50 watt Synrad CO 2 laser. Laser fusion analysis: Reactive gases removed during a 1.5 minute reaction with 2 SAES GP-50 getters, 1 operated at ~450°C and 1 at 20°C. Gas also exposed to a W filament operated at ~2000°C and a cold finger operated at -140°C.

Analytical parameters: Electron multiplier sensitivity averaged 1.65x10 -16 moles/pA for NM-181 and 1.40x10 -16 moles/pA for NM-172 data sets. NM-172 total system blank and background averaged: 25, 0.17, 0.01, 0.18, 0.10 x 10-17 moles for masses 40, 39, 38, 37, 36, respectively. NM-181 total system blank and background averaged: 13, 1.1, 0.05, 0.08, 0.07 x 10-17 moles for masses 40, 39, 38, 37, 36, respectively.

J-factors determined to a precision of ± 0.1% by CO 2 laser-fusion of 8-20 single crystals from each of 6-9 radial positions around the irradiation tray. Correction factors for interfering nuclear reactions were determined using K-glass and CaF 2 and are as follows: 40 39 36 37 39 37 ( Ar/ Ar)K = 0±0.0004; ( Ar/ Ar)Ca = 0.000280±0.000005; and ( Ar/ Ar)Ca = 0.00070±0.00002.

155 Table 2 Representative major and trace element geochemical data for Santa Rosa-Calico volcanic field silicic units

Sample MB00-8 MB00-2 MB02-46 MB01-15 MB00-18 MB03-54 MB00-15A MB02-19 MB02-61 MB02-71 MB02-33 MB02-41 MB01-73 MB01-31 MB01-40 MB00-32B MB03-33 MB03-10A MB00-33 MB01-60A MB03-45 MB01-56 MB03-46 Location WSC WSC WSC WSC WSC WSC WSC WSC WSC WSC WSC WSC WSC ESC ESC ESC ESC ESC ESC ESC ESC ESC ESC Unit Thc Thc Thc Twsc Tpr Tpr Tpr Tpr Tbr Tbr Tem Tem Tem Tcm Tcm Tcm Tcm Tom Tom Tct Tct Tct Tbl Type LF LF LF LF LF I I LF I LF LF LF LF AF AF LF LF V LF AF V AF V Northing 454927 456372 459051 451415 450931 455848 451415 456953 454363 456159 458646 459726 455793 474210 475572 475284 475304 465783 469083 471094 476386 471990 459051 Easting 4612880 4611475 4605818 4615976 4615665 4610821 4615976 4618371 4610200 4615605 4632364 4632826 4626275 4628834 4633075 4626584 4620116 4636455 4638444 4615936 4612278 4615070 4605818

SiO2 67.05 70.48 75.45 64.8 69.96 71.97 74.13 75.89 75.61 75.99 64.81 69.81 72.99 66.3 67.96 71.42 73.98 75.74 76.27 69.85 73.94 74.42 71.92

TiO2 0.69 0.44 0.18 0.92 0.53 0.3 0.16 0.1 0.08 0.08 0.74 0.44 0.28 0.79 0.65 0.27 0.27 0.12 0.16 0.61 0.13 0.2 0.35

Al2O3 14.16 13.83 12.56 14.81 13.9 13.06 11.75 12.03 12.61 12.3 15.02 15.62 14.87 13.76 13.9 12.87 13.81 11.95 11.84 14.37 13.26 13.25 13.89

Fe2O3 4.52 2.88 1.1 6.16 3.33 2 1.52 1.08 1.22 1.28 6.84 1.52 1.09 5.85 4.5 2.95 0.83 1.72 2.04 3.52 1.49 1.6 2.08 MnO 0.09 0.05 0.02 0.12 0.06 0.04 0.03 0.01 0.02 0.02 0.11 0.01 0.01 0.07 0.03 0.05 0.01 0.02 0.02 0.06 0.05 0.04 0.08 MgO 1.4 0.34 0.15 1.1 0.72 0.37 0.29 0.1 0.13 0.1 0.68 0.07 0.03 1.88 0.34 0.18 0.03 0.18 0.02 0.43 0.05 0.12 0.27 CaO 2.51 0.99 0.54 2.9 2.11 1.5 0.95 0.67 0.46 0.64 2.51 1.75 0.95 3.72 1.8 1.08 0.87 0.36 0.11 1.99 0.74 0.87 1.14

Na2O 3.85 3.95 3.61 3.88 3.34 3.07 2.72 3.16 3.22 3.81 4.13 4.52 4.2 3.39 3.98 3.32 3.84 3.67 3.76 4.19 3.59 3.78 4.08

K2O 4.29 5.17 5.19 3.8 4.51 4.66 5.03 5 4.92 4.81 3.86 4.74 5.27 3.7 4.33 5.15 5.17 4.79 4.82 4.2 5.03 5.22 4.65

P2O5 0.23 0.08 0.04 0.31 0.18 0.07 0.04 0.15 0.03 0.04 0.27 0.13 0.05 0.28 0.13 0.06 0.06 0.05 0.05 0.18 0.02 0.06 0.08 LOI 0.6 0.29 0.33 0.38 1.55 2.75 2.53 2.19 0.52 0.56 0.55 0.36 0.08 0.63 1.51 2.63 0.38 0.93 0.52 0.54 0.45 0.6 0.24 TOTAL 99.4 98.51 99.16 99.18 100.19 99.8 99.14 100.4 98.82 99.62 99.52 98.95 99.83 100.38 99.15 99.99 99.25 99.54 99.62 99.94 98.75 100.14 98.78

Ni 1673343 2112322244 23 12 3323 Co 731843 4333632156 22 21 2122 Cr 2 49

Notes: Major element concentrations are reported as weight percent oxides and expressed as raw data; trace element concentrations are reported in ppm. Major element analyses analyzed by the techniques outlined in Katoh et al., 1999 at Miami University by DCP-AES (Direct Current Argon Plasma Atomic Emission Spectrometry). All trace elements were analyzed by XRF (X-ray fluorescence) at Franklin and Marshall College by techniques outlined in Mertzman, 2000. Abbreviations are the following: Location: WSC=Western SC, ESC= Eastern SC; Unit: Thc= Hinkey-Coal Pit units, Twsc= Western margin units, Tpr= Porphyritic rhyolite, Tbr= Banded rhyolite, Tem= Eightmile Mountain units, Tcm= Calico units, Tom= Odell Mountain units, Tct= Coyote-Zymns units, Tbl= Black dome units; Type: V=vent, I= shallow intrusive body, LF= lava flow, AF= ash flow. UTM Northing and Easting values are displayed and correspond to UTM Zone 11 T and the NAD 27 CONUS map datum. Appendix 1.

40Ar/39Ar analyses of single crystal sanidines from volcanic rocks from the Santa Rosa-Calico volcanic field.

Sanidine single crystals were analyzed by the laser fusion method at the New Mexico Geochronology Research Laboratory (NMGRL), NM Tech, Socorro, NM. The analytical methods are summarized in Table 1 with complete isotopic data given in Table 2. Probability diagrams for the age data are presented for the 13 samples in Figures (1a – m). For each sample, 15 crystals were analyzed and the 40Ar/39Ar data are typical for pristine sanidine crystals. Most crystals are highly radiogenic and yield precise apparent ages (Table 2). Four samples (MB02-44, MB00-18, MB01-54A and MB00-32B) had minor plagioclase contamination that is easily recognized by their low K/Ca value and low age precision (Table 2). Sample MB01-76 apparently has a single xenocrystic contaminant grain that yields an apparent age of 28.27 Ma that is much older than the remainder of the crystals. Weighted mean ages for samples range from 15.40 to 16.55 Ma (Table 1). Nearly all of the samples yield simple age distributions as defined by acceptable MSWD values (Table 2). We consider an MSWD as acceptable if it falls within the 95% confidence window for N-1 degrees for freedom as defined by Mahon (1996). Most of the weighted means have 15 analyses (14 degrees of freedom) and thus an acceptable age population at the 95% confidence level would yield an MSWD value between 0.40 and 1.86 (Mahon 1996). Samples MB00-33 (Fig. 1e), MB01-54A (Fig. 1g), MB01- 76 (Fig. 1h), and MB02-55 (Fig. 1l) each have MSWD values that are slightly above the 95% confidence window and indicate some scatter that cannot be explained by analytical error alone. These samples could yield slightly scattered age data due to minor alteration (Deino and Potts, 1992), from glass adhering to the sanidine crystals, or perhaps from excess argon within melt inclusions (Winick et al., 2001). Each weighted mean age is considered a reliable eruption age. We do not believe that the scatter in the four samples that have slightly high MSWD values severely compromises this interpretation. Based on this interpretation these samples define an eruption history that ranges

157 between about 15.4 and 16.6 Ma thereby lasting approximately 1.2 Ma. Table 3 displays geographic information for where the newly dated samples were collected.

References cited.

Deino, A., and Potts, R., 1992. Age-probability spectra from examination of single-crystal 40Ar/39Ar dating results: Examples from Olorgesailie, southern Kenya rift, Quat. Int., 13/14, 47-53. Mahon, K.I., 1996. The New “York” regression: Application of an improved statistical method to geochemistry, International Geology Review, 38, 293-303. Steiger, R.H., and Jäger, E., 1977. Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planet. Sci. Lett., 36, 359-362. Taylor, J.R., 1982. An Introduction to Error Analysis: The Study of Uncertainties in Physical Measurements,. Univ. Sci. Books, Mill Valley, Calif., 270 p. Winick, J.A., McIntosh, W.C., Dunbar, N.W., 2001. Melt-inclusion-host excess 40Ar in quartz crystals of the Bishop and Bandelier magma systems, Geology, 29, no. 3, 275-278.

158 10 (a) MB02-44 (b) MB02-19 K/Ca 0.1 20

10 N

0 16.164 ± 0.020, MSWD = 1.39 16.184 ± 0.019, MSWD = 1.01 Relative Probability

10 (c) MB00-18 (d) MB03-10B K/Ca 0.1 20

10 N

0 ± 16.234 ± 0.021, MSWD = 1.24 16.431 0.032, MSWD = 1.53 Relative Probability

15 16 17 15 16 17 Age (Ma) Age (Ma)

Figure 1 (a-d). Relative age probability and K/Ca diagrams for sanidine single crystal laser fusion data. N = number of grains analyzed. Open symbols are data not included in weighted mean calculations and are typically plagioclase crystals as reveal by relatively low K/Ca values. Errors are 1s and include error in J-factor.

159 10 (e) MB00-33 (f) MB03-45 K/Ca 0.1 20

10 N

0 16.547 ± 0.025, MSWD = 2.87 16.396 ± 0.038, MSWD = 0.65 Relative Probability

10 (g) MB01-54A (h) MB01-76 K/Ca 0.1 20

10 N

0 16.440 ± 0.027, MSWD = 3.96 16.475 ± 0.022, MSWD = 1.92 Relative Probability

15 16 17 15 16 17 Age (Ma) Age (Ma)

Figure 1 cont'd (e-h). Relative age probability and K/Ca diagrams for sanidine single crystal laser fusion data. N = number of grains analyzed. Open symbols are data not included in weighted mean calculations and are typically plagioclase crystals as reveal by relatively low K/Ca values. Errors are 1s and include error in J-factor. 10 (i) MB01-27 (j) MB00-32B K/Ca

0.1 20

10 N

0 16.504 ± 0.021, MSWD = 0.98 16.510 ± 0.024, MSWD = 1.53 Relative Probability

10

K/Ca (k) MB03-36A (l) MB02-55 0.1 20

10 N

0

15.401 ± 0.044, MSWD = 1.71 15.460 ± 0.022, MSWD = 2.38 Relative Probability

15 16 17 15 16 17 Age (Ma) Age (Ma)

Figure 1 cont'd (i-l). Relative age probability and K/Ca diagrams for sanidine single crystal laser fusion data. N = number of grains analyzed. Open symbols are data not included in weighted mean calculations and are typically plagioclase crystals as reveal by relatively low K/Ca values. Errors are 1s and include error in J-factor.

161 (m) MB03-26F 10 K/Ca 0.1 20

10 N

0 15.747 ± 0.082, MSWD = 0.38 Relative Probability

15 16 17 Age (Ma)

Figure 1 cont'd (m). Relative age probability and K/Ca diagram for sanidine single crystal laser fusion data. N = number of grains analyzed. Open symbols are data not included in weighted mean calculations and are typically plagioclase crystals as reveal by relatively low K/Ca values. Errors are 1s and include error in J-factor.

162 Table 1. Summary of assigned ages and analytical methods.

Summary of Sanidine results.

Sample Unit Location L# Irrad MSWD n K/Ca ±1s Age ±1s MB02-44 Tpr Southern margin 54404 NM-172 1.39 12 41.4 5.9 16.16 0.02 MB02-19 Tpr Western coulée 54400 NM-172 1.01 15 57.3 4.8 16.18 0.02 MB00-18 Tpr Southern coulée 55041 NM-181B 1.24 14 33.1 6.5 16.23 0.02 MB03-10B Tom Klondike Canyon 55043 NM-181B 1.53 15 28.9 3.1 16.43 0.03 MB00-33 Tom Odell Mountain 54403 NM-172 2.87 15 33.6 3.9 16.55 0.02 MB03-45 Tct Zymns Butte 55047 NM-181B 0.65 15 23.9 3.0 16.40 0.04 MB01-54A Tct Coyote Mt. 54405 NM-172 3.96 13 17.9 3.8 16.44 0.03 MB01-76 Tcm Southern Calico Mts. 54407 NM-172 1.92 15 12.3 1.9 16.47 0.02 MB01-27 Tcm Capitol Peak 54401 NM-172 0.98 15 29.0 4.2 16.50 0.02 MB00-32B Tcm Mahogany Pass 54402 NM-172 1.53 13 11.9 3.1 16.51 0.02 MB03-36A Cold SpringsTuff Hardscrabble Basin 55046 NM-181B 1.71 15 8.7 1.7 15.40 0.04 MB02-55 Cold SpringsTuff Cold Springs Butte 54406 NM-172 2.38 15 9.0 2.1 15.46 0.02 MB03-26F Cold SpringsTuff Holloway Meadows 55044 NM-181B 0.38 15 3.6 2.3 15.75 0.08

L# = Lab number Irrad = Irradiation number K/Ca error is 1s standard deviation of grains used for weighted mean age. n = number of grains included in weighted mean calculation

Methods

Sample preparation and irradiation: Minerals separates provided by Matt Brueseke. Separates were loaded into machined Al discs and irradiated for either 7 (NM-172) or 15 hours (NM-181) in D-3 position, Nuclear Science Center, College Station, TX. Neutron flux monitor Fish Canyon Tuff sanidine (FC-2). Assigned age = 28.02 Ma (Renne et al., 1998).

Instrumentation: Mass Analyzer Products 215-50 mass spectrometer on line with automated all-metal extraction system.

Single crystal sanidines fused by a 50 watt Synrad CO2 laser. Laser fusion analysis: Reactive gases removed during a 1.5 minute reaction with 2 SAES GP-50 getters, 1 operated at ~450°C and 1 at 20°C. Gas also exposed to a W filament operated at ~2000°C and a cold finger operated at -140°C.

Analytical parameters: Electron multiplier sensitivity averaged 1.65x10-16 moles/pA for NM-181 and 1.40x10-16 moles/pA for NM-172 data sets. NM-172 total system blank and background averaged: 25, 0.17, 0.01, 0.18, 0.10 x 10-17 moles for masses 40, 39, 38, 37, 36, respectively. NM-181 total system blank and background averaged: 13, 1.1, 0.05, 0.08, 0.07 x 10-17 moles for masses 40, 39, 38, 37, 36, respectively.

J-factors determined to a precision of ± 0.1% by CO2 laser-fusion of 8-20 single crystals from each of 6-9 radial positions around the irradiation t

Correction factors for interfering nuclear reactions were determined using K-glass and CaF2 and are as follows: 40 39 36 37 39 37 ( Ar/ Ar)K = 0±0.0004; ( Ar/ Ar)Ca = 0.000280±0.000005; and ( Ar/ Ar)Ca = 0.00070±0.00002.

163 Table 2. 40Ar/39Ar analytical data.

40 39 37 39 36 39 39 40 ID Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Age ±1σ (x 10-3) (x 10-15 mol) (%) (Ma) (Ma)

MB02-44 sa, A6:172, Sanidine, single crystal, J=0.0007545±0.10%, D=1.005±0.001, NM-172, Lab#=54404 # 55 12.26 2.623 2.320 0.311 0.19 96.2 16.01 0.24 63 11.91 0.0095 0.1973 3.641 53.9 99.5 16.066 0.035 58 11.96 0.0147 0.3052 4.717 34.8 99.3 16.092 0.038 61 11.96 0.0097 0.1439 4.768 52.6 99.7 16.149 0.035 59 12.88 0.0130 3.238 12.410 39.1 92.6 16.157 0.033 50 12.02 0.0136 0.2978 8.048 37.6 99.3 16.171 0.029 60 12.15 0.0136 0.7238 9.957 37.7 98.2 16.172 0.032 53 11.96 0.0137 0.0776 3.472 37.2 99.8 16.174 0.041 62 12.40 0.0133 1.550 9.531 38.3 96.3 16.180 0.031 64 11.97 0.0123 0.1019 9.662 41.3 99.8 16.185 0.027 56 11.99 0.0119 0.1420 9.241 42.9 99.7 16.187 0.028 57 12.00 0.0124 0.1966 8.907 41.1 99.5 16.189 0.034 52 12.16 0.0125 0.6919 8.286 40.8 98.3 16.197 0.031 # 54 12.24 1.640 1.449 0.985 0.31 97.6 16.213 0.090 # 51 12.08 2.649 1.172 0.463 0.19 98.9 16.23 0.17 Mean age ± 1σ n=12 MSWD=1.39 41.4 ±5.9 16.164 0.020

MB02-19 sa, A1:172, Sanidine, single crystal, J=0.0007569±0.10%, D=1.005±0.001, NM-172, Lab#=54400 54 11.91 0.0090 0.1841 4.383 56.7 99.5 16.120 0.046 62 11.88 0.0083 0.0769 5.203 61.8 99.8 16.122 0.039 55 11.98 0.0086 0.3918 5.906 59.4 99.0 16.133 0.039 56 12.64 0.0086 2.605 6.673 59.7 93.9 16.135 0.041 64 11.98 0.0082 0.3000 6.188 62.5 99.3 16.171 0.036 53 12.06 0.0093 0.5398 6.734 54.6 98.7 16.174 0.047 58 11.99 0.0111 0.2971 8.578 46.0 99.3 16.175 0.032 51 12.00 0.0100 0.3054 12.359 50.8 99.3 16.188 0.029 60 11.98 0.0085 0.2414 9.332 59.7 99.4 16.189 0.027 57 12.00 0.0086 0.3187 8.439 59.5 99.2 16.191 0.038 63 11.97 0.0099 0.2205 9.935 51.7 99.5 16.191 0.029 50 11.94 0.0092 0.0969 6.121 55.2 99.8 16.195 0.043 52 12.02 0.0080 0.3510 5.634 63.7 99.1 16.202 0.043 59 12.02 0.0086 0.2895 7.581 59.1 99.3 16.224 0.031 61 12.01 0.0086 0.2054 8.650 59.6 99.5 16.243 0.029 Mean age ± 1σ n=15 MSWD=1.01 57.3 ±4.8 16.184 0.019

164 Table 2. 40Ar/39Ar analytical data.

40 39 37 39 36 39 39 40 ID Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Age ±1σ (x 10-3) (x 10-15 mol) (%) (Ma) (Ma)

MB00-18, sanidine, J=0.0016177±0.10%, D=1.0037±0.0005, NM-181B, Lab#=55041 # 09 5.862 2.129 1.894 0.918 0.24 93.5 15.94 0.52 10 5.606 0.0157 0.1736 6.585 32.5 99.1 16.144 0.081 08 5.652 0.0166 0.3251 5.203 30.7 98.3 16.146 0.096 13 5.592 0.0159 0.0576 27.623 32.1 99.7 16.201 0.024 11 5.606 0.0137 0.0881 11.332 37.3 99.6 16.216 0.048 04 5.604 0.0134 0.0801 14.442 38.2 99.6 16.217 0.038 07 5.675 0.0202 0.3143 14.284 25.2 98.4 16.222 0.041 01 5.616 0.0128 0.1051 11.338 39.8 99.5 16.229 0.047 15 5.592 0.0114 0.0215 9.747 44.6 99.9 16.230 0.057 12 5.631 0.0194 0.1368 3.823 26.3 99.3 16.25 0.14 06 5.621 0.0179 0.0794 8.961 28.5 99.6 16.266 0.060 05 5.634 0.0171 0.1225 10.773 29.8 99.4 16.267 0.051 03 5.637 0.0236 0.1220 4.976 21.6 99.4 16.28 0.10 02 5.636 0.0132 0.0837 17.419 38.8 99.6 16.306 0.033 14 5.660 0.0135 -0.1152 4.025 37.8 100.6 16.54 0.13 Mean age ± 1σ n=14 MSWD=1.24 33.1 ±6.5 16.234 0.021

MB03-10B, sanidine, J=0.0016187±0.10%, D=1.0037±0.0005, NM-181B, Lab#=55043 02 5.712 0.0185 0.3258 11.719 27.6 98.3 16.330 0.045 13 5.741 0.0193 0.3929 3.495 26.4 98.0 16.36 0.14 11 5.842 0.0161 0.7093 6.853 31.8 96.4 16.378 0.074 01 5.703 0.0173 0.2307 11.006 29.4 98.8 16.383 0.050 06 5.693 0.0170 0.1537 2.866 29.9 99.2 16.42 0.17 03 5.722 0.0172 0.2316 7.686 29.7 98.8 16.440 0.068 12 5.743 0.0175 0.2512 3.061 29.1 98.7 16.48 0.16 15 5.696 0.0162 0.0718 3.905 31.5 99.7 16.50 0.13 10 5.793 0.0191 0.3694 5.843 26.7 98.1 16.526 0.092 05 5.742 0.0155 0.1799 7.794 33.0 99.1 16.542 0.067 14 5.729 0.0175 0.1227 2.986 29.1 99.4 16.55 0.17 04 5.731 0.0244 0.1080 2.491 20.9 99.5 16.57 0.20 08 5.793 0.0153 0.1872 4.381 33.3 99.1 16.68 0.12 09 6.011 0.0190 0.9222 1.358 26.8 95.5 16.69 0.36 07 5.780 0.0177 -0.0828 2.367 28.9 100.4 16.88 0.21 Mean age ± 1σ n=15 MSWD=1.53 28.9 ±3.1 16.431 0.032

165 Table 2. 40Ar/39Ar analytical data.

40 39 37 39 36 39 39 40 ID Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Age ±1σ (x 10-3) (x 10-15 mol) (%) (Ma) (Ma)

MB00-33 sa, A5:172, Sanidine, single crystal, J=0.0007549±0.10%, D=1.005±0.001, NM-172, Lab#=54403 57 12.24 0.0145 0.5440 1.818 35.2 98.7 16.384 0.056 58 12.26 0.0167 0.5317 1.188 30.6 98.7 16.416 0.082 53 12.54 0.0203 1.457 1.919 25.1 96.6 16.423 0.060 63 12.36 0.0168 0.7850 4.224 30.4 98.1 16.443 0.040 60 12.28 0.0169 0.3816 2.793 30.2 99.1 16.503 0.041 51 12.30 0.0145 0.3987 2.985 35.1 99.1 16.518 0.044 61 12.34 0.0135 0.5271 2.808 37.7 98.7 16.524 0.040 64 12.39 0.0153 0.6454 3.677 33.5 98.5 16.535 0.039 55 12.28 0.0131 0.2126 2.057 39.1 99.5 16.559 0.051 52 12.31 0.0142 0.3089 5.020 36.0 99.3 16.562 0.036 62 12.48 0.0152 0.8590 7.003 33.6 98.0 16.575 0.035 59 12.32 0.0148 0.2501 2.376 34.5 99.4 16.605 0.049 56 12.33 0.0128 0.2590 4.403 39.7 99.4 16.612 0.036 54 12.57 0.0164 1.038 2.417 31.0 97.6 16.623 0.046 50 12.37 0.0158 0.3386 7.785 32.2 99.2 16.639 0.033 Mean age ± 1σ n=15 MSWD=2.87 33.6 ±3.9 16.547 0.025

MB03-45, sanidine, J=0.0016127±0.10%, D=1.0037±0.0005, NM-181B, Lab#=55047 13 5.680 0.0241 0.5365 1.499 21.1 97.2 16.00 0.32 11 5.678 0.0211 0.3612 1.951 24.2 98.2 16.14 0.27 08 5.665 0.0190 0.1779 4.646 26.8 99.1 16.26 0.11 12 5.694 0.0232 0.2599 1.043 22.0 98.7 16.28 0.46 15 5.779 0.0242 0.5038 1.032 21.1 97.5 16.31 0.49 06 5.674 0.0201 0.1340 3.854 25.4 99.3 16.32 0.13 09 5.697 0.0257 0.1968 3.596 19.9 99.0 16.34 0.14 14 5.667 0.0212 0.0540 5.252 24.1 99.7 16.37 0.10 05 5.685 0.0196 0.0765 9.214 26.1 99.6 16.405 0.056 03 5.694 0.0256 0.0998 3.854 20.0 99.5 16.41 0.13 02 5.692 0.0179 0.0805 2.438 28.4 99.6 16.42 0.21 10 5.711 0.0202 0.0195 4.891 25.3 99.9 16.53 0.10 01 5.759 0.0203 0.1519 2.868 25.1 99.2 16.55 0.18 04 5.728 0.0247 0.0160 1.834 20.7 100.0 16.58 0.27 07 5.788 0.0178 0.0676 1.930 28.7 99.7 16.71 0.25 Mean age ± 1σ n=15 MSWD=0.65 23.9 ±3.0 16.396 0.038

166 Table 2. 40Ar/39Ar analytical data.

40 39 37 39 36 39 39 40 ID Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Age ±1σ (x 10-3) (x 10-15 mol) (%) (Ma) (Ma)

MB01-54A sa, A8:172, Sanidine, single crystal, J=0.0007541±0.10%, D=1.005±0.001, NM-172, Lab#=54405 # 62 12.34 0.9319 2.183 0.394 0.55 95.4 15.95 0.22 # 64 13.43 1.013 5.665 0.307 0.50 88.2 16.04 0.25 51 12.15 0.0258 0.5924 1.347 19.8 98.6 16.217 0.067 61 12.21 0.0261 0.6026 2.280 19.6 98.6 16.296 0.049 63 12.25 0.0309 0.6943 1.949 16.5 98.3 16.320 0.056 58 12.25 0.0232 0.5432 3.415 22.0 98.7 16.373 0.039 59 12.37 0.0252 0.8800 2.075 20.3 97.9 16.399 0.048 57 12.29 0.0238 0.5357 4.231 21.4 98.7 16.428 0.033 53 12.32 0.0724 0.6581 4.428 7.0 98.5 16.432 0.034 55 12.42 0.0327 0.9507 4.900 15.6 97.8 16.437 0.035 54 12.25 0.0278 0.3490 6.554 18.4 99.2 16.457 0.031 52 13.16 0.0268 3.337 6.641 19.1 92.5 16.490 0.038 50 12.31 0.0260 0.4006 8.484 19.6 99.1 16.511 0.031 56 12.92 0.0316 2.432 2.468 16.2 94.5 16.523 0.050 60 12.35 0.0304 0.4117 3.532 16.8 99.0 16.568 0.042 Mean age ± 1σ n=13 MSWD=3.96 17.9 ±3.8 16.440 0.027

MB01-76 sa, A10:172, Sanidine, single crystal, J=0.0007546±0.10%, D=1.005±0.001, NM-172, Lab#=54407 61 12.61 0.0488 1.787 3.600 10.5 95.8 16.381 0.042 52 12.41 0.0418 1.095 4.170 12.2 97.4 16.389 0.032 58 12.38 0.0372 0.8984 2.796 13.7 97.9 16.428 0.043 57 12.42 0.0365 0.9913 5.537 14.0 97.7 16.443 0.035 56 12.24 0.0389 0.3682 2.555 13.1 99.1 16.444 0.048 60 12.77 0.0461 2.126 3.477 11.1 95.1 16.457 0.043 51 12.23 0.0344 0.2757 4.208 14.8 99.4 16.470 0.037 63 13.64 0.0448 5.002 4.580 11.4 89.2 16.483 0.056 53 12.31 0.0385 0.4761 3.054 13.2 98.9 16.492 0.041 59 12.32 0.0409 0.4721 1.474 12.5 98.9 16.511 0.064 64 12.59 0.0437 1.383 4.277 11.7 96.8 16.512 0.039 50 12.36 0.0494 0.5979 6.063 10.3 98.6 16.512 0.037 62 12.70 0.0351 1.732 6.689 14.5 96.0 16.527 0.036 55 12.45 0.0648 0.8524 7.923 7.9 98.0 16.535 0.035 54 12.39 0.0368 0.5670 3.139 13.9 98.7 16.563 0.042 # 65 21.13 -0.0087 0.7010 0.337 - 99.0 28.27 0.25 Mean age ± 1σ n=15 MSWD=1.92 12.3 ±1.9 16.47 0.02

167 Table 2. 40Ar/39Ar analytical data.

40 39 37 39 36 39 39 40 ID Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Age ±1σ (x 10-3) (x 10-15 mol) (%) (Ma) (Ma)

MB01-27 sa, A2:172, Sanidine, single crystal, J=0.0007566±0.10%, D=1.005±0.001, NM-172, Lab#=54401 53 12.51 0.0151 1.458 6.505 33.8 96.6 16.417 0.046 56 12.41 0.0140 1.066 0.903 36.5 97.5 16.43 0.15 60 12.30 0.0177 0.6651 4.824 28.9 98.4 16.447 0.041 62 12.30 0.0203 0.6191 2.601 25.1 98.5 16.465 0.067 61 12.62 0.0172 1.675 4.682 29.6 96.1 16.479 0.049 55 12.26 0.0177 0.4510 1.841 28.9 98.9 16.481 0.082 58 12.28 0.0165 0.5260 2.329 30.8 98.7 16.482 0.066 52 12.47 0.0173 1.152 4.681 29.5 97.3 16.486 0.046 64 12.25 0.0241 0.3997 4.985 21.2 99.1 16.494 0.044 50 12.24 0.0207 0.2970 4.717 24.7 99.3 16.519 0.040 63 12.29 0.0172 0.4324 2.244 29.6 99.0 16.534 0.079 59 13.94 0.0144 6.004 1.248 35.3 87.3 16.54 0.13 51 12.30 0.0182 0.4052 4.716 28.0 99.0 16.557 0.040 57 12.32 0.0214 0.4253 3.232 23.8 99.0 16.572 0.053 54 12.36 0.0173 0.5567 5.519 29.5 98.7 16.574 0.039 Mean age ± 1σ n=15 MSWD=0.98 29.0 ±4.2 16.504 0.021

MB00-32B sa, A4:172, Sanidine, single crystal, J=0.0007557±0.10%, D=1.005±0.001, NM-172, Lab#=54402 # 60 12.99 0.5309 3.785 0.937 0.96 91.7 16.18 0.11 63 12.47 0.0395 1.372 1.590 12.9 96.8 16.376 0.066 54 12.28 0.0843 0.7054 1.721 6.0 98.4 16.398 0.063 64 12.23 0.0377 0.4654 1.413 13.5 98.9 16.419 0.067 58 12.60 0.0427 1.591 2.314 12.0 96.3 16.465 0.051 53 12.25 0.0392 0.3835 3.472 13.0 99.1 16.481 0.043 51 12.49 0.0395 1.169 5.284 12.9 97.3 16.481 0.036 # 52 12.88 0.9314 2.703 0.579 0.55 94.4 16.51 0.15 50 12.22 0.0361 0.1805 2.470 14.1 99.6 16.516 0.046 59 12.51 0.0788 1.126 1.597 6.5 97.4 16.536 0.058 62 12.41 0.0529 0.7008 2.439 9.6 98.4 16.560 0.046 61 12.27 0.0361 0.2446 3.393 14.1 99.4 16.561 0.041 57 12.38 0.0318 0.5797 2.405 16.0 98.6 16.570 0.048 55 12.33 0.0554 0.4148 1.163 9.2 99.0 16.572 0.079 56 12.68 0.0351 1.597 3.428 14.6 96.3 16.573 0.044 Mean age ± 1σ n=13 MSWD=1.53 11.9 ±3.1 16.510 0.024

168 Table 2. 40Ar/39Ar analytical data.

40 39 37 39 36 39 39 40 ID Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Age ±1σ (x 10-3) (x 10-15 mol) (%) (Ma) (Ma)

MB03-36A, sanidine, J=0.0016138±0.10%, D=1.0037±0.0005, NM-181B, Lab#=55046 01 5.313 0.0555 0.2809 6.084 9.2 98.5 15.176 0.084 13 5.361 0.0561 0.3781 3.136 9.1 98.0 15.23 0.16 10 5.344 0.0565 0.3225 1.184 9.0 98.3 15.23 0.40 03 5.345 0.0469 0.2655 5.400 10.9 98.6 15.280 0.092 15 5.346 0.0641 0.2419 3.052 8.0 98.8 15.31 0.16 04 5.343 0.0693 0.1460 8.874 7.4 99.3 15.382 0.058 12 5.351 0.0640 0.1257 2.022 8.0 99.4 15.42 0.25 09 5.354 0.0799 0.0887 3.369 6.4 99.6 15.47 0.15 07 5.358 0.0454 0.0874 2.020 11.2 99.6 15.47 0.25 11 5.401 0.0756 0.1772 1.620 6.7 99.1 15.52 0.30 02 5.363 0.0612 0.0043 3.526 8.3 100.1 15.56 0.14 06 5.352 0.0657 -0.0510 3.205 7.8 100.4 15.58 0.15 08 5.383 0.0588 -0.0201 3.369 8.7 100.2 15.64 0.15 05 5.368 0.0694 -0.0779 5.643 7.4 100.5 15.644 0.088 14 5.571 0.0419 0.3465 0.410 12.2 98.2 15.9 1.2 Mean age ± 1σ n=15 MSWD=1.71 8.7 ±1.7 15.401 0.044

MB02-55 sa, A9:172, Sanidine, single crystal, J=0.0007543±0.10%, D=1.005±0.001, NM-172, Lab#=54406 62 11.47 0.0479 0.5261 1.499 10.6 98.7 15.335 0.062 59 11.43 0.0489 0.3658 2.218 10.4 99.1 15.353 0.047 58 11.45 0.0418 0.3230 2.186 12.2 99.2 15.392 0.044 51 11.51 0.0428 0.5109 2.791 11.9 98.7 15.396 0.039 53 11.51 0.0950 0.5054 2.242 5.4 98.8 15.401 0.047 63 11.46 0.0573 0.2626 3.114 8.9 99.4 15.435 0.038 55 11.48 0.0705 0.3175 2.393 7.2 99.2 15.437 0.043 60 11.45 0.0437 0.2014 3.861 11.7 99.5 15.444 0.035 64 11.49 0.0624 0.3131 3.000 8.2 99.2 15.450 0.040 61 11.57 0.0609 0.5193 4.128 8.4 98.7 15.473 0.037 50 11.48 0.0682 0.2135 5.945 7.5 99.5 15.483 0.036 57 11.60 0.0848 0.5639 1.566 6.0 98.6 15.505 0.060 54 11.54 0.0581 0.3008 2.868 8.8 99.3 15.528 0.044 52 11.55 0.0553 0.2695 4.445 9.2 99.3 15.542 0.032 56 11.75 0.0576 0.9571 5.071 8.9 97.6 15.542 0.033 Mean age ± 1σ n=15 MSWD=2.38 9.0 ±2.1 15.460 0.022

169 Table 2. 40Ar/39Ar analytical data.

40 39 37 39 36 39 39 40 ID Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Age ±1σ (x 10-3) (x 10-15 mol) (%) (Ma) (Ma)

MB03-26F, sanidine, J=0.0016175±0.10%, D=1.0037±0.0005, NM-181B, Lab#=55044 02 5.486 0.2683 1.006 1.095 1.9 95.0 15.14 0.42 05 5.337 0.1809 0.3434 0.846 2.8 98.4 15.26 0.57 14 5.527 0.2230 0.6962 1.236 2.3 96.6 15.52 0.39 03 5.424 0.1846 0.2801 1.947 2.8 98.8 15.56 0.24 07 5.431 0.0918 0.1882 1.529 5.6 99.1 15.64 0.31 10 5.498 0.2860 0.4451 0.641 1.8 98.0 15.66 0.73 12 5.494 0.1384 0.2355 2.397 3.7 98.9 15.80 0.21 15 5.470 0.0644 0.1029 1.491 7.9 99.5 15.82 0.33 01 5.416 0.3925 0.0029 2.860 1.3 100.6 15.83 0.18 04 5.454 0.1074 0.0379 2.393 4.8 100.0 15.84 0.21 08 5.492 0.0574 0.1430 0.762 8.9 99.3 15.85 0.63 11 5.463 0.2602 0.0790 1.610 2.0 100.0 15.87 0.29 06 5.458 0.2610 0.0418 0.762 2.0 100.2 15.89 0.62 09 5.550 0.1854 0.1452 0.477 2.8 99.5 16.05 0.99 13 5.528 0.1745 -0.0045 1.100 2.9 100.3 16.11 0.44 Mean age ± 1σ n=15 MSWD=0.38 3.6 ±2.3 15.747 0.082

Notes: Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reaction Ages calculated ralative to FC-1 Fish Canyon Tuff sanidine interlaboratory standard at 28.02 Ma. Errors quoted for individual analyses include analytical error only, without interfering reaction or J uncertainties. Mean age is weighted mean age of Taylor (1982). Mean age error is weighted error of the mean (Taylor, 1982), multiplied by the square root of the MSWD where MSWD>1, and also incorporates uncertainty in J factors and irradiation correction uncertainties. Decay constants and isotopic abundances after Steiger and Jäger (1977). # symbol preceding sample ID denotes analyses excluded from mean age calculations. Discrimination = 1.0037 ± 0.0005 Correction factors: 39 37 ( Ar/ Ar)Ca = 0.0007 ± 5e-05 36 37 ( Ar/ Ar)Ca = 0.00028 ± 1e-05 38 39 ( Ar/ Ar)K = 0.0129 40 39 ( Ar/ Ar)K = 0 ± 0.0004

170 Table 3. Sampling locations for newly dated samples Sample Unit Location Northing Easting MB02-44 Tpr Southern margin 462103 4607365 MB02-19 Tpr Western coulée 456953 4618371 MB00-18 Tpr Southern coulée 450931 4615665 MB03-10B Tom Klondike Canyon 465783 4636455 MB00-33 Tom Odell Mountain 475304 4620116 MB03-45 Tct Zymns Butte 476386 4612278 MB01-54A Tct Coyote Mt. 470826 4614029 MB01-76 Tcm Southern Calico Mts. 475462 4624446 MB01-27 Tcm Capitol Peak 474243 4631219 MB00-32B Tcm Mahogany Pass 475284 4626584 MB03-36A Cold SpringsTuff Hardscrabble Basin 470782 4608204 MB02-55 Cold SpringsTuff Cold Springs Butte 463995 4621788 MB03-26F Cold SpringsTuff Holloway Meadows 462850 4627544 Notes: UTM Northing and Easting values are displayed and correspond to UTM Zone 11 T and the NAD 27 CONUS map datum.

171 CHAPTERT 4:

GEOLOGY AND PETROLOGY OF THE MID-MIOCENE SANTA ROSA-CALICO VOLCANIC FIELD, NORTHERN NEVADA

INTRODUCTION Mid-Miocene to recent magmatism on and adjacent to the Oregon Plateau of the northwestern U.S.A. is and has been dominated by bimodal, basalt-rhyolite volcanism. The most voluminous volcanism in this timespan occurred from ~17 to ~14 Ma and was characterized by eruptions of the Steens-Columbia River flood basalts and the coeval formation of the dominantly silicic Oregon Plateau volcanic fields (e.g. McDermitt, Lake Owyhee; hereafter referred to as the Oregon Plateau volcanic province) (Fig. 1a, b; Zoback et al. 1994; Wallace and John 1998; Cummings et al. 2000; John and Wallace 2000; John et al. 2000; John 2001; Camp et al. 2003; Wallace 2003; Camp and Ross 2004; Jordan et al. 2004). Concurrent with this volcanism associated with the Yellowstone-Newberry melting anomaly, was tectonic activity along a focused zone of extension, primarily located against the western margin of the North American craton (Zoback et al. 1994; Cummings et al. 2000). An ~1000 km broadly north-south trending zone that encompasses the feeder dikes of the Steens-Columbia River basalts, the Oregon-Idaho graben, and the northern Nevada rift(s) is the manifestation of this short lived, laterally focused extensional event (Fig. 1a). To better understand this extensional event, particularly the interplay between upwelling mantle melt and the formation of multi-vent Oregon Plateau volcanic fields, it is essential to decipher the evolution of these complex volcanic systems. To this end, a detailed field, chronologic, geochemical, and petrologic study of the mid-Miocene Santa Rosa- Calico volcanic field of northern Nevada has been performed (Fig. 1a, b). Throughout the remainder of this contribution this volcanic field will simply be referred to as the SC. The SC provides an ideal physical setting to better understand the evolution of the Oregon Plateau volcanic province and the volcanotectonic processes that affected this region in the mid-Miocene due to its location within the focused zone of mid-Miocene extension and at the approximate common intersection of the northwest trending High Lava Plains - Newberry and northeast trending Snake River Plain - Yellowstone volcanic trends. Furthermore, unlike other

172 mid-Miocene Oregon Plateau volcanic fields, the SC was characterized by abundant intermediate composition volcanism during its >2 Ma duration.

GEOLOGIC SETTING The SC lies along the southern margin of the Oregon and Owyhee plateaus. This Idaho- Oregon-Nevada tri-state region has long been associated with the initial manifestation of the Yellowstone melt anomaly (Pierce and Morgan 1992). At ~16.7 Ma, this region was affected by the initial outpouring of regional flood basalt volcanism (Steens) and the development and activity of large silicic volcanic fields temporally and spatially associated with this flood basalt event (Figure 1a, b). While the ultimate cause of this Pacific Northwest magmatic event is debatable, this portion of the Oregon Plateau is the only location where the flood basalt lava flows and the dominantly silicic volcanic fields formed/erupted coevally. Since this initial mid- Miocene event, the Oregon Plateau has experienced continuous basaltic volcanism and is the only region in the Pacific Northwest where this complete record of ~16.7 Ma activity is present (Hart 1985; Shoemaker 2004). During the initial flood basalt event, basalt through basaltic andesitic lava flows erupted from scattered loci across the Oregon Plateau, coeval with eruption and emplacement of the Columbia River Basalt Group (Brueseke et al. in review). The most studied eruptive loci and package of Oregon Plateau flood basalt is exposed at Steens Mountain, where ~1 km of lava flows are cut by numerous feeder dikes (Hart and Carlson 1985; Carlson and Hart 1987; Camp et al. 2003). Other packages of chemically distinct mid-Miocene mafic lava flows and intrusive bodies around the Oregon Plateau are found as far east as the Jarbidge Mountains, Nevada (Seventy-Six basalt; Hart and Carlson 1985; Rahl et al. 2002), as far west as the Honey Lake region of western California (Lovejoy basalt; Wagner et al. 2000), and as far south as the Roberts Mountain region of central Nevada (Zoback et al. 1994). Regional mid-Miocene silicic volcanism initiated at ~16.6 Ma and continued to ~12 Ma. The initial stage of regional silicic activity was characterized by the eruption of voluminous ash flows from large caldera complexes (e.g. McDermitt, Northwest Nevada, and Lake Owyhee volcanic fields; Noble et al. 1970; Greene and Plouff 1981; Rytuba and McKee 1984; Ach and Swisher 1990; Rytuba et al. 1991; Bussey 1995; Castor and Henry 2000). These volcanic systems were present across the entire Oregon Plateau. Also present were numerous small volume rhyolite dome complexes (Fig. 1a, b; Coats 1968; Walker 1969; 1974; MacLeod et al.

173

1975; Legge 1988; Manley and McIntosh 2002; Brueseke et al. 2004). By ~14 - 12 Ma, Oregon Plateau silicic activity was waning and the locus of silicic activity diverged to the northwest and northeast, forming the High Lava Plains - Newberry and Snake River Plain - Yellowstone trends (Walker 1969; 1974; MacLeod et al. 1975; Pierce and Morgan 1992; Christiansen et al. 2002; Jordan et al. 2004). Concurrent with the extensive, dominantly bimodal basalt-rhyolite volcanism, the Oregon Plateau region was affected by a focused “event” of lithospheric extension. The formation of a an ~1000 km zone of continental extension spanning from Washington south into central Nevada commenced at ~16.5 Ma and ended at ~14 Ma (Fig. 1a, b; Zoback and Thompson, 1978; McKee and Noble, 1986; Carlson and Hart 1987; Hart and Carlson 1987; Blakely and Jachens 1991; Zoback et al. 1994; Cummings et al. 2000; John et al. 2000; Ponce and Glen 2002; Glen and Ponce 2002). This event was characterized by low net lateral extension compared to other continental rifts (John et al. 2000) and is best expressed as subsurface gravity and aeromagnetic anomalies and the surficial exposure of approximately north - south trending mafic dike swarms (Zoback et al. 1994; John et al. 2000). Throughout the northern Nevada rift this extension led to the emplacement of basaltic magmas into the upper crust and has had a profound effect on the formation and genesis of the SC. Abundant and often isolated basins and lacustrine systems formed across the southern Oregon Plateau concurrent with mid-Miocene rift related extension, and were sometimes associated with epithermal precious metal mineralization (Vikre 1985a; Vikre 1987; Castor and Henry 2000; Wallace 2003). These deposits provide a detailed record of the regional silicic volcanic record as a result of the numerous tephra horizons preserved within the basin sediments (Perkins et al. 1998; Perkins and Nash 2002; Gilbert et al. 2003). At ~11 Ma, Basin and Range style extensional tectonism started affecting the region, coeval with the formation of the western Snake River Plain graben, and the onset of small- volume, Oregon Plateau-wide HAOT (low-K, high-alumina olivine tholeiite) volcanism (Hart 1985; Shoemaker and Hart 2002; Bonnichsen et al. 2004; Colgan et al. 2004; Jordan et al. 2004; Colgan et al. 2006). This same tectonic regime has persevered to the present-day; faults along the eastern Santa Rosa range front have been active in the last 15 Ka and < 2 Ma HAOT and mildly-alkaline basaltic lava flows are found across the Oregon Plateau and western Snake River Plain (Hart 1985; Jordan et al. 2004; Personius et al. 2004; Shoemaker 2004).

174

GENERAL SANTA ROSA-CALICO GEOLOGY Work in the SC area falls into four broad categories: [1] studies of the metamorphic and granitoid rocks that form the core of Santa Rosa Range (e.g., Compton 1960; Shieh and Taylor 1969; Wodzicki, 1971; Stuck 1993), [2] reconnaissance studies along the margins of the Santa Rosa Range (western SC from Hinkey Summit north through Buckskin Mountain and in the southern Santa Rosa range) (e.g., LeMasurier 1965, 1968; Larson et al. 1971; Stewart and Carlson 1976; Hart and Carlson 1985; Carlson and Hart 1987; Mellott 1987); [3] studies of local mid-Miocene epithermal mineralization (e.g., Winchell 1912; Lindgren 1915; Roberts 1940; Willden 1964; Vikre 1985a, b), and [4] studies of local and regional eruptive loci and fault patterns (e.g., King 1984; McCormack 1996; Colgan et al. 2004; Personius et al. 2004). This study represents the first attempt to fully define and integrate the areal extent, chronostratigraphy, lithostratigraphy, chemostratigraphy, physical volcanology, and petrologic and tectonic processes of the multi-vent volcanic field that herein is formally defined as the Santa Rosa - Calico volcanic field (Fig. 2). The SC lies within the northern projection of the northern Nevada rift along the southern Oregon Plateau boundary almost due north of Winnemucca, NV and between two volcanic fields associated with the Yellowstone-Newberry melting anomaly, the McDermitt and “Owyhee- Humboldt” volcanic fields (Fig. 1b). The volcanic field is roughly oval in shape bounded by the northern portion of the Santa Rosa mountain range (western boundary), a basin bounding normal fault at the north end of Paradise Valley (southern boundary), the Calico Mountains (eastern boundary), and the Owyhee Plateau (northern boundary) (Fig. 2). Elevations in the SC range from over 2500 m along its margins to less than 1500 m in the central portion, a topographic depression called the Goosey Lake depression. The SC lies mainly in Humboldt-Toiyabe National Forest and within Bureau of Land Management administered lands and access is limited to intermittently maintained unpaved roads. Rugged topography is present throughout the SC and primarily results from post- and syn-SC faulting and Pleistocene glaciation. Flooring the package of SC derived mid-Miocene volcanic rocks is a diverse assemblage of older material. Intensely deformed Triassic (Norian) metasedimentary strata are intruded by ~102 - 85 Ma granitoid plutonic bodies (Compton 1960; Willden 1964; Stuck 1993; Wyld et al. 2001). Following Cretaceous magmatism, a cessation in local activity occurred until the mid- Eocene, when most of Nevada experienced an ~20 Ma period of andesite-rhyolite volcanism

175

related to subduction and extensional processes, often referred to as the “Cenozoic ignimbrite flare-up” (Coney 1978; Armstrong and Ward 1991; Christiansen and Yeats 1992; Humphreys 1995). In northern Nevada, lava and ash flows related to this magmatic event are best exposed in the vicinity of the Tuscarora volcanic field, east of the SC and poorly exposed (if present) in northwestern Nevada (Castor et al. 2003; Colgan et al. 2006). At ~22 Ma, volcanism in the northern Great Basin became dominated by lava flows and intrusive bodies of dominantly intermediate composition that were emplaced as part of an extensive arc system along the North American margin (Christiansen and Yeats 1992). These dominantly calc-alkaline western andesite assemblage arc-related lava flows are found across the eastern Oregon Plateau and include the Steens Mountain volcanics that directly underlie the mid-Miocene Steens Basalt at Steens Mountain (Fuller 1931; John 2001). On the Oregon Plateau, this magmatic event ended at ~20 - 19 Ma. At ~16.7 Ma, SC volcanic activity initiated (Table 1), coeval with regional Oregon Plateau bimodal basalt-rhyolite volcanism (this study). The up to >1500 m-thick package of mafic through silicic lava flows, eruptive loci, and hypabyssal bodies associated with this event are well exposed in the SC and are the focus of the current study. At minimum, volcanic products derived from vents within the SC occur over an ~840 km2 area. Using a minimum thickness of 150 m, this represents ~126 km3 of volcanic products, not including unidentified outflow. This volume estimate may be closer to 400 km3 because much of the SC pile is buried. Previous work indicates that magma generation and emplacement associated with the western Santa Rosa-Calico volcanic field took place between approximately ~16.5 - 14 Ma (e.g., Larson et al. 1971; Vikre 1985b; Carlson and Hart 1983; Hart and Carlson 1985; Carlson and Hart 1987; Mankinen et al. 1987; Mellott 1987). The same time span of local eruptive activity is recorded across the Oregon Plateau in the McDermitt, Northwest Nevada, and Lake Owyhee volcanic fields. (Noble et al. 1970; Greene and Plouff 1981; Rytuba and McKee 1984; Ach and Swisher 1990; Rytuba et al. 1991; Bussey 1995; Perkins et al. 1998; Castor and Henry 2000; Perkins and Nash 2002). Also, fallout and flow facies likely associated with caldera forming eruptions in the McDermitt volcanic field are preserved along the western flanks of the SC (Mellott, 1987) and have been suggested to exit within the study area (King, 1984; Rytuba and McKee, 1984; Vikre, 1985a, b). Also present along the western flanks is a thick (at least 200 m; Larson et al. 1971) pile of basalt derived from eruptions at or adjacent to Steens Mountain (Larson et al. 1971;

176

Carlson and Hart, 1983; Hart and Carlson 1985; Mankinen et al. 1987). Previous workers have also suggested that the enitre central SC (Goosey Lake depression) formed in response to caldera-forming volcanism and represents a central caldera structure (Ekren et al. 1984; Vikre 1985a). Bordering the eastern margin of the SC is the Owyhee Plateau, an intermontane plateau covered by <11 Ma basaltic lava flows and shield volcanoes (Shoemaker 2004). Where canyons have cut into these younger basalts, silicic lava flows and lava-like ignimbrites are well exposed. These ~14 - 12 Ma silicic units onlap SC outflow in some locations along the western SC margin, and have been used by past workers to help define the southern Owyhee Plateau as the “Owyhee-Humboldt” eruptive center (Bonnichsen 1985; Manley and McIntosh 2002; Brueseke et al. 2004; this study).

SAMPLING STRATEGY AND ANALYTICAL METHODS Over two hundred and seventy representative samples were collected from throughout the area depicted in Figure 2 in five field seasons from 2000 to 2004 (see Appendix 2; Detailed Sample Locations and Descriptions). Sample collection was guided by the earlier work of LeMasurier (1965, 1968), Vikre (1985a, b), Mellot (1987), and preliminary reconnaissance data of W.K. Hart (Pers. Comm.). The sampling strategy was designed to maximize information on stratigraphic and spatial relationships throughout the volcanic field. Vertical and horizontal sampling traverses by foot provided the necessary means for sample collection and the gathering of stratigraphic data in most cases. Topographic highs were also targeted in the event that outstanding stratigraphy may be present nearby and to identify whether these high regions were SC eruptive loci. All regional highs with the exception of McConnell Peak (along the northern SC margin, north of the National mining district) were accessed via often long and arduous traverses. Known mining districts (e.g. National, Buckskin-National, Spring City region) were not targeted for sampling and minimal time was spent in these locations due to the large amount of post-magmatic alteration and mineralization present in outcropping units. Figure 3 is a cartoon that schematically depicts the generalized volcanic stratigraphy of the SC. This diagram illustrates the overall stratigraphic complexity found across the SC and the different types of eruptive products that crop out in the volcanic field. It is important to note that at any given time in its history, SC volcanism was characterized by a spectrum of differing magma compositions that led to different local eruptive histories across the volcanic field.

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Field relationships guided the selection of 250 samples for geochemical analytical work. Prior to analysis, weathering rinds were removed from all samples and then each was split carefully examined to make sure that only the freshest material possible would be further powdered and prepared for analytical work. No attempt was made to remove all potential syn- magmatic contaminants from each sample prior to geochemical analyses, however, obvious contaminants were removed by hand-picking prior to processing. Major element analyses were performed by DCP-AES at Miami University following the procedures outlined in Katoh et al. (1999). Trace element analyses were performed at Franklin and Marshall College by XRF (techniques outlined in Mertzman, 2000) and a subset of samples were analyzed at Washington State University by ICP-MS. Based on data from the above techniques a suite of samples was chosen for Sr, Nd, and Pb isotopic measurements. Approximatly 0.1 - 0.2 g of whole rock powder was dissolved in HF-HNO3, prior to chemical separation. Except for Sm - Nd separation, these procedures follow Walker et al. (1989). Sm - Nd separations were performed by methods similar to Pin and Zalduegui (1997), using EiChrom Ln-Spec resin. Isotopic compositions were measured by thermal ionization mass spectrometry (TIMS) at Miami University, using a Finnigan Triton mass spectrometer. Strontium isotopic ratios were fractionation corrected using 86Sr/88Sr = 0.1194. Sixty-eight measurements of the NBS 987 strontium standard gave an average of 87Sr/86Sr = 0.710236 ± 0.000014 (1 SD). Neodymium isotopic ratios were fractionation corrected using 143Nd/146Nd = 0.7219. Sixty-one measurements of the LaJolla neodymium standard gave an average of 143Nd/144Nd = 0.511846 ± 0.000007 (2 SD). 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb were fractionation corrected by 0.1% per amu based on NBS 981 data from Todt et al. (1996). Errors on measured values of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb were ± 0.01, ± 0.02, and ± 0.06 respectively. Geochronology was performed by the 40Ar/39Ar method at the New Mexico Geochronological Research Laboratory, under the direction of Matthew T. Heizler (Table 1). Complete data are presented in Appendix 3 (Data) and a detailed discussion of the analytical procedures and methods used is present in Appendix 1 (Analytical Methods). Mafic and intermediate units were dated by whole-rock methods when possible to minimize the use of potential xenocrystic plagioclase feldspars. For the silicic units, fifteen sanidine crystals were analyzed by the laser fusion method. Errors associated with 40Ar/39Ar ages discussed herein are given as two sigma. All major element data reported in the following discussion have been normalized to 100% anhydrous after adjusting the

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FeO/Fe2O3 as outlined by LeMaitre (1976). Additionally, the newly reported data from this study have been combined with SC geochemical data collected by Mellot (1987) and W.K. Hart (Pers. Comm.) to form one master database. SC units discussed below are defined based on their physical and petrographic characteristics. Silicic units across the SC are physically and petrographically distinct from each other, while morphologic differences among mafic and intermediate composition units are much less pronounced (Fig. 4). However, geochemical characteristics allow these units to be distinguished from each other. Figure 5 illustrates the complete compositional spectrum of basalt through high-silica rhyolite present within the SC. Of the 286 analyses shown on Figure

5a, 26% lie between 52 - 63 wt. % SiO2, indicating that the SC is not just a bimodal basalt- rhyolite volcanic field. Geochemical differences present between SC silicic units mirror the physical differences and also help define these units. Additionally, physical and geochemical data suggest that at least one widespread unit present within the SC volcanic assemblage was not

locally derived (Tp1; peralkaline ignimbrite). Figure 6 depicts the generalized geology of the SC and illustrates the location of Pre-SC units, SC vents and vent alignments, and highlights the overall complexity of the volcanic field.

PHYSICAL AND PETROGRAPHIC CHARACTERISTICS OF LOCAL UNITS This section introduces and defines the local SC lithologies (units) primarily on the basis of spatial, temporal, and physical (outcrop to microscopic scale) features. As previously mentioned, geochemical properties also have played an important role in these definitions and these properties will be discussed in detail in the following section. The combined features of the SC volcanic suite defy simple unit designations that define mappable field assemblages, and coherent chronostratigraphic levels, and petrographically and geochemically unique materials. In other words, the SC volcanic pile is very complex with, for example, physically and chemically distinct units that are substantially time transgressive. In order to provide a reasonable representation of the lithologies mappable in any given area as well as the complexities present, two separate but interrelated unit designations are used throughout the remainder of this paper; [1] Map units – units that serve as the basis for a generalized geologic map and stratigraphic framework (Fig. 6b), and [2] Petrologic units – units that represent the primary observed physical, petrographic, and geochemical heterogeneities. The discussions in

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this and the following section define the petrologic unit designations and the materials so designated. Information presented in Table 2 and in the key to Figure 6 illustrates how the defined petrologic units link to the designated map units. For example, the Ta petrologic unit (Tertiary andesite of Black Ridge) is widely distributed in space and time throughout the study area, thus can be found locally within the following map units; Thc, Twsc, Tgl, Tcm (Table 2 and Fig. 6b).

Pre-SC Units A diverse package of Triassic to Neogene metamorphic and igneous rocks stratigraphically underlies the SC volcanic suite. The designations for these pre-SC units defined below are the same as those utilized in the simplified geologic map of Figure 6b. Triassic metasedimentary strata (Trms) The oldest package of rocks exposed in the SC is over ~6 km of metamorphosed Triassic (Norian) sedimentary back-arc basinal strata. These rocks were formed in an outer submarine fan to basin plain environment and these deep marine sediments were transported from continental sources to the east (Wyld et. al. 2001). Compton (1960) divided this assemblage into six units: the Grass Valley, Winnemucca, O’Neill, Singas, Andorno, and Mullinix Formations (up-section). Slates and phyllite dominate each unit and make up ~80 % of the metasedimentary package (Wyld et al. 2001). Also present within each unit are interlayered quartzite and calcareous shales and siltstones (Wyld et al. 2001). Following deposition, the original sedimentary strata were deformed and metamorphosed during two regional Jurassic compressional events related to the formation of the Luning-Fencemaker fold-thrust belt (Wyld et al. 2001). Metamorphism during these deformation events reached greenschist grade and subsequent contact metamorphism overprinted this regional event during the intrusion of the ~102 to 85 Ma Santa Rosa granitoid suite (Compton, 1960; Rogers, 1999; Wyld, 2001). During this intrusive event, upper amphibolite facies were reached in some locations (Compton, 1960). In the SC, these rocks are best exposed in the vicinity of Hinkey Summit and along the southern SC margin, as well as along the southwestern SC margin (Fig. 4a; 6b). No metasedimentary rocks were encountered in the eastern SC at the base of the Calico Mountains; however, King (1984) suggested the presence of these rocks in this location.

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Cretaceous granitoid (Kg) Granitoid intrusions are best exposed in the main Santa Rosa range, where they make up the high (>2400 m) crest of the mountain range in most locations. At least six, surficially isolated plutonic intrusive bodies are present locally; the largest is the Santa Rosa pluton in the center of the main Santa Rosa range (Fig. 4b; Compton, 1960; Stuck, 1993). Wyld et al. (2001) dated the Andorno stock by U-Pb methods that yield an age of 102.4 ± 1.0 Ma, similar to the earlier chronologic results of Smith et al. (1971) and Stuck (1993). Rb-Sr isochron dating of the Granite Peak stock by Stuck (1993) yields an age of ~85 Ma. These temporal differences mirror chemical and isotopic differences that illustrate that Santa Rosa range granitoids make up at least two petrogenetically unrelated intrusive suites (Stuck, 1993). Santa Rosa range granitoids include biotite granodiorite, quartz monazite, quartz monzodiorite, biotite granite, and granite. Aplite dikes are also present throughout the larger plutonic bodies (Stuck, 1993). The ~102 Ma 87 86 Santa Rosa-Andorno group ( Sr/ [email protected] Ma = 0.7048 to 0.7058) is interpreted as the end result of mantle melts that underwent crystal fractionation and crustal assimilation in a continental arc 87 86 setting (Stuck, 1993). The ~85 Ma Granite Peak-Sawtooth group ( Sr/ [email protected] Ma = 0.7061 to >0.7070) is interpreted to have either a crustal source (melting of terriginous sediment) or an enriched mantle source (Stuck, 1993). Aplite dikes are also present that cross-cut both types of 87 86 granitoid ( Sr/ [email protected] Ma = 0.7095 to >0.7400). Granitoid rocks are only exposed in three locations within the SC (Fig. 6b): (1) at Granite Peak (Fig. 4a, g; ~85 Ma Granite Peak-Sawtooth group), (2) at the base of the Calico Mountains (Fig. 4c; ~102 Ma Santa Rosa-Andorno group), and along the southern SC margin (~102 Ma Santa Rosa-Andorno group). Cenozoic calc-alkaline arc lava flows (Tarc) Underlying SC eruptive products in the southern part of the volcanic field is a suite of Cenozoic, dominantly calc-alkaline, lava flows (Fig. 4a, d; Fig. 6b). Andesitic to dacitic varieties of this suite can be thick (up to ~150 m) and are characterized by occasional columnar jointing that weathers to platy talus (Fig. 4d and similar to platy weathering Tct in Fig. 4p), vesiculated flow tops, spheroidal weathering, tan to red color, and thick (often > 20 m) basal and upper brecciated zones. Within these breccias, abundant dark glassy clasts are present and

resemble clasts associated with upper breccias of SC silicic units (e.g. Tad2 and Tem). Occasional flow-on-flow stratigraphy where cooling units are separated by breccia zones are also observed, such as that exposed along the fault scarp defining the southern margin of the SC

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(northern boundary of Paradise Valley). Plagioclase (An30-58), orthopyroxene, clinopyroxene, olivine, and oxides dominate the groundmass while either olivine or plagioclase and orthopyroxene are present as phenocrysts (Mellot, 1987). Mellot (1987) first discovered that Tarc lava flows (his acid andesite 1 and 3 groups) are physically and compositionally distinct from the younger SC intermediate composition units. He also demonstrated that east dipping Tarc lava flows are found along the western side of Paradise Valley (along the eastern side of the main Santa Rosa range) and along Indian Creek near Hinkey Summit. K-Ar ages from Tarc lava flows obtained by Mellot (1987) yield eruptive ages of ~19.6 - 22.8 Ma. Tarc lava flows are temporally correlative with the Steens Mountain Volcanics which underlie Steens Basalt at Steens Mountain. New reconnaissance mapping illustrates that Tarc lava flows are present along the western SC margin between Granite Peak and Buckskin Mountain and are also present as far east as Black Dome (Fig. 6b). Additionally, at least one highly eroded Tarc eruptive center may be present just north of Granite Peak along the western SC margin. North of this location, Tarc lava flows along the western margin uncomfortably overlie Trms and include mafic (basalt to basaltic andesite) lava flows. New 40Ar/39Ar ages confirm the earlier ages of Mellot (1987) and expand the age range represented by local Tarc lava flows (Table 1). A basal Tarc lava flow exposed along the western margin of the SC directly overlying Trms yields a 40Ar/39Ar age of 35.52 ± 0.34 Ma. This is the oldest Cenozoic volcanic material documented in this portion of the Oregon Plateau. The overlying Tarc lava flow is a basaltic andesite and yields a 40Ar/39Ar age of 23.50 ± 0.24 Ma, similar to the older ages of Mellot (1987). At Hinkey Summit, a Tarc lava flow directly underlying the basal package of ~16.7 Ma SC derived Tba lava flows yields a 40Ar/39Ar age of 22.48 ± 0.08 Ma. 87 86 Tarc lava flows are calc-alkaline and have Sr/ Sr@16 Ma = 0.7040 to 0.7050 (Mellot, 1987). Mellot used the geochemical and isotopic characteristics of these lava flows to conclude that they were the end result of subduction related mantle melting and differentiation. This interpretation temporally fits with regional models of northwestern United States tectonomagmatism and similar interpretations have been made for other temporally identical regionally exposed calc-alkaline volcanic suites (Dickinson, 2004). In addition to their physical characteristics, major and trace element chemical variations are used to distinguish Tarc lava flows from the younger SC-derived units.

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Mafic Units Santa Rosa-Calico mafic lava flows, shallow intrusive bodies, and eruptive loci resemble mafic units exposed across the Oregon Plateau and are divided into distinct units on the basis of

chemical composition. Included within these mafic SC units are the Ta1 and Ta2 units of Vikre (1985a, b) and the Tb and Tbi units of LeMasurier (1965, 1968). Tertiary basalt (Tb) Mafic lava flows of unit Tb have only been found in the southwestern SC in the greater Hinkey Summit-Granite Peak region (Fig. 6b). Exposures are up to ~10 m-thick and characterized by a tortoise-shell like fracturing and red/black ophiomottling. Visible ~1 - 4 mm plagioclase crystals and barely visible olivine (<1 mm) are typically found in a dark gray/bluish, dense to crystalline matrix. Plagioclase (An45-60), clinopyroxene, olivine, and oxide are present in this unit. Minor (<1 - 2 %) disequilibrium textures (resorbed plagioclase phenocryts) are also occasionally found. In two locations, siliceous sinter and red-green mineralization (hot springs deposits) appear to be stratigraphically below Tb lava flows, and likely reflect the same stratigraphic horizon. One dike of Tb was also sampled and strikes 65º. This dike is observed in close proximity to three of the four locations where Tb lava flows were sampled and provides evidence for a local origin of these lava flows. Two 40Ar/39Ar ages were obtained on lava flows from this unit and yield a weighted mean age of 16.36 ± 0.82 Ma (Table 1). Tertiary locally-erupted Steens Basalt (Tba) The second mafic unit present is exposed as lava flows (Fig. 4f), shallow intrusive bodies (e.g dikes and plugs; Fig. 4g, h), and eruptive loci with accompanying pyroclastic deposits (e.g. bombs and scoria). Tba lava flows are best exposed in the western SC, however; at least one Tba lava flow crops out in the eastern SC and was likely locally sourced from within the Calico Mountains region (Fig. 6b). Tba lava flows are also thin (~5 - 10 m) and resemble Tb lava flows, but lack the tortoise-shell weathering. Upper flow surfaces typically show pahoehoe features (ropes) and the basal portions of these lava flows are brecciated, similar to regionally exposed Steens Basalt lava flows. Plugs and dikes crop out between Hinkey Summit and Buckskin Mountain, along the western SC margin. The best exposed shallow intrusive body is Chocolate Mountain, a large-dome shaped plug that dominates the view to the west-northwest while driving north along Forest Service road 084 north of Hinkey Summit (Fig. 4g, h). At least three other smaller stocks are exposed just north of Chocolate Mountain, suggesting that the

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greater Hinkey Summit-Granite Peak region was a focus of localized Tba magmatism. Measured Tba intrusive bodies strike across a range from 266º to 15º. Mafic dikes tentatively grouped into Tba are also present in the vicinity of the National Mining district (Vikre, 1985a, b). Eroded Tba eruptive loci and pyroclastic deposits are present along the western SC margin on the range-crest of the northern extension of the Santa Rosa range. This observation, coupled with the fact that over 97% of all Tba and Tb exposures in the SC occur along the western margin of the field, suggests that the western portion of the SC was the focal point of local mafic magma upwelling. Interestingly, all sampled Tba products lack the ~1 - 4 cm plagioclase crystals that characterize plagioclase-phyric Steens Basalt lava flows (Brueseke et al. under review). A well exposed ~240 m thick package of mafic lava flows that was tentatively correlated with the Steens Basalt at Steens Mountain by paleomagnetic methods (Larson et al. 1971) occurs along the western margin of the SC downslope and just west of Buckskin Mountain. Furthermore, Carlson and Hart (1983) and Hart and Carlson (1985) used isotopic data to suggest that this package erupted from a source near Steens Mountain. Within this package there are plagioclase- phyric lava flows that resemble the plagioclase-phyric variety of Steens Basalt so commonly observed elsewhere on the Oregon Plateau (Brueseke et al. under review). The lack of plagioclase-phyric products in Tba and the presence of plagioclase-phyric lava flows along the western SC margin, further suggests that this poorly studied package of Steens lava flows was derived from a regional Steens eruptive center and flowed up against and onlapped the topographic high created by the northern extension of the main Santa Rosa range. Tba lava flows and intrusive bodies texturally resemble the intermediate-plagioclase and aphyric petrographic types of regionally exposed Steens Basalt (Brueseke et al under review).

Plagioclase (An25-62), olivine, clinopyroxene, and oxide are the dominant phases present. Disequilibrium textures are often present and found as crystal clots of clinopyroxene ± plagioclase ± oxide and highly resorbed/spongy-sieved plagioclase crystals, like those present in SC intermediate units depicted in Figure 7. Plagioclase separates from a Tba lava flow near the base of the mid-Miocene section at Hinkey Summit yield a 40Ar/39Ar age of 16.45 ± 0.34 Ma (Table 1). This is similar to a whole rock 40Ar/39Ar age obtained on the basal mafic lava flow exposed in the Hinkey Summit region (overlies Triassic metasedimentary rocks) which yields an age of 16.73 ± 0.04 Ma (Alan Wallace, Pers. Comm.; Table 1) The Tba lava flow exposed in the Calico Mountains is also at least 16.5 Ma based on its stratigraphic position and relationship to

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locally exposed, newly dated eastern SC units. Tba exposed in the Chocolate Mountain plug yields a 40Ar/39Ar age of 15.97 ± 0.20 Ma (Alan Wallace, Pers. Comm.; Table 1). Chemically and physically similar plugs and lava flows crop out nearby, suggesting that a period of heightened Tba activity occurred at ~16 - 15.8 Ma. Additional younger Tba is found along the western SC margin north of Hinkey Summit, where a lava flow associated with a Tba vent yields a 40Ar/39Ar age of 14.35 ± 0.38 Ma. These new 40Ar/39Ar ages and field relationships illustrate that Tba lava flows were being emplaced and locally erupting throughout the entire duration of SC volcanism.

Intermediate Units Santa Rosa-Calico intermediate composition units are defined by geographic, temporal, and major and trace element chemical differences. Like SC mafic units, textural differences among these units are slight and it is often impossible to distinguish one unit from another in the field. Andesite and dacite lava flows are usually thick (>10 m) and characteristically possess thick upper and lower flow-margin brecciated zones (Francis and Oppenheimer, 2004; Schmincke, 2004). The older Pre-SC arc lava flows that in places underlie the SC volcanic assemblage illustrate these “typical” intermediate-composition flow features quite well. However, most broadly andesitic SC lava flows are usually thin (<5 - 7 m thick) and resemble simple (one cooling unit) mafic lava flows (Fig. 4i, j). Regionally this type of lava flow is associated with both Steens and HAOT basalt eruptions. Consequently, “flood andesite” may be an appropriate term to describe their overall physical character. Only one intermediate composition volcano was identified in the field, however, field relationships combined with chronologic and geochemical data allow for the identification of vent regions for the three Tad units. Volume estimates for Tad units were calculated by measuring a minimum areal extent using the known outcrop pattern via ArcView GIS 3.2 and multiplying by a minimum thickness of 150 m, which is the approximate average thickness of the Tad2 and Tad3 packages based on our field observations and the mapping of Vikre (1985b). These calculations yield volumes of 3 18.5, 17.7, and 15.8 km for Tad1, Tad2, and Tad3 respectively. These estimates indicate that combined, Tad units may represent as much as 40% of the entire volume of SC products. Included with these units are the Tql (Tertiary quartz latite) units of Vikre (1985b) which are now much better defined geochemically and chronologically. LeMasurier (1965, 1968) grouped

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all “andesitic” material into one unit (Ta) and did not differentiate between pre-SC Tarc lava flows and SC derived intermediate eruptive products. Tertiary andesite of Black Ridge (Ta) Black Ridge andesite lava flows are found across the SC and are the most areally extensive intermediate unit in the map area (Fig. 4 i, j; included in Tcm, Thc, Twsc, and Tgl of Fig. 6b). The best exposures are found in the vicinity of Holloway Meadows and where Forest Service road 531 cuts Black Ridge in the North Fork of the Little Humboldt river canyon (Fig. 2). Here, and further south along Black Ridge, these lava flows are interbedded with and 40 39 underlie Tad1 lava flows (Fig. 4j). One Ta lava flow from Black Ridge yields a Ar/ Ar age of 14.94 ± 0.24 Ma (Table 1). Just north of Holloway Meadows, Ta lava flows onlap flow lobes of <15.8 Ma Tem lava flows (Fig. 4i). Ta lava flows are also found at the base of the exposed section in the southern Calico Mountains, interbedded with the package of >16.2 Ma Tba, Tad1 and Thc lava flows exposed in the greater Hinkey Summit region, and found along the western

SC margin stratigraphically under Tpr lava flows (within the Tql2 of Vikre, 1985b). Ta lava flows are also occur across the eastern Goosey Lake Depression where they are the stratigraphically oldest encountered unit. Ta lava flows are thin (<6 m) and are simple flows with highly vesiculated upper flow tops (vesicles are usually stretched and up to 1 - 3 cm long) and rubbly bases. Thicker Ta lava flows often have a massive, spheriodially weathered interior, while thin flows are highly vesiculated throughout. These thin andesitic lava flows physically resemble lava flows associated with flood basalts and are easily mistaken for Tba lava flows in the field. Ta shallow intrusive bodies were sampled and chemical similarities suggest at least one source of Ta lava flows was a low shield volcano located between Coyote Mountain and the southern Calico Mountains, along the eastern SC margin (hill 6648 on the Coyote Mountain 1:24,000 U.S.G.S quadrangle). Lobes of Ta lava flows also emanate from this location. This vent appears to be onlapped in places by ~16.4 Ma Tct ash flows, again illustrating that Ta lava flows were time-transgressive throughout the duration of SC volcanism. Given the complex stratigraphic relationships outlined above and wide areal distribution of Ta lava flows, it is likely that hill 6648 was not the sole Ta eruptive locus. Plagioclase, clinopyroxene, orthopyroxene, olivine, and oxide are the dominant phases present in Ta lava flows. Ta flows are texturally variable; some lava flows are much finer grained than others regardless of bulk chemical composition. This likely reflects differences in

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cooling rates and thickness between individual Ta lava flows. Additionally, Ta lava flows show variable evidence of disequilibrium. Like some Tba products, some Ta lava flows contain crystal clots of plagioclase ± clinopyroxene ± orthopyroxene ± oxide and occasional highly resorbed/sieved plagioclase phenocrysts (when present, less than 2% modally). These resemble the clots depicted in Figure 7e-h.

Tertiary andesite-dacites of Hinkey Summit-Coal Pit Peak (Tad1) Hinkey Summit-Coal Pit Peak intermediate lava flows are the most voluminous of the four SC intermediate units. The thickest exposures (over 300 m) are found in the vicinity of

Coal Pit Peak, along the southern SC margin (Fig. 6b). Tad1 lava flows crop out at the top of Coal Pit Peak and are the stratigraphically oldest unit exposed just to the north in the Martin

Creek canyon. Tad1 lava flows overlie ~16.4 Ma Thc rhyolite flows at Hinkey Summit and north of Hinkey Summit and this relationship provides a maximum age for the oldest exposed Tad1 lava flows. Throughout the greater Hinkey-Coal Pit region, they also underlie 16.2 Ma Tpr rhyolite lava flows. Tad1 lava flows are also exposed in patches along the eastern margin of the main Santa Rosa Range (in Paradise Valley) overlying Tarc. Along Black Ridge between Coal

Pit Peak and where it is cut by the North Fork of the Little Humboldt river, Tad1 lava flows are interbedded with ~14.9 Ma Ta lava flows (Figure 4j). The stratigraphically highest exposed Tad1 lava flow from Coal Pit Peak yields a 40Ar/39Ar age of 13.90 ± 0.60 Ma, which is the youngest age reported for any SC eruptive product (Table 1). These ages and stratigraphic relationships demonstrate that Tad1 lava flows were time-transgressive and erupted throughout most of the SC’s history. In the Coal Pit Peak region, no direct evidence for local eruption/magma emplacement was encountered. However, topographic highs in the SC were extensively glaciated during the Pleistocene and the youngest, near-vent material may have been eroded

during recent glaciation. Tad1 lava flows are only present in the southern portion of the SC with increasing abundance toward Coal Pit Peak. Additionally, all exposures thicken toward Coal Pit

Peak. Based on these spatial relationships, the eruptive locus for Tad1 lava flows was in the Coal

Pit Peak region and likely was a large shield volcano. Like Ta lava flows, Tad1 lava flows are thin (<6 m) and characterized by a thin (<2 m) rubbly base and more massive, often vesiculated, interior. Also like Ta lava flows, these are easily misidentified as more mafic units in the field.

When exposed, Tad1 flow surfaces are highly oxidized and show abundant ramp structures.

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However, their overall outcrop pattern is similar to Ta lava flows and resembles simple basalt flows.

Tad1 lava flows were originally named acid andesite 3 by Mellot (1987) who distinguished them from the earlier arc lava flows by their abundant disequilibrium features.

These features distinguish Tad1 lava flows from the three other SC intermediate groups. Plagioclase, orthopyroxene, clinopyroxene, olivine, and oxide are the dominant phases present in

Tad1 lava flows. The most apparent textural characteristic of this unit are the abundant disequilibrium features, typically ~8 - 15 % modally. Extremely resorbed/sieved and sometimes complexly zone plagioclase crystals are ubiquitous (Fig. 7). Macro and microscopic xenoliths are also abundant and are found as three types. The most common are (1) plagioclase and potassium feldspar dominated xenoliths that resemble granitoid and (2) crystal clots of plagioclase (often oscillatory zoned) ± clinopyroxene ± orthopyroxene ± oxide, similar to what is

found in other SC units (Fig. 7b, d, e-g). However, one sampled Tad1 lava flow is characterized by the presence of an ~1 cm long metamorphic xenolith, the only Trms xenolith observed in an SC unit (Fig. 8a, b). While the metamorphic xenolith-host contact is fairly abrupt overall, in some places it is diffuse and appears that partial melting of the xenolith along this contact occurred (Fig. 8b).

Tertiary andesite-dacites of Staunton Ridge (Tad2)

Lava flows of Tad2 are exposed in the northwestern SC underlying Tem lava flows (Fig.

4m). These lava Tad2 flows were originally included by Vikre (1985b) in his Tql1 and Tql2 map units and are best exposed at Staunton Ridge, where a section of ~190 m of flow on flow stratigraphy is present (Figs. 2, 6b). No shallow intrusive bodies or near-vent facies were

observed that could be associated with this unit, however, Tad2 lava flows are not found anywhere else in the volcanic field and this limited spatial occurrence suggests the presence of a localized magmatic system. Within this package, the 5th flow stratigraphically down from the 40 39 top of Staunton Ridge yields a Ar/ Ar age of 15.76 ± 0.28 Ma. Here, Tad2 lava flows are 4 -

12 m thick and in places, much platier in appearance than Ta, Tad1, and Tad3 lava flows. Upper portions of Tad2 flows weather to plates, which are occasionally vertical. Above this platy zone, the lava flows are vesiculated and then grade into an upper flow-top oxidized breccia. Included within these brecciated zones are dense, glassy clasts, similar to those found within the pre-SC Tarc units. The central, more massive portions of these flows are platy, but jointing is

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horizontal. Morphologically, these Tad2 lava flows exhibit characteristics that are more similar to “typical” intermediate composition lava flows. Texturally, these lava flows are much more aphyric than other SC intermediate units. In

hand sample, crystals are barely visible and Tad2 pieces fracture conchoidally when broken by a hammer. In thin section, plagioclase, clinopyroxene, orthopyroxene, and oxide are the dominant phases present. Disequilibrium textures are sparse (typically less than 1% modally) but include resorbed, sieved, and embayed plagioclase. Xenocrystic olivine (~1 - 1.5 mm euhedral and iddingsitized phenocrysts) was observed in one sample and rims of <1 mm pyroxene crystals on these xeno/phenocrysts are occasionally present. The ubiquitous plagioclase ± clinopyroxene ±

orthopyroxene ± oxide clots that are present in other SC units were not observed in Tad2 lava flows.

Tertiary andesite-dacites of the Calico Mountains (Tad3) Within the lower half of the ~400 m thick ~16.5 Ma package of lava and ash flows exposed in the Calico Mountains are Tad3 lava flows. These flows were mapped by King (1984)

and included into his u1 and u2 map units. Like Tad2 lava flows, no shallow intrusive bodies, near-vent facies, or eruptive loci were identified that could be associated with this unit. On the

eastern side of the Calico Mountains, a Tad3 lava flow unconformably overlies ~102 Ma granitoid and yields a 40Ar/39Ar age of 16.54 ± 0.16 Ma (Fig. 4c). Lava flows and ash flows

exposed in the upper portion of the Calico volcanic pile that overlie Tad3 lava flows yield identical ~16.5 Ma ages. Additionally, these Tad3 lava flows are not exposed anywhere else in

the SC. These observations indicate that Tad3 lava flows erupted from a magmatic system that was separate from those associated with other SC intermediate units. Tad3 lava flows are typically ~10 - 20 m thick and when exposed, are characterized by basal and upper brecciated zones that are highly vesiculated and monolithologic. Occasional auto-injection squeeze-ups are also present in association with Tad3 upper brecciated zones. Above the lower breccia, a platy

zone typically grades into a massive and jointed interior, similar to Tad2 lava flows.

Tad3 lava flows are extremely fine-grained and again resemble Tad2 lava flows. In hand sample, 1 - 3 mm feldspars are visible. In thin section, plagioclase, clinopyroxene, orthopyroxene, and oxide are the dominant phases present. Like other SC intermediate units,

disequilibrium textures are present in Tad3 lava flows. Both 1 - 5 mm granitoid xenoliths and <1

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mm plagioclase ± clinopyroxene ± orthopyroxene ± oxide clots (like other SC units) are present and modally make up <5 % by volume (Fig. 7).

Silicic Units SC silicic eruptive loci were primarily focused along its eastern and western margins and were dominated by domal and fissural sources. Silicic units in the SC are divided into ten physically and chemically distinct units. In summary, five silicic units are found only in the western SC: Thc, Tbr, Twsc, Tpr, and Tem. These units erupted over an ~1 Ma duration and represent the products of separately evolving magmatic systems. In the eastern SC, silicic volcanism occurred from ~16.5 - 16.4 Ma and three silicic units are present: Tom, Tcm, and Tct. Two other silicic units that represent the only products of caldera-forming volcanism present in

the SC are best exposed in the Goosey Lake depression. Tp1 ash flows are the only peralkaline silicic material present within the SC and likely were sourced in the nearby McDermitt volcanic field. Tcst ash flows were locally derived, are the youngest SC silicic eruptive products, and the only identified SC derived products associated with caldera-forming volcanism. Typical ignimbrite flow unit stratigraphy is identifiable in some SC ash flows; however, these relationships are often obscured and difficult to identify (Fig. 9). The following provides brief summaries of these silicic units based on the more detailed discussion presented in Brueseke et al. (accepted). Silicic units that crop out around Hinkey Summit and were earlier defined by LeMasurier (1965) have been included within these units. Tertiary Hinkey Summit-Coal Pit Peak dacites-rhyolites (Thc) Thc lava flows are best exposed between Hinkey Summit and Coal Pit Peak, where they form a massive ~100 m wall and overlie the basal package of Tba and Ta lava flows (Fig. 4a). This same package is exposed south of Granite Peak in the vicinity of Solid Silver Creek where it was down dropped to the south along the basin bounding normal fault that defines the southern SC margin (Figs. 2, 4a, 6b). Included within Thc is the Trd lava flows of LeMasurier (1965). These lava flows are also exposed just north of Hinkey Summit along Forest Service road 084 at the turn-off for Lye Creek campground and in the region between Lye Creek and Buckskin Mountain (Figs. 2, 6b). The lava flow exposed at the Lye Creek turn-off yields a 40Ar/39Ar age of 16.36 ± 0.10 Ma (Alan Wallace, Pers. Comm; Table 1). When fully exposed, these lava flows are between ~15 to 120 m thick and are thickest between Hinkey Summit and Coal Pit Peak. No

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Thc near-vent facies or eruptive loci were encountered, however their source is inferred to be in the Hinkey-Coal Pit region based on the thick local exposures. Thc lava flows are cliff-forming and characterized by ubiquitous flow banding and upper flow margin breccias and jointing, causing them to weather into piles of thin plates. Because of their platy appearance, Thc lava flows resemble Tarc lava flows. However, Thc lava flows are easily distinguished by the presence of abundant phenocrysts of feldspar in a light gray to purple matrix. Quartz, biotite, clinopyroxene, and orthopyroxene are also present, in addition to plagioclase and potassium feldspar. Tertiary porphyritic rhyolite (Tpr) Porphyritic rhyolite lava flows and shallow intrusive bodies are exposed throughout the southern and southwestern SC (Figs. 2, 6b). LeMasurier (1965) included these in his Tr and Tri groups. The best exposures are found just south of Hinkey Summit along Forest Service road 084 as it climbs into the SC (Fig 4f, h, k), and along the southern and western margins of the Goosey Lake depression (Fig. 4l). South of Hinkey Summit, a large shallow intrusive body is exposed and intrudes the pre-SC igneous and metamorphic assemblage (Figs. 4f, k). Along the western and southern Goosey Lake depression margins, two Tpr flow-dome complexes (coulées) are well exposed (Fig. 6b). The western flow can be easily traced back to its source along the western SC margin; an eroded plug with underlying near-vent bedded pyroclastic fall deposits (Fig. 4l). The source for the southern flow is identified based on aerial imagery (Figs. 2, 6b). Also present throughout this portion of the SC are numerous north-northwest trending Tpr dikes that strike between 280º and 23º (Figs. 4f, g, h). The weighted mean of four 40Ar/39Ar ages from Tpr intrusive bodies and lava flows is 16.19 ± 0.04 Ma (Table 1), and is consistent with stratigraphic relationships. Because both the southern and western Tpr lava flows flowed eastward into a topographic low and the western flow overlies a sedimentary package, these ages also provide a minimum age for the formation of the Goosey Lake depression. Tpr lava flows and intrusive bodies are extremely crystal rich (~25 to 50% modally) and are the only amphibole bearing unit present in the SC suite. Plagioclase feldspar, quartz, potassium feldspar, biotite, oxides, and pyroxene are also present as phenocrysts and microphenocrysts. Microscopic (1 - 10 mm) granitoid xenoliths and plagioclase ± clinopyroxene ± orthopyroxene ± oxide clots are present in Tpr flows and intrusive bodies (Fig. 7e, f). Additionally, 1 - 5 cm mafic xenoliths are visible in some Tpr dikes.

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Tertiary Eightmile Mountain lava flows (Tem) Eightmile Mountain area lava flows were originally mapped by Vikre (1985b) as the

Rhyolite of Buckskin Mountain (Tr3, Trt, Tbx) and crop out to the north and east of Buckskin Mountain (Figs. 4m, 4i, 6b). Near Eightmile Mountain, oxidized pyroclastic deposits are present in close spatial association with dikes that strike between 5º and 35º. These features and the thickest exposures of Tem represent a near-vent facies. These field relationships and the lack of any type of associated domal body suggest a fissural origin for Tem lava flows. A similar eruptive style has been proposed for ~11 Ma lava flows exposed along the southern margin of

the western Snake River Plain graben (Bonnichsen et al. 2004). Tem lava flow(s) overlie Tad2 lava flows in the vicinity of Buckskin Mountain, while near Odell Mountain they overlie Tom lava flows and eruptive loci (Fig. 4m). This stratigraphic relationship with Tad2 lava flows indicates that Tem lava flows are younger than ~15.8 Ma. Like Tarc, Thc, and Twsc lava flows, Tem lava flows are typically associated with large platy talus slopes due to their highly jointed nature (like Fig. 4p). Macroscopically they resemble Thc lava flows and are characterized by 1 - 3 mm feldspars in an aphanitic, purple matrix. Microscopically, plagioclase feldspar, oxide, apatite, and zircon are present. While disequilibrium textures are present in Tem lava flows (resorbed and sieved plagioclase, plagioclase ± clinopyroxene ± orthopyroxene ± oxide clots, and fractured Mg-rich olivine), they are much less abundant than other SC silicic units (<1 %). Tertiary flow-banded rhyolite lava (Tbr) Tbr lava flows and shallow intrusive bodies are well exposed in the southwestern SC (Fig. 6b; Thc, Tsi, and Tpr map units). These shallow intrusive bodies cut older SC units and Tbr lava flows are found overlying Tpr lava flows. Their most distinguishing characteristic is a lack of phenocrysts and abundant ~1 - 3 mm laminations in a white aphanitic open-textured matrix. Shallow intrusive bodies are commonly columnar jointed and extremely flow-banded. Microscopically, Tbr lava flows and shallow intrusive bodies are extremely fine grained and <1 mm crystals of plagioclase and potassium feldspar, quartz, biotite, oxide, and apatite are present. These units also lack disequilibrium textures. Like Tpr lava flows and shallow intrusive bodies, these were not differentiated by LeMasurier (1965) as a separate unit and were included with his Tr and Tri units.

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Tertiary western margin lava flows (Twsc) Western margin lava flows are exposed between Buckskin Mountain and Granite Peak (Fig. 6b). Macroscopically, low-silica Thc lava flows resemble SC intermediate units. However most Twsc lava flows resemble Thc and Tem and crop out as ~25 - 35 m cliffs and/or slopes of platy talus similar to that depicted in Figure 4p. Vikre (1985b) included Twsc lava flows in his crystal-lithic rhyolite tuff unit (Tr2) and LeMasurier (1965) included them in his Trd unit. One Twsc dike was sampled and strikes 353º. Stratigraphically, Twsc lava flows overlie ~16.5 Ma Ta and Tba lava flows, but their relationship with ~16.2 Ma Tpr lava flows is obscured by recent glaciation. Plagioclase (with and without disequilibrium textures) and potassium feldspar, clinopyroxene, orthopyroxene, biotite, and quartz are present in this unit. Tertiary Odell Mountain area silicic units (Tom) Exposed in the vicinity of Odell Mountain is ~295 m of silicic lava and ash flows that are overlain by younger SC silicic units (Fig. 6b). At least one eroded rhyolite dome that was likely the source for some Tom flows is exposed at the head of Klondike Canyon (Fig. 4n). Stratigraphically, the lowest units that crop out in this region are rhyolite lava flows which are best exposed along the South Fork of the Quinn River. At Odell Mountain, these lava flows are overlain by a series of interbedded ash flows, air-fall tuffs, and lava flows. The uppermost unit (flow banded lava flow) yields a 40Ar/39Ar age of 16.55 ± 0.04 Ma. The eroded rhyolite dome yields a similar 40Ar/39Ar age of 16.43 ± 0.06 Ma (Table 1). Sanidine, quartz, plagioclase feldspar, oxide, pyroxene (opx and cpx), zircon, and apatite are present in Tom units. Sparse (<0.4 %) biotite was also observed in one Tom lava flow. Additionally, resorbed sanidine and quartz were also observed in Tom products. Tertiary Calico Mountains area silicic units (Tcm) Tcm lava and ash flows are exposed in the Calico Mountains along the eastern SC border (Fig. 6b). These units are physically diverse and include lava flows, ash flows, and lava-like ignimbrites. Commonly, Tcm products are interbedded with Tad3 lava flows. Within the ~ 400 m-thick package of SC products that overlie Cretaceous granitoid in the Calico Mountains, Tcm products in the lower 230 m of section are typically lava-like ignimbrites. In the upper portion of this section, the ~130 m-thick Capitol Peak ash flow underlies a package of rhyolite lava flows. These lava flows, best exposed at Capitol Peak, physically resemble and are chemically identical to Tom lava flows. They are likely sourced from near Odell Mountain and are included with

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Tom for further discussions. Sanidine separates from the uppermost of these lava flows yield a 40Ar/39Ar age of 16.50 ± 0.04 Ma. To the south, thick exposures of rhyolite lava crop out along

the range crest. One of these lava flows is exposed at Mahogany Pass where it overlies Tad2 and Tba lava flows. This flow yields a 40Ar/39Ar age of 16.51 ± 0.04 Ma and erupted from a small eroded dome, just to the north of Mahogany Pass (Hill 7502 on the Capitol Peak 1:24,000 quadrangle). Further south, more rhyolite lava flows overlie the Mahogany Pass rhyolite. Sanidine crystals from one of these flows yield a 40Ar/39Ar age of 16.47 ± 0.04 Ma (Table 1). Based on this age and on stratigraphic relationships, it appears that these southern Calico Tcm lava flows were locally derived. One Tcm dike was sampled on the eastern side of the Calico Range and strikes 350º, further confirming a local Tcm source. Tcm silicic units are petrographically variable. Tcm ash flows are xenolith-rich (plagioclase ± clinopyroxene ± orthopyroxene ± oxide clots and xenoliths that resemble SC Tba and Ta lava flows), feldspar bearing, and in thin section, characterized by abundant disequilibrium textures. In most cases, flattened and compressed glass shards related to welding are visible and occasionally textures that are consistent with magma mixing/mingling are also present (e.g. banding and round blobs; Fig. 7c). Tcm lava flows resemble each other and are difficult to distinguish petrographically from each other. Plagioclase and potassium feldspar, quartz, oxide, pyroxene, apatite, and zircon are all present in these lava flows. Additionally, granitoid xenoliths are also occasionally present. Tertiary Coyote Mountain-Zymns Butte area silicic units (Tct) Coyote Mountain, Zymns Butte, and Black Dome are three silicic eruptive centers found in the southeastern portion of the SC (Figs. 2, 6b). Physiographically, Black Dome is the most prominent of these three loci, situated at the southern end of Black Ridge. However, Coyote Mountain is the best exposed eruptive center. The highly welded Coyote Mountain ash flow is exposed on the flanks of Coyote Mountain and in its vicinity (Fig. 10). Sanidine from block and ash deposits exposed on Coyote Mountain that stratigraphically overlie the rheomorphic Coyote Mountain ash flow yield a 40Ar/39Ar age of 16.44 ± 0.06 Ma (Table 1). Black Dome outflow overlies the Coyote Mountain ash flow. The stratigraphic relationship between Zymns Butte and Coyote Mountain outflow is hard to discern, but it appears that Zymns Butte is younger than Coyote Mountain. Outflow from all three loci exhibit disequilibrium textures including resorbed

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and sieved feldspars, and plagioclase ± clinopyroxene ± orthopyroxene ± oxide clots (like Fig. 7 e-h). Tertiary Cold Springs tuff (Tcst) Two pyroclastic flow units that represent local and regional pyroclastic volcanism are best exposed in the Goosey Lake depression. The Cold Springs tuff is one of these units with its type locality in the vicinity of Cold Springs Butte, along the eastern margin of the western Goosey Lake depression (Figs. 2, 8, 11, 12). Because of the post-SC faulting that was concentrated within the central portion of the Goosey Lake depression, using stratigraphic and petrographic data (e.g. elevation differences, welding facies, etc.) to reconstruct the areal extent and distribution of distinct Tcst flow units is difficult. Additionally, fluvial systems associated with local Pleistocene glaciers disturbed much of the original Tcst outcrop pattern in portions of the Goosey Lake depression. In this context, at least two separate depositional flow units are present in the central SC, and composite Tcst sections including these units reach a maximum thickness of ~50 m (Fig. 11). However, the maximum Tcst thickness is likely greater. Where exposed elsewhere, Tcst deposits range from ~12 - 45 m in thickness and these variations may reflect the influence of substantial pre-Tcst local topographic irregularity. Just east of Cold Springs Butte, the basal portion of one Tcst flow unit overlies inferred bedded Tcst fall deposits (Fig. 11, 12). These fall deposits overlie buff to tan lacustrine sedimentary strata, similar to deposits exposed in small patches across the Goosey Lake depression and along the eastern SC margin. Above these fall deposits is ~2 m of a poorly welded ground surge that grades upward into the lower ~5 m of a Tcst main ignimbrite body (Fig. 12a). This lower portion of the main body is a poorly welded, crystal rich unit that lies below ~5 m of extremely welded Tcst. Within this zone of extremely welded deposits, a lower ~1.5 m spheriodially weathering and dark glassy zone grades into a blocky lighter zone (<1 m thick), that directly grades into a thin (<1 m), highly welded white-colored zone. This upper highly welded zone texturally resembles other portions of highly welded Tcst deposits. Above these highly welded deposits is ~20 m of poorly welded Tcst that physically resemble and are interpreted to be main body deposits. These stratigraphic-based changes in welding are inferred to reflect a more densely welded zone near the central portion of the main ignimbrite body (classic welding profile of a simple ignimbrite cooling unit; Best and Christiansen, 2001). Just south of these exposures and south of Cold Springs Butte is an oval ~2.5 x ~3.5 km region of low

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(relative to its surroundings) topography which is visible both in the field and on aerial imagery (Figs. 2, 6). Where fully exposed, measured Tcst thicknesses are greatest near this depression and the only observed lithic fragments within Tcst deposits are locally present. Also, rare imbricated Tcst pumice clasts were observed at one location directly north of this depression and reflect a northward directed flow. While no near-vent lag breccia was observed, these other geologic features together suggest that this depression may have been the source for Tcst pyroclastic deposits. In this scenario, this small, localized depression is likely the remnant of a caldera; the only caldera identified within the SC. Conversely, the Tcst pyroclastic deposits could have been derived from “trap-door” style subsidence or fissural-style eruptions (Lipman, 1997; Bonnichsen et al. 2004) along an active zone of faulting within the central Goosey Lake Depression. However, the observed geologic data suggest the caldera-source interpretation (Cold Springs caldera). As mentioned previously, Tcst welding and facies changes make linking exposures across the SC difficult. Observed Tcst deposits are found as far south as the Hardscrabble basin (Fig. 2, 6, 9, 12c) where at least two flow units are exposed and separated by at least 70 m of interbedded lacustrine strata and air-fall tuffs. The uppermost Tcst flow unit at this location yields a 40Ar/39Ar age of 15.40 ± 0.08 Ma, constraining the age of the underlying deposits (Table 1). At Cold Springs Butte, a poorly welded main body deposit from an ~30 m-thick Tcst outcrop exposed along the eastern side of the inferred caldera yields a 40Ar/39Ar age of 15.46 ± 0.04 Ma (Table 1). These two ages likely reflect the same eruptive event. Just north of Holloway Meadows, ~18 m of Tcst is exposed lying on a package of Ta lava flows. Here, the basal poorly welded portion of the outcrop yields a 40Ar/39Ar age of 15.75 ± 0.16 Ma (Table 1). Above this poorly welded zone, the Tcst deposit gets increasingly more welded (and de-vitrified) up-section and texturally is identical to the welding variations present near Cold Springs Butte described earlier (Fig. 11). However, unlike the highly welded zone near Cold Springs Butte, this Holloway Meadows highly welded zone lacks overlying main body deposits. Texturally, Tcst deposits vary based on their degree of welding and de-vitrification. The highly welded zones are also the most de-vitrified. At an outcrop scale, poorly welded Tcst main-body deposits are monotonous and no normal or reverse grading was observed (Fig. 12a). Approximately 1 - 7 cm gray pumice clasts are often present and in some cases flattened (Fig. 12a). Also present in these deposits are 1 - 3 cm blocky glass fragments that resemble rhyolite

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vitrophyre. As welding increases, the pumice fragments become more flattened and in highly- welded Tcst zones, all ignimbritic textures are obliterated and the deposit physically resembles a rhyolite lava flow. North of Cold Springs Butte in the Groundhog Meadows region (eastern Goosey Lake depression), extremely welded Tcst deposits are locally present up to ~18 m-thick and are lithophysae-rich (Fig. 12b). Tcst glass shards are light brown under plane light. Sanidine, anorthoclase, and plagioclase feldspar are present, in addition to clinopyroxene (pigeonite), oxides, apatite, and zircon. Disequilibrium textures are present in Tcst pheno/xenocrysts and include resorbed and highly zoned plagioclase and ~1 - 3 mm crystal clots of plagioclase ± potassium feldspar ± clinopyroxene ± orthopyroxene ± oxide clots. Typically, these clots resemble those found in other SC units (Fig. 7e-h).

Tertiary ignimbrite (Tp1)

Tp1 outcrops are found across the Goosey Lake depression and in other scattered

locations in the western SC (Fig. 12d). Where fully exposed, Tp1 outcrops are typically ~1 - 5 m-thick and appear to represent one simple depositional flow unit. However, exposures up to 25 m-thick are present along the northern SC margin, east of Odell Mountain. Tp1 ash flows are characterized by the ubiquitous presence of ~1 - 3 cm long fiamme in a highly welded, gray/beige/yellow/pink matrix. Additionally, this unit is the only peralkaline silicic unit exposed in the SC. Just northwest of Hinkey Summit (NW ¼ of section 25, Hinkey Summit 1:24,000

U.S.G.S. quadrangle), an ~3 m-thick exposure of Tp1 is present on a topographic bench at

~8020’ elevation. Here, the Tp1 that crops out appears to be welded following a classic welding profile of a single cooling unit. In other locations in the Odell Mountain region and on Eightmile

Mountain, Tp1 outcrops include an exposed basal vitrophyre. The vitrophyre is less welded than overlying deposits and is comprised of >95 % flattened pumice. Sanidine from a Tp1 deposit exposed in the southern portion of the eastern Goosey Lake depression that overlies sedimentary 40 39 strata and Tad1 lava flows yields a Ar/ Ar age of 16.45 ± 0.12 Ma. This age is in agreement with locally observed stratigraphic relationships, and recent 40Ar/39Ar age determinations on silicic units from the McDermitt volcanic field demonstrate that peralkaline McDermitt activity

was occurring prior to 16.3 Ma (Castor and Henry, 2000). Clasts of Tp1 ash flows are also present in ~15.4 Ma Tcst deposits in at least one location, also suggesting that Tp1 ash flows were locally present prior to Tcst volcanism. However, in other locations in the Goosey Lake

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depression, Tp1 directly overlies Tcst ash flows and Tem lava flows, implying that these Tp1 deposits are much younger than the ~16.5 Ma age obtained on the dated Tp1 ash flow. Together, these field and chronologic relationships indicate that more than one Tp1 flow unit is exposed within the SC. At least three voluminous McDermitt ash flows erupted from ~15.7 to ~15.5 Ma (the 15.7 Ma Tuff of Double-H, the 15.6 Ma Tuff of Long Ridge, and the 15.5 Ma Tuff of

Hoppin Peaks; Rytuba and McKee, 1984) and Tp1 deposits in the SC could be related to these in addition to older McDermitt volcanism. No local SC source for Tp1 flow units can be deduced or inferred from the field observations.

Tp1 deposits are highly welded, have undergone post-emplacement devitrification, and are crystal poor. Occasional ~1 mm sanidine and highly altered pyroxene crystals are present and make up <2 % by mode. Flattened ~2 - 4 cm long pumice are present in all Tp1 deposits and are typically devitrified, leaving a “ghost” texture.

Tertiary air-fall tuffs (Tp2) Tuffaceous horizons are exposed throughout the SC in a variety of stratigraphic contexts. The best exposures are interbedded with sedimentary strata in the basin fill of the Goosey Lake depression, however, silicic fall deposits are also found interstratified with lava and ash flows in the SC volcanic pile and underlying locally erupted rhyolite lava flows. Gilbert et al. (2003) performed bulk glass purification and major and trace element geochemical analyses on a sub-set of locally exposed fall deposits. Gilbert et al. (2003) chemically correlated subalkaline vitric fall deposits found within an ~18 m thick package of volcanogenic sedimentary strata that stratigraphically underlie Tcst ash flows (just south of the Cold Springs caldera) with 15.84 - 15.76 Ma Buffalo Canyon type ash beds and the 15.6 Ma Virgin Valley 1 ash bed of Perkins and Nash (2002). These interpretations agree with local stratigraphic data and illustrate that this portion of the Goosey Lake depression was a sedimentary depo-center by ~15.8 Ma. Along the western Goosey Lake depression margin, fall deposits directly underlie the eastern lobe of the western Tpr lava flow. These deposits overlie sedimentary strata, are chemically similar to the Tpr flow, and further illustrate that at least a portion of the Goosey Lake depression defined a region of low topography and sedimentation as early as ~16.2 Ma.

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PETROLOGIC CHARACTERISTICS OF SC UNITS SC products range from basalt to high-silica rhyolite (Fig 5; Table 3) and the entire suite approximates a subalkaline differentiation trend. Major and trace element data are used to distinguish pre-SC Tarc lava flows from SC derived units (Fig. 13a, b). SC mafic and intermediate units are dominantly tholeiitic, but span the tholeiitic/calc-alkaline boundary (Fig. 13b). SC silicic units are predominantly peraluminous to metaluminous and the only peralkaline unit (Tp1) was likely not derived locally (Fig. 13c). SC units are characterized by a wide range in REE concentrations and are isotopically diverse. The following sections first introduce and discuss the major and trace element characteristics of the observed mafic, intermediate, and silicic units, then present the isotopic characteristics of the suite as a whole, followed by preliminary petrogenetic interpretations based in part on these data.

Mafic Units Tertiary basalt (Tb)

Tb lava flows are the least evolved materials erupted within the SC and have high Al2O3

(avg. 17.13 wt. %), low K2O (avg. 0.32 wt. %), and low TiO2 (avg. 1.35 wt. %) and incompatible (in a mafic liquid) element concentrations (Fig. 14a, b; Table 3; Appendix 3). These and other major and trace element characteristics are similar to regionally exposed HAOT lava flows, which are the dominant basalt type erupted across the Oregon Plateau over the last 11 Ma (Hart, 1985; Shoemaker, 2004). Prior to identifying these Tb lava flows as HAOT, the oldest documented HAOT-like lava flows and eruptive loci identified were ~13 Ma, from the Oregon- Idaho graben (Camp et al. 2003). HAOT-like dikes cut the granitoid exposed in the main Santa Rosa range and may be related to these newly identified mid-Miocene units, rather than younger Mio-Pliocene HAOT volcanism (Stuck, 1993). The Tb lava flows form a cluster on all major and trace element Harker diagrams. No discernable trends are present within the Tb group. However, because Tb is the most mafic material encountered in this study, it represents the mafic end-member on all Harker plots (Figs. 14a, b). Figure 15a depicts elemental variations of SC mafic units normalized to mid-ocean ridge basalt (MORB) (Pearce, 1983). Mafic magmas with large ion lithophile element (LILE) enrichments and high field strength element (HFSE) depletions are commonly associated with subduction zone environments (affected by subduction-related fluids and/or sediment) or were

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derived from a subduction modified source; these characteristics are observable on this type of diagram (Hawksworth et al, 1993; Plank and Langmuir, 1998; Davidson et al. 2005). One Tb sample is depicted and it is slightly enriched in LILE, with the exception of a pronounced positive Ba anomaly. This pattern is nearly identical to younger, Oregon Plateau area HAOT (Shoemaker, 2004). Tb exhibits a flat light rare earth element (LREE) pattern and LREEs are slightly more enriched than heavy rare earth elements (HREE) (Figure 16a). Tertiary locally-erupted Steens Basalt (Tba) Tba lava flows and shallow intrusive bodies are much more evolved than Tb, as illustrated by the major and trace element plots on Figures 14a and b. These lava flows are dominantly tholeiitic, though they straddle the tholeiitic to calc-alkaline boundary (Fig. 13b). Compositionally, Tba resembles regionally exposed Steens Basalt lava flows (Fig. 17). Comparisons of major and trace element data illustrate that average Tba is slightly more evolved than average Oregon Plateau Steens Basalt, however, the compositional range represented by

Tba overlaps with that found in the Steens Basalt (~46 - 56 wt. % SiO2, ~0.9 - 3.3 wt. % TiO2,

~3.0 - 7.8 wt. % MgO, ~7.7 - 13.5 wt. % FeO*, ~6.0 - 10.1 wt. % CaO, ~0.7 - 2.5 wt. % K2O; Fig. 14). Based on these compositional similarities and the fact that Tba volcanism occurred throughout the duration of SC volcanism, Tba activity is interpreted as local Steens basalt volcanism. The Tba lava flows are compositionally diverse (Figs. 14a, b), and define arrays that in some cases are suggestive of multiple parental liquids and/or early differentiation histories (e.g.,

FeO*, P2O5, Ni, and Sr vs. SiO2). Normalized to MORB (Figure 15a), Tba lava flows have pronounced enrichments in LILE but not in the HFSE. This elemental pattern is similar to regional Oregon Plateau flood basalt lava flows (as previously mentioned; Fig 17). Tba LREE are moderately enriched relative to their HREE concentrations. Together, LREE and HREEs define a smooth pattern (Fig. 16a).

Intermediate Units Tertiary andesite of Black Ridge (Ta) Ta lava flows are chemically similar to the most evolved Tba lava flows and are dominantly tholeiitic basaltic andesites to andesites (Fig. 5a, 13c). They lie across the basic/acid boundary in the andesite classification scheme of Gill (1981) (Fig. 5b). Ta major element

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concentrations overlap with the most evolved samples of Tba and “cluster” between Tba and the three Tad groups (Figs. 14a, b). Trace element variations also overlap with Tba. For example, the ranges in Ni, Sc, Sr, Rb, Zr, La, and Nb concentrations present in Ta lava flows are identical to those of Tba. However, some elements (e.g. Ba and the REE) are more enriched in Ta relative to Tba. While there are slight major and trace element variations within the Ta group, Ta lava flows appear to be a more evolved extension of the dominant mafic SC magma type (Tba). Ta lava flows are enriched in LILE and have HFSE “troughs” when normalized to MORB (Fig. 15). Their overall MORB-normalized pattern is also nearly identical to Tba lava flows and with the exception of Ba (slightly more enriched), overlaps with the most enriched Tba. REE patterns for Ta illustrate moderate LREE enrichment with a slight negative Eu anomaly (Fig. 16b). These REE patterns overlap with the most enriched Tba and are consistent with crystal fractionation from a less evolved, Tba-like magma.

Tertiary andesite-dacites of Hinkey Summit-Coal Pit Peak (Tad1)

Tad1 lava flows are classified as high-K andesites (Fig. 5). However, lower FeO*/MgO ratios at a given SiO2 content define these lava flows as calc-alkaline and distinguish them from all other SC intermediate composition units (Fig. 13). Other temporally, compositionally, and physically similar calc-alkaline lava flows are exposed within and to the north of the Oregon- Idaho graben in northeastern Oregon (Robyn, 1979; Hooper et al. 1995; Cummings et al. 2000; Camp et al. 2003). Harker plots of major and trace element concentrations also help distinguish

Tad1 lava flows from other SC intermediate composition groups (Figs. 14a, b). Tad1 lava flows have lower HFSE, REE, Sr and higher Ni and Sc concentrations than other SC intermediate units over the same range in silica (Fig. 14b). Like their temporal, spatial, and petrographic attributes, these Tad1 chemical characteristics indicate that they were derived from a magmatic system(s) separate from other SC intermediate units.

The Tad1 lava flows are divided into low (~59 - 62 wt. % SiO2) and high (~62 - 64 wt. %)

silica groups (Fig. 14a; see MgO vs. SiO2) that do not appear to be stratigraphically controlled.

On a MORB normalized multi-element diagram (Fig. 15c) Tad1 lava flows display overall

patterns similar to other SC intermediate units (Ta, Tad2, Tad3) and Tba. Tad1 lava flows have slightly more enriched LILE than Tba, with the exception of Sr. They also have pronounced negative HFSE and P anomalies. Other trace element concentrations normalized relative to

MORB overlap with Tba; though Tad1 has lower P, Ti and higher Y than Tba. Tad1 lava flows

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possess moderate LREE enrichment and obvious negative Eu anomalies (Fig. 16b). This pattern overlaps with Tba and is consistent with fractional crystallization from a Tba-like magma.

Tertiary andesite-dacites of Staunton Ridge (Tad2)

Tad2 lava flows are classified as high-K andesites (Fig. 5), but unlike Tad1 lava flows, are

tholeiitic. On Harker variation plots of major and trace element concentrations, Tad2 lava flows are compositionally most similar to Tad3 lava flows, but are slightly more alkaline. The similarity to Tad3 is best seen on plots of SiO2 vs. MgO, P2O5, and Ni (Figs. 14a, b). However, different HFSE and REE concentrations at similar silica contents easily differentiate these two units (Fig. 14b). Major and trace element variations also help illustrate that Tad2 are different than Tad1 lava flows (e.g. lower MgO and higher P2O5, TiO2, LILE, HFSE, and REE). Age relationships substantiate these chemical differences and preclude these units from being petrogenetically related (Table 1).

Like Tad1, some Tad2 major and trace element concentrations vary slightly with silica

while others remain relatively constant (Figs. 14a, b). Tad2 have more enriched overall trace element patterns than Tba and Tad1, when normalized to MORB (Fig. 15d). Sr, Ti, and P are the exceptions. Like all other SC mafic and intermediate units, they also exhibit LILE > HFSE relative enrichments. The shape of their chondrite normalized REE pattern is similar to other SC intermediate units, however, Tad2 lava flows have overall higher REE abundances than Ta and

Tad1 (Fig. 16b). Tad2 lava flows also have a slight negative Eu anomaly and this anomaly and the overall REE enrichments are consistent with fractional crystallization of a less evolved mafic magma.

Tertiary andesite-dacites of the Calico Mountains (Tad3)

Tad3 lava flows are also classified as high-K andesites (Fig. 5) and are tholeiitic. On major element Harker variation plots, they have similar concentrations as Tad2 lava flows

(slightly higher TiO2, MgO, and CaO, and slightly lower K2O; Fig. 13a). Their trace element concentrations are also similar; however, Tad3 lava flows have higher Sr, HFSE, and REE

concentrations than both Tad2 and Tad1 lava flows (Fig. 13b). These chemical differences agree with substantial spatial and temporal differences. Together these observations strongly suggest that the individual Tad units erupted from distinct SC magmatic systems.

Tad3 lava flows have nearly identical MORB normalized elemental patterns as Tad2 and have more enriched trace element concentrations than Tba (Figs. 15d). They have the

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characteristic LILE enrichments and HFSE “troughs” that are found in all SC intermediate units, but they possess the highest concentrations of the HFSE. Their REE pattern is most similar to

Tad2 (Fig. 16b) and again displays a slight Eu anomaly.

Silicic Units Tertiary Hinkey Summit-Coal Pit Peak dacites - rhyolites (Thc) Thc lava flows are dacitic to rhyolitic based on their concentrations of total alkalies and silica (Fig. 5). Within this unit, at least three subgroups are present, with two falling between 67

- 74 wt. % SiO2 (Figs. 18a, b). These two subgroups are evident in the Zr vs. silica plot and together define a single array in element-element space. The high Ba concentrations (up to 5500 ppm) that characterize low-silica Thc lava flows are unique within the SC silicic suite and distinguish these from other silicic units. The third subgroup of Thc lava flows is characterized by >74 wt. % SiO2 and crops out along the southern SC margin in the massive cliffs north of Paradise Valley (Figs. 2, 6). Major and trace element characteristics help delineate Thc lava flows from other SC silicic units and support physical, spatial, and geochronologic differences suggesting that Thc lava flows represent outflow from a distinct SC silicic system. Figure 19 depicts trace element variations of SC silicic lava flows normalized to upper continental crust (Taylor and McLennan, 1985). Overall, Thc lava flows (and most other SC silicic units) exhibit relative elemental enrichments compared to average upper continental crust (Fig. 19d). However, Thc has pronounced negative Ta, Nb, Ti, and Sr anomalies and slight positive Zr, Hf, and Ba anomalies. Figure 16f illustrates the REE behavior of Thc. Like the previously described SC intermediate units, the Thc displays a LREE enriched pattern. However unlike other units, Thc has a slightly positive Eu anomaly. This, coupled with the strong negative Sr anomaly (Fig. 19d), suggests that processes more complex than crystal fractionation alone are likely responsible for the evolution of this unit. Tertiary porphyritic rhyolite (Tpr) Tpr lava flows and shallow intrusive bodies are rhyolitic and span a range in silica from ~69 to ~78 wt.%. Within this range, two subgroups are present: 1) a low silica subgroup

between ~69 - 71 wt. % SiO2 and 2) a high silica subgroup, with concentrations over 72 wt. % (Figs. 18a, b). All samples that make up the low silica subgroup were collected from the Tpr lava flow exposed along the southern SC margin, while samples from the western margin Tpr

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flow fall within the higher silica subgroup (Figs. 18a, b). Tpr intrusive bodies including the dikes between Hinkey Summit and Buckskin Mountain and the large hypabyssal body southeast of Hinkey Summit also fall within the higher silica subgroup. Differences between these subgroups are apparent in their trace element compositions (e.g. the low silica subgroup has lower Rb, higher Zr and Sr). The high silica subgroup is more compositionally similar to Tbr lava flows and intrusive bodies than any other SC silicic unit. Normalized to upper continental crust, Tpr lava flows and intrusive bodies show relative enrichments that vary depending on mode of occurrence (Fig. 19a). The two Tpr samples depicted on Figure 19a were collected from the western and southern margin Tpr lava flows. The low silica southern margin Tpr lava flow is less enriched relative to upper continental crust and has small negative Sr and Ti anomalies and a small positive Ba anomaly. However the western margin higher silica Tpr lava flow is more enriched than the southern flow overall and has substantial negative Ba, Sr, and Ti anomalies that likely reflect substantial fractionation of phases such as feldspar and magnetite. Tpr REE patterns are characterized by LREE enrichment (Fig. 16c), with the degree of relative enrichment following the relationships discussed above. Both varieties have a negative Eu anomaly; however the high silica variety has a much more pronounced anomaly. Tertiary Eightmile Mountain lava flows (Tem) Tem pyroclastic material and lava flows are dacitic to rhyolitic and span a range in silica from ~65 to ~74 wt. % (Figs. 18a, b). On Harker diagrams, Tem samples typically define liner patterns, that with decreasing silica, project back toward Tad2. This potential petrogenetic link is supported by the previously discussed spatial and temporal relationships between these two units. As silica increases, CaO, MgO, FeO*, TiO2, P2O5, Rb, HFSE, and REE increase while

K2O, Na2O, Sr, and Sc decrease (Figs. 18a, b). Tem eruptive products have higher Na2O, Al2O3,

Sr, and lower TiO2 and MgO than most other SC silicic units. Overall, they are most compositionally similar to Twsc, but trace element differences (e.g. Zr) distinguish these two units, as do field and chronologic relationships. Tem lava flows have a similar upper continental crust-normalized elemental pattern as Thc (Fig. 19d). Tem has negative Eu, Ti, Ta, and Nb anomalies and a positive Ba anomaly. The Tem REE pattern is smooth and there is a slight range in enrichment in HREE within the Tem group (Fig. 16f). Like most other SC silicic units, Tem also has a negative Eu anomaly.

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Tertiary flow-banded rhyolite lava (Tbr)

Tbr lava flows and intrusive bodies are high silica rhyolites (>76 wt. % SiO2) and are easily distinguished from other SC silicic units by their limited bulk composition range (Figs. 18a, b). This chemical distinction is not tied into any type of petrographic or physical distinction. Additionally, the abundant obsidian clasts in Holocene to Recent SC sedimentary deposits are chemically identical to Tbr. While no obsidian domes were identified in the field, this suggests that at least one may have been present. On figure 19, Tbr lava flows and shallow intrusive bodies exhibit elemental patterns similar to those of Tpr lava flows and intrusive bodies. When normalized to upper continental crust, the most characteristic elemental features of Tbr are the large negative Ba, Sr, and Ti anomalies (Fig. 19a). Chondrite normalized Tbr REE patterns differ slightly from Tpr because the overall Tbr pattern is nearly flat with the exception of the pronounced negative Eu anomaly (Fig. 16c). The very pronounced negative Ba, Sr, Ti, and Eu anomalies exhibited by Tbr suggest a significant role for crystal fractionation in the petrogenesis. Tertiary western margin lava flows (Twsc) Twsc lava flows are dominantly dacites to rhyolites, with one sample falling along the andesite-dacite boundary. Although they physically resemble Tem lava flows, the Twsc lava flows are readily distinguished based on many major and trace element characteristics (Fig. 18a, b) Their trace element concentrations also distinguish them from similarly evolved Thc flow units as noted, for example, on a plot of SiO2 vs. Zr (Fig. 18b). One low silica Twsc lava flow was analyzed for a full suite of trace and REE concentrations. On MORB normalized trace element plots, it has a nearly identical pattern to

Tad3 including enriched LILEs and HFSE “troughs” that characterize other compositionally similar SC units (Fig. 15d). Low silica Twsc displays a LREE enriched pattern and a slight

negative Eu anomaly (Fig. 16b). This pattern is nearly identical to Ta, Tad2, and Tad3. Tertiary Odell Mountain area silicic units (Tom) The Tom lava and ash flows are rhyolitic with the highest silica contents of all eastern SC silicic units (> 73 wt. % SiO2; Figs. 18a, b). Tom lava flows are also distinguished from other eastern SC silicic units by their higher HFSE and lower LILE concentrations (Fig. 18b). Two distinct subgroups are present within the Tom chemical group as differentiated by their silica

content (<74 wt. % SiO2 and >75 wt. % SiO2). Mafic xenolith-rich Tom ash flows make up this

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lower silica group, while the higher silica group includes the stratigraphically youngest Tom lava flows and the eroded rhyolite dome at the mouth of Klondike Canyon. When normalized to upper continental crust, Tom lava and ash flows have the most pronounced negative Sr anomalies of any SC unit together with pronounced negative Ba and Ti anomalies (Fig. 19b). Tom lava and ash flows exhibit LREE enriched patterns with large negative Eu anomalies (Fig. 16d). Tertiary Calico Mountains area silicic units (Tcm)

Tcm lava and ash flows range from ~63 - 76 wt. % SiO2 (Figs. 18a, b). This compositional variation roughly follows stratigraphic position, where the most evolved samples are at the highest stratigraphic level. Within this range, Tcm subgroups are present and represent: [1] the xenolith bearing ash flows exposed below the Capitol Peak ash flow (<70 wt.

% SiO2), [2] the lava flow(s) exposed at Mahogany Pass (~71 - 73 wt. % SiO2), and 3) the southern Calico rhyolite lava flows that overlie the Mahogany Pass rhyolite (~73 - 76 wt. %

SiO2). The stratigraphically highest units at Capitol Peak (above the Capitol Peak ash flow) also

have high SiO2, however, their overall major and trace element composition are virtually identical to Tom lava and ash flows and are likely related (and therefore should be classified as Tom). On Harker variation diagrams, the xenolith bearing ash flows have higher MgO, Ni, and lower Ba concentrations than other Tcm silicic units (Figs. 18a, b). As a group, Tcm silicic units have nearly constant HFSE and REE concentrations (Fig. 18b). This lack of variation is particularly apparent on plots of SiO2 vs. La and Nb. Tcm lava and ash flows have similar elemental patterns on multi-element plots (Figs. 16, 19b), although a lower Tcm ash flow lacks the negative Eu anomaly (Fig. 16e). When normalized to upper continental crust, Tcm lava and ash flows have variable, but typically negative Sr, and Ta anomalies and positive Ba anomalies (Fig. 19b). The lower Tcm ash flows do not have a negative Ti anomaly as compared to the upper units. Tertiary Coyote Mountain-Zymns Butte area silicic units (Tct) Major and trace element data illustrate that Tct silicic products are broadly similar to Tcm lava and ash flows (Figs. 18a, b). Additionally, the products of all three loci (Coyote Mountain, Zymns Butte, and Black Dome) show the same approximate compositional range as Tcm. Like in the Calico Mountains, the stratigraphically lowest Tct silicic products are the least evolved. Samples collected from the Coyote Mountain ash flow define a range in silica (~69 - ~75 wt. %)

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that likely reflects pre-eruption chemical zonation in the magmatic system. In addition to the upper portion of the Coyote Mountain ash flow, higher silica Tct products (>74 wt. %) include pyroclastic deposits exposed at Coyote Mountain and Zymns Butte. Normalized to upper continental crust, Tct deposits have pronounced negative Sr and Ti anomalies and overall, have similar patterns to Tcm (Fig. 19c). Overall, Tct lava and ash flows have smooth REE patterns and are enriched in LREE relative to HREEs (Fig. 16e). Each sample analyzed is characterized by a negative Eu anomaly and the overall range in REE enrichment roughly corresponds to silica content (e.g. the most enriched Tct has the highest silica content). Tertiary Cold Springs tuff (Tcst) Bulk analyses of Tcst deposits span a range in silica from ~67 - 77 wt. % (Figs. 18a, b). Major and trace element data illustrate that the Tcst is chemically dissimilar from other SC silicic units and likely represents a separate magmatic system. This point is best illustrated by the concentrations of K, Al, Zr, Nb, and the REE. Pure glass separates were extracted from three bulk Tcst samples spanning the observed silica range. These pure glass separates defined a range

from ~73 - 76 wt. % SiO2 (Knight et al. 2004), at the upper end of the range defined by whole rock analyses. Additionally, pumice clasts from Tcst main body deposits exposed at Cold

Springs Butte are less silicic than the purified glass shards (~71.5 wt. % SiO2). Together, these bulk and purified Tcst chemical differences illustrate that the Tcst magmatic system was chemically heterogeneous. Because of these major and trace element variations, using bulk chemical characteristics to correlate Tcst outcrops is difficult. Knight et al. (2004) demonstrated that some of the bulk Tcst variation is a function of crystal content and that up-section welding variations often are accompanied by up-section chemical variations. However, welding variations in the Tcst near Cold Springs Butte are not accompanied by substantial (>1 wt. %

SiO2) chemical variation. Tcst magma temperatures were estimated using the relationship derived by Watson and Harrison (1983) as modified by Perkins et al. (1995): 10,000/T = -1.02 +

1.27 * ln(500,000/XZr), where T= degrees Kelvin and XZr = zirconium concentration in glass shards (ppm). Tcst magma temperatures for the purified glass separates and pumice calculated using this expression range from ~1047 to 1093°C, consistent with magma temperatures from regional mid-Miocene silicic eruptive systems (>1000°C; Perkins and Nash, 2002). Elemental variations of Tcst ash flows normalized to upper continental crust are depicted on Figure 19d. Here, it is apparent that Tcst has high concentrations of Zr, Hf, Ta, Nb, and

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LREE suggesting that zircon (and other trace phases) were not major fractionating phases. Tcst REE patterns are among the most enriched of all SC silicic units (Fig. 16f). High silica Tcst has an identically shaped, but more enriched pattern relative to low silica Tcst, and they both have small negative Eu anomalies. These within-Tcst characteristics are consistent with a major role for fractional crystallization in the Tcst magmatic system.

Tertiary ignimbrite (Tp1)

Tp1 deposits are peralkaline to mildly metaluminous and range from ~73 - 77 wt. % SiO2

(Figs. 18a, b). The one high silica sample (~76.6 wt %) overlies less welded Tp1 that falls within a more limited silica range defined by all other samples (~73 to ~75 wt. % SiO2). Its high SiO2 content may reflect secondary silicification. Tp1 major element geochemical data illustrate that these ash flows are chemically distinct from other SC silicic units (Fig. 13c). Chemical variation

within the Tp1 group is present and also makes chemical correlation of Tp1 outcrops difficult.

Tp1 ash flows are peralkaline, as are the voluminous >16 Ma ash flows that were erupted during

caldera-forming events associated with the McDermitt volcanic field. However, the Tp1 deposits have not been conclusively linked to any major McDermitt units. Also, no chemical data for the ~15.5 Ma Tuff of Hoppin Peaks exists in the literature, so comparing this unit with similar aged

Tp1 ash flows is not possible. In summary, Tp1 deposits across the SC are likely the remnants of more than one regionally derived peralkaline ash flow sheet(s).

Tertiary air-fall tuffs (Tp2) A subset of SC tephra was studied by Gilbert et al, (2003). Pure glass separates were analyzed to determine their major and trace element composition. These tuffs are rhyolitic and subalkaline. Gilbert et al. (2003) compared these tephra chemically to published analyses of regional mid-Miocene pyroclastic units and lava flows. They demonstrated that at least some of the tephra could have a local derivation, while others were chemically similar to material sourced in the McDermitt and Owyhee-Humboldt volcanic fields. Now with more SC data in hand it is apparent that links to other SC units can be made. For example, a tuff exposed in sedimentary strata directly underlying the western Tpr lava flow is chemically similar to Tpr lava flows. This tuff is also pumiceous and relatively thick, consistent with a nearby eruptive source. As previously discussed, tuffs exposed in the sparse sedimentary pile in the Goosey Lake depression near the Cold Springs caldera are chemically similar to regionally exposed tuffs (e.g. ~15 to ~16 Ma tephra; Perkins and Nash, 2002). A stratigraphically older tuff exposed within a sedimentary

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pile along the southern Goosey Lake depression is tentatively correlated with the Tcm rhyolitic lava flows exposed in the southern Calico range. Thus, our current state of understanding indicates that SC tephra represent the products of proximal and distal explosive eruptions.

Isotopic Variations of SC Units To further constrain the petrologic characteristics of the SC, a subset of samples was chosen for Sr, Nd, and Pb isotopic analyses (Table 3). This subset was chosen to represent the chemically diverse SC eruptive package and emphasis was placed on representing the previously defined petrologic units. These data are combined with Sr-Nd-Pb isotopic measurements of Mellot (1987) and W.K. Hart (unpublished data) to better represent the SC eruptive assemblage, although not all samples in this combined data base have a full suite of Sr-Nd-Pb isotopic values. 87 86 Also, in Table 3, initial Sr/ Sr ratios (Sri) are not reported for certain high Rb/Sr silicic units due to post-eruptive modification of this ratio due to hydrothermal alteration. The combined Sr-Nd isotopic characteristics of the units defined in this study are 87 86 depicted on Figure 20. Initial Sr/ Sr ratios (Sri) range from 0.70378 to 0.70764 and initial 143 144 Nd/ Nd ratios (Ndi) from 0.51241 to 0.51288 (Table 3). SC mafic units have Sri and Ndi values of 0.70378 to 0.70445 and 0.51263 to 0.51288, respectively, with Tba exhibiting higher

Sri and lower Ndi values than Tb (Fig. 20). SC intermediate units have Sri = 0.70440 to 0.70583

and Ndi = 0.51263 to 0.51273. Of the four intermediate composition petrologic groups, Tad1 has the highest Sri values; however, Tad1 has Ndi values similar to the other groups (Fig. 20). SC silicic units have Sri = 0.70479 to 0.70764 and Ndi = 0.51241 to 0.51275 (Fig. 20). Figures 21a and b illustrate these Sr-Nd variations vs. wt. % SiO2. These diagrams also illustrate that like some of the local granitoid basement (Granite Peak-Sawtooth group), some SC silicic units are characterized by Sri >0.706 (Tpr, Tbr, and Tcst). When considered together on Figure 21, SC mafic units and Ta form an array of

increasing SiO2 and Sri and decreasing Ndi to ~58 wt. %, ~0.7045 and ~0.51268, respectively.

With continued increasing SiO2, Ndi for the majority of the intermediate and silicic units remains

nearly constant. The Sri behavior is more complicated; most units define a relatively flat array as

with Ndi whereas Tad1 has Sri of ~0.7058 up to 62 wt.% SiO2. Lying off of the above defined

arrays are Tcst and Tbr/Tpr with lower Ndi and higher Sri values.

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Pb isotopic ratios also vary within and between SC units. Figure 23 depicts the measured 206Pb/204Pb, 207Pb/204Pb (Fig. 23a), and 208Pb/204Pb (Fig. 23b) ratios of SC units. Overall, 206Pb/204Pb ranges from 18.71 to 19.25, 207Pb/204Pb from 15.80 to 15.66, and 208Pb/204Pb from 38.54 to 39.10 (Fig. 23). Excluding the Tb and Tcst, the remaining units define a much tighter array in Pb isotope space (Fig. 23). Considerably greater overlap exists in Pb isotope space between the SC mafic and more evolved units than is noted in Sr-Nd isotope space (Fig. 20 and

23). On the other hand, some similar characteristics emerge; intermediate unit Tad1 contains the most radiogenic Pb isotope ratios while also standing out as different from most of the other SC units with respect to Nd-Sr isotope characteristics (Figs. 20 and 22). The two Tcst samples analyzed for Pb isotopes have nearly identical 206Pb/204Pb values (~18.7), but higher 208Pb/204Pb and similar 207Pb/204Pb relative to all other SC silicic units. Figures 23a and 23b illustrate the combined 206Pb/204Pb, 143Nd/144Nd, and 87Sr/86Sr characteristics of the SC units. A number of observations are apparent: [1] Tcst is isotopically distinct (Sr-Nd-Pb) from all other SC units; [2] Tpr (Nd-Pb) and Tbr (Sr-Nd-Pb) are isotopically similar and also appear to be distinct from all other SC units; [3] Tom and Tcm have similar Sr-

Nd-Pb characteristics but the other main eastern SC silicic unit, Tct, is distinct; [4] Tad1, Tad2, and Tad3 are isotopically distinct from each other and form “tight” groups when multiple samples are represented; [4] Tb and Tba are isotopically distinct. Figure 24 compares the initial isotopic characteristics of SC units with regionally erupted Steens Basalt (Carlson and Hart, 1987) and the known isotopic composition of local crust that may have interacted with or produced SC magmas (Kg and Tarc, calculated at 16.5 Ma; Mellot, 1987; Stuck, 1993). Tarc Sr and Nd isotopic ratios overlap with Tba, but define an array that extends to higher 87Sr/86Sr and lower 143Nd/144Nd. Fields are drawn to depict the Sr-Nd isotopic characteristics of the two main Cretaceous granitoid groups (~102 Ma Santa Rosa-Andorno and ~85 Ma Granite Peak-Sawtooth; Stuck, 1993). Also depicted are the Sr and Nd isotope characteristics of Cretaceous aplitic dikes that cross-cut the larger plutonic bodies. These dikes form an array extending to more radiogenic Sr at a relatively constant 143Nd/144Nd, similar to that which characterizes Santa Rosa-Andorno group granitoid. Two important observations can be made from this figure: [1] Tcst has Sr and Nd isotopic compositions similar to Granite Peak- Sawtooth group granitoid; [2] nearly all other analyzed SC intermediate and silicic samples fall within the field denoting the Sr-Nd isotopic values of Santa Rosa-Andorno group granitoid. The

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overall Pb isotopic composition of local granitoid is poorly constrained except for two samples from the Santa Rosa and Bloody Run plutons (Santa Rosa-Andorno group granitoid). Their measured Pb isotopic values are similar to SC silicic units (206Pb/204Pb = 19.016, 207Pb/204Pb = 15.626 to 15.634, and 208Pb/204Pb = 38.621 to 38.650; Wooden et al. 1999). The Pb isotope composition of SC units and Santa Rosa-Andorno group granitoid also overlap with regional Pb data obtained from Mesozoic and Tertiary plutonic and volcanic rock rocks exposed west of the Carlin mineral trend, located just east of the SC (Wooden et al. 1998). As mentioned previously, Figure 21 depicts the Sr and Nd isotopic characteristics of SC units as a function of silica content. It is evident that the isotopic compositions vary with silica, although they do not define any single, simple trend. These relationships preclude closed system crystal fractionation of a mafic parental magma(s) as the sole explanation for the geochemical and isotopic diversity of SC magmas. Moreover, the relationships depicted in figure 21 require a substantial role for crustal reservoirs in either the melting or upper level differentiation histories of SC magmatic systems, or both. Returning to figure 24 it is apparent that local Kg upper crust could represent such reservoirs. Direct evidence for this is observed in the form of Kg xenoliths in some SC lava flows, particularly those that appear to have erupted from vents in close spatial association with present-day exposures of the local granitoid basement. Summary of geochemical and isotopic relationships The following points summarize the pertinent geochemical and isotopic relationships and allow for a first-order discussion of the physical and petrologic evolution of the SC. To help illustrate these points, figure 25 depicts SC units normalized to an average Tba composition. Major and trace element variations suggest that geochemical heterogeneity is present within the Tba group therefore, chemical variations present in other SC units relative to a specific Tba composition could partially be a function of this heterogeneity. However, Tba provides the best estimate for the volumetrically dominant mafic component and endmember involved in SC petrogenesis, therefore an average Tba composition is used in this normalization. 1. SC mafic volcanism is characterized by two distinct tholeiitic groups: a less voluminous, more primitive, and chemically homogenous group that resembles regional HAOT (Tb) and the volumetrically dominant, more evolved Tba (Steens Basalt). The chemical difference between these groups is evident on figure 25a. Additionally, Tba has higher

Sri and lower Ndi as well as different Pb isotopic values (Fig. 23).

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2. SC intermediate volcanism is characterized by at least four distinct units. Ta lava flows

are the least evolved of these units and are dominantly tholeiitic, like Tad2 and Tad3.

Tad1 lava flows are more evolved than Ta and are the only dominantly calc-alkaline mafic to intermediate unit exposed in the SC. Relative to average Tba, these units all have similar major and trace element enrichments and depletions (Fig. 25b). Isotopic differences do exist between these units (Fig. 23) and these and other geochemical variations support field, physical, and chronologic observations that suggest these intermediate units represent products from different magmatic systems. 3. Overall, SC silicic units are metaluminous to peraluminous and unlike other regional volcanic systems, not peralkaline. Major element variations as well as variations in Ba, Sr, Ti, Eu and other HFSE and REE help distinguish between SC silicic units and likely reflect specific petrogenetic processes. Within group chemical variations are present in most units and likely reflect heterogeneous differentiation processes. Figure 25 clearly illustrates some of these within and between group variations. Isotopic differences also exist between units; however most units (except Tpr, Tbr, and Tcst) define a field in Sr- Nd-Pb isotopic space that is also shared by SC intermediate units (Fig. 23). The isotopic differences that are exhibited by Tpr, Tbr, and Tcst enforce the spatial, physical, and chronologic data which distinguish these units from other SC silicic activity.

Implications of SC Physical and Petrologic Characteristics The diverse physical and petrologic characteristics of SC units illustrates that a complex set of petrogenetic processes must have played a role in SC magma generation and evolution. Previous work directed toward understanding the petrogenesis of western SC units was performed by LeMasurier (1965; 1968) and Mellot (1987). LeMasurier (1965; 1968) invoked crystal fractionation from Tb-like basaltic parental magma to explain the chemical characteristics of Tarc, Tba, Ta, Tad1, Thc, Tpr, and Tbr. However, his specific Tb-like parental magma was later demonstrated to be <10 Ma, thus unrelated to SC magmatism (Hart et al. 1984). Mellot (1987) provided the first isotopic evidence that processes other than crystal fractionation were required and called upon open system magmatic processes to explain the isotopic heterogeneity. Additionally, Maloy et al. (2004) demonstrated that assimilation-fractional crystallization (AFC) could be a viable mechanism to explain the chemical and isotopic characteristics of Tad1. The

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physical, chemical, and isotopic data presented in this study from the entire SC support these previous interpretations and allow for additional details to be inferred. Prior to discussing the petrogenesis of SC silicic and intermediate units, it must be restated Tba appears to exhibit geochemical heterogeneity (Fig. 14a, b). Therefore, if differentiation from a more mafic liquid (Tba) is one of the important petrogenetic processes that acted in the SC to generate intermediate and silicic liquids, a variety of potential mafic endmembers likely were involved. Generation and evolution of SC intermediate units – first order models Four major intermediate composition units are present across the SC. Two of these are

time-transgressive (Ta and Tad1), erupting throughout the duration of SC volcanism. The other two, Tad2 and Tad3 likely representing two additional and separate magmatic systems, erupted at ~15.8 Ma and ~16.5 Ma and are spatially associated with coeval silicic products (Table 1). All four units exhibit disequilibrium mineralogies and textures, though as previously mentioned,

Tad1 is dominated by these features at a microscopic scale. These include xenoliths of Kg in

Tad1 and Tad3 and one observed xenolith of Trms in Tad1. Whole rock major and trace element data provide evidence that crystal fractionation occurred during the formation of SC magmas, particularly within individual units. Although the overall pattern of the arrays present on Harker diagrams (Figs. 14 and 18) are fairly linear, inflections are present within groups that likely reflect changes in the crystal fractionation process. For example, on plots of CaO, MgO, and Sc

vs. SiO2, there is an obvious inflection at ~62 wt. % within Tad1 which likely reflects the role of increased clinopyroxene fractionation. Sr and Al2O3 also decrease with increasing SiO2 for SC intermediate and mafic units, consistent with plagioclase fractionation. Also, the pronounced negative Eu, Sr, and Ba anomalies of many SC units as illustrated on Figs. 16, 19, and 25 are consistent with plagioclase and potassium feldspar fractionation. These patterns are best seen on Figure 25 where SC units are normalized relative to an average Tba. As silica content increases across the SC suite, the negative anomalies become more pronounced. However, the disequilibrium mineralogies and textures preserved in SC units and the overall trace element and isotopic diversity within and between units require a role for open-system processes. Figures 26 and 27 illustrate this for SC mafic and intermediate units. Figure 26 illustrates the variation of Zr with Ba and K2O/MgO for SC mafic and intermediate units and also depicts a compositional field for locally exposed Kg. If closed-system crystal fractionation of a more mafic parental

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magma was the primary petrogenetic process from which SC intermediate units were derived, HFSE (Zr) and LILE (K, Ba) concentrations should increase with increased fractionation as a continuation of the array defined by Tba (Fig. 26, labeled crystallization). However, the arrays exhibited by SC intermediate units differ considerably and exhibit substantial Ba and K2O/MgO variations over a narrow range in Zr, which cannot easily be explained by crystal fractionation but can be explained by open-system processes (e.g. magma mixing and/or assimilation- fractional crystallization). It is also evident that some Tba samples lie off of the mafic “crystallization array” and appear to lie along mixing lines with the intermediate units. This supports physical and isotopic evidence that at least some of the chemical variation present within Tba is a function of open-system processes. Furthermore, Tad1 lava flows form an array that heads toward (and intersects) the field of local granitoid, while Tad2 and Tad3 do not. This suggests that Tad1 may have interacted with local Kg upper crust, which is validated by the occurrence of Kg xenoliths in Tad1 (Fig. 7a). The chemical and isotopic variations illustrated on figure 27 substantiate the relationships depicted on figure 26a. Here, K/P and 87Sr/86Sr increase with increasing differentiation, consistent with the addition of heterogeneous evolved crustal material or magma to crystallizing mafic melt (AFC). This covariance (including increasing 87 86 SiO2 with Sr/ Sr) suggests that the “crystallization array” exhibited by SC mafic units on figure 26 may actually represent fractional crystallization plus assimilation of local crust. Additionally, there is a positive relationship between the increased proportion of xenolithic and 87 86 xenocrystic material in Tad1 and elevated K/P and Sr/ Sr. Moreover, Tad1 is the only calc- alkaline composition intermediate unit in the SC. Similar LILE enriched and HFSE depleted calc-alkaline magmas were also generated in extensional settings north of the SC (e.g. in the Miocene Powder River volcanic field in northeastern Oregon) where their genesis has been attributed to decompression melting of previously enriched (from prior subduction) lithospheric mantle (Hooper et al. 1995). Similar scenarios have been proposed for the generation of calc- alkaline magmas that erupted across the Basin and Range prior to the mid-Miocene (Gans et al. 1989; Leeman and Harry, 1993). Alternatively, the addition of Fe-poor material (local upper crust and/or silicic melts) can explain the lack of Fe-enrichment in Tad1. A similar explanation was proposed to explain the chemical characteristics of younger calc-alkaline intermediate lava flows that are associated with the Oregon-Idaho graben (Camp et al. 2003). The presence of granitoid xenoliths in Tad1, together with the geochemical and isotopic information illustrated on

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figures 26 and 27 strongly suggests that crustal assimilation and/or interaction with more evolved material played a role in the genesis of this unit and likely contributed to its calc-alkaline nature.

The increased K/P ratio of Tad1 relative to other intermediate units depicted on figure 27 and arrays present on figure 26 could therefore be a function of increased crustal interaction and bulk assimilation of Kg (± interaction with silicic melts). Granitoid xenoliths are also present in some

Tad3 lava flows, further illustrating that those SC units that erupted in close proximity to and/or through granitoid outcrops interacted with these bodies. However, this interaction may not have been sufficient to substantially change their bulk chemical signatures. Another potential open system process that could have acted during the generation of SC magmas is magma mixing. For example, the nearly vertical arrays exhibited by SC intermediate units (and to some extent Tba) on figure 26 are also consistent with magma mixing. Two locations where mixing may have occurred are the Calico Mountains and in the Staunton Ridge/Eightmile Mountain region (Figs. 2, 6). Here, intermediate and silicic lava/ash flows form thick packages consisting of numerous individual flows and flow units that all erupted nearly simultaneously (~16.5 Ma in the Calico pile and ~15.8 Ma in the Staunton Ridge/Eightmile region; Table 1). The physical characteristics of silicic units from both locations and their vent patterns also indicate that local extensional tectonism was occurring simultaneously (Brueseke et al. accepted). Similar associations of intermediate and silicic outflow and coeval faulting are present along the margins of and on the eastern Snake River Plain in Idaho (Leeman, 1982; Honjo and Leeman, 1987; McCurry et al. 1999) and at the Duck Butte eruptive center in southeastern Oregon (Johnson and Grunder, 2000). Furthermore, the onset of SC mafic activity and a period of heightened mafic volcanism (e.g. Chocolate Mountain and related intrusive bodies/lava flows) temporally coincide with the formation and eruption of these two intermediate systems. While physical evidence indicates that some Calico lava flows interacted with local

basement granitoid, Tad3 and Tcm lava/ash flows typically define linear trends on Harker diagrams consistent with magma mixing (Figs. 14 and 18). Similar trends and an overall lack of inflections that could reflect magma mixing are also present on Harker diagrams of Tad2 and Tem (Figs. 14 and 18). To test whether binary mixing is a viable process in the evolution of SC eruptive units from these two locations, a simple least squares binary mixing model was applied toward two crystal poor samples from both locations. Table 4 illustrates the results of these calculations. On

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the left side of the table, a Tcm ash flow (MB01-50) is modeled as a mix of 63.4% of a Tad3 (MB01-47) lava flow and 36.6% of a Tcm lava flow (MB00-32B). The lower portion of figures 28a and b illustrates the nearly identical elemental patterns of the calculated mix (thick solid line), relative to the observed characteristics (MB01-50) and parental liquids. Additionally, the modeled ash flow is extremely rheomorphic and has textural characteristics suggesting substantial open system behavior, including magma mixing/mingling (Fig. 7c). The right side of

table 4 shows the results of mixing a Tad2 lava flow with the most evolved Tem to generate less silicic Tem. Elemental similarities between the calculated mixture and the observed composition

are depicted in figures 28a and b. The modeled Tem lava flow (and nearly all other Tad2/Tem samples) lacks granitoid xenoliths and is characterized by less disequilibrium textures than other similarly evolved SC units. Like Tad3/Tcm, Tad2 and Tem lava flows appear to have erupted nearly coevally. These simple calculations illustrate that magma mixing is a feasible open system process that can explain the bulk chemistry of certain SC units. To summarize, the combined physical, major, trace, and isotopic characteristics of SC intermediate units are clearly consistent with their derivation through open-system magmatic processes. In the intermediate systems that were active during the entire duration of SC volcanism (e.g. Tad1 and likely Ta), fractional crystallization of less evolved magmas (Tba) combined with assimilation of local crust was likely the dominant magma differentiation 87 86 process. The relationship of increasing SiO2 with increasing Sr/ Sr and decreasing 143 144 Nd/ Nd (Fig. 21) is also mirrored by increases in K/P, consistent with interaction with K2O rich material such as the local upper crust or melts thereof. In the case of Tad1, continued and increased interaction with Fe-poor, calc-alkaline Kg may have been the primary contributor to its calc-alkaline composition. In contrast, magma mixing likely played a much more important role in the evolution of those intermediate systems that were active for only a short duration (Tad2 and Tad3). In Tad3, the presence of Kg xenoliths does indicate upper crustal interaction, however for both magmatic systems, the relative paucity of inclusions of Kg and Trms, the intimate field relationship with coeval silicic and mafic activity, the short duration of volcanism, and the nearly linear relationships on elemental plots suggest a greater role for magma mixing in their genesis.

Furthermore, the silicic eruptive products that Tad2 and Tad3 lava flows are either interbedded with or directly underlie (e.g. Tcm and Tem, respectively) appear to have erupted from primarily

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fissural or fissure-related sources. This highlights and documents another example of the important role of contemporaneous faulting as a catalyst for magma mixing processes (Johnson and Grunder, 2000). Generation of SC silicic magmas – first order models Silicic units in the SC are the products of diverse physical processes and multiple eruptive loci (Brueseke et al. accepted). Possible explanations for the generation of SC silicic magmas include prolonged fractionation of a more mafic parent coupled with crustal assimilation or mixing of more evolved material, anatectic melting of local crust, or mixing between chemically and isotopically heterogeneous mantle and crustal melts (Mellot, 1987). All of these processes may have occurred to some degree; however, the physical, chemical, and isotopic data reported in this study help shed light on whether one or more of these potential processes dominated the generation and evolution of SC silicic magmas. Recent work on silicic melt generation has illustrated the role that dehydration melting of local crust plays in the generation of silicic melts. Essentially, fluid-absent incongruent melting of hydrous mineral phases forms melt and residual minerals (Beard and Lofgren, 1991; Rapp et al. 1991; Wolf and Wyllie, 1994; Patiño Douce, 1995; Patiño Douce and Beard, 1995; Skjerlie and Johnston, 1996; Patiño Douce, 1999; Petcovic and Grunder, 2003). Most of these studies dealt with dehydration melting of amphibole or mica rich protoliths (typically amphibolite) that might occur at mid to lower crustal pressures (>10 kbar). In these studies, melt compositions vary by silica concentration, but are typically rhyolitic and the restite products are dominated by feldspar (plagioclase or potassium feldspar) ± clinopyroxene ± orthopyroxene ± oxide ± quartz ± garnet mineral assemblages. Pectovic and Grunder (2003) studied the in-situ relationship between shallow crustal melt generation (<1 kbar; 2 - 2.5 km depth) induced by the emplacement of Columbia River Basalt Group dikes into biotite-hornblende granitoid rocks in the Wallowa Mountains. Here, partially quenched mafic dikes that are compositionally similar to Tba have melted portions of the granitoid wallrock they intruded and generated melt within a restite composed of plagioclase + pyroxene + magnetite (Petcovic and Grunder, 2003). This scenario of mafic magma intruding granitoid upper crust is likely very similar to what occurred in the SC when upwelling Tba came in contact with Kg. The Wallowa melt compositions range from metaluminous to peraluminous and are K-rich (Petcovic and Grunder, 2003). Additionally, their observations indicate that as the degree of melting increased, the resulting melts became less

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evolved and less K-rich (Petcovic and Grunder, 2003). Furthermore, Petcovic and Grunder (2003) also note that these melt compositions resemble regional silicic volcanic units. The high silica, low degree melts resemble high silica rhyolite domes and ash flows exposed across central Oregon (Fig. 1a, the Oregon High Lava Plains), while the low silica melts produced by higher degrees of melting resemble the metaluminous rhyodacitic lava flows exposed at the ~10.4 Ma Duck Butte, OR eruptive center that were interpreted as crustal melts by Johnson and Grunder (2000). Petcovic and Grunder (2003) suggest that in the Wallowa Mountains, melt production only occurred at specific loci where mafic intrusion was long-lived and sustained. As a result, these results have important implications for the generation of SC silicic magmas and also illustrate that if silicic melts were produced in the SC through similar processes, granitoid upper crust and long-lived voluminous mafic magmatism were necessary local components. Partial melting of upper crustal granitoid to generate SC silicic magmas also implies that mafic upwelling was occurring throughout the periods that these silicic units were erupting. Furthermore, it suggests that a large volume of mafic magma must have been emplaced to shallow crustal levels beneath the SC to sustain silicic melt generation. Local evidence for shallow emplacement is present (e.g. the numerous Tba plugs in the vicinity of Hinkey Summit; Fig. 4g, h) and regional studies of Steens and Columbia River Basalt Group lava flows indicate that they resided in upper crustal magma chambers prior to eruption (Johnson et al. 1996; Durand and Sen, 2004). In this scenario, the paucity of voluminous local mafic volcanism would likely have been inhibited by the presence of less dense, intermediate and silicic melts in the subsurface. Similar scenarios are present in other volcanic fields dominated by silicic products (e.g. Yellowstone; Christiansen, 2001). As previously mentioned, silicic melts in the SC are peraluminous to metaluminous and

span a wide range in SiO2. The most peraluminous units are exposed in the western SC (Tbr and Tpr), while those present in the eastern SC (e.g. Tom, Tcm, and Tct) are dominantly metaluminous (Fig. 13c). Additionally, the Sr and Nd isotopic composition of most SC silicic units overlap with both Santa Rosa-Andorno group Kg (all but Tpr, Tbr, and Tcst) and Granite Peak-Sawtooth group Kg (Tcst) (Fig. 24). Also, Sr and Nd isotopic characteristics of Tbr and Tpr are similar to Granite Peak-Sawtooth group Kg as well as the Kg aplite dikes (Fig. 24). Although the Pb isotopic composition of Kg is poorly constrained, it appears that the Pb isotopic

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composition of Santa Rosa-Andorno group Kg resembles those SC silicic units that also have similar Sr and Nd isotopic compositions. Eastern SC silicic units erupted at ~16.5 to ~16.4 Ma, essentially coeval with the onset and most voluminous period of local and regional mafic flood basalt volcanism. In the western SC, Tem, Twsc, and Thc are physically and chemically distinct from Tpr and Tbr, and all erupted in close temporal association with a period of enhanced mafic activity. The least evolved compositions within these units are all high-K and metaluminous, and with the exception of Tom, dacitic to rhyodacitic; compositionally similar to the melts studied by Petcovic and Grunder (2003) that resulted from higher degrees of melting of Kg-like granitoid by CRBG magmas. Figure 29 depicts major element characteristics of SC silicic units and two (18 and 47 volume % melt) of the melt compositions studied by Petcovic and Grunder (2003). Evident in these diagrams is their overall similarity. These similarities coupled with the identical isotopic characteristics of these SC silicic units and locally exposed Santa Rosa-Andorno group Kg, suggest that crustal anatexis of Santa Rosa-Andorno group Kg was the primary magma generation process for these silicic units (i.e. Tcm, Tom, Tct, Thc, Twsc, and Tem). The more evolved, “A-type granite” like chemical characteristics of Tom may simply be a function of smaller degrees of partial melting of the same Santa Rosa-Andorno group or aplitic Kg sources. In this scenario, within unit chemical and minor isotopic variation reflects heterogeneous magmatic processes (e.g. assimilation upon ascent and eruption and/or magma mixing/mingling) affecting individual systems, or in the case of eastern SC activity, portions of a larger system (Brueseke et al. accepted). A similar process may have generated Tcst, Tpr and Tbr magmas, just with different protoliths(s). Initial Tcst volcanism occurred at ~15.8 Ma, soon after the heightened period of local mafic activity that is associated with Chocolate Mountain. Tcst is also isotopically similar to Granite Peak-Sawtooth group Kg. Combined, these data suggest that the generation of Tcst magmas by crustal anatexis of Granite Peak-Sawtooth group Kg is plausible. Similar processes may have led to the generation of Tpr and Tbr magmas. Their identical Pb - Nd isotopic characteristics differentiate them from all other SC silicic units (Figs. 22, 23). If Tbr and Tpr were generated through crustal anatexis, possible isotopically distinct protoliths could include local Trms as well as Kg aplitic bodies. To summarize, the bulk chemical compositions and isotopic characteristics of SC silicic units are consistent with their derivation through partial melting of local granitoid upper crust by

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upwelling mafic magma, likely followed by subsequent crystal fractionation and interaction with less evolved crust and liquids. As a first-order test of this hypothesis, a simple batch melting calculation was performed using the melt volumes (Stage 3, 18% and Stage 5, 47%) and restite assemblages reported by Petcovic and Grunder (2003) as proxies for the % melting and restite assemblage produced during the melting of locally exposed Kg. These values are used because they reflect the % melting that generated the quenched liquids studied by Petcovic and Grunder (2003) that are compositionally similar to SC silicic units (Fig. 29). Seven Kg bulk rock compositions were selected to reflect the compositional diversity of Kg for use in these calculations. Appropriate distribution coefficients were selected from the Geochemical Earth Reference Model (GERM) Kd database (http://earthref.org/GERM/index.html) when possible. Distribution coefficients were estimated based on similar elemental behavior for those elements that lacked distribution coefficient values in the GERM Kd database. The distribution coefficients and parental compositions used in the batch melting calculations are displayed in

Appendix 3. Figure 30 illustrates the variation in Zr, Ba, and K2O/MgO of the calculated melts, SC silicic units, and also depicts fields for the Kg samples used in the batch melting calculations.

Zr, Ba, and K2O/MgO values are depicted because the relationships between calculated melts, their Kg “parents”, SC silicic units, and the arrays exhibited by SC mafic and silicic units that suggest open-system behavior (Fig. 26) are readily observed by variations in these elements. This diagram illustrates that melting of Kg can produce liquids with elemental characteristics that overlap with those of certain SC silicic units. Different distribution coefficients and changes in the percent melting could explain some of the variability between calculated melts and SC silicic units. Additionally, post-melt crystal fractionation and assimilation/mixing with local crust and SC mafic and intermediate magmas could also account for some of this variability and the depicted arrays. Figure 31 utilizes the same information shown on figure 30 in order to provide a qualitative summary of the fundamental petrogenetic processes affecting SC units: 1) differentiation of Tba accompanied by melting of granitoid

upper crust (increasing Zr, Ba, and K2O/MgO), 2) interaction of differentiated mafic magma with Kg and silicic melts of Kg giving rise to the intermediate units, and 3) fractional crystallization of silicic melts (decreasing Ba, Zr, and increasing K2O/MgO). The arrows on the diagrams are only depicted to illustrate how specific processes (melting of Kg to generate SC silicic magmas; fractional crystallization of SC mafic magmas; mixing/assimilation of mafic magmas with Kg

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and/or SC silicic magmas; fractional crystallization of SC silicic magmas) can account for the

observed variations in Zr, Ba, and K2O/MgO. Their positions do not imply any specific evolutionary path. For example, fractional crystallization of zircon and feldspar would decrease the Zr and Ba concentrations in the silicic magmas and move the melt composition along a path in a similar direction as the arrow in figure 31d. While most SC units do not have pronounced negative Zr anomalies on multi-element plots (Fig. 19), zircon is observed as a trace phase in most units which suggests that this process occurred. The voluminous eruptions of Steens magmas across the Oregon Plateau and the volumetric dominance of local Steens volcanism (Tba) suggest that upwelling Steens, not upwelling HAOT-like magmas (Tb) induced local crustal melting and subsequent silicic melt generation. The detailed work of Pectovic and Grunder (2003) provides an outstanding example of how dehydration melting of biotite and hornblende bearing granitoid can produce silicic melts that are compositionally similar to those that erupted across the SC. Their work, combined with this study also helps to illustrate that similar upper-crustal melting processes were likely occurring across the Oregon Plateau during the mid-Miocene flood basalt event in locations where upwelling Steens-Columbia River Basalt Group magmas came in contact with the upper crustal remnants of major Mesozoic granitoid bodies. Similar processes likely occurred in the Owyhee Mountains where Steens Basalt eruptive loci and lava flows crop out in close proximity to coeval intermediate-silicic eruptive loci and lava flows, as well as Mesozoic granitoid (Panze, 1975; Halsor et al. 1988; Cupp, 1989). Presence of crystal clots – the smoking gun for open system behavior Microscopic xenoliths and crystal clots are ubiquitous in most SC intermediate and silicic units (Fig. 7). Some of the larger and coarser grained inclusions texturally resemble resorbed and partially melted Kg (Fig. 7a, b, c), while others are identical to Tba and Ta (Fig 7c). A third type is composed primarily of intergrown feldspar ± pyroxene ± oxide crystals that form crystal clots. Additionally, plagioclase phenocrysts in most intermediate and silicic units are typically resorbed and often much more sodic than groundmass plagioclase in the host, suggesting that these sodic plagioclase crystal are xenocrysts associated with the same original material as the associated crystal clots. As previously mentioned, plagioclase feldspar ± orthopyroxene ± clinopyroxene ± oxide restites were the products of the reactions studied by Petcovic and Grunder (2003). Similar restitic material has also been identified in other natural and

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experimental studies of melt generation (Beard and Lofgren, 1991; Rapp et al. 1991; Wolf and Wyllie, 1994; Patiño Douce and Beard, 1995; Knesel and Davidson, 1996; Skjerlie and Johnston, 1996). In most of these other studies, quartz, alkali feldspar, and at higher pressures, garnet, are also part of the restite assemblage. However, none of these minerals have been observed in SC clots. Inclusions and xenoliths that are texturally similar to the Tba/Ta-like mafic xenoliths and feldspar ± pyroxene ± oxide clots found in SC units are commonly reported in studies of silicic and intermediate volcanic systems and typically fall into two categories: [1] inclusions of less evolved magma (in some cases partially to fully disaggregated) entrained into a more silicic host prior to ascent and eruption (Bacon and Metz, 1984; Bacon, 1986; Clynne, 1989; 1999; Stimac et al. 1990; Tepley et al. 1999; Johnson and Grunder, 2000) and [2] inclusions of crust that were assimilated and partially disaggregated into the host magma (Grove et al. 1988; Green, 1994; Dungan and Davidson, 2004; Dungan, 2005; Beard et al. 2005). In the latter, inclusions vary from much older granitoid country rock (Grove et al. 1988; Beard et al. 2005) to subvolcanic intrusions related to earlier phases of volcanism at the same volcanic center (Dungan and Davidson, 2004; Dungan, 2005). Tepley et al. (1999) studied the Chaos Crags silicic lava domes of the Lassen volcanic center, to better understand the genesis of basaltic andesite inclusions that are present within the silicic outflow. They proposed a model that explains the origin of these inclusions as resulting from basaltic magma intruding into a rhyodacitic magma chamber. Subsequently, hybrid basaltic andesite inclusions formed along the basalt-rhyodacite boundary due to the entrainment of rhyodacitic magma into the basaltic magma and subsequent mingling (Tepley et al. 1999). The phenocryst assemblages of these inclusions are a mixture of crystals that originally formed in the mafic and silicic magmas (Tepley et al. 1999). Based on the Sr crystal isotope stratigraphy of plagioclase crystals in all three materials, Tepley et al. (1999) demonstrate that disaggregation of the intermediate inclusions physically recycled plagioclase crystals that originally formed in the silicic magma, back into the silicic magma. Similar processes of crystal formation, entrainment in a different magma, and subsequent liberation/disaggregation into the new (or old) host have been observed in Oligocene silicic ash flows from the Ethiopian Plateau (Barbey et al. 2005). The experimental studies of Huppert and Sparks (1988) also demonstrate that a similar process can occur during the partial melting of granitoid country rock by an intruding mafic sill.

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A zone of silicic partial melt forms between the sill and the country rock and continues to grow with more mafic magma emplacement (Huppert and Sparks, 1988). As this silicic body grows at the expense of the granitoid basement, refractory phenocrysts from the granitoid are entrained, partially to fully melted, and in some cases, reprecipitated in the silicic host (Huppert and Sparks, 1988). The end result of this process is the coexistence of granitoid restite mineralogies and igneous phenocrysts in the source region of an evolving silicic magma and the availability of these materials for incorporation into that magma prior to ascent and eruption (Huppert and Sparks, 1988). Crystal growth in an evolving silicic magma body followed by subsequent mixing with a different magma, restite generation through biotite ± amphibole dehydration of local granitoid crust followed by assimilation, disaggregation, and melting/recrystallization in an evolved liquid, and also possibly assimilation of the plutonic roots of locally erupted Tarc lava flows could all generate the feldspar ± pyroxene ± oxide crystal clots observed in SC silicic products. Detailed study of these crystal clots and their relationship with SC units and local crust should provide conclusive evidence for the specific type(s) of open-system process(s) that acted upon individual SC magmatic systems. However, their presence illustrates that SC eruptive units provide the physical record of a complex magmatic history. The physical, chemical, and isotopic data from this study provide first-order constraints on the petrogenesis of a complexly developed volcanic field and indicate that a combination of magmatic processes led to the generation of the SC magmas including: 1. Generation and ascent (± eruption) of mafic magma. This event is interpreted to be the consequence of regional, Steens-Columbia River flood basalt volcanism. 2. Anatectic melting of granitoid upper crust. Upper crustal ascent of Tb and Tba magmas initiated melting processes similar to those observed regionally (e.g. the Wallowa Mountains; Petcovic and Grunder, 2003). This process led to the generation of silicic liquids that were able to erupt, interact with crust and less evolved melts, and undergo further differentiation. 3. Interaction of Tba with more evolved crustal melts and local upper crust. This

interaction, dominated by magma mixing (Tad2 and Tad3) and assimilation-fractional

crystallization (Tad1), led to the generation of the temporally, spatially, and compositionally distinct intermediate volcanic units that crop out across the SC.

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The timing of SC volcanism and spatial distribution of known and inferred eruptive loci suggests an intimate relationship existed between melt production and coeval extensional tectonism. Extension would have stimulated magma-mixing processes and facilitated crustal ascent and eruption of SC magmas. With these petrogenetic constraints, the overall architecture and geologic development of the SC can be discussed and placed in a relevant context.

FORMATION AND EVOLUTION OF THE SANTA ROSA – CALICO VOLCANIC FIELD Chrono- and Chemostratigraphic Summary Comprehensive field observations and new 40Ar/39Ar geochronology allow for the reconstruction of SC eruptive events throughout the volcanic field. Figure 31 illustrates detailed stratigraphic relationships at ten different locations across the SC and provides a glimpse into the complex development of this volcanic field. The sections at these map locations are composited from numerous studied and measured sections exposed in the vicinity of each location. The locations and geographic descriptions of the numbered sections and the general geologic relationships are provided in figure 33, which consolidates information previously presented in Figs. 2 and 6. SC products overlie a thick package of older rocks at Hinkey Summit (section #1). Here, ~23 Ma Tarc lava flows are well exposed and are highly brecciated. These overlie Trms rocks dominated by platy weathering phyllites. Overlying Tarc lava flows is the package of SC units. Less than 5 m of interbedded tuffaceous sedimentary strata and tuffs is directly overlain by ~16.7 Ma Tba lava flows, with at least three flows present in this ~20 - 30 m thick package. Just to the east toward Granite Peak, Tb and Ta lava flows are present within this package and are interbedded with Tba lava flows. These are then overlain by another thin and discontinuous zone of tuffaceous strata (<5 m thick) that in turn is overlain by a dominantly silicic sequence that includes Ta and Tad1 lava flows. The thick cliff-forming units exposed at Hinkey Summit are ~16.4 Ma Thc lava flows, also part of this package. These flows dip away from Hinkey Summit to the east-northeast and are exposed along the southern SC margin toward Coal Pit Peak (see section #10) and along Forest Service Road 084 before it drops into the Goosey Lake depression.

Above these silicic lava flows is a thin package of Ta and Tad1 lava flows. These Tad1 lava flows thicken toward their inferred Coal Pit Peak source to the east. Younger Tpr and Tba

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intrusive bodies cut this package. The best examples of these intrusive bodies are the ~16.2 Ma Tpr hypabyssal body south of Hinkey Summit, the spectacularly jointed and arched Tpr dike that baked Tba lava flows and tuffaceous strata at Hinkey Summit, and ~16.0 Ma Chocolate Mountain, the best exposed mafic (Tba) shallow intrusive body present in the SC. Further to the northwest along the western SC margin is section #2. Here, similar relationships to those present at Hinkey Summit are encountered. Again, Tarc lava flows crop out and unconformably overlie Trms rocks. Within this Tarc package are basaltic and basaltic andesite lava flows, including the stratigraphically oldest flow encountered (the ~35 Ma basalt flow discussed previously). SC derived units are exposed above this almost ~200 m thick package of Tarc rocks. Like at Hinkey Summit, a thin (<5 m) zone of tuffaceous sedimentary strata and air-fall tuff lies directly below mafic lava flows. This package of dominantly mafic lava flows is much thicker than at Hinkey Summit (>120 m) and includes ~16.7 Ma Tb, Tba, and Ta lava flows. Also present within this package are tuffaceous strata that overlie the package of Ta lava flows. Associated with this sedimentary package are sinter deposits and minor mineralization that together occur in a less than 5 m-thick interval. Near the top of this section is a package of Twsc silicic lava flows that physically resemble Thc lava flows. These are exposed along the range-crest of the northern Santa Rosa range. However, this entire package of SC- derived material is cut by younger Tba dikes and the stratigraphically youngest unit here is a small and eroded ~14.4 Ma Tba vent, exposed near the topographic base of the section. Section #3 is located just north of Holloway Meadows at Quinn Pasture. Here, some of the youngest SC units crop out. In this area, Tem and Ta lava flows and Tcst ash flows are exposed in northwest trending fault blocks. Additionally, the distal lobe of a Tem lava flow is onlapped by a series of Ta lava flows and illustrates that some Ta lava flows in the SC are >15.8 Ma. The Tem lava flow thins toward the south and is interpreted to be the direct result of it flowing from sources near Eightmile Mountain down into the actively forming Goosey Lake Depression. Overlying these lava flows are outcrops of ~15.5 - 15.8 Ma Tcst that vary in thickness, but are thinner than Tcst exposures near Cold Springs Butte. Section #4 summarizes the relationships observed at the southeast end of Odell

Mountain. Here, ~255 m of ~16.5 Ma Tom lava and ash flows are overlain by ~ 5 - 10 m of Tp1 deposits. Most of the stratigraphic thickness present at this location is made up of rhyolite lava flows, some of which likely erupted from the eroded vent at the head of Klondike Canyon. The

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interbedded ash flows are chemically similar to the lava flows and may represent a pyroclastic

episode preceding lava flow eruption from the same vent. The Tp1 deposits in this region are the thickest in the SC and this observation and their presence suggests that they were either erupted from a nearby source or flowed into the SC via this region from a source outside of the SC (e.g.

McDermitt). Directly east of section #4, Tp1 deposits overlie lobes of Tem. Directly south, thick exposures of variably welded and, in some cases, lithophysae-rich Tcst deposits overlie Tem and

Tom lava/ash flows. Like locally exposed Tp1 ash flows, these Tcst deposits appear to have flowed into a local region of low topography. Section #5 is located along the eastern margin of the Calico Mountain range (and the SC). Here, along the base of the Calico range, highly jointed ~102 Ma Santa Rosa-Andorno group Kg is overlain by a ~400 m thick package of locally derived effusive and pyroclastic materials. Highly weathered outcrops of Kg are exposed at the base of the section, just east of Capitol Peak

(Fig. 2). Unconformably overlying this granitoid are ~16.5 Ma Tad2 and Tcm lava and ash

flows. In the lower portion of this section, Tad2 lava flows dominate the assemblage; however, further up-section Tcm ash flows become more prevalent, but are still interbedded with Tad2 lava flows. The Tcm ash flows present in this section are highly welded, rheomorphic, and lava-like. In the upper ~175 m of section, the Capitol Peak ash flow is overlain by a series of silicic units that include the capping ~16.5 Ma Tom lava flows exposed at Capitol Peak. Section #6 is located near the center of the SC, in the Goosey Lake depression along the North Fork of the Little Humboldt River. Here, the stratigraphic relationships that typify those present throughout the depression are well exposed. Throughout the depression, Tcst ash flows are found overlying intermediate lava flows. Here, ~80 m of Tcst overlies ~140 m of lava flows. This package of Ta lava flows is well exposed in the cliffs on both sides of the North Fork of the

Little Humboldt, and in some places, interlayered with Tad1 lava flows. Conformably overlying this sequence of lava flows are Tcst and Tp1 ash flows. Both ash flow exposures are

discontinuous and locally, either Tcst or Tp1 (or both), may be present directly overlying Ta lava flows. This relationship is common across the Goosey Lake depression where these ash flows are exposed. The most complete exposures of ~16.4 Ma Coyote Mountain tuff (Tct) crop out in the vicinity of their Coyote Mountain derived source at section #7. Here, Tct lava flows are well exposed and interfinger with locally derived Tct ash flows at the base of the assemblage.

226

Conformably overlying these lava and ash flows is the Coyote Mountain tuff and associated pyroclastic deposits. Tct pyroclastic materials include the block and ash deposits exposed in patches along the flanks of Coyote Mountain and the welded fall deposits that underlie the Coyote Mountain tuff. Overall, the composite thickness of these units varies, however, the Coyote Mountain tuff is ~35 m thick at the section location. Section #8 is located along the southeastern margin of the SC in the Hardscrabble Basin. Here, the thickest locally exposed package of mid-Miocene sedimentary strata crops out. This

sedimentary package was deposited on a surface of Tad1 lava flows and is over 100 m thick. Within the sedimentary pile, at least 14 distinct air-fall tuff horizons and two Tcst ash flows crop out. The Tcst ash flows are much thinner than those present to the northwest near Cold Springs Butte (<5 m thick) and likely represent a distal facies. They are also separated by ~80 m of section and illustrate that multiple eruptive events characterized Tcst volcanism. The air-fall

tuffs are all between ~1 and ~2 m thick, much thicker than other SC fall deposits (Tp2). The sedimentary strata that is interbedded between the volcanic horizons is similar to that present beneath the Cold Springs tuff near Cold Springs Butte. No apparent fluvial material crops out; instead, the entire sedimentary sequence appears to be comprised of tuffaceous, buff to salmon colored massive silts and silty clays. Physically, these resemble mid-Miocene lacustrine strata found across the southern Oregon Plateau and in the central SC (Wallace, 2003; this study). In places, occasional root casts are present within the sediment and suggest that the entire sedimentary sequence was not solely the result of lacustrine depositional processes. The limited areal extent and overall physical characteristics of the sedimentary strata as well as the interbedded fall deposits and ash flows, suggest that these materials were deposited in an actively forming basin that was a continuous topographic low during SC volcanism. Section #9 is located along the southern boundary of the SC between Black Dome and

Coal Pit Peak. Here, at least 330 m of Tad1 lava flows are well exposed. This flow on flow stratigraphy is best exposed in the Martin Creek canyon, where reconnaissance sampling up- section toward Coal Pit Peak (and down-section, from Coal Pit Peak) indicates that these lava

flows are all Tad1. No attempt was made to document the exact number of lava flows that crop out here, but it is estimated that there are at least 40 flows present. This large number of

compositionally similar lava flows is the thickest continuous package of Tad1 lava flows present in the SC and provides evidence that the Tad1 source, likely a shield volcano, was nearby in the

227

vicinity of Coal Pit Peak. Further from this location, the overall thickness of the Tad1 package decreases and Tad1 lava flows are often found interbedded with other SC units. Within the package, the basal lava flows exposed near Martin Creek are andesitic, while the overlying lava flows are broadly dacitic. Section #10 is located along the southern SC boundary, between Hinkey Summit and Coal Pit Peak. More complex stratigraphic relationships are present at this locality than those exposed at section #9. Here, Tarc and SC derived eruptive products were emplaced over Kg. This relationship, like elsewhere in the SC, implies that at least local exhumation and uplift of these Cretaceous plutons occurred prior to initial Tarc volcanism (~35 Ma). Overlying the Kg is a package of Tarc lava flows that vary in thickness along the southern SC margin. In the vicinity of section #10, Tba lava flows crop out directly above the Tarc package, similar to the situation at Hinkey Summit and north of Granite Peak (Wes LeMasurier, Pers. Comm.). However, at section #10, >100 m of ~16.4 Ma Thc lava flows overlie Tarc lava flows. Above this silicic package is ~150 m of Tad1 lava flows. This Tad1 package thins along the southern margin,

toward Hinkey Summit. Both the Thc and some of the Tad1 lava flows are cut by north- northwest trending ~16.2 Ma Tpr dikes (Fig. 6b) and hypabyssal bodies, some of which are also exposed near the top of the section (topographically). This discussion illustrates one key point that is essential to understanding the eruptive history of the SC: prior to and during SC volcanism, regions of high topography were locally present. The variable pre-SC topography likely had a profound influence on the areal distribution and subsequent outcrop pattern of SC volcanic units. Pre-existing paleovalleys formed topographic lows that helped confine SC eruptive units. During SC activity, the eruption of units from multiple vents and on-going faulting also contributed to a dynamic environment of changing topography. SC pyroclastic units were highly influenced by these variations. These paleotopographic variations have contributed to the complexity of the volcanic field stratigraphy by making simple topographic-based stratigraphic correlations nearly impossible. Coupled with the physically and chemically variable SC eruptive package, the end result of SC volcanism is a complexly developed volcanic field with a wide array of eruptive products.

228

Eruptive History Summary To reconstruct the active SC volcanic field, it is necessary to summarize the most important physical observations: [1] SC eruptive loci are numerous and concentrated along the margins of the volcanic field; [2] combined field, physical, temporal, and chemical characteristics of SC units suggest that multiple magmatic systems dominated SC volcanism, rather than one (or a few) large magmatic system; [3] substantial intermediate composition suites were erupted during SC activity and appear to result from multiple open system processes; [4] SC eruptive loci are characterized by fissure-sources, domes, and at least one caldera; [5] unlike other regional mid-Miocene volcanic fields, no regional and areally extensive eruptive products appear to be locally derived. Figure 34 illustrates these observations and relationships and provides a summary of Santa Rosa-Calico volcanic activity through time. Figure 35 illustrates the compositional variation of SC eruptive products through the same time slices depicted in figure 34. Additionally, figure 36 is a cartoon that provides a cross-sectional view illustrating how magma generation and eruptive processes may have occurred across the SC in the subsurface. Pre-SC (Figs. 34a, 35a) Regional <11 Ma and younger Basin and Range block faulting and volcanism has made reconstructing the mid-Miocene paleogeography of the southern Oregon Plateau difficult. The observed stratigraphic relationships present in the SC provide first-order constraints on the local pre-SC paleogeography. The unconformities between SC and pre-SC units which exist primarily along the southern, southeastern, and eastern SC margins require the presence of variable pre-SC topography. The presence of Tarc lava flows as old as 35 Ma directly overlaying Triassic metasedimentary rocks (Fig. 32, Sections #1, 2, 10) also illustrates that this region was characterized by high topography prior to the onset of SC volcanism. Documented Tarc lava flows and probable near-vent facies also illustrate that at least some of these lava flows were derived locally from loci near Hinkey Summit, as illustrated in figure 34a. Other regions of high topography that existed prior to the onset of SC volcanism include the Calico Mountains, where

Tad2 lava flows locally overlie Cretaceous granitoid and near Black Dome. ~16.7 to 16.3 Ma, initial SC volcanism (Figs. 34b, 35b, 36a) At ~16.7 Ma, SC volcanism initiated. The oldest eruptive products are the Tb and Tba lava flows exposed in the vicinity of Hinkey Summit that overlie Tarc lava flows. Here and to

229

the north past Buckskin Mountain, Tba lava flows erupted from north-northwest trending fissural sources (Fig. 6b) coeval with the most voluminous pulse of regional Steens-Columbia River flood basalt volcanism. Just north of Granite Peak, at least one small Tb eruptive loci and flow field erupted and formed. Silicic volcanism in this portion of the SC initiated at ~16.4 Ma and led to the emplacement of Thc lava flows from a source(s) along the southern SC margin. Discrete silicic eruptive loci also were present along the northwestern margin of the SC, in the vicinity of Buckskin Mountain and the National Mining district (Vikre, 1985a, b). Almost simultaneously, volcanism initiated along the northeastern margin of the SC. Here, Tom and Tcm rhyolite domes formed and erupted along with pyroclastic flows along broadly north-south

trending fissural systems parallel to the eastern SC margin. Tad2 and Tba lava flows also erupted locally in the Calico Mountains. Further south, Coyote Mountain, Zymns Butte and Black Dome were active and their eruptive products flowed over a surface floored by slightly older intermediate composition (Tct and Tad1) lava flows. During this time period, the Tad1 magmatic system initiated in the greater Coal Pit Peak region. Sedimentary deposition in the Goosey Lake depression also was occurring prior to ~16.2 Ma, however the areal extent and maximum age of sedimentary strata is unknown. ~16.2 to 16 Ma (Figs. 34c, 35c, 36b) By ~16.2 Ma, SC volcanism was focused in the southwestern portion of the volcanic field and volcanic activity in the eastern SC had ceased. At ~16.2 Ma, Tpr magmas were emplaced and locally erupted from discrete loci along the southern and western SC margin. Tpr lava flows flowed down into the Goosey Lake depression, where continued sedimentary deposition was occurring. Tba volcanism continued in the greater Hinkey Summit region, Tad1 lava flows continued to be erupted from their source near Coal Pit Peak and Ta lava flows continued to erupt from unknown loci (likely fissural). ~16 to 14 Ma (Figs. 34d, 35d, 36c) During the last two million years of SC volcanism, activity was mainly concentrated in a northwest trending zone, broadly cutting through the center of the Goosey Lake depression.

Sporadic Tba volcanism also continued along the western SC margin. At ~15.8 Ma, Tad2 lava flows erupted from a source near Staunton Ridge. Shortly following the eruption of these intermediate composition lava flows, Tem lava flows erupted from fissures at and just east of Eightmile Mountain. Due to post-SC erosion, the pre-erosion areal extent of Tem lava flows is

230

hard to discern, but may have been similar to that depicted in Figure 34d. Tem lava flows did flow to the south and east into the Goosey Lake depression, where they were onlapped by Ta lava flows. At ~15.8 to ~15.5 Ma, the Cold Springs tuff magmatic system was active and Tcst ash flows erupted from the small caldera present in the eastern Goosey Lake depression. Tcst ash flows flowed over earlier SC products and also into the sedimentary depocenters present in and peripheral to the Goosey Lake depression (e.g. the Hardscrabble Basin). Additional Tad1 volcanism continued to ~14 Ma, when SC volcanic activity ceased. During this waning stage of SC activity, Swisher Mountain-tuff equivalent ash flows erupted from unknown regional sources and onlapped eastern SC volcanic units (Fig. 6b). Additional <16 Ma silicic activity was also present outside of the SC along its southeastern margin (Alan Wallace, Pers. Comm.).

SUMMARY AND CONCLUSIONS This study provides a detailed examination of the physical evolution of a complex continental volcanic field, the Santa Rosa – Calico volcanic field, and provides first-order information on the nature of the magmatic processes responsible for the physical, geochemical, and isotopic diversity documented by the products of this volcanic field. In the SC, numerous mafic through intermediate eruptive loci and shallow intrusive bodies crop out along north-south and northwest-southeast trending alignments, coincident with northward projection of the northern Nevada rift (Figs. 1, 6). Across the volcanic field, at least sixteen physically and compositionally distinct units are present and represent the products of over ~2 Ma of volcanism. Local mafic units physically and chemically resemble regional Steens Basalt and HAOT and erupted over the duration of SC volcanism. SC andesite to dacite lava flows are found throughout the volcanic field and represent the products of at least four temporally, geographically, and chemically distinct magmatic systems. Complex open-system magma generation and evolution processes including both magma mixing and assimilation-fractional crystallization gave rise to these intermediate units. Contemporaneous faulting likely stimulated these processes via further emplacing mafic magma into the upper crust and also by providing a mechanism that could drive magma mixing. SC silicic products vary physically, chemically, and isotopically according to geographic location and again, require the presence of numerous magmatic systems throughout the >2 Ma duration of the SC. The physical, chemical, and isotopic characteristics of SC silicic units are also consistent with their derivation via crustal

231 anatexis of local Kg upper crust. Similar processes of mafic magma upper crustal intrusion and coeval silicic melt generation appear to have occurred in other regions across the Pacific Northwest during the mid-Miocene flood basalt event. Caldera-derived eruptive products are present in the SC (e.g. the Cold Springs tuff); however voluminous caldera-forming events did not typify SC activity. Rather, a paucity of caldera-forming silicic volcanism distinguishes the SC from coeval mid-Miocene Oregon Plateau volcanic fields (e.g. McDermitt) as well as other silicic-dominated volcanic fields that are typically associated with the younger Snake River Plain-Yellowstone volcanic province. The styles of volcanism and compositional spectrum of products that characterize the SC provide outstanding examples of the diversity that can be present within intracontinental volcanic fields. Furthermore, the SC provides an outstanding opportunity to study the generation of intermediate and silicic magmas in a continental setting, as well as the interaction between multiple magma and crustal types. Because of its intimate link to the regional mid-Miocene volcanotectonic history of the Pacific Northwest (U.S.A.), the SC is an example of how regional mafic magmatism and ongoing tectonism interacted and its geologic history is a model for the development of physically and compositionally similar volcanic systems. Also, the results of this study provide a comprehensive framework that will help future workers better understand the relationship between local and regional precious metal mineralization and mid-Miocene volcanism. In summary, the diverse magmatic products and styles of volcanism that define the SC are interpreted to result from the interplay between regional flood basalt volcanism, focused mid-Miocene extension, and diverse lithospheric processes and assemblages. Accordingly, this study provides a direct link between regional lithospheric extension, Steens-Columbia River flood basalt volcanism, and the development of coeval Oregon Plateau volcanic fields.

232

Figure 1. (a) Shaded relief map of the northwestern United States depicting select Cenozoic tectonomagmatic features. Green shading depicts the region where mid-Miocene flood basalt lava flows crop out (after Hart and Carlson 1985; Camp and Ross 2004). The major flood basalt dike swarms/eruptive loci are depicted as red lines, the Oregon-Idaho graben and magnetic anomalies corresponding to zones of lithospheric extension/mafic magma emplacement are depicted as black lines (Cummings et al. 2000; Glen and Ponce 2002). The SC is labeled and surrounded by a black box. The purple ovals are the silicic-dominated volcanic systems of the Snake River Plain-Yellowstone province; BJ, Bruneau-Jarbidge (~12.5 - <11 Ma); TF, Twin Falls (~10 - 8.6 Ma); PC, Picabo (~10 Ma); HS, Heise (~6.7 - 4.3 Ma); and YS, Yellowstone (<2.5 Ma), and the purple lines are the age isochrons associated with Oregon High Lava Plains silicic activity (N, Newberry Volcano; after Jordan et al. 2004). The initial 87Sr/86Sr 0.706 and 0.704 isopleths are depicted as dashed blue lines (after Armstrong et al. 1977; Kistler and Peterman 1978; Leeman et al. 1992; Crafford and Grauch 2002). These are commonly interpreted to define the western edge of the Precambrian North American craton (the 0.706 isopleth) and a zone of transitional lithosphere between the older craton and Mesozoic accreted terranes to the west (between the two isopleths). (b) Shaded relief map of the southwestern Oregon Plateau depicting mid-Miocene silicic volcanic systems in close proximity to the Santa Rosa-Calico volcanic field (SC). MD, McDermitt; LO, Lake Owyhee; and NWNV, Northwest Nevada (e.g. Virgin Valley, High Rock, Hog Ranch, and unnamed calderas) volcanic fields are depicted in purple. Blue circles are less voluminous mid-Miocene silicic centers and include the HVLM, Hawks Valley-Lone Mountain dome complex; SI, Silver City-Delamar dome complex; JM, Juniper Mountain volcanic center; SS, Snowstorm Mountains dome complex; J, Jarbidge. Unidentified circles are other mid-Miocene silicic centers. Other abbreviations are as follows: HLP, High Lava Plains; SM, Steens Mountain; OIG, Oregon-Idaho graben; WSRP, western Snake River Plain; OP, Owyhee Plateau; NNR, northern Nevada rift and related lineaments. Other Mid-Miocene extensional features from figure 1a are depicted as black lines.

233 o o

o

44 40

ID

114

J

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SS

JM

SI

SC

NNR

OP

OIG

LO

MD NWNV HLP

HVLM

o

B

NV

OR

120

YS

o

WY

111

A

HS

6.7-4.3

MT

1010

PC

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o

UT

114

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NV

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12.5-<1 12.5-<11

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117

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10

0.706 line

Sr isotope

CRB Dikes

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123

CA

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46 42 Figure 1 Figure 2. Satellite image and map of the Santa Rosa-Calico volcanic field depicting physical landmarks. The left is a Landsat 7 image rendered so bands 7, 4, and 3 are red, green, and blue colors. Orange dashed line is approximate boundary of the SC; SC outflow extends past this line in all directions. Dashed yellow lines on the image are major roads, which are also depicted and numbered on the map to the right of the image. Yellow symbols that resemble hills are identical to those in black depicted in the map and denote topographic highs (named on the map). The rugged SC topography is extremely visible on this image and the large bright red patch near Odell Mountain is a recent burn area. The black and white map on the right depicts SC topographic high-points, rivers/streams, and geographic features. The boxed numbers denote the location and number of the stratigraphic sections depicted in Figure 31.

235

Legend

Topographic High Benchmark Elevation (m) Mining District State Road Marker Forest Service River or Light Duty Detailed Stratigraphic Road Marker Creek Road, Dirt Section 4

3

792

y

2966

Ì

E

FS 531

y

River

1817

1990

River

Zymns Butte

E

2102

1706

km

Capitol Peak Mountains

y

y Owyhee 5

5km 5

E

Mahogany Calico Little Pass

y

Humboldt 2167

2394

2549

E

River Hardscrabble 8

Coyote Mountain Quinn Goosey Lake y y Basin Flat 7

1558

1792 4 Fork

2222

.Fork E. E.

Eastern Goosey Lake Depression

E

Mountain Little Ridge Black

y lnieCanyon Klondike

Black 2134 Martin Crk. Odell

2460 9

Spring City

2165

1859 S531 FS 2082

E

y

2228 6

E

2307

y

Ì

y 3

E

2152

Coal Pit Peak

y

N.Fork

Cold Springs Butte

Western Goosey Lake Depression

E

Quinn Pasture

Trouble 10

2372

.

Staunton Ridge Crk. Mountain FS 084

Santa

NV.

792

2296

Hinkey Summit

2607 E Range Rosa

Ì

Cabin

the

Paradise Valley

2400

E Field, NV Crk. Indian

E

of the Santa

Buckskin -National

Windy Gap

y 1

E

Eightmile 1615

2665 otenSanta Northern Crk.

Ì 2

Volcanic Field,

Crk. y

McConnell Peak

Features

Buckskin Mountain

National

o Volcanic

2666

Crk. E

Granite Peak

2966 Solid Silver Crk. Eightmile Crk.

Crk.

Threemile Canyon Crk.

Flat Crk. N

1433

Rosa-Calic Geographic Features of Rosa-Calico

y

E

km

5km 5

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7 Image

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.

NV.

Rosa-Calico

Landsat 7

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Santa

the

N

RenderedRendered Landsat of the Santa Rosa-Calic Volcanic of Volcanic Field, Figure 2 Figure 3. Generalized stratigraphy of the SC. Ages for Cretaceous granitoids are from Wyld et al. (2001) and Stuck (1993). All other ages are based on the new 40Ar/39Ar geochronology presented in this study and are calculated to a 28.02 Ma age for the FC-2 Fish Canyon Tuff sanidine interlaboratory standard.

237

Western SC- Greater Hinkey Summit Region 16.0 Ma 14.3 Ma Eastern SC- Greater Central SC- Goosey Lake Depression Calico Mountains Region 16.2Ma

13.9 Ma 15.5 Ma 16.5 Ma 16.4 Ma

14.9 to 15.8 Ma VVVVVVV 16.7Ma VVVVVVVV 16.5 Ma 22.5 Ma VVVVVVVVVVV SC Derived 23.5 Ma VVVVVV >16Ma 16.5 Ma 35.4 Ma

Pre-SC

Fluvial-lacustrine deposits ~19-24 Ma calc-alkaline VVV with interbedded tuffs andesite assemblage Rhyolite ~43-28 Ma basalt to Porphyritic rhyolite rhyolite assemblage Ash flow ~85 Ma Granite peak group granitoids Rhyodacite ~102 Ma Santa Rosa Dacite

Pre-Santa Rosa -Calico units

Santa Rosa-Calico group granitoids derived units Andesite Norian (Late Triassic) Basalt & Metasediments Basaltic andesite

238 Figure 3 Figure 4. Photographs of SC units and geologic features (a) View toward Granite Peak up Solid Silver Creek. Well exposed dipping beds in photograph are SC lava flows (mostly Thc), identical to those exposed at Hinkey Summit and dropped down to the south along the normal fault that bounds the northern edge of Paradise Valley. Poorly exposed dipping strata below cliffs include Tarc lava flows that underlie SC-derived units. Typical exposure of Trms is labeled. (b) Looking southwest toward the main Santa Rosa range. High peaks (R-L) are Santa Rosa Peak (2957 m), Paradise Peak (2878 m), and Singas Peak (2870 m). Low hills in the foreground are Trms. (c) ~102 Ma granitoid exposed along the eastern base of the Calico Mountains. Owyhee Plateau is in the background. Inset shows Tcm lava flow overlying Kg looking to the left (west) of the outcrop depicted in the main picture. (d) Ramp structures and platy jointing in Tarc lava flow exposed just west of Hinkey Summit. (e) Thick package of lava flows exposed along the western SC margin near Buckskin Mountain. This package includes regionally erupted Steens Basalt lava flows, SC-derived units, and possibly Tarc lava flows. (f) Looking to the northeast at Hinkey Summit. Mafic lava flow in the foreground is a basal-SC Tba lava flow. Lava flows exposed at Hinkey Summit are the same as those shown in (a) and are mostly Thc. Spectacular Tpr shallow intrusive bodies are present just above and to the right of the road (diagonal scar visible below Hinkey Summit). (g) View from Forest Service road 084 past the Lye-Creek turn-off looking to the southwest. Granite Peak is the light-colored mountain on the skyline, Chocolate Mountain is the Tba plug in the center of the picture, and a north trending Tpr dike is in the foreground (light colored). Most of the orange colored rugged hills throughout the picture are Thc lava flows. Near the summit of Chocolate Mountain, Tarc lava flows crop out. (h) Looking toward Buckskin Mountain from the Tpr dike exposed at Hinkey Summit. Chocolate Mountain is in the center of the picture and Buckskin Mountain is the high point along the skyline. The hills between Chocolate and Buckskin Mountains are primarily Tpr lava flows and flow-dome complexes. The rubbly outcrop to the right of the dike is a Tba lava flow. (i) A distal lobe of a Tem lava flow exposed just north of Holloway Meadows. Onlapping this lobe is a package of at least three Ta lava flows. (j) A package of Ta and Tad1 lava flows overlying Black Dome outflow on Black Ridge. This picture was taken below the summit of Black, looking toward the northwest along Black Ridge. (k) View from Hinkey Summit looking to the southeast across Paradise Valley. All of the vertical, typically light colored rock is part of the massive Tpr shallow intrusive body exposed just southeast of Hinkey Summit. The Hinkey

239

Summit Tpr dike (not pictured is its distinguishing arch) is the massive body along the left side of the picture. Tarc lava flows crop out in the small hill at the center of the picture (surrounded by Tpr intrusives). (l) View toward Buckskin Mountain from a high point near the Lye Creek turn-off. The large hill in the center of the picture is the middle portion of the western Tpr flow- dome complex and its flow margin is exposed near the right side of the picture. Here, this lava flow overlies mid-Miocene Goosey Lake depression sedimentary strata. (m) Looking west toward the high-point of Eightmile Mountain, from Eightmile Mountain. Here, a glacially

eroded Tem lava flow (highly weathered reddish talus slopes and summit) overlies Tad2 lava flows (bedded units). The Quinn River valley is in the background. (n) An eroded Tom rhyolite dome exposed at the mouth of Klondike Canyon. Highly oxidized (red) Tom dikes are exposed in the foreground at the center and right side of the picture. Across the canyon, vertical to sub- horizontal banding/dikes are present. The up arrow is in the center of the conduit (vertical dikes). Directly behind where this picture was taken are near-vent carapace deposits. (o) View looking north toward the Hill 7502 eroded rhyolite dome in the Calico Mountains. This locus was a source for Mahogany Pass-type Tcm rhyolite lava flows. The circular outcrop of vitrophyric rhyolite present in the center of this locus is evident. These features combined with abundant 15 - 20 cm clasts to boulders of pumice help define this feature as an eroded dome. (p) Typical weathering pattern of SC silicic lava flow. Notice the platy jointing, which is also present in Tarc lava flows and can make distinguishing Tarc lava flows from SC units difficult. This outcrop is a portion of the Black Dome lava flow (Tcm) on Black Ridge.

240

Granite Peak

Thc Tarc

Trms

A B

Tcm

Owyhee Plateau Kg

C D

Hinkey Summit

Thc

Tpr Tba

E F

Granite Peak Buckskin Mountain Chocolate Mountain (Tba) Tpr

Thc

Tpr G H Tba

241 Figure 4 Tem Ta

Ta &Tad1

Tct (Black Dome outflow) I J

Tpr flow-dome

K L

Tem

Tad M 2 N

O P

242 Figure 4 cont... Figure 5. (a) Total alkalies vs. silica volcanic rock classification of LeBas et al. (1986). Notice the complete compositional spectrum from basalt through rhyolite, defined by SC units. B, basalt; TB, trachybasalt; BA, basaltic andesite; BTA, basaltic trachyandesite; A, andesite; TA, trachyandesite; D, dacite; TD, trachydacite; RD, rhyodacite; R, rhyolite. (b) Plot of K2O vs SiO2 (wt. %) illustrating andesite types of Gill (1981). Symbol key shown below is used for all other chemical plots reported in this study.

243

7 KO 10 n=286 6 2 TD 8 TA 5 BTA 4 6 R TB ACID D 3 BASIC 22 4 High-K

Na O+K O A RD BA 2 2 B Medium-K A 1 0 Low-K B 45 50 55 60 65 70 75 0 46 50 54 58 62 66 70 74 78 SiO2

SiO2

Tad1 Tad2 Tad3 Tem Tbr Tct Tcst Tp1 Tb

Tba Ta Twsc Thc Tpr Tcm Tom Tp2

Figure 5

244 Figure 6. (a) Black and white Landsat 7 image of the Santa Rosa-Calico volcanic field. Geographic features within the SC are: CP, Capitol Peak; CPP, Coal-Pit Peak; HS, Hinkey Summit; GLD, Goosey Lake depression; BM, Buckskin Mountain; EM, Eightmile Mountain; OM, Odell Mountain; CM, Calico Mountains; HB, Hardscrabble Basin. Features peripheral to the SC are: PV, Paradise valley; MSR, main Santa Rosa range; QR, Quinn river valley; OP, Owyhee Plateau. Northward projection of the northern Nevada rift into the SC is shown by black dashed lines and the dashed white box depicts the region in figure 2b. (b) General geology of the SC (after LeMasurier 1965; 1968; Vikre 1985b, Mellott 1987; King 1984; this study). SC dike orientations depicted on the rose diagram are from LeMasurier (1965, 1968), Vikre (1985b), Mellot (1987), and this study. Symbols are depicted in the legend and a generalized stratigraphy is provided (a). Major mid-Miocene epithermal mining districts from south to north are Spring City, Buckskin-National, and National and geographic features not present on (a) are: GP, Granite Peak; ZB, Zymns Butte; BD, Black Dome. A detailed key to map units is provided.

245

Tsmt

Qsed

ZB

Kg

Tcm Tcm Mountains

Tct

SC dike orientation Calico n=62

Lava flow direction

CT

Qsed

Tgl

Tsed

Qsed

OM

GLD

Tgl

BD

Tom

Thc

Thc

Silicic/mafic dike zone

Forest Service Road Marker

Thc FS 531

CPP

State Road Marker

Mafic & Intermediate vent/vent region

Major mine

FS 531

792

Ì

Tem

GLD

Tgl

uff

Kg Twsc FS 084

Tpr

Twsc

Tsed

Thc EM

Qsed

Tem

Ì

HS

Tsi

792

Possible caldera for Cold Springs T

Major normal fault

Tpr

Improved Road

BM

Silicic vent/vent region

Thc

Ì

Tsi

Tsb

Thc Tarc

Tsb

Tsi Tarc

Kg

GP

5km FS 084

Twsc

B

Twsc

Trms

N

Qsed

A

SC

10 km

N

Rift

Kg

Tct

Tpr

Tsb

Tem Tom

Tarc

Tsmt

Trms

Qsed

CP

Nevada

Northern

CT

HB

OM

Tcm

CPP

PV

Tgl

Thc

Twsc

EM

HS

BM

Tsi

Tsed

Tmi

QR

MSR

Tsb

Generalized Stratigraphy

NV

OR Figure 6 Tbr

’Neill,

alley Formations.

Tp1 interbeds(Miocene).

Andorno, Singas, O

Generalized Stratigraphy

Tb, Tba, Ta, Thc, Tad1, Tpr, Tp1, and lava flows, shallow intrusive bodies, and Tct ash flows, ash flows, and pyroclastic Tcm, Tad3, Ta, and Tba lava & ash flows, Tom and Tp1 ash flows, lava flows, and Regionally and locally derived Steens Basalt Tarc lava flows and potential near-vent deposits (Miocene). eruptive loci, and eruptive loci (Miocene) (Tba; Miocene). pyroclastic deposits (late Eocene to Granitoid (Cretaceous) - ~102 Ma Santa eruptive loci (Miocene). Triassic metasediments (Norian) - undivided Miocene). Rosa/Andorno and ~85 Ma Granite Peak/Sawtooth plutonic bodies. Mullinix, Winnemucca, and Grass V

SC

Kg

Tct

Tsb

Thc

Tcm

Tom

Tarc

Trms

Kg

Tct

Tpr

Tsb

Tem Tom

Tarc

Tsmt

Trms

Qsed

Tcm

Tpr

Tp1 lava

Tp1 lava flows,

Tgl

Thc

Twsc

Tp2 (Miocene).

Tcst caldera.

f (Miocene) - lava-like

Tsi

Tsed Tmi

Undivided sediment (Holocene and Pleistocene). Swisher Mountain tuf Undivided lacustrine and fluvial basinal Tem lava flows and eruptive loci (Miocene). Ta, Tad1, Tad2, Thc, Tcst, and Tba shallow intrusive bodies (includes Silicic intrusive bodies, includes Tpr lava flow/dome complexes (Miocene). ignimbrites derived from ~14.4 - 12 Ma strata, Tcst, Tp1, and flows, ash flows, and Chocolate Mt; Miocene). (Miocene). Juniper Mountain eruptive center other Owyhee Plateau loci.

Tb, Tba, Ta, Twsc, Tad2, and ash flows, shallow intrusive bodies, and eruptive (Miocene).

Tsi

Tgl

Tpr

Tmi

Tem

Tsed

Tsmt

Twsc

Qsed

247 Figure 6 cont.. Figure 7. Photomicrographs of common SC disequilibrium textures (a) Granitoid xenolith with

abundant resorbed feldspars in a Tad1 lava flow (cross polarized light). (b) Granitoid xenolith with amphibole to microcrystalline oxide + pyroxene + feldspar dehydration products present in

a Tad3 lava flow (plane polarized light). (c) Complex textural relationships in a lava-like Tcm ash flow. Mafic xenolith (labeled) resembles Ta lava flows. Dark round blob in lower right and contorted banding may result from welding or magma mingling processes (plane polarized light). (d) Same view as (b), in cross polarized light. (e) Typical feldspar + pyroxene + oxide clot present in SC intermediate and silicic units. In this case, it is found in a Tpr lava flow (plane polarized light). (f) Same view as (e), in cross polarized light. (g) Feldspar + pyroxene + oxide clot in Tad1 lava flow. (h) Same view as (g), in cross polarized light. Notice the oscillatory zoned feldspar and two types of pyroxene.

248

500µ m

A B 1mm

500µ m mafic xenolith

amph dehydration?

banding or C mingling? D 1mm

plag

ox

500µ m 500µ m E F

cpx

opx

500µ m 500µ m G H

249 Figure 7 Figure 8. Photomicrographs of metmamorphic xenolith in a Tad1 lava flow. (a) Xenolith-host contact. Notice abrupt contact near the bottom of the picture, but diffuse contact near the top (polarized light). (b) Same view in plane light illustrating possible melting of the metamorphic xenolith (dark brown glass along the contact which also extends into the xenolith).

250

A Trms Tad 1

500µ m

B

500µ m

Figure 8 251 Figure 9. Idealized ash flow (ignimbrite) flow unit. Zone 1 = Ground layer/surge; deposit laid down at the flow front prior to main body. Zone 2 = Main body of ash flow; 2a is basal ash layer and 2b is main body of flow. Zone 3 = deposit from overriding ash cloud; co-ignimbrite ash. Photograph of distal Cold Springs tuff exposure from Hardscrabble Basin illustrating possible link between idealized ignimbrite profile and this unit. In this exposure, a zone of mantled lapilli fall deposits lies at the base of the ash flow and separates upper from lower ash flow and fall.

252

Underlying ash flow

2a/b

1

Cross Bedding Planar Bedding

Normal grading of lithics

Reverse grading of pumice

Reverse grading of ash

Ash

Ash Cloud Main Basal Layer Ground Layer Body

Lithic Fragments Pumice

Emplacement Surface

Emplacement direction

1

3

2a

2b Figure 9 Zones Figure 10. Sketch depicting the physical features present in the Coyote Mountain ash flow, tied into idealized flow unit depicted in Figure 9.

254

Block and ash flow with upper lapilli tuff

Upper massive to banded zone of densely welded ash-flow tuff

Lower platy, flow folded 2a/b and locally brecciated zone of densely welded ash-flow tuff

Basal moderately welded

glassy vitrophyre 1

Coyote Mountain ash flow

Basal poorly welded vitrophyric lapilli ash

Ground Layer

Pumiceous lapilli tuff (fall) -darker, more welded upper -lighter less welded basal

Precursor Fall

Intermediate lava flows

Figure 10 255 Figure 11. Sketch depicting Cold Springs tuff composite stratigraphic section in the vicinity of Cold Springs Butte to Holloway Meadows. Lower densely welded zone is compositionally similar to over- and underlying poorly welded deposits. Upper densely welded zone is compositionally more evolved than underlying less welded deposits.

256

50m Blocky, white, densely welded 47m Spheroidal, welded -pumice increases and coarsens upward

43m Platy, welded -high lithic content

40m

Poorly welded -highest lithic and pumice content (2a/b)

12m Blocky, white, 10m densely welded Spheroidal, welded

Stratigraphically lowest flow unit Basal ash layer 5m platy and poorly welded (1)

Ground surge, poorly welded Precursor bedded fall deposits 0m Goosey Lake depression lacustrine strata

Figure 11 257 Figure 12. Photographs of SC tuffaceous units. (a) Close-up of Cold Springs tuff main body deposits at Cold Springs Butte. Light oval features are pumice clasts. (b) Lithophysael zone in a highly welded portion of the Cold Springs tuff at Groundhog Meadows. (c) Basal Cold Springs tuff deposits near Cold Springs Butte. Tcst fall deposits overlie lacustrine strata and are overlain

by Tcst ground surge and main body deposits. (d) Valley filling Tp1 ash flow (light colored) overlying Tom lava flows (dark colored) near Odell Mountain. Tp1 outcrop is ~ 5 to 10 m thick. (e) View to the northwest toward Black Dome across Hardscrabble Basin. Cliff forming units among rounded hills are Tcst ash flows interbedded with sedimentary strata. Martin Creek has cut through this sedimentary dominated sequence.

258

Zone 2a

Zone 1

Fall

Lacustrine A B C strata

Black Dome

Tom

Tcst Tcst

D Tp1 E

Figure 12 259 Figure 13. (a) Plot of ppm Sr vs. wt. % silica that is used to distinguish Tarc lava flows from SC units. (b) Tholeiitic vs. Calc-alkaline discrimination diagram of Miyashiro (1974) illustrating the

primarily calc-alkaline nature Tarc lava flows. Also evident is the calc-alkaline nature of Tad1 lava flows. (c) Plot of A/NK (molecular Al2O3/Na2O+K2O) vs. A/CNK (molecular

Al2O3/CaO+Na2O+K2O).

260

5 800 Sr FeO*/MgO Tholeiitic 4 600 3

400 2

200 1 Calc-alkaline A B 0 0 46 50 54 58 62 66 70 74 78 48 53 58 63 SiO SiO2 2

Tarc Tad1 Tad2 Tad3 Tem Tbr Tct Tcst Tp1

Tb Tba Ta Twsc Thc Tpr Tcm Tom Tp2

1.5

1.4

1.3

1.2

A/NK

1.1

Metaluminous Peraluminous 1.0 Peralkaline C 0.9 0.8 0.9 1.0 1.1 1.2 1.3 A/CNK

261 Figure 13 Figure 14. Harker diagrams illustrating major element (a) and trace element (b) variations of SC mafic to intermediate units. Chemical distinctions within and between SC mafic through intermediate composition units are apparent from these diagrams. For example, notice the Sr,

La, Zr, and Nb differences between SC Tad units (Tad1, Tad2, Tad3; green filled triangles, red filled circles, and blue filled squares respectively).

262

8 13

6 11

4 9

2 7

MgO FeO* 0 5

10 17

8 16

6 15

4 14

CaO Al23 O 2 13

KO2 Na2 O 4

4 3

2 3

1

0 2

TiO2 PO25

3 0.7

2 0.5

1 0.3

0 0.1 46 48 50 52 54 56 58 60 62 46 48 50 52 54 56 58 60 62 64

SiO2 SiO2

Tad1 Tad2 Tad3 Tb A Tba Ta Twsc Figure 14

263 200 40 Ni Sc

150 30

100

20 50

0 10

Ba Sr

1500 500

1000 400

500 300

0 200

Rb Zr

150 300

100 200

50 100

0 0

La Nb 40

20 30

20 10

10

0 0 46 48 50 52 54 56 58 60 62 46 48 50 52 54 56 58 60 62 64

SiO2 SiO2

Tad1 Tad2 Tad3 Tb B Tba Ta Twsc

Figure 14 264 Figure 15. Trace element variation diagram illustrating the characteristics of SC mafic and intermediate units normalized to MORB (Pearce, 1983). (a) Tb and Tba. (b) Ta. (c) Tad1. (d)

Tad2, Tad3, and Twsc. Gray shaded field on (a), (b) and (c) is the range of Tba depicted in (a).

265

Rock/MORB; Pearce, 1983 100

10

Tba 1

A B .1

10

1

C D .1 Sr K RbBa Th TaNbCe P Zr HfSm Ti Y Yb Sr K RbBa Th TaNbCe P Zr HfSm Ti Y Yb

Tad1 Tad2 Tad3 Tb Tba Ta Twsc

Figure 15

266 Figure 16. Rare earth element variations of SC units normalized to chondrite (Sun and

McDonough, 1989). (a) Tb and Tba. (b) Ta, Tad1, Tad2, Tad3, and Twsc. (c) Tpr and Tbr. (d) Tom and Tcm. (e) Tct. (f) Thc, Tem, and Tcst.

267

Rock/Chondrites; Sun and McDonough, 1989

100

10

Tba

1 A B

100

10

1 C D

100

10

1 E F La Pr Pm Eu Tb Ho Tm Lu La Pr Pm Eu Tb Ho Tm Lu Ce Nd Sm Gd Dy Er Yb Ce Nd Sm Gd Dy Er Yb

Tad1 Tad2 Tad3 Tem Tbr Tct Tcst Tb Tba Ta Twsc Thc Tpr Tcm Tom

268 Figure 16 Figure 17. Trace element variation diagram comparing regional Steens Basalt with locally erupted Steens Basalt (Tba). Data for regional Steens Basalt is from Johnson et al. (1998) and Brueseke et al. in review.

269

100 Modified Pearce, 1983 Rock/MORB

10

1 Steens Basalt Local Tba lava flows

.1 Sr K Rb Ba Nb La P Zr Ti Y

Figure 17

270 Figure 18. Harker diagrams illustrating major element (a) and trace element (b) variations of SC silicic units. Chemical distinctions within and between SC silicic units are apparent from these diagrams. For example, notice the high Ba concentrations present in some Thc lava flows (green open triangles). Cold Springs tuff samples are also easily distinguished from older SC silicic units by their elevated Zr, La, and Nb concentrations (yellow filled crosses).

271

2.5 7

6 2.0 5

1.5 4

1.0 3 2 0.5 1 MgO FeO* 0.0 0

16 4 15

3 14

2 13 12 1 11 CaO Al23 O 0 10

KO2 Na2 O

6 4

5

3 4

3 2

0.8 0.3

0.6 0.2 0.4

0.1 0.2

TiO2 PO25 0.0 0.0 64 66 68 70 72 74 76 64 66 68 70 72 74 76 78

SiO2 SiO2

Tem Tbr Tct Tcst Tp1 A Twsc Thc Tpr Tcm Tom Tp2

272 Figure 18 40 30 Ni Sc

30 20

20

10 10

0 0 Ba Sr 5000 300 4000

3000 200

2000 100 1000

0 0

Rb Zr 600 300 500

400 200 300

200 100 100

0 0 La Nb 60 90 50

70 40

50 30 20 30 10

10 0 64 66 68 70 72 74 76 64 66 68 70 72 74 76

SiO2 SiO2

Tem Tbr Tct Tcst Tp1 B Twsc Thc Tpr Tcm Tom Tp2

273 Figure 18 cont... Figure 19. Trace element variation diagram illustrating the characteristics of SC silicic units normalized to the upper continental crust of Taylor and McLennan (1985). (a) Tpr and Tbr. (b) Tom and Tcm. (c) Tct. (d) Thc, Tem, and Tcst.

274

Rock/Upper Continental Crust; Taylor and McLennan, 1985 10

1

.1

AA B .01 C D

1

.1

C D .01 Cs Ba U Ta La Sr Hf Sm Y Lu Cs Ba U Ta La Sr Hf Sm Y Lu Rb Th K Nb Ce Nd Zr Ti Yb Rb Th K Nb Ce Nd Zr Ti Yb

Tem Tbr Tct Tcst Thc Tpr Tcm Tom

Figure 19 275 Figure 20. Initial Sr and Nd isotopic characteristics of SC units.

276

0.5129

0.5128

0.5127

0.5126

0.5125 143Nd/ 144 Nd 0.5124 0.703 0.704 0.705 0.706 0.707 87Sr/ 86 Sr

Tad1 Tad2 Tad3 Tem Tbr Tct Tcst Tb Tba Ta Twsc Tcm

Figure 20

277 Figure 21. Initial Sr and Nd isotopic characteristics of SC units as a function of wt. % SiO2.

278

0.708 87Sr/ 86 Sr 0.707

0.706

0.705

0.704

A 0.703 B 0.5128

0.5127

0.5126

0.5125 143Nd/ 144 Nd 0.5124 46 50 54 58 62 66 70 74 78

SiO2

Tad1 Tad2 Tad3 Tem Tbr Tct Tcst Tb Tba Ta Twsc Thc Tpr Tcm Tom

Figure 21

279 Figure 22. Variations in Pb isotopic compositions of SC units. (a) 206Pb/204Pb vs. 207Pb/204Pb. (b) 206Pb/204Pb vs. 208Pb/204Pb.

280

15.66

15.64

15.62

15.60

207 204 Pb/ Pb A 15.58 208Pb/ 204 Pb B 39.1

38.9

38.7

38.5 18.7 18.8 18.9 19.0 19.1 19.2 19.3 206Pb/ 204 Pb

Tad1 Tad2 Tad3 Tem Tbr Tct Tcst Tb Tba Ta Twsc Thc Tpr Tcm Tom

Figure 22

281 Figure 23. Combined Sr, Nd, and Pb isotope characteristics of SC units. (a) 206Pb/204Pb vs. initial 87Sr/86Sr. (b) 206Pb/204Pb vs. initial 143Nd/144Nd.

282

0.707

0.706

0.705

0.704

87 86 Sr/ Sr A 0.703

143Nd/ 144 Nd B 0.5128

0.5127

0.5126

0.5125

0.5124 18.7 18.8 18.9 19.0 19.1 19.2 19.3 206Pb/ 204 Pb

Tad1 Tad2 Tad3 Tem Tbr Tct Tcst Tb Tba Ta Twsc Thc Tpr Tcm Tom

Figure 23

283 Figure 24. Variations in local Sr and Nd isotopic composition. Initial Sr and Nd isotope ratios plotted for SC units, whereas isotope ratios calculated at 16.5 Ma are plotted for Kg and Tarc. Tarc and granitoid ratios from Mellot (1987) and Stuck (1993). SRA= Santa Rosa-Andorno group granitoid; GPS= Granite Peak-Sawtooth group granitoid. Data defining Steens Basalt from Carlson and Hart (1987).

284

0.5129 Steens 0.5128 Basalt SRA Group 0.5127

0.5126 Aplite Dikes (0.710-0.750) 0.5125 GPS Group 143Nd/ 144 Nd 0.5124 0.703 0.704 0.705 0.706 0.707 87Sr/ 86 Sr

Tarc Tad1 Tad2 Tad3 Tem Tbr Tct Tcst Tb Tba Ta Twsc Thc Tpr Tcm Tom

Figure 24

285 Figure 25. Trace element variation diagram illustrating the characteristics of SC units normalized to the average composition of the five SC Tba lava flows and intrusive bodies depicted in Figure 16a. (a) Tb and Tba. (b) Ta, Tad1, Tad2, Tad3, and Twsc. (c) Tpr and Tbr. (d) Tom and Tcm. (e) Tct. (f) Thc, Tem, and Tcst.

286

Rock/SC Tba; Average Santa Rosa-Calico Tba 100

10

1

.1

A B .01

10

1

.1

C D .01

10

1

.1

E F .01 Cs Ba Th K La Nd Zr Ti Eu Yb Y Cs Ba Th K La Nd Zr Ti Eu Yb Y Rb Sr U Ta Ce Nb Hf Sm Tb Lu Rb Sr U Ta Ce Nb Hf Sm Tb Lu

Tad1 Tad2 Tad3 Tem Tbr Tct Tcst Tb Ta Twsc Thc Tpr Tcm Tom

287 Figure 25 Figure 26. Zr vs (a) K2O/MgO and (b) Ba for SC mafic and intermediate units. The blue fields (and arrow) depict the compositional variation of Kg bodies (Stuck, 1993).

288

K O/MgO 4 2 A Mixing/AFC

Kg Aplite dikes 3

2 SRA & GPS Group Kg 1

Crystallization 0

Ba B

1500 Mixing/AFC

SRA & GPS 1000 Group Kg

500 Crystallization Kg Aplite dikes 0 0 100 200 300 400 Zr

Tad1 Tad2 Tad3 Tb Tba Ta Twsc

Figure 26

289 87 86 Figure 27. Weight % SiO2 vs. K/P and initial Sr/ Sr for SC mafic and intermediate units. Field for regionally exposed Steens Basalt also depicted (Carlson and Hart, 1987).

290

50 K/P 40

30

20

10

0 87Sr/ 86 Sr

0.706

0.705

0.704

Steens Basalt 0.703 46 48 50 52 54 56 58 60 62 64

SiO2

Tad1 Tad2 Tad3 Tb Tba Ta Twsc

Figure 27 291 Figure 28. Rare earth element concentrations and the results of binary mixing models for Tcm and Tem. Also depicted are the mixing endmebmers. (a) Illustrates compositions normalized to Average SC Tba while (b) illustrates compositions normalized to chondrite.

292

Rock/SC Tba; Average Santa Rosa-Calico Tba Rock/Chondrites; Sun and McDonough, 1989 100

10

100 1

.1

.01 A B 10

10

100 1

.1

.01 10 Rb Sr K La Nd Zr Ti Eu Yb Y La Ce Nd Sm Eu Gd Dy Er Yb Ba Ce Nb Sm

MB01-63 MB01-47 MB01-73 MB00-32B Calculated mix MB02-31B MB01-50

Figure 28

293 Figure 29. Comparison between SC silicic units and 18 and 47% volume melt compositions of

Petcovic and Grunder (2003). (a) Plot of A/NK (molecular Al2O3/Na2O+K2O) vs. A/CNK

(molecular Al2O3/CaO+Na2O+K2O). (b) SiO2 vs. TiO2. (c) SiO2 vs. FeO* (d) SiO2 vs. K2O.

294

1.5 1.0 B 0.8 1.3

0.6

A/NK

1.1 0.4 Metaluminous Peraluminous Peralkaline 0.2 0.9 A 0.8 0.9 1.0 1.1 1.2 1.3 TiO2 A/CNK 0.0 D C 6 6 5

4 5 3

2 4 1

KO2 FeO* 3 0 64 66 68 70 72 74 76 64 66 68 70 72 74 76 78

SiO2 SiO2

Tem Tbr Tct Tcst Twsc

Thc Tpr Tcm Tom Tp1 Petcovic and Grunder, 2003 Stage 3 (18 vol. % melt) Petcovic and Grunder, 2003 Stage 5 (47 vol. % melt)

295 Figure 29 Figure 30. Comparison between SC silicic units, Kg bodies (blue fields and arrows), and calculated melt compositions derived via batch melting of Kg (light orange fields). (a) Zr vs.

K2O/MgO. (b) Zr vs. Ba.

296

30 30

K2 O/MgO Aplite melts 25 Aplite dikesKg 25

20 20

15 15

10 10 Calculated melts 5 5 A 0 0

B Ba

3000 SRA & GPS 3000 Group Kg

Calculated melts 2000 2000

1000 1000

Aplite melts Kg Aplite dikes 0 0 0 200 400 600 800 Zr

Tem Tbr Tct Tcst Thc Tpr Tcm Tom

Figure 30

297 Figure 31. Comparison between SC units, Kg bodies (same fields as in Figs. 26 and 30), and calculated melt compositions derived via batch melting of Kg (same fields as in Fig. 30). Mix/AFC, mixing and/or assimilation-fractional crystallization; FC, fractional crystallization.

298

30 30

K2 O/MgO K2 O/MgO 25 25

20 20

15 FC 15

10 Kg melting 10 Mix/AFC 5 5 FC 0 0

Ba Ba

3000 3000

2000 Kg melting 2000 Mix/AFC FC 1000 1000

FC 0 0 0 200 400 600 800 0 200 400 600 800 Zr Zr

Figure 31

299 Figure 32. Detailed stratigraphic sections from ten locations across the SC. Section locations depicted in figures 2 and 33. Petrologic units same as in text and Table 2.

300

23.5 Ma 35.4 Ma

14.4 Ma

~16.7 - 16.5 Ma

Section #2

total t=349 m

Tarc Tba Tb Tb Tba/Tb Trms

Ta Twsc

60 m

16.2 Ma

16.4 Ma

16.7 Ma

16.0 Ma

~20-23Ma arc suite

Section #1

total t=299 m

Thc Tarc Tba Tpr Trms

Ta Tba Tad1

Triassic Metased.

Andesite

Basalt/Basaltic Cretaceous Dacite andesite Granitoid

Pre-SC Arc Suite

Ash flow Sedimentary strata

Porphyritic Rhyolite rhyolite Covered Interval

Air-fall Tuff

Figure 32 16.4 Ma

>15.5 Ma

<16 Ma

15.5 - 15.8 Ma

11 m

Section #6

Section #7

total t=1

total t=220 m

Tct Ta Tcst Ta Tcst

Thc Tp1

60 m

16.5 Ma

16.5 Ma

Section #5

total t=406 m

c Tom Tcm Tcm/Tad2 Kg

60 m

<15.8 Ma

16.5 Ma

15.8 Ma

Section #3

Section #4

total t=265m

total t=198 m

Tem Tp1

Tom

Ta Tcst Figure 32 cont.. Tpr

16.2 Ma

~20-23Ma arc suite

16.4 Ma

Section #10

total t=290 m

Tad1 Thc Tarc Kg

60 m

13.9 Ma

<15.8 Ma

Section #9

total t=330 m Tad1

60 m

15.4 Ma

Section #8

total t=157 m

Tsed Tsed Tad1

Tcst Tcst

Figure 32 cont.. Figure 33. Generalized SC geology and geographic features depicting the stratigraphic section locations depicted in figure 32.

304

GeographicFeatures of the Santa y Qsed EOdell

5km Twsc Rosa-CalicoVolcanic Field, NV. Mountain River 1817 Tom 2165 Calico Qsed OM Tem McConnellE2296 y2152 4 Quinn Twsc Tcm Peak Fork Qsed E. Fork 2394y

EM Eightmile Calico Eightmile Crk. EMountain Kg 1433 Tsb y 2400 Quinn y 5 2307 Pasture 2549 Capitol Ì 3 Klondike Canyon E Tsi National 2228 Peak Tgl Threemile y Eastern Goosey Lake Depression Little Tem Crk. Goosey Lake Flat Owyhee Buckskin Rosa Range N.Fork Twsc GLD Mahogany River Qsed Mountain Pass Tsb E Staunton Canyon Crk. Mountains BM Mountains 2665 Ì Ridge Little FS 531 Crk. Buckskin 6 FS 084 1792 Twsc -National FS 531 y

2082 Black Windy Tcm E Gap FS 084 Cold Tpr HumHumboldt Springs Butte y Tgl boldt 2102 Tsi Cabin FS 084 Crk. 2134 Tgl y Tsed GLD Tsmt Flat Crk. Ridge River Thc Crk. 1859 2666 Northern Santa y Tarc Tct y Trms Thc 792 Western Goosey CT 2 Lake Depression 2167 7 E Zymns Black Coyote GP Hinkey Butte HS 2966 1 Martin Crk. E Mountain Kg Thc BD E ESummit ZB Granite 2222 E Tsi 2607 1990 Tpr Peak 9 Thc SC dike Coal Pit orientation 8 Hardscrabble Basin Thc Indian Crk. PeakE Tsed Solid Silver Crk. Trouble 2460 CPP 10 N 1615 792 y E N 2372 y Tarc Crk. Paradise Spring Qsed Kg Thc 1558y 1706 n=62 Valley Ì City

Qsed Legend Tsmt E Topographic High Silicic vent/vent region Mafic & Intermediate vent/vent region Tsed Tgl Tem Tsi y Benchmark Tmi Tpr Possible caldera Silicic dike zone Twsc SC for Cold Springs Tuff Tct 2966 Elevation (m) Thc Tcm Tsb Tom Mining District Ì River or Major normal Lava flow Tsb Creek fault direction Generalized 792 State Road Marker Tarc Stratigraphy Kg Forest Service Detailed Stratigraphic FS 531 Trms Road Marker Improved Road 3 Section

Figure 33

305 Figure 34. Cartoon illustrating the geologic development of the SC. Upside-down triangles depict regions of higher elevation. Red lines depict basaltic dike swarms/zones of eruptive loci. Orange ovals depict silicic domes/outflow. Black lines depict silicic dikes and dike swarms. Green circles depict Tad eruptive loci, except for (a), where they depict probable Tarc loci. Shaded beige regions depict Goosey Lake depression sedimentary depocenters and in (d), the Tcst caldera is illustrated by blue circle with inward-facing hachures. (a) Prior to SC volcanism illustrating potential Tarc eruptive loci near Hinkey Summit and regions of high topography. Blue abbreviations as in Figure 6. (b) ~16.7 to 16.3 Ma SC activity. (c) ~16.2 to 16 Ma SC activity. (d) ~16 to 14 Ma SC activity.

306

Pre-SC OM ~16.7- (A) 16.3 Ma (B) EM CP

BM GLD

CT HS

CPP 5km

~16.2- ~16- 16 Ma 14 Ma (C) (D)

Figure 34 307 Figure 35. Total alkalies vs. silica volcanic rock classification of LeBas et al. (1986) illustrating the compositional variations of local eruptive products over the same time slices in Figure 25. (a) Pre- SC Tarc volcanism. (b) ~16.7 to 16.3 Ma SC activity. (c) ~16.2 to 16 Ma SC activity. (d) ~16 to 14 Ma SC activity. Notice the compositional spectrum of SC units being erupted throughout as well as the time-transgressive character of some SC units.

308

~16.7- 10 Pre-SC 16.3 Ma (A) TD (B) TD 8 TA TA BTA BTA R R 6 TB TB D D 22 4

Na O+K O A RD A RD BA BA 2 B B

0 ~16.2- ~16- 10 16 Ma 14 Ma (C) TD (D) TD 8 TA TA BTA BTA R R 6 TB TB D D 22 4

Na O+K O A RD A RD BA BA 2 B B

0 45 50 55 60 65 70 75 45 50 55 60 65 70 75

SiO2 SiO2

Tarc Tad1 Tad2 Tad3 Tem Tbr Tct Tcst Tb Tba Ta Twsc Thc Tpr Tcm Tom

309 Figure 35 Figure 36. Cartoon illustrating subsurface magmatic processes that occurred in the SC through time. Note, faulting due to broadly east-west directed extension is implied on this diagram, but not depicted. W SC, western SC; E SC, eastern SC; unit abbreviations are the same as in text. (a) ~16.7 to 16.3 Ma SC activity. (b) ~16.2 to 16 Ma SC activity. (c) ~16 to 14 Ma SC activity.

310

~16.7 - Eastern SC mafic-silicic 16.3 Ma package Thc Tad Ta (A) Tba 1

Tarc

Trms

Kg

WSC Extension ESC

~16.2 - 16 Ma Tpr (B)

WSCExtension ESC

~16 - Tcst 14 Ma (C)

Figure 36 311 TABLE 1. SUMMARY OF ASSIGNED 40Ar/39Ar AGES

Single crystal results Sample Petrologic Unit Location MSWD n K/Ca ±1σ Age ±1σ (Ma) (Ma) MB03-36A Tcst Hardscrabble Basin 1.71 15 8.7 1.7 15.40 0.04 MB02-55 Tcst Cold Springs Butte 2.38 15 9.0 2.1 15.46 0.02 MB03-26F Tcst Holloway Meadows 0.38 15 3.6 2.3 15.75 0.08 MB02-44 Tpr Southern margin 1.39 12 41.4 5.9 16.16 0.02 MB02-19 Tpr Western coulée 1.01 15 57.3 4.8 16.18 0.02 MB00-18 Tpr Southern coulée 1.24 14 33.1 6.5 16.23 0.02 MB03-45 Tct Zymns Butte 0.65 15 23.9 3.0 16.40 0.04 MB03-10B Tom Klondike Canyon 1.53 15 28.9 3.1 16.43 0.03 MB01-54A Tct Coyote Mt. 3.96 13 17.9 3.8 16.44 0.03

MB00-23 Tp1 G. Lake Depression 0.63 15 7.6 2.4 16.45 0.06 MB01-76 Tcm Southern Calico Mts. 1.92 15 12.3 1.9 16.47 0.02 MB01-27 Tcm Capitol Peak 0.98 15 29.0 4.2 16.50 0.02 MB00-32B Tcm Mahogany Pass 1.53 13 11.9 3.1 16.51 0.02 MB00-33 Tom Odell Mountain 2.87 15 33.6 3.9 16.55 0.02

Whole rock results Sample Unit Location MSWD Steps 39Ar Age ±1σ (%) (Ma) (Ma)

MB02-43* Tad1 Coal Pit Peak 1.30 Isochron N/A 13.90 0.30 MB01-6A Tba Western margin 1.66 5 66.0 14.35 0.19 MB02-53* Ta Black Ridge 2.30 Isochron N/A 14.94 0.12

MB01-65 Tad2 Staunton Ridge 1.98 6 48.8 15.76 0.14 MB01-12 Tb Western margin 0.43 9 100 16.07 0.46 $ MB00-13 Tba Hinkey Summit 1.26 8 75.2 16.45 0.17

MB01-47 Tad3 Central Calico Mts. 3.36 4 54.5 16.54 0.08 MB01-24 Tb Western margin 2.21 8 100 16.95 0.65 MB02-5 Tarc Hinkey Summit 1.93 7 81.5 22.48 0.04 MB01-2 Tarc Western margin 0.96 2 46.9 23.50 0.12 MB01-1 Tarc Western margin 3.77 4 83.9 35.52 0.17 *Isochron age is interpreted to be eruption age $Plagioclase seperates used, see Appexndix X for whole rock age

Unpublished data collected by A. Wallace, U.S.G.S (presented in Brueseke et al., 2003) Sample Unit Location Age ±1σ (Ma) (Ma) 00LH-3 -- Southeastern margin 15.45 0.04 00HS-6 Tba Chocolate Mt. 15.97 0.10 00HS-8 Tpr Hinkey Summit dike 16.18 0.05 00HS-7 Thc Lye Creek turn-off 16.36 0.05 00HS-X Tba Hinkey Summit 16.73 0.02

All ages reported normalized to a 28.02 Ma age for the Fish Canyon Tuff sanidine (FC-2) standard.

312 TABLE 2. SUMMARY OF SC ERUPTIVE UNITS

Petrologic Unit Map Unit Age SiO2 (wt. %) TAS Nomenclature Tb Thc ~16.7 Ma 47.4 - 47.7 basalt

Tba Tsb, Tmi, Thc, Twsc, Tcm ~16.7 - 14 Ma 46.3 - 56.2 basalt, basaltic andesite, trachybasalt, basaltic trachyandesite

Ta Thc, Twsc, Tgl, Tcm ~16.7 - 14 Ma 55.5 - 60.1 basaltic andesite, andesite, basaltic trachyandesite, trachyandesite

Tad1 Thc, Tgl ~16.4 - 14 Ma 59.3 - 64.1 andesite, dacite, trachyandesite

Tad2 Twsc ~15.8 Ma 61.1 - 63.9 trachyandesite, trachydacite

Tad3 Tcm, Tct ~16.5 Ma 60.9 - 62.9 trachyandesite, trachydacite

Twsc Twsc ~16.7 - 15.8 Ma 62.6 - 74.1 trachydacite, rhyolite

Tpr Tpr, Tsi, Thc ~16.2 Ma 69.2 - 78.7 rhyolite

Tem Tem >15.8 Ma 64.4 - 73.1 trachydacite, rhyolite

Tct Tct, Tgl ~16.5 - 16.4 Ma 64.3 - 75.1 dacite, trachydacite, rhyolite

Tcm Tcm ~16.5 Ma 63.5 - 75.3 dacite, trachydacite, rhyolite

Tom Tom, Tcm ~16.5 Ma 73.0 - 76.6 rhyolite

Thc Thc ~16.4 Ma 66.9 - 76.1 trachydacite, rhyolite

Tbr Thc, Tsi ~16.7 to 16 Ma 76.3 - 77.9 rhyolite

Tcst Tgl, Tsed, Tcm, Tct ~15.8 - 15.4 Ma 67.9 - 76.7 trachydacite, rhyolite

Tp1 Tgl, Twsc, Thc, Tom, Tem ~16.5 - <15.4 Ma 71.6 - 76.6 rhyolite

Tp2 Tsed, Thc, Twsc, Tom, Tcm ~16.7 - 15.4 Ma 72.4 - 76.6 rhyolite

313 TABLE 3. REPRESENTATIVE MAJOR AND TRACE ELEMENT GEOCHEMICAL DATA FOR SANTA ROSA-CALICO UNITS

Sample MB02-77 MB01-46A MB01-46B MB01-1 MB01-2 MB02-5 MB03-51 MB01-24 MB01-12 MB01-6A MB01-33 MB01-25 Pet. Unit Kg Kg Kg Tarc Tarc Tarc Tarc Tb Tb Tba Tba Tba Map Unit Kg Kg Kg Tarc Tarc Tarc Tarc Thc Thc Thc Tcm Tmi

SiO2 57.39 67.98 76.66 49.21 54.67 63.33 61.50 47.20 48.12 54.56 55.66 49.34

TiO2 0.92 0.38 0.05 2.18 1.09 0.69 0.89 1.29 1.37 1.65 1.05 1.98

Al2O3 16.99 16.24 13.69 14.98 16.39 16.37 16.15 17.02 16.81 15.57 14.32 14.70

Fe2O3 6.90 2.65 0.35 11.31 7.58 5.08 6.02 11.51 12.12 10.32 8.47 13.83 MnO 0.11 0.07 0.01 0.17 0.12 0.09 0.10 0.17 0.18 0.16 0.13 0.22 MgO 4.65 1.18 0.03 8.03 4.46 2.19 2.91 8.65 8.74 5.53 6.31 5.43 CaO 5.95 2.97 0.78 7.44 6.79 4.03 4.74 10.14 9.99 7.55 7.50 8.70

Na2O 3.66 4.59 4.37 2.95 3.60 4.14 3.97 2.68 2.66 2.82 2.54 3.20

K2O 2.53 2.45 4.15 1.83 2.50 3.59 3.23 0.32 0.33 1.82 2.16 1.33

P2O5 0.33 0.16 0.02 0.78 0.45 0.31 0.47 0.14 0.14 0.24 0.29 0.50 LOI 0.75 1.02 0.44 2.24 3.19 0.29 0.02 0.50 0.65 0.52 0.70 0.26 TOTAL 100.18 99.68 100.53 101.12 100.85 100.10 99.99 99.61 101.11 100.74 99.13 99.50

Ni 50 5 <1 59 64 22 30 157 150 75 155 21 Cr 146 6 <1 233 108 45 66 97 102 66 326 81 Sc 18 6 1.7 20 17 9 13 25 26 20 23 32 V 168 40 <1 226 162 88 108 244 261 197 193 347 Ba 1826 1072 61 770 943 1391 1220 135 166 425 439 443 Rb 84 67 120 36 76 100 93 2.0 3.7 51 60 25 Sr 1036 459 43 885 699 557 595 422 410 350 311 437 Zr 204 126 40 185 195 253 263 71 73 165 129 141 Y 18 11 6 24 19 17 19 21 27 27 23 35 Nb 8.00 5.40 8.50 22 13 13 15 3.70 3.20 11 8.40 7.50 Ga 20 20 19 20 18 19 18 19 19 20 18 22 Cu 56 9 4 34 51 39 22 127 135 80 77 59 Co 19 5 2 37 24 12 16 53 54 39 36 44 Zn 90 62 17 116 79 102 102 80 82 95 86 120 Pb 14 17 40 5.8 9.0 15 14 4.1 1.0 6.2 10 4.9 U 2.0 3.0 1.1 2.8 1.4 1.7 3.1 2.1 <1 1.5 2.3 <1 Th 10 4.6 13 4.4 7.0 10 9.1 <2 <2 4.4 5.7 2.5 Cs ------0.30 1.47 2.75 0.69 Hf ------1.96 4.04 3.48 3.50 Ta ------0.27 0.73 0.70 0.48 La 25.6 13.1 1.40 33.4 31.3 41.5 40.0 4.00 6.25 17.7 18.4 18.6 Ce 54.7 37.1 19.9 65.2 57.7 76.7 92.0 24.4 14.8 36.2 35.7 39.4 Pr ------2.16 4.47 4.35 5.19 Nd ------11.2 19.8 18.6 24.2 Sm ------3.52 5.32 4.84 6.49 Eu ------1.31 1.66 1.27 2.06 Gd ------4.07 5.37 4.73 6.76 Tb ------0.69 0.88 0.77 1.1 Dy ------4.26 5.25 4.56 6.83 Ho ------0.87 1.04 0.93 1.37 Er ------2.29 2.72 2.38 3.71 Tm ------0.32 0.39 0.34 0.52 Yb ------1.91 2.31 2.08 3.18 Lu ------0.30 0.36 0.33 0.50 87 86 Sr/ Srm ------0.703791 --- 0.704139 --- 87 86 Sr/ Sri ------0.703784 --- 0.704012 --- 143 144 Nd/ Ndm ------0.512897 --- 0.512649 --- 143 144 Nd/ Ndi ------0.512876 --- 0.512746 --- epsilon Ndi ------5.1 --- 2.6 --- 206Pb/204Pb ------18.941 --- 19.044 --- 207Pb/204Pb ------15.580 --- 15.623 --- 208Pb/204Pb ------38.541 --- 38.762 ------Notes: Major element concentrations are reported as weight percent oxides and expressed as raw data; trace element concentrations are reported in ppm. Major element analyses were analyzed by DCP-AES (Direct Current Argon Plasma Atomic Emission Spectrometry) at Miami Univeristy. All trace elements were analyzed by XRF (X-ray fluorescence) at Franklin and Marshall College with the following exceptions: 1) For those samples with Hf, Ta, and Cs data, the reported Hf, Ta, Cs, Rb, Pb, U, Th, La, and Ce values are inductively coupled plasma mass spectrometry (ICP-MS) data run at Washington State University; 2) the trace element data reported for MB01-54B and all Tp2 samples are DCP-AES values run at Miami University; 3) Rb and Sr concentrations in bold are isotope dilution results. All isotopic data was obtained by TIMS (thermal ionization mass spectrometry) at Miami University. Initial87 Sr/86Sr values are not listed for certain high Rb/Sr samples due to post-solidification modification of this ratio due to hydrothermal alteration. Unit abbreviations are the same as in the text (e.g. Pet. unit; petrologic unit).

314 TABLE 3. Continued

Sample MB01-78 MB01-85 MB02-9B MB02-53 MB02-43 MB00-1 MB00-17 MB00-16 MB01-65 MB00-44 MB02-33 MB01-63 Pet. Unit Ta Ta Ta Ta Tad1 Tad1 Tad1 Tad1 Tad2 Tad2 Tad2 Tad2 Map Unit Tcm Tgl Thc Tgl Thc Thc Thc Thc Twsc Twsc Twsc Twsc

SiO2 58.08 58.44 56.30 57.63 62.83 60.65 62.62 59.57 62.18 60.57 64.81 61.83

TiO2 1.22 1.69 1.72 1.64 0.86 0.95 0.90 1.13 1.04 1.20 0.74 1.00

Al2O3 14.87 14.58 15.84 14.21 14.17 14.57 14.63 14.68 15.20 15.15 15.02 15.10

Fe2O3 9.09 9.85 10.03 9.96 6.53 7.49 6.83 8.51 8.16 8.62 6.84 7.88 MnO 0.14 0.18 0.15 0.17 0.12 0.14 0.11 0.16 0.15 0.15 0.11 0.14 MgO 4.05 2.76 2.52 2.68 2.27 3.06 2.35 3.53 1.25 1.14 0.68 1.12 CaO 6.57 5.87 5.65 5.62 4.33 5.47 4.41 6.09 3.77 3.88 2.51 3.75

Na2O 3.16 3.51 3.59 3.55 3.36 3.10 3.32 3.08 3.63 4.14 4.13 3.70

K2O 2.36 2.75 2.81 2.71 3.16 3.06 3.10 2.79 4.01 3.31 3.86 3.88

P2O5 0.27 0.70 0.50 0.63 0.19 0.18 0.20 0.24 0.43 0.52 0.27 0.33 LOI 0.75 0.24 0.34 0.34 0.84 0.69 0.90 0.61 0.87 0.33 0.55 1.23 TOTAL 100.56 100.56 99.44 99.14 98.66 99.37 99.38 100.39 100.68 99.00 99.52 99.96

Ni3815914111711203433 Cr 66 6 10 7 17 20 19 18 <1 9 <1 <1 Sc 20.5 22.2 19.5 21.4 19.1 23.9 21.6 26.1 14.9 16.3 13.7 15.8 V 192 196 194 180 160 194 165 225 43 122 22 41 Ba 1023 1724 1070 1499 1042 1099 979 1168 1210 1204 1341 1262 Rb 66 67 71 71 94 82 93 72 103 96 111 104 Sr 354 377 375 349 264 264 259 275 347 444 292 346 Zr 226 248 253 247 180 166 180 184 262 285 276 264 Y 354343443335343545404044 Nb 13 12 15 12 11 10 11 8.9 17 23 17 17 Ga 20 20 23 20 19 20 19 20 21 22 21 21 Cu 46 29 33 28 26 49 26 44 14 12 11 13 Co 28 20 23 22 19 26 19 29 11 14 6 10 Zn 107 123 110 113 89 94 90 100 117 122 106 122 Pb 12 13 16 14 17 35 18 15 17 16 19 17 U 2.2 2.5 1.8 1.5 4.2 <1 <1 1.2 <1 <1 <1 3.6 Th 7.4 8.2 7.7 10 12 12 12 8.4 8.8 11 11 10 Cs 2.22 1.44 ------2.53 ------4.34 Hf 5.90 6.22 ------4.97 ------6.87 Ta 0.79 0.69 ------0.78 ------1.04 La 29.2 32.7 31.8 34.2 30.6 25.6 30.8 27.2 38.4 44.4 42.6 39.4 Ce 57.4 65.2 62.8 63.9 57.3 56.2 56.9 54.8 70.2 80.2 70.4 77.2 Pr 6.93 7.99 ------6.53 ------9.31 Nd 29.4 34.7 ------26.0 ------39.1 Sm 7.34 8.74 ------6.25 ------9.39 Eu 1.89 2.39 ------1.35 ------2.36 Gd 7.07 8.43 ------5.96 ------8.85 Tb 1.12 1.36 ------1.00 ------1.42 Dy 6.73 8.19 ------6.16 ------8.60 Ho 1.34 1.65 ------1.28 ------1.74 Er 3.57 4.48 ------3.51 ------4.72 Tm 0.50 0.64 ------0.52 ------0.69 Yb 3.07 3.96 ------3.29 ------4.28 Lu 0.49 0.62 ------0.52 ------0.69 87 86 Sr/ Srm 0.704515 0.705068 0.704624 ------0.705020 87 86 Sr/ Sri 0.704397 0.704963 0.704501 ------0.704839 143 144 Nd/ Ndm 0.512722 0.512702 0.512719 ------0.512701 143 144 Nd/ Ndi 0.512705 0.512687 0.512729 ------0.512686 epsilon Ndi 1.8 1.4 2.2 ------1.4 206Pb/204Pb 19.010 19.146 19.072 ------19.10 207Pb/204Pb 15.633 15.642 15.637 ------15.64 208Pb/204Pb 38.776 38.846 38.745 ------38.79

315 TABLE 3. Continued

Sample MB01-45 MB01-47 MB03-34 MB01-41 MB00-18 MB03-54 MB00-15A MB02-19 MB02-44 MB02-61 MB02-71 MB01-31 Pet. Unit Tad3 Tad3 Tad3 Tad3 Tpr Tpr Tpr Tpr Tpr Tbr Tbr Tcm Map Unit Tcm Tcm Tcm Tcm Tpr Tsi Tsi Tpr Tpr Tsi Tpr Tcm

SiO2 60.47 62.89 59.89 63.59 69.96 71.97 74.13 75.89 74.80 75.61 75.99 66.3

TiO2 1.41 1.17 1.45 1.21 0.53 0.3 0.16 0.1 0.30 0.08 0.08 0.79

Al2O3 14.70 15.44 14.64 14.98 13.9 13.06 11.75 12.03 13.27 12.61 12.3 13.76

Fe2O3 8.31 7.81 8.89 6.35 3.33 2 1.52 1.08 2.12 1.22 1.28 5.85 MnO 0.13 0.14 0.13 0.13 0.06 0.04 0.03 0.01 0.03 0.02 0.02 0.07 MgO 1.57 1.24 1.98 1.36 0.72 0.37 0.29 0.1 0.24 0.13 0.1 1.88 CaO 4.15 3.64 4.68 3.78 2.11 1.5 0.95 0.67 1.48 0.46 0.64 3.72

Na2O 3.88 4.14 3.45 3.53 3.34 3.07 2.72 3.16 3.34 3.22 3.81 3.39

K2O 3.15 3.52 3.00 3.95 4.51 4.66 5.03 5 4.42 4.92 4.81 3.7

P2O5 0.53 0.46 0.48 0.40 0.18 0.07 0.04 0.15 0.11 0.03 0.04 0.28 LOI 1.07 0.54 1.10 1.78 1.55 2.75 2.53 2.19 0.41 0.52 0.56 0.63 TOTAL 99.36 101.01 99.70 101.05 100.19 99.8 99.14 100.4 100.51 98.82 99.62 100.38

Ni548343 2<12<1224 Cr 9 <1 4 2 3 2 7 2 2 <1 <1 51 Sc 17 11 20 15 7.7 5.0 3.3 2.7 4.3 4.4 3.5 14 V 114.19 71.38 165 74.9 43 24 10.2 <1 17.3 <1 <1 83.5 Ba 1136 1179 925 1302 1278 645 236 83 669 24 19 497 Rb 90 102 97 108 154 179 229 295 194 364 355 126 Sr 435 427 381 423 196 116 30 14 130 5 12 181 Zr 278 309 231 288 217 154 126 110 155 83 149 200 Y 384037393026506422767337 Nb 21.8 23.4 13.4 22.2 13.2 11.5 12.8 14.9 11.2 16.4 24.2 16.3 Ga 22.3 22.4 21.5 22.1 17.4 15.5 19.8 21.5 17.5 23.1 25.9 19.3 Cu 14 13 115 11 8 22 9 6 9 11 8 21 Co1310181143 4333315 Zn 119 120 115 122 63 49 53 55 47 62 87 89 Pb 16 18 15 17 24 27 27 31 27 29 37 19 U 3.1 3.5 2.3 <1 6.3 9.4 <1 11 <1 <1 10 <1 Th 9.4 10 9.5 12 17 24 24 31 28 19 30 15 Cs 2.87 2.78 ------4.34 ------13.7 ------9.14 --- Hf 7.17 7.91 ------6.27 ------4.68 ------7.01 --- Ta 1.33 1.41 ------1.01 ------1.59 ------2.05 --- La 43.3 46.0 32.0 44.8 37.0 36.0 42.0 54.6 39.9 21.1 23.2 40.7 Ce 84.7 90.7 70.0 83.9 66.5 82.0 78.7 108.0 70.1 46.1 49.2 72.9 Pr 10.5 11.0 ------7.67 ------12.4 ------6.96 --- Nd 44 45 ------30 ------47 ------30 --- Sm 10.3 10.5 ------6.76 ------11.3 ------10.2 --- Eu 2.67 2.63 ------1.23 ------0.12 ------0.03 --- Gd 9.30 9.36 ------6.12 ------10.2 ------10.6 --- Tb 1.38 1.40 ------0.98 ------1.78 ------2.01 --- Dy 7.92 8.05 ------5.98 ------11.1 ------12.7 --- Ho 1.54 1.58 ------1.22 ------2.29 ------2.59 --- Er 3.92 4.11 ------3.35 ------6.36 ------7.22 --- Tm 0.54 0.58 ------0.51 ------0.96 ------1.11 --- Yb 3.24 3.55 ------3.18 ------5.98 ------7.12 --- Lu 0.50 0.56 ------0.53 ------0.90 ------1.08 --- 87 86 Sr/ Srm 0.705074 0.704876 ------0.706386 ------0.706973 --- 87 86 Sr/ Sri 0.704943 0.704724 ------*see note ------*see note --- 143 144 Nd/ Ndm 0.512668 0.512672 ------0.512579 ------0.512609 --- 143 144 Nd/ Ndi 0.512652 0.512657 ------0.512563 ------0.512587 --- epsilon Ndi 0.7 0.8 ------1.0 ------0.6 --- 206Pb/204Pb 19.01 19.02 ------19.13 ------19.14 --- 207Pb/204Pb 15.61 15.63 ------15.63 ------15.65 --- 208Pb/204Pb 38.71 38.77 ------38.82 ------38.85 ---

316 TABLE 3. Continued

Sample MB01-40 MB00-32B MB01-76 MB03-33 MB01-54A MB01-54B MB01-60A MB03-45 MB01-56 MB03-46 MB03-10B MB00-33 Pet. Unit Tcm Tcm Tcm Tcm Tct Tct Tct Tct Tct Tct Tom Tom Map Unit Tcm Tcm Tcm Tcm Tct Tct Tct Tct Tct Tct Tom Tom

SiO2 67.96 71.42 75.37 73.98 71.97 70.91 69.85 73.94 74.42 71.92 75.17 76.27

TiO2 0.65 0.27 0.26 0.27 0.20 0.39 0.61 0.13 0.2 0.35 0.11 0.16

Al2O3 13.9 12.87 13.36 13.81 12.99 13.76 14.37 13.26 13.25 13.89 12.15 11.84

Fe2O3 4.5 2.95 1.01 0.83 1.45 2.21 3.52 1.49 1.6 2.08 1.64 2.04 MnO 0.03 0.05 0.01 0.01 0.06 0.08 0.06 0.05 0.04 0.08 0.02 0.02 MgO 0.34 0.18 0.05 0.03 0.16 0.31 0.43 0.05 0.12 0.27 0.03 0.02 CaO 1.8 1.08 0.78 0.87 0.74 1.20 1.99 0.74 0.87 1.14 0.41 0.11

Na2O 3.98 3.32 3.68 3.84 3.51 3.64 4.19 3.59 3.78 4.08 3.75 3.76

K2O 4.33 5.15 5.19 5.17 5.32 5.02 4.2 5.03 5.22 4.65 4.90 4.82

P2O5 0.13 0.06 0.07 0.06 0.06 0.09 0.18 0.02 0.06 0.08 0.02 0.05 LOI 1.51 2.63 0.31 0.38 --- 3.21 0.54 0.45 0.6 0.24 0.93 0.52 TOTAL 99.15 99.99 100.07 99.25 96.46 100.82 99.94 98.75 100.14 98.78 99.14 99.62

Ni4233<11 332332 Cr<12<1<1 2 2<1<1<1<1<1 Sc 7.7 3.7 5.2 7.0 5.4 7.6 8.0 6.0 4.8 7.0 2.0 <1 V 27 6 4 11 <1 2 21 9 2 12 5 14 Ba 1339 1663 1419 1535 1601 1660 1486 585 1357 1575 65 49 Rb 142 162 177 161 157 149 122 170 172 144 237 241 Sr 249 88 84 91 58 124 208 46 60 11 86 Zr 337 342 343 360 244 412 364 178 255 409 274 404 Y 383536384742433146445653 Nb 23 18 18 17 9.1 19 18 15 19 18 33 49 Ga 21 20 22 19 --- 18 19 16 18 17 22 25 Cu 13 9 6 137 --- 6 7 <2 6 <2 <2 18 Co6232---2 2<12221 Zn 89 70 49 50 93 70 89 53 61 77 93 100 Pb 21 26 26 26 --- 25 22 25 25 25 47 21 U <1 5.6 <1 5.3 --- 6.8 <1 5.2 <1 6.7 9.3 7.3 Th 17 17 18 17 --- 18 17 19 20 18 29 26 Cs --- 5.36 ------3.94 --- 3.86 --- 2.74 6.95 6.28 Hf --- 9.48 ------10.4 --- 5.95 --- 9.94 10.2 12.9 Ta --- 1.22 ------1.25 --- 1.16 --- 1.27 2.96 2.88 La 49.0 46.1 59.5 43.0 --- 42.1 38.8 52.7 41.3 42.4 66.5 66.5 Ce 88.2 86.6 86.9 93.0 --- 79.5 69.7 82.6 73.8 78.7 129.4 137.0 Pr --- 9.92 ------9.07 --- 11.1 --- 9.10 14.2 14.8 Nd --- 39.0 ------35.9 --- 41.7 --- 35.7 54.0 56.1 Sm --- 8.85 ------8.31 --- 8.44 --- 8.21 11.9 12.0 Eu --- 1.28 ------1.57 --- 1.02 --- 1.59 0.20 0.25 Gd --- 7.75 ------7.73 --- 6.51 --- 7.5 9.93 9.82 Tb --- 1.25 ------1.27 --- 1.02 --- 1.26 1.64 1.61 Dy --- 7.54 ------7.93 --- 5.99 --- 7.77 9.67 9.56 Ho --- 1.49 ------1.64 --- 1.18 --- 1.62 1.88 1.90 Er --- 4.05 ------4.69 --- 3.30 --- 4.55 5.08 5.24 Tm --- 0.59 ------0.71 --- 0.50 --- 0.71 0.76 0.77 Yb --- 3.72 ------4.56 --- 3.23 --- 4.57 4.72 4.85 Lu --- 0.58 ------0.75 --- 0.50 --- 0.74 0.71 0.76 87 86 Sr/ Srm --- 0.705371 ------0.706043 ------0.705829 0.705939 87 86 Sr/ Sri --- *see note ------0.705327 ------*see note *see note 143 144 Nd/ Ndm --- 0.512637 ------0.512686 ------0.512688 0.512677 143 144 Nd/ Ndi --- 0.512622 ------0.512670 ------0.512673 0.512663 epsilon Ndi --- 0.1 ------1.1 ------1.1 0.9 206Pb/204Pb --- 19.06 ------19.21 ------19.02 19.03 207Pb/204Pb --- 15.62 ------15.66 ------15.62 15.62 208Pb/204Pb --- 38.76 ------38.91 ------38.73 38.77

317 TABLE 3. Continued

Sample MB01-27 MB02-41 MB01-73 MB02-31B MB00-8 MB00-2 MB02-46 MB02-55 MB02-63B MB03-26A MB03-26F MB03-36A Pet. Unit Tom Tem Tem Tem Thc Thc Thc Tcst Tcst Tcst Tcst Tcst Map Unit Tom Tem Tem Tem Thc Thc Thc Tgl Tgl Tgl Tgl Tgl

SiO2 73.65 69.81 72.99 64.21 67.05 70.48 75.45 68.08 72.92 75.24 70.98 70.09

TiO2 0.18 0.44 0.28 0.38 0.69 0.44 0.18 0.45 0.32 0.32 0.33 0.35

Al2O3 11.90 15.62 14.87 14.36 14.16 13.83 12.56 13.32 12.23 12.47 12.06 12.53

Fe2O3 2.15 1.52 1.09 4.36 4.52 2.88 1.1 3.73 2.61 1.28 3.24 2.98 MnO 0.03 0.01 0.01 0.11 0.09 0.05 0.02 0.07 0.05 0.01 0.06 0.06 MgO 0.07 0.07 0.03 0.43 1.4 0.34 0.15 0.45 0.06 0.08 0.25 0.44 CaO 0.54 1.75 0.95 1.95 2.51 0.99 0.54 2.24 0.83 0.45 0.90 1.22

Na2O 2.89 4.52 4.2 3.45 3.85 3.95 3.61 3.24 2.77 3.53 2.65 2.39

K2O 5.94 4.74 5.27 4.51 4.29 5.17 5.19 5.35 6.08 5.51 5.66 5.73

P2O5 0.03 0.13 0.05 0.20 0.23 0.08 0.04 0.10 0.04 0.05 0.06 0.23 LOI 2.43 0.36 0.08 --- 0.6 0.29 0.33 2.70 2.71 0.44 2.88 3.44 TOTAL 99.81 98.95 99.83 93.96 99.4 98.51 99.16 99.73 100.62 99.38 99.07 99.46

Ni 2 2 2 5 16 7 3 5 3 <1 3 4 Cr 5 <1 <1 <1 2 49 <1 5 <1 <1 <1 <1 Sc <1 9.1 8.9 10 12 8.4 4.0 4.4 3.3 5.0 4.0 6.0 V <1 8 4 13 49 16 <1 16 2 11 13 18 Ba 55 1173 1918 1875 3792 1376 240 2188 1148 1235 1185 1095 Rb 219 155 173 131 114 130 186 156 192 185 181 172 Sr 5 234 142 229 104 24 9 119 44 45 50 62 Zr 455 307 333 305 378 561 212 600 632 668 664 531 Y 603233434445427098939574 Nb 31.6 17.7 17.7 16.4 16.5 19.2 20.7 40.6 62.3 51.3 48.5 41.7 Ga 24.1 20.9 21.2 18.4 16.8 18.2 17.8 21.0 23.3 22.4 21.3 20.0 Cu 7 8 5 <2 25 12 6 16 8 <2 <2 62 Co232373 142223 Zn 107 63 107 93 87 97 57 87 117 97 116 84 Pb 32 24 23 20 19 23 26 24 28 30 29 29 U 8.1 <1 5.6 5.1 <1 <1 <1 4.6 <1 5.7 5.7 6.8 Th 22 15 15 14 16 16 23 19 25 24 23 24 Cs 7.33 --- 3.86 5.21 ------3.41 --- 2.83 ------Hf 12.5 --- 8.61 7.86 ------16.6 --- 18.4 ------Ta 2.17 --- 1.15 1.09 ------2.28 --- 3.38 ------La 62.6 48.0 48.9 44.5 42.9 41.8 55.2 70.9 100.0 100.7 89.0 86.0 Ce 120.6 67.9 88.8 79.3 76.6 57.3 80.2 133.7 149.5 178.0 191.0 182.0 Pr 13.8 --- 10.4 9.81 ------15.0 --- 21.8 ------Nd 54.1 --- 41.5 38.9 ------58.5 --- 85.1 ------Sm 12.2 --- 9.50 8.99 ------13.1 --- 19.3 ------Eu 0.28 --- 1.71 1.79 ------2.57 --- 1.9 ------Gd 10.8 --- 8.53 8.24 ------12.1 --- 17.7 ------Tb 1.75 --- 1.37 1.34 ------2.02 --- 3.00 ------Dy 10.6 --- 7.95 8.22 ------12.6 --- 18.5 ------Ho 2.13 --- 1.53 1.66 ------2.55 --- 3.71 ------Er 5.78 --- 4.01 4.60 ------7.04 --- 10.2 ------Tm 0.85 --- 0.58 0.68 ------1.06 --- 1.49 ------Yb 5.32 --- 3.56 4.32 ------6.56 --- 9.14 ------Lu 0.83 --- 0.53 0.69 ------1.02 --- 1.38 ------87 86 Sr/ Srm 0.705110 --- 0.705713 0.705685 ------0.706918 --- 0.706424 ------87 86 Sr/ Sri *see note --- 0.705009 0.705349 ------0.706080 --- *see note ------143 144 Nd/ Ndm 0.512764 --- 0.512670 0.512682 ------0.512429 --- 0.512476 ------143 144 Nd/ Ndi 0.512634 --- 0.512655 0.512667 ------0.512415 --- 0.512462 ------epsilon Ndi 0.4 --- 0.8 1.0 ------3.9 --- -3.0 ------206Pb/204Pb 19.024 --- 19.08 19.10 ------18.72 --- 18.71 ------207Pb/204Pb 15.616 --- 15.62 15.65 ------15.64 --- 15.61 ------208Pb/204Pb 38.710 --- 38.74 38.84 ------39.10 --- 38.97 ------

318 TABLE 3. Continued

Sample MB00-23 MB03-23 MB00-27 MB00-21B MB00-38D MB02-17A MB02-57A1 MB02-57B MB02-58A MB02-58B MB02-58C MB02-69 Pet. Unit Tp1 Tp1 Tp1 Tp2 Tp2 Tp2 Tp2 Tp2 Tp2 Tp2 Tp2 Tp2 Map Unit Tgl Tgl Tgl Tgl Tom Tsed Tsed Tsed Tsed Tsed Tsed Tgl

SiO2 75.16 73.67 75.27 70.90 67.73 72.07 68.08 70.58 69.59 69.07 67.20 69.95

TiO2 0.31 0.32 0.36 0.29 0.48 0.23 0.33 0.20 0.19 0.33 0.34 0.35

Al2O3 11.84 12.01 12.67 11.53 11.93 11.06 11.86 11.05 10.86 11.22 11.03 11.48

Fe2O3 2.95 2.70 2.04 2.83 4.37 1.95 3.10 2.37 2.36 3.57 3.91 3.39 MnO 0.02 0.03 0.02 0.05 0.06 0.10 0.06 0.04 0.04 0.07 0.08 0.06 MgO 0.14 0.19 0.04 0.05 0.24 0.09 0.21 0.01 0.01 0.14 0.11 0.09 CaO 0.30 0.26 0.17 0.76 1.55 0.26 1.18 0.45 0.45 1.04 1.16 1.01

Na2O 4.14 4.11 4.32 2.96 2.58 1.87 2.08 2.73 2.78 2.73 2.93 2.71

K2O 5.02 5.01 5.20 4.87 4.60 6.27 4.75 5.82 5.52 4.76 4.54 4.90

P2O5 0.06 0.03 0.04 0.03 0.08 0.15 0.01 0.02 0.04 0.08 0.01 0.20 LOI 0.82 1.12 0.44 TOTAL 100.75 99.47 100.56 94.26 93.62 94.04 91.66 93.27 91.85 92.99 91.32 94.14

Ni22234<1332926<1 Cr <1 <1 4 8 ------9 41 5 4 --- Sc 1.5 <1 1.7 5.3 7.3 5.6 7.4 <1 <1 6.8 7.6 6.3 V 6 11 15 --- 12 <1 ------<1 Ba 256 370 263 1373 1204 107 1658 727 709 1350 1405 1283 Rb 185 199 185 145 150 217 133 104 126 140 136 148 Sr 14 6 9 55 135 5 120 8 8 83 90 81 Zr 418 491 398 396 376 445 373 581 564 422 484 318 Y 57 82 69 51 49 78 52 106 104 59 69 48 Nb 25 27 24 17 11 16 13 34 35 26 27 6.4 Ga 25 24 25 ------Cu 8 <2 7 ------Co 2 2 3 ------Zn 129 171 110 99 98 117 98 169 154 98 114 93 Pb 17 29 24 ------U <1 9.0 <1 ------Th 19 19 18 ------Cs ------Hf ------Ta ------La 45 56 53 ------Ce 71 114 73 ------Pr ------Nd ------Sm ------Eu ------Gd ------Tb ------Dy ------Ho ------Er ------Tm ------Yb ------Lu ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

319 TABLE 4. RESULTS OF TWO COMPONENT MIXING CALCULATIONS FOR TWO SC SAMPLES

Part A Part B Part A Part B 63.4% 36.6%= Mix 52.2% 47.8% = Mix Obs. Calc. Obs. Calc. MB01-47 MB00-32B MB01-50 R MB01-73 MB01-63 MB02-31B R

SiO2 63.09 73.58 66.79 66.92 1.00 73.26 63.13 68.66 68.44 1.00

TiO2 1.18 0.28 0.78 0.85 1.09 0.28 1.02 0.4 0.63 1.58

Al2O3 15.49 13.26 14.7 14.67 1.00 14.93 15.41 15.36 15.16 0.99

Fe2O3 7.05 2.74 5.32 5.47 1.03 0.98 7.24 4.2 3.97 0.95 MnO 0.14 0.05 0.11 0.11 1.00 0.01 0.15 0.12 0.08 0.67 MgO 1.25 0.18 1.11 0.86 0.77 0.03 1.15 0.46 0.56 1.22 CaO 3.66 1.12 2.92 2.72 0.93 0.96 3.83 2.08 2.33 1.12

Na2O 4.15 3.42 3.77 3.88 1.03 4.22 3.78 3.69 4.01 1.09

K2O 3.53 5.3 4.29 4.18 0.97 5.29 3.96 4.82 4.66 0.97

P2O5 0.46 0.06 0.22 0.32 1.45 0.05 0.34 0.22 0.19 0.86 Σr2 0.167 0.325

Ni 4 2 5 3 0.62 23530.52 Sc 11 3.7 11 8.4 0.76 8.9 16 10 12 1.24 V 71 6 49 48 0.97 4 41 13 22 1.70 Ba 1179 1663 1372 1385 1.01 1918 1262 1884 1619 0.86 Rb 102 162 129 127 0.98 142 346 132 141 1.07 Sr 427 88 298 307 1.03 4 41 230 243 1.06 Zr 309 342 321 328 1.02 333 264 307 303 0.99 Y 40 35 38 39 1.03 33 44 44 39 0.89 La 46.0 46.1 44.5 46.9 1.05 48.9 39.4 44.7 44.8 1.00 Ce 90.7 86.6 93.2 90.9 0.98 88.8 77.2 79.7 84.1 1.06 Nd 45.4 39.0 44.2 43.8 0.99 41.5 39.1 39.1 40.8 1.04 Sm 10.5 8.85 8.90 10.0 1.12 9.50 9.39 9.00 9.60 1.07 Eu 2.63 1.28 2.10 2.20 1.05 1.71 2.36 1.80 2.00 1.11 Gd 9.36 7.75 8.50 8.90 1.05 8.53 8.85 8.30 8.80 1.06 Dy 8.05 7.54 6.80 8.00 1.18 7.95 8.60 8.30 8.40 1.01 Er 4.11 4.05 3.80 4.20 1.11 4.01 4.72 4.60 4.40 0.96 Yb 3.55 3.72 3.50 3.70 1.06 3.56 4.28 4.30 4.00 0.93 Obs.= observed concentrations; Calc.= calculated concentrations. R= the ratio of calculated daughter to the observed values. For major elements, the sum of the squared residuals is shown in bold. Major element concentrations are anhydrous values calculated by the "MIXING" module of IgPet2000.

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APPENDIX 1: ANALYTICAL METHODS

SAMPLE PREPARATION All rock samples were prepared for analysis through the following methods. Fist-sized hand samples were reduced using a hydraulic rock splitter and a representative sample of each was saved for reference. Smaller, split fragments were cut for thin sections and standard (27 x 46 mm), standard microprobe-polished, or large format (51 x 75 mm) thin sections were obtained commercially. Weathering rinds were then trimmed from the remaining sample and post- magmatic mineralization in void space within the sample was removed by the saw. A silicon carbide belt grinder was used to remove metal shavings from trimmed fragments prior to crushing through a jaw crusher. Prior to being passed through a Braun Chipmunk brand jaw crusher (steel crushing plates), rock fragments were washed in deionized water and kept at room temperature until dry. Once through a primary crush, samples were handpicked when necessary to remove any other post-magmatic mineralization and passed through again. The entire jaw- crushed sample was then passed through the alumina ceramic crushing plates of a Braun brand pulverizer (“disk-mill”). The entire sample was then divided by repeated “cone and quartering” to obtain a homogenous, 30 ml aliquot. This 30 ml aliquot was then run through a Spex brand alumina ceramic shatterbox for 10 minutes to ensure a fine grained powder for major and trace element flux fusions. Final powders were placed in borosilicate glass vials, dried uncapped at 100º C for 12-14 hours, and then stored in a desiccator (capped).

Laboratory Procedures for Silicic Glass Purification Tephra were processed initially closely following magnetic separation techniques detailed in Irvin (2002) and Katoh et al. (1999). In all cases, the dominant more felsic glass population was isolated through these methods. In special cases where two populations were observed petrographically, the magnetic properties of the samples seemed to substantiate their dual population component. For these samples, the more felsic population was chosen. Further purification methods listed below:

335 Purification of silicic glass using Lithium Hetero-polytungstates (LST) Density Liquid • Purified 80-100 mesh glass was crushed using a Boron Carbide mortar and pestle to <325 mesh (<45µ)

• LST – quartz distilled H O mixture prepared between 2.38 g/cm3 and 2.45 g/cm3 r 2.45 2

g/cm3 and 2.50 g/cm3 • Sample was centrifuged in LST to separate light fraction of glass at the top from the heavy fraction of microlitic impurities

• Light fraction (glass) was decanted, washed with quartz distilled H2O and dried at 60ºC

Outlined below are the specific laboratory preparation procedures used on each sample MB02-58A • Crush and sieve, recording mass of separate size fractions • DIAA wash and acid wash 80-100 mesh (180-150 µm) with 10% HCl for at least five minutes in ultrasonic bath, etch with 7% HF for approximately three minutes, rinse with deionized water and dry • Frantz 5 grams of 80-100 mesh (180-150 µm) at 0.25 A to remove magnetic materials and lithic fragments. Run non magnetic at 0.25 A at 1.0 A to remove feldspars. Run ~2.0 grams of 0.25-1.0 A at 0.5 A. Run 0.5-1.0 A at 0.55 A (3X), then 0.55-1.0 A at 0.95 A. Working split is 0.55-0.95 A

• Density liquid separation yielding 0.55-0.95 LST (L) @ 2.38 g/cm3 - 2.45 g/cm3 • Major and trace element run on DCP MB02-58C • Crush and sieve, recording mass of separate size fractions • DIAA wash • Acid wash both 60 to 80 ( 250-180 µm) and 80-100 (180-150 µm) mesh with 10% HCl for at least five minutes in ultrasonic bath, etch with 7% HF for approximately three minutes, rinse with deionized water and dry • Removal of ferromagnetic minerals with hand magnet • Frantz 60-80 size at 0.5 A; equal separation, so 80-100 size used • Run 80-100 size at 0.25 A (2X). Removed magnetic minerals and lithic fragments. Run at 0.75 A. Removed feldspars. Run at 0.27 A to remove lithics and glass with inclusions of

336 magnetic minerals. Stock saved as 0.27-0.65 A. Extract ~2 grams and run at 0.37 A (5X) and run again at 0.65 A (2X). • Handpick 0.37-0.65 A working fraction to remove altered glass and other contaminant material MB02-58B • Crush and sieve, recording mass of separate size fractions • DIAA wash and acid wash 80-100 mesh (180-150 µm) fraction with 10% HCl for at least five minutes in ultrasonic bath, etch with 7% HF for approximately three minutes, rinse with deionized water and dry • Removal of ferromagnetic minerals with hand magnet • Frantz 80-100 size (5 grams) at 0.25 A to remove magnetic minerals and lithic fragments. Run at 1.0 A and save 0.25-1.0 A as stock. Try run at 0.5 A; equal separations. Try run at 0.35A; sample appears to be mostly non-magnetic. Run non-magnetic material at 0.8 A. Working split 0.35-0.8 A. • Handpick ~1.0 grams of working split to remove altered glass and other contaminant material • Major and trace element run on DCP MB02-57A • Crush and sieve, recording mass of separate size fractions • DIAA wash and acid wash 80-100 mesh (180-150 µm) fraction with 10% HCl for at least five minutes in ultrasonic bath, etch with 7% HF for approximately three minutes, rinse with deionized water and dry • Removal of ferromagnetic minerals with hand magnet • Frantz 80-100 size (5 grams) at 0.25 A to remove magnetic minerals and lithic fragments. Run at 1.0 A to remove feldspars. Save 0.25-1.0 A as stock. Run ~ 2 grams of stock at 0.35 A. Run at 0.9 A. Small amount of feldspars and other contaminants removed. Try run at 0.75 A. Not a good split. Run at 0.8 A. Sample not as clean as possible. Run at 0.38 A. Working split is 0.38-0.8 A. • Crude handpicking of entire working split. Lots of crystals so proceed to LST procedure

• Density liquid separation yielding (1) 0.38-0.8 LST (L) @ 2.38 g/cm3 - 2.45 g/cm3; heavy

materials from (1) separated to yield (2) 0.38-0.8 LST (L) @ 2.45 g/cm3 - 2.50 g/cm3

337 • Major and trace element run on DCP MB00-21B • Crush and sieve, recording mass of separate size fractions • DIAA wash and acid wash 80-100 mesh (180-150 µm) fraction with 10% HCl for at least five minutes in ultrasonic bath, etch with 7% HF for approximately three minutes, rinse with deionized water and dry • Frantz 80-100 size (5 grams) at 0.25 A to remove magnetic minerals and lithic fragments. Run at 1.0 A to remove feldspars. Save 0.25-1.0 A as stock. Run ~ 2 grams of stock at 0.35 A. Run at 0.9 A. Run magnetic at 0.9 A at 0.33 A. Working split 0.33-0.9 A • Handpick ~1 gram of working split to remove obvious contaminants

• Density liquid separation yielding 0.33-0.9 LST (L) @ 2.45 g/cm3-2.50 g/cm 3 • Major and trace element run on DCP MB02-57B • Crush and sieve, recording mass of separate size fractions • DIAA wash and acid wash 80-100 mesh (180-150 µm) fraction with 10% HCl for at least five minutes in ultrasonic bath, etch with 7% HF for approximately three minutes, rinse with deionized water and dry • Frantz 80-100 size (5 grams) at 0.1 A (minor magnetics); run at 0.25 A (minor magnetics). Extract ~2 grams of non magnetic at 0.25 A for working split. Run working split at 0.55 A (3X). Run non magnetic at 0.55 A at 0.55 A (3X). Working fraction is 0.55-0.95 A. • Density liquid separation yielding 0.55-0.95 A LST (L) @ 2.45 g/cm3-2.50 g/cm3 • Major and trace element run on DCP MB02-17A • Crush and sieve, recording mass of separate size fractions • DIAA wash and acid wash 80-100 mesh (180-150 µm) fraction with 10% HCl for at least five minutes in ultrasonic bath, etch with 7% HF for approximately three minutes, rinse with deionized water and dry • Removal of ferromagnetic minerals with hand magnet • Frantz 80-100 size (~3.7 grams) at 0.25 A to remove magnetic minerals and lithic fragments. Run at magnetic at 0.25 A at 1.0 A to remove feldspars. Run at magnetic at

338 1.0 A at 0.35 A. Run at 0.8 A. Sample appears to have two populations or a complex mixture. Run magnetic at 0.35 A. Run 0.35-0.8 A at 0.58 A. Final split is 0.58-0.8 A • Major and trace element run on DCP MB02-69 • Crush and sieve, recording mass of separate size fractions • DIAA wash and acid wash 80-100 mesh (180-150 µm) fraction with 10% HCl for at least five minutes in ultrasonic bath, etch with 7% HF for approximately three minutes, rinse with deionized water and dry • Frantz 80-100 size (5 grams) at 0.25 A to remove magnetic minerals and lithic fragments. Run at 1.0 A to remove feldspars. 3 grams of 0.25-1.0 A run at 0.35 A. Run at 0.9 A and 0.8 A. Sample appears to have two populations. Try magnetic at 0.8 A at 0.55 A; majority of the glass is magnetic. Working split now 0.35-0.55 A. • Major and trace element run on DCP MB00-38D • Crush and sieve, recording mass of separate size fractions • DIAA wash and acid wash 80-100 mesh (180-150 µm) fraction with 10% HCl for at least five minutes in ultrasonic bath, etch with 7% HF for approximately three minutes, rinse with deionized water and dry • Frantz 80-100 size (5 grams) at 0.25 A to remove magnetic minerals and lithic fragments. Run at 1.0 A to remove feldspars. 3 grams of 0.25-1.0 A run at 0.35 A. Run at 0.9 A and 0.8 A. Sample appears to have two populations. Try magnetic at 0.8 A at 0.55 A (2X). Working split now 0.35-0.55 A • Major and trace element run on DCP MB01-55 • Crush and sieve, recording mass of separate size fractions • DIAA wash and acid wash 80-100 mesh (180-150 µm) fraction with 10% HCl for at least five minutes in ultrasonic bath, etch with 7% HF for approximately three minutes, rinse with deionized water and dry • Frantz 80-100 size (5 grams) at 0.25 A to remove magnetic minerals and lithic fragments. Run at 1.0 A to remove feldspars. 3 grams of 0.25-1.0 A run at 0.9 A. Magnetic material

339 from 0.9 A run at 0.3 A (2X). Run non magnetic at 0.3 A at 0.5 A, then 0.6 A. Working split now 0.6-0.9 A • Handpick 600 milligrams of sample to remove obvious contaminants • Major and trace element run on DCP MB01-54A • Crush and sieve, recording mass of separate size fractions • DIAA wash and acid wash 80-100 mesh (180-150 µm) fraction with 10% HCl for at least five minutes in ultrasonic bath, etch with 7% HF for approximately three minutes, rinse with deionized water and dry • Frantz 80-100 size (5 grams) at 0.25 A to remove magnetic minerals and lithic fragments. Run at 1.0 A to remove feldspars. 3 grams of 0.25-1.0 A run at 0.9 A. Magnetic material from 0.9 A run at 0.3 A (2X). Run 0.3-0.9 @ 0.6 A to yield a working split of 0.6-0.9 A • Handpick 600 milligrams of sample to remove obvious contaminants • Major and trace element run on DCP

LOSS ON IGNITION (LOI) LOIs were determined for each rock sample analyzed for major and trace element chemistry. ~1 gram of sample was weighed into a ceramic crucible and the weight was recorded. The powder and crucible was heated to 950º C for one hour, cooled at room temperature, and then weighed.

The LOI was calculated by the equation: LOI = ((weightinitial - weightfinal)/weight initial)*100 which solves for the total loss on ignition (weight percent).

MAJOR AND TRACE ELEMENT ANALYSIS New whole-rock major and trace element analyses were performed using direct current argon plasma atomic emission spectroscopy (DCP-AES) at Miami University and X-ray fluorescence (XRF) at Franklin and Marshall College. REE and a suite of trace elements were also analyzed on a subset of samples by Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) at Washington State University.

340 Whole rock Samples analyzed by DCP-AES analysis were prepared by manually mixing 200 mg of sample

with 600 mg of purified LiBO2 flux (Spectroflux 100A, manufactured by Johnson Mathey Materials Technology). This mixture was then fused in a graphite crucible at 950º C for 20 minutes. The molten bead generated during the fusion process was dropped into 50 ml of a 6%

HNO3 solution spiked with 3000 ppm Li, 10 ppm Ge, and 20 ppm Cd and shaken vigorously until dissolved. This solution stood at room temperature for at least 12 hours and was then ready for analysis (the trace element stock solution; TESS). A 1 ml aliquot of the TESS was added to

25 ml of a 6% HNO3 solution spiked with 3000 ppm Li and 30 ppm Ge. Li was present as a plasma enhancer and matrix suppressant, while the Cd (major) and Ge (trace) spikes are used as internal references that within-run background corrected element intensities are normalized to for reduction of in-run machine/plasma drift. In every major element run, one procedural blank

(LiBO2 only) and the same 10 international rock standards (JG-2, RGM-1, GSP-1, MA-N, SDC- 1, W-2, BE-N, SY-2, DNC-1, and JGb-1) were prepared and analyzed with the batch of unknown samples. Throughout the procedure, precise masses of all measured solids and solutions were recorded so that dilution factors and final concentrations could be determined accurately. During each analytical run, each sample solution was analyzed three times and the standard solutions and blank were analyzed four times. A multi-element cassette was used to allow simultaneous determination of major elements. Data collection and reduction were performed by specific software written for the DCP-AES at Miami University. Major element concentrations were determined by comparison of the intensity ratio if an unknown to calibration curves generated by the rock standards run with a given set of samples. XRF trace element analyses (Rb, Sr, Y, Zr, Nb, Ni, Ga, Cu, Zn, U, Th, Co, Pb, Sc, Cr and V. La, Ce, and Ba) were performed on all samples in this study except purified glass separates (those discussed earlier) by Dr. Stan Mertzman at Franklin and Marshall College, following the methods described in Mertzman (2000). These are outlined and described at the following Internet site: http://server1.fandm.edu/departments/earthandenvironment/facilities/xrf/index.html. ICP-MS trace element analyses (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ba, Th, Nb, Y, Hf, Ta, U, Pb, Rb, Cs, Sr, Sc, Zr) were performed on a suite of samples at the

341 GeoAnalytical laboratory at Washington State University following the methods of Crock and Lichte (1982), Lichte et al. (1987), and Doherty (1989). These are outlined and described at the following Internet site: http://www.wsu.edu:8080/~geology/geolab/note/icpms.html. In Appendix 3, all reported trace element data are XRF values with the following exceptions: 1) For those samples with Hf, Ta, and Cs data, the reported Hf, Ta, Cs, Rb, Pb, U, Th, La, and Ce values are inductively coupled plasma mass spectrometry (ICP-MS) data run at Washington State University; 2) the trace element data reported for MB01-54B and all Tp2 samples are DCP-AES values run at Miami University; 3) Rb and Sr concentrations in bold are isotope dilution results.

Purified glass separates Tephra were analyzed via a slightly different method than whole rock samples. 0.200 ± 0.001 g of purified and dried <325 mesh (<45 µ) glass was combined with 0.600 ± 0.005 LiBO2 in a graphite crucible. This mixture was then fused at 950º C for 15 minutes. Trace element analyses on purified glass separates were performed by DCP-AES at Miami University via a standard addition technique. Further details are found in Katoh et al. 1999

ISOTOPE ANALYSIS A subset of samples was chosen for whole rock Sr, Nd, and Pb isotopic analyses. All isotopic compositions were measured by a Thermo-Finnigan Triton multi-collector thermal ionization mass spectrometer (TIMS) at Miami University. Approximately 0.1 - 0.2 g of whole rock powder was dissolved in HF-HNO3, prior to chemical separation. Except for Sm - Nd separation, these procedures follow Walker et al. (1989) and are summarized in Snyder (2005). Because of the high Rb/Sr ratio of some SC silicic units, Isotope Dilution was utilized for a subset of SC samples. For this subset, Rb concentrations were determined by ICP-MS in the Department of Chemistry at Miami University (see note at end of previous section). Sm - Nd separations were performed by methods similar to Pin and Zalduegui (1997), using EiChrom Ln- Spec resin (also summarized in Snyder, 2005). Strontium isotopic ratios were fractionation corrected using 86Sr/88Sr = 0.1194. Sixty-eight measurements of the NBS 987 strontium standard gave an average of 87Sr/86Sr = 0.710236 ± 0.000014 (2 SD). Neodymium isotopic ratios were

342 fractionation corrected using 143Nd/146Nd = 0.7219. Sixty-one measurements of the LaJolla neodymium standard gave an average of 143Nd/144Nd = 0.511846 ± 0.000007 (2 SD). 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb were fractionation corrected by 0.1% per amu based on measured ratios in NBS 981 from values in Todt et al. (1996). Errors on measured values of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb (2 SD) based on fifty-eight measurements of NBS 981 were ± 0.01, ± 0.02, and ± 0.06 respectively.

For a detailed description of the 40Ar/39Ar method geochronology employed in this study, see Appendix 4.

343 APPENDIX 2: SAMPLE LOCATIONS AND DESCRIPTIONS

Sample locations, descriptions, and select petrographic descriptions for those samples collected ant analyzed for elemental/isotopic geochemistry. Representative samples were point counted using a Swift Point Counter with a target of 1000 counts per thin section and a step interval of two. Microscopic textures and point counted modes directly follow field descriptions for this subset. Samples are listed in the order they were collected. Northing and Easting correspond to UTM Zone 11 T and the map datum is NAD 27 CONUS.

Sample ID: MB00-1 Petrologic Type: Tad1 Map Type: Thc Northing: 456539 Easting: 4611169 Description: Vesicular, dark gray matrix with abundant small plagioclase phenocrysts (up to 3 mm).

Sample ID: MB00-2, 3 Petrologic Type: Thc Map Type: Thc Northing: 456372 Easting: 4611475 Description: Rhyolite, massive with gray/purple altered matrix. Abundant feldspars. Appears to be upper part of flow, cut by dike of similar material. MB00-3 is fresher sample of the same.

Sample ID: MB00-5 Petrologic Type: Tpr Map Type: Tpr Northing: 456173 Easting: 4611864 Description: Abundant feldspar and amphibole, no xenoliths. Some dark brown/golden feldspar present? Further east along ridge, this outcrop connects to a definite dike of xenolith-rich Tpr. The dike trends N5W at its chilled contact with a Thc lava flow. Xenoliths are dark green mafic material and up to ~10 cm. Abundant rounded feldspars are also present

Sample ID: MB00-6 Petrologic Type: Tad1 Map Type: Thc Northing: 455606 Easting: 4612602 Description: From massive zone of rubbly outcrop. Vesicular in places with stretched vesicles. Fine grained gray matrix with abundant feldspar crystals and also mafic minerals (olivine/pyroxene) present. Very fresh.

Sample ID: MB00-7 Petrologic Type: Ta Map Type: Thc Northing: 455548 Easting: 4612610 Description: Extremely fine grained dark matrix, sampled from approximately 2 m down section/below potential vent. “Vent” characterized by extremely scoriaceous (small), vesicular and oxidized material. Even massive blocks appear oxidized.

344 Sample ID: MB00-8 Petrologic Type: Thc Map Type: Thc Northing: 454927 Easting: 4612880 Description: Sample is massive fine grained gray/blue matrix. Small (1-2mm) crystals of feldspar, quartz, and mafic mineral (pyroxene?). No flow banding and no large equant feldspars.

Sample ID: MB00-9 Petrologic Type: Thc Map Type: Thc Northing: 454983 Easting: 4613020 Description: Sample is flow banded, equant feldspar-rich silicic lava flow.

Sample ID: MB00-10 Petrologic Type: Twsc Map Type: Twsc Northing: 454910 Easting: 4613135 Description: Sample of upper platy intermediate flow from massive portion underlying vesicular zone. Dark gray, fine-grained matrix. Small equant (1-2 mm) plagioclase crystals present in sample, otherwise void of crystals.

Sample ID: MB00-11 Petrologic Type: Tba Map Type: Thc Northing: 454865 Easting: 46113197 Description: From upper mafic lava flow. Abundant in microphenocrysts of plagioclase, gray dark matrix. Trachytic texture and possible olivine. Flow 5-6 m thick.

Sample ID: MB00-12 Petrologic Type: Tba Map Type: Thc Northing: 454850 Easting: 4613207 Description: Trachytic plagioclase microphenocrysts in dark gray matrix; olivine also present.

Sample ID: MB00-13 Petrologic Type: Tba Map Type: Thc Northing: 454851 Easting: 4613247 Description: Course ground, plagioclase-phyric mafic lava flow. At least the third flow, possibly basal. Gray, open-textured matrix, abundant alteration, lots of plagioclase crystals (up to 1 cm long). Possible olivine.

Sample ID: MB00-15A Petrologic Type: Tpr Map Type: Tsi Northing: 454679 Easting: 4612908 Description: Northeast vitrophyre/chilled margin of dike. Real fresh. Abundant equant feldspars and quartz in dark black, glassy matrix. Xenoliths of basalt and green material present within the chilled zone. Crystals of feldspar up to 0.5 cm long.

Sample ID: MB00-16 Petrologic Type: Tad1 Map Type: Thc Northing: 464148 Easting: 4614200 Description: Gray, fine-grained matrix. Almost appears to be slightly open textured. 1-2 mm plagioclase/feldspar microphenocrysts in the matrix.. No visible mafic minerals. Sample taken from blocky material near base of exposure. Entire outcrop is spheroidally weathered. Vesicles are also present throughout the outcrop and are found up to 10 cm in length.

Sample ID: MB00-17 Petrologic Type: Tad1 Map Type: Thc Northing: 464148 Easting: 4614200 Description: Sample taken from upper unit. Fine grained and glassy compared to lower sample (MB00- 16). Black-gray matrix with abundant feldspars.

345 Sample ID: MB00-18 Petrologic Type: Tpr Map Type: Tpr Northing: 462074 Easting: 4613975 Description: Abundant feldspar crystals up to 1.5 cm long and 1 cm wide. Amphibole present as small laths (0.4 cm) and abundant quartz crystals (~2 cm) are also present. Small biotite crystals appear to be present (<1cm). All found in a glassy gray matrix. Overall, very coarse-grained. Hypocrystalline phaneritic matrix. Inequigranular crystals. Rounded quartz, sieved and resorbed plagioclase, zoned plagioclase (oscillatory), rounded amphibole, poikilitic biotite and opx in plagioclase and zircon in orthopyroxene. Plagioclase + orthopyroxene + quartz clots. Plagioclase composition ranges from An9 to An67. Biotite up to 1.25 mm, sanidine and anorthoclase (microcline?) up to 1 mm, plagioclase up to 2 mm, hornblende up to 2 mm, hypersthene, clinopyroxene, zircon, apatite, quartz, up to 2 mm. Mode: Plag 13.58%; oxides 1.16%; potassium feldspar 2.50%; orthopyroxene 1.33%; clinopyroxene 0.58%; zircon 0.41%; apatite 0.25%; quartz 4.25%; biotite 2.41%; amphibole 1.08%; clots and resorbed plagioclase 18.67%; matrix 53.75%.

Sample ID: MB00-20 Petrologic Type: Tp1 Map Type: Tgl Northing: 462775 Easting: 4614605 Description: Welded-ash flow. Pink matrix with large flattened pumice fragments (4+ cm long). Glassy/rock fragments abundant; gray in color. Feldspars present (1-2 mm). Outcrop is ~2 m thick.

Sample ID: MB00-21B Petrologic Type: Tp2 Map Type: Ttuf Northing: 462704 Easting: 4614424 Description: Bedded fall deposits from ~4 m thick exposure. Underlies MB00-20. Medium grained, indurated glass shards. Pumiceous shards are present and are dominantly clear. <2% darker glass. Lying directly below is a coarse, crystal-rich zone that is present to the base of the exposure.

Sample ID: MB00-22 Petrologic Type: Tpr Map Type: Tpr Northing: 462412 Easting: 4612636 Description: From margin of Tpr lava flow. Glassy matrix (frothy) that alternates to zones of more massive, rocky matrix. Also appears to be altered.

Sample ID: MB00-23 Petrologic Type: Tp1 Map Type: Tgl Northing: 462452 Easting: 4615107 Description: Welded ash flow. Massive light gray matrix, conchoidally fracture in places. Fiamme-rich and small feldspars (1-2 mm) are also present.

Sample ID: MB00-24 Petrologic Type: Tad1 Map Type: Thc Northing: 462424 Easting: 4615127 Description: Very dark glassy matrix with microphenocrysts of plagioclase. Appear to be some pyroxenes.

Sample ID: MB00-26A Petrologic Type: Tad2 Map Type: Twsc Northing: 454596 Easting: 4623796 Description: Dark, fine-grained matrix from block in an overall very platy outcrop. Aphyric with sparse plagioclase phenocrysts and maybe pyroxene in the groundmass.

346 Sample ID: MB00-26B Petrologic Type: Tad2 Map Type: Twsc Northing: 454596 Easting: 4623796 Description: Pink, very fine-grained matrix. Some feldspars and maybe altered mafic crystals. Seems to be coarser-grained than MB00-26B. Sample collected from oxidized breccia that overlies 26A lava flow.

Sample ID: MB00-27 Petrologic Type: Tp1 Map Type: Tgl Northing: 456235 Easting: 4622454 Description: Welded ash flow. Light gray/white matrix with abundant large pumice shards (up to 4+ cm). Quartz and abundant feldspar crystals (2-3 mm).

Sample ID: MB00-28 Petrologic Type: Tpr Map Type: Tsi Northing: 454534 Easting: 4623632 Description: ~3 m thick Tpr dike that trends 8-10º. Light gray to pink/purple matrix with abundant feldspar (2-3 mm and less), biotite (1-2 mm), and quartz (1-2 mm).

Sample ID: MB00-29 Petrologic Type: Twsc Map Type: Twsc Northing: 454184 Easting: 4623801 Description: Extremely fine grained groundmass. Dark gray to black with some small (<1mm) feldspars in the matrix. Occasional, sparse slightly large feldspars (1-2 mm wide) are also present.

Sample ID: MB00-30B Petrologic Type: Tba Map Type: Tsb Northing: 453783 Easting: 4624034 Description: Upper most exposed mafic lava flow. Coarse grained, dark gray matrix with abundant alteration. ~1cm plagioclase phenocrysts present but the flow is not plagioclase-phyric. Numerous amygdaloidal quartz/chalcedony filled vesicles. Small feldspar laths present in matrix.

Sample ID: MB00-31 Petrologic Type: Tad2 Map Type: Twsc Northing: 453219 Easting: 4624512 Description: Dark gray/blue with slight tinge of red matrix. Sparse feldspars (1-2mm). Also appears that pyroxene or biotite is present. Overall outcrop is ~10 m thick.

Sample ID: MB00-32B Petrologic Type: Tcm Map Type: Tcm Northing: 475284 Easting: 4626584 Description: Basal vitrophyre of Mahogany Pass lava flow. Dark black vitrophyric matrix with white to light gray feldspars and quartz. Perlitic fracture in places and slight pervasive, devitrification. Perlitic fracturing, holohyaline matrix. Rounded and embayed quartz (± reaction rims), embayed sanidine, rounded/resorbed/sieved plagioclase. Plagioclase + cpx + oxide clots up to 2 mm. Granitoid xenoliths with kinked (twinning) plagioclase also present. Plagioclase phenocryst composition= An11; plagioclase from clots= An60. Augite, hypersthene, plagioclase up to 1 mm, quartz up to 0.5 mm, sanidine up to 0.7 mm, apatite, oxides, and zircon. Mode: plag 1.41%; oxides 0.41%; sanidine 1.66%; hypersthene 0.08%; augite 0.25%; zircon 0.25%; apatite 0.33%; quartz 2.25%; clots + xenoliths 9.91%; void 10.41%; matrix 73.00%.

347 Sample ID: MB00-33 Petrologic Type: Tom Map Type: Tom Northing: 469083 Easting: 4638444 Description: Blue/gray to white spotty matrix with abundant 1-3 mm feldspars. Slight hint of an open texture in in the matrix. <2 mm quartz and oxidized mafic minerals also present. Spherulitic, hypocrystalline matrix, with some resorbed plagioclase. Poikilitic zircon in orthopyroxene. Also, some pyroxene is highly altered. Quartz up to 1.5 mm, plagioclase up to 0.75 mm, sanidine up to 2 mm, orthopyroxene up to 0.5 mm, clinopyroxene up to 0.5 mm, oxides up to 0.5 mm, apatite and zircon. Mode: Plag 0.50%; oxide 0.91%; sanidine 8.58%; orthopyroxene 1.46%; clinopyroxene 0.08 %; quartz 2.75%; zircon 0.25%; apatite 0.41%; void 2.83%; matrix 82.91%.

Sample ID: MB00-35 Petrologic Type: Tom Map Type: Tom Northing: 467954 Easting: 4638094 Description: Material is dark gray, find grained with abundant feldspars. ~2-5 cm mafic xenoliths are also present.

Sample ID: MB00-36B Petrologic Type: Tom Map Type: Tom Northing: 468450 Easting: 4637062 Description: Pink to light purple banded coarse matrix with abundant feldspars. Some quartz appears to be present also.

Sample ID: MB00-37 Petrologic Type: Tom Map Type: Tom Northing: 468472 Easting: 4636691 Description: Dark gray-black vitrophyre. Abundant 1-3 mm feldspars and flow banding is also present in matrix. Vitrophyre of lava flow.

Sample ID: MB00-38D Petrologic Type: Tp2 Map Type: Tom Northing: 468546 Easting: 4636597 Description: From at least 5 m thick exposure of tuff. MB00-38D is from upper, fine-grained (fining upward) non-laminated zone. Abundant gray glass.

Sample ID: MB00-41 Petrologic Type: Tom Map Type: Tom Northing: 468822 Easting: 4635586 Description: Highly welded ash flow. Flow banded matrix with mafic xenocrysts. Abundant 1-2 mm feldspars. While present, xenoliths are much less abundant than ~12 m downsection in the same flow.

Sample ID: MB00-42 Petrologic Type: Tp1 Map Type: Tgl Northing: 462879 Easting: 4632110 Description: Welded ash flow. At least 3 m thick outcrop. Light gray matrix with abundant devitrified fiamme. Abundant 1-3 mm feldspar crystals.

Sample ID: MB00-44 Petrologic Type: Tad2 Map Type: Twsc Northing: 460814 Easting: 4635614 Description: Sampled massive zone in rubbly outcrop. Gray, very fine-grained matrix. Conchoidal fracture in places. 1-3 mm feldspars are present but sparse.

348 Sample ID: MB00-45 Petrologic Type: Tad2 Map Type: Twsc Northing: 461000 Easting: 4636921 Description: Very fine-grained gray to purple matrix. Lots of alteration in vesicles. Some 1-3 mm feldspars and the overall outcrop is extremely platy. Platy jointing is vertical in some cases.

Sample ID: MB00-46 Petrologic Type: Tp1 Map Type: Tgl Northing: 452842 Easting: 4631002 Description: From welded ash-flow outcrop that is at least 30 m thick. Gray-green platy breaking matrix. Abundant 2-5 cm fiamme and 1-3 mm feldspars.

Sample ID: MB00-47 Petrologic Type: Tba Map Type: Twsc Northing: 453183 Easting: 4630664 Description: Lowermost exposed mafic lava flow in valley. Dark gray, open-textured matrix with abundant orange alteration. Plagioclase and olivine present, no discrete phenocrysts of either.

Sample ID: MB01-1 Petrologic Type: Tarc Map Type: Tarc Northing: 449632 Easting: 4617566 Description: Fine to medium grained matrix. Abundant <2 mm plagioclase microphenocrysts and no noticeable mafic crystals. Outcrop has slight columnar jointing and pahoehoe flow-tops in places and is a small knob that directly overlies metamorphic basement.

Sample ID: MB01-2 Petrologic Type: Tarc Map Type: Tarc Northing: 450151 Easting: 4617302 Description: Fine-grained, sugary dark gray matrix. Sparse <1 mm plagioclase crystals. Very fresh matrix. Blocky flow with brown weathering rind, overlies metamorphic basement.

Sample ID: MB01-4 Petrologic Type: Tarc Map Type: Tarc Northing: 450510 Easting: 4617135 Description: Within package of four flows, sampled from autoinjection coming through upper flow-top breccia. Very fine-grained blue-gray platy matrix. No vesicles present, weathers to brown. Flow appears to be 20 – 30 m thick.

Sample ID: MB01-5 Petrologic Type: Tarc Map Type: Tarc Northing: 450604 Easting: 4616873 Description: Above flow top breccia of underlying lava flow. Extremely fine-grained sugary texture and conchoidal weathering. Very platy and weathers blue-brown.

Sample ID: MB01-6A Petrologic Type: Tba Map Type: Thc Northing: 450844 Easting: 4616690 Description: 1.5 - 3 m high vertical spires of monolithologic breccia. Glassy and dark, aphyric material in matrix and also lighter, ash-like material present also. Tephra deposits locally exposed also. Highly oxidized tephra in places and oxidized scoriaceous material is abundant. Appears to be eroded mafic vent. From knob within spires, fine-medium grained dark matrix with slight open- texture. Plagioclase microphenocrysts also present in places. Hypocrystalline, intersertal matrix. Subophitic cpx + plag, resorbed/sieved plag, some with rims, olivine with cpx (?) rims. Plagioclase up to 0.6 mm, some of these are rounded. Olivine up to 0.25 mm, oxides, clinopyroxene up to 0.2 mm, and apatite (in matrix). Plagioclase composition ranges from An25 to An45. Mode: Plag 38.67%; oxides 6.33%; clinopyroxene 13.99%; olivine 3.83%; matrix 36.67%.

349 Sample ID: MB01-8 Petrologic Type: Tba Map Type: Thc Northing: 451111 Easting: 4616443 Description: Dense, black/gray massive matrix. Abundant plagioclase crystals. Lava flow overlies tuffs exposed in a landslide scarp, appears that the tuffaceous material was baked by flow.

Sample ID: MB01-9 Petrologic Type: Tba Map Type: Thc Northing: 451159 Easting: 4616397 Description: Dark gray matrix with slight open texture. Abundant plagioclase microphenocrysts that are up to 0.5 mm long. Slight columnar jointing present.

Sample ID: MB01-10 Petrologic Type: Ta Map Type: Thc Northing: 451265 Easting: 4616351 Description: Reddish-black platy weathering outcrop. Light gray, slightly open-textured matrix with abundant vesicles. Plagioclase microphenocrysts present.

Sample ID: MB01-12 Petrologic Type: Tb Map Type: Thc Northing: 451323 Easting: 4616329 Description: Mafic lava flow that overlies baked tuffaceous material and siliceous sinter. ~3 - 4 m thick with tortoise shell fracturing. Dark gray/blue very sugary fine-grained matrix. Abundant plagioclase laths (1 - 3 mm) and red/black ophiomottling. Holocrystalline, ophitic to subophitic clinopyroxene and plagioclase. One large resorbed plagioclase (up to2 mm) present; composition is An65. Plagioclase up to 2 mm (composition varies between An46 to An60), olivine up to 0.5 mm, oxides, clinopyroxene, apatite and “clay” in matrix. Mode: Plag 56.83%; oxides 7.25%; olivine 14.50%; clinopyroxene 15.83%; resorbed plag crystal 1.16%; matrix 4.42%.

Sample ID: MB01-13 Petrologic Type: Tb Map Type: Thc Northing: 451406 Easting: 4616233 Description: Medium-grained, slightly open textured matrix. Plagioclase microphenocrysts (~1 mm) are abundant.

Sample ID: MB01-14 Petrologic Type: Twsc Map Type: Twsc Northing: 451401 Easting: 4616036 Description: From platy massive, interior portion of lava flow. Abundant 1 - 3 mm feldspar laths in a very fine-grained dark gray matrix. Brown/red rusty weathering rind. Pink and flow-banded near the top of the lava flow.

Sample ID: MB01-15 Petrologic Type: Twsc Map Type: Twsc Northing: 451415 Easting: 4615976 Description: Platy, but not oxidized outcrop. Abundant plagioclase laths present (1 - 2) mm in a very fine-grained blue/gray matrix. Sampled above highly oxidized zone.

Sample ID: MB01-16 Petrologic Type: Tarc Map Type: Tarc Northing: 451256 Easting: 4615493 Description: Block to platy weathering outcrop. Highly brecciated in places also with numerous cobble sized glassy clasts present in the breccia. Very fine-grained and banded matrix from sampled material.

350 Sample ID: MB01-17 Petrologic Type: Tb Map Type: Thc Northing: 450868 Easting: 4615494 Description: Mafic dike. Trends 65°. Sugary, open-textured matrix that is plagioclase and olivine rich. Dike terminates at mound of brecciated and highly oxidized material suggesting an eroded vent.

Sample ID: MB01-18 Petrologic Type: Tarc Map Type: Tarc Northing: 450931 Easting: 4615665 Description: Very fine-grained black matrix with abundant plagioclase microphenocrysts. Conchoidal fracture also. Sampled from four clasts within a monolithologic breccia. In this breccia, some clasts are oxidized, some are not. Breccia appears to be upper breccia of lava flow.

Sample ID: MB01-19 Petrologic Type: Tarc Map Type: Tarc Northing: 450979 Easting: 4615829 Description: Aphanitic dense gray matrix. Feldspar microphenocrysts also present and the matrix fractures conchoidally.

Sample ID: MB01-20 Petrologic Type: Tarc Map Type: Tarc Northing: 451089 Easting: 4615776 Description: Exposure of platy weathering intermediate unit. Sample from a blockier zone, medium gray matrix with abundant orange alteration. Sparse, reddish plagioclase crystals that are up to 3 mm long are also present.

Sample ID: MB01-21 Petrologic Type: Tarc Map Type: Tarc Northing: 451336 Easting: 4615481 Description: Contact between Tba dike and Tarc lava flow. Sample is from within a Tarc breccia. Extremely aphyric black, dense matrix. Sparse plagioclase also present.

Sample ID: MB01-22 Petrologic Type: Tba Map Type: Tmi Northing: 451336 Easting: 4615481 Description: Tba dike that trends 10°. Fine-grained, blue matrix with abundant plagioclase crystals (~1 mm).

Sample ID: MB01-23 Petrologic Type: Tba Map Type: Thc Northing: 452556 Easting: 4615837 Description: Fine-grained vesicular dark gray matrix. Slight alteration appears present.

Sample ID: MB01-24 Petrologic Type: Tb Map Type: Thc Northing: 452668 Easting: 4615722 Description: Plagioclase-phyric basalt flow. Plagioclase laths up to 0.5 cm long with some olivine also. Gray matrix. Lava flow dips to the east/northeast at 20°.

351 Sample ID: MB01-25 Petrologic Type: Tba Map Type: Tmi Northing: 452204 Easting: 4615387 Description: Mafic stock that resembles Chocolate Mountain. Massive, very fine-grained sugary matrix with plagioclase microphenocrysts between 1 - 2 mm. Columnar jointing present in places and another similar stock is exposed locally (this is the southernmost of the pair). Hypocrystalline, slightly altered matrix (calcite present). Subophitic plagioclase + clinopyroxene. Plagioclase up to 0.5 mm, clinopyroxene (Ti-augite?) up to 0.25 mm, oxides up to 0.1 mm, olivine up to 0.5 mm, apatite (in matrix). Mode: Plag 37.75%; oxides 8.00%; clinopyroxene 12.33%; olivine 3.83%; matrix 38.08%.

Sample ID: MB01-26 Petrologic Type: Tom Map Type: Tcm Northing: 473596 Easting: 4630042 Description: Stratigraphically (and topographically) highest unit exposed in the Calico Mountains. Sampled from summit of Capitol Peak. Gray/pink matrix with abundant feldspars and opaline quartz (blue sheen present). Orange mafics (pyroxene) also present in hand sample. Appears to be slightly altered.

Sample ID: MB01-27 Petrologic Type: Tom Map Type: Tom Northing: 474243 Easting: 4631219 Description: From lava flow exposed below MB01-26. Glassy vitrophyric matrix with numerous 1 - 3 mm feldspar phenocrysts. Slightly devitrified, hypocrystalline matrix. Resorbed quartz and sanidine, poikilitic oxides in zircon. Sanidine up to 3 mm, quartz up to 1 mm, clinopyroxene up to 0.4 mm, orthopyroxene up to 1.5 mm, clinopyroxene up to 0.4 mm, zircon up to 0.1 mm, oxides and apatite. Mode: Plag 0.08%; Oxide 1.25%; Sanidine 10.58%; orthopyroxene 1.33%; clinopyroxene 0.83%; quartz 3.08%; zircon 0.16%; apatite 0.33%; void 3.66%; matrix 78.66%.

Sample ID: MB01-28A Petrologic Type: Tcm Map Type: Tom Northing: 474014 Easting: 4631060 Description: From upper portion of Capitol Peak ash flow. Highly oxidized and welded matrix with abundant feldspar and quartz. Sparse mafics also present.

Sample ID: MB01-28B Petrologic Type: Tcm Map Type: Tcm Northing: 474014 Easting: 4631060 Description: Banded, but similar to MB01-28B. Gray/blue altered matrix with abundant feldspar and mafic (basalt/andesite) lithic fragments.

Sample ID: MB01-31 Petrologic Type: Tcm Map Type: Tcm Northing: 474210 Easting: 4628834 Description: Highly welded, rheomorphic ash flow. Very fine-grained gray matrix, abundant green mafic xenoliths as well as rounded <1 cm feldspars.

352 Sample ID: MB01-32 Petrologic Type: Tcm Map Type: Tcm Northing: 474210 Easting: 4628834 Description: Extremely fine-grained matrix with abundant orange stained feldspars. Quartz phenocrysts also present. Devitrified pyroclastic (shards present) matrix. Clots of plag + orthopyroxene, rims of chlorophaeite (+ oxide?) along the clots, sieved sanidine. Plagioclase up to 1.2 mm, sanidine up to 0.6 mm, clinopyroxene up to 0.2 mm, oxides up to 0.21 mm, olivine (xenocryst) up to 0.2 mm, orthopyroxene up to 0.15 mm, mafic xenolith up to 0.25 mm, apatite. Mode: Plag 1.00%; oxides 0.58%; sanidine 0.25%; orthopyroxene 1.92%; clinopyroxene 0.16%; mafic xenolith 0.16%; clots 0.83%; void 0.16%; matrix 94.91%.

Sample ID: MB01-33 Petrologic Type: Tba Map Type: Tcm Northing: 474210 Easting: 4628834 Description: Dark black conchoidally breaking matrix. Sparse plagioclase microphenocrysts also present. Overall, outcrop has a blocky to rubbly appearance. This is exposed in a saddle stratigraphically underlying MB01-32/34. Pilotaxitic, holocrystalline matrix. Resorbed plagioclase, mafic xenoliths (very fine-grained olv + cpx + plag), olivine rimmed by pyroxene, swallowtail olivine (iddingsitized). Plagioclase up to 0.35 mm, clinopyroxene up to 0.11 mm, olivine up to 0.52 mm, orthopyroxene, apatite in matrix. Mode: Plag 7.58%; oxides 0.91%; orthopyroxene 1.41%; clinopyroxene 1.08%; olivine 4.66%; xenolith/clot 0.16%; void 3.66%; matrix 80.50%.

Sample ID: MB01-34 Petrologic Type: Tcm Map Type: Tcm Northing: 474210 Easting: 4628834 Description: Platy portion of silicic lava flow overlying lower vitrophyre. Gray/pink frothy matrix with abundant quartz and feldspar phenocrysts. From margin of flow.

Sample ID: MB01-35 Petrologic Type: Tcm Map Type: Tcm Northing: 474210 Easting: 4628834 Description: Vitrophyre at top of eroded rhyolite dome (Hill 7502 on the Capitol Peak 1:24,000 quadrangle; Mahogany Pass rhyolite vent). Glassy, but slightly devitrified. Abundant feldspars present within vitrophyre. Locally present are 15 - 20 cm buff to white crystal-rich pumiceous clasts as well as concentric ridges of vertical fins of vitrophyre.

Sample ID: MB01-36 Petrologic Type: Tcm Map Type: Tcm Northing: 474210 Easting: 4628834 Description: Capitol Peak ash flow exposure along the eastern range front. Gray, banded white matrix with abundant 1 - 4 cm mafic xenoliths (with 1 - 2 mm reaction rims) and < 2 cm rounded feldspars. Highly welded and rheomorphic.

Sample ID: MB01-38 Petrologic Type: Tcm Map Type: Tcm Northing: 475254 Easting: 4633209 Description: Extremely fine-grained black matrix. Abundant feldspars and pyroxene crystals, slight layering in center of the outcrop. Lava flow.

Sample ID: MB01-39 Petrologic Type: Tcm Map Type: Tcm Northing: 475373 Easting: 4633178 Description: Rhyolite dike that trends 350°. Extremely coarse-grained and light gray/blue matrix. Some mafic xenoliths present (< 1 cm) and abundant feldspar and quartz. Sampled near abandoned adit. Dike cuts intermediate lava flows and lava-like silicic ash flows.

353 Sample ID: MB01-40 Petrologic Type: Tcm Map Type: Tcm Northing: 475572 Easting: 4633075 Description: Platy and highly deformed lava-like ash flow. Extreme rheomorphism and flow-folding. Dark burgundy to red matrix, abundant feldspars. Underlain by tuffaceous material.

Sample ID: MB01-41 Petrologic Type: Tad3 Map Type: Tcm Northing: 475641 Easting: 4633053 Description: Very fine-grained glassy matrix with sparse feldspars.

Sample ID: MB01-43 Petrologic Type: Tad3 Map Type: Tcm Northing: 475799 Easting: 4632976 Description: Very fine-grained black/gray matrix with sparse 1 - 3 mm feldspars. Slight yellow ophiomottling. Outcrop is mostly breccia.

Sample ID: MB01-44 Petrologic Type: Tad3 Map Type: Tcm Northing: 475978 Easting: 4632952 Description: Very fine-grained massive dark gray matrix. Sparse 1 - 3 mm feldspars. Brecciated.

Sample ID: MB01-45 Petrologic Type: Tad3 Map Type: Tcm Northing: 476030 Easting: 4632892 Description: Very fine-grained gray matrix, conchoidal fracture. Occasional feldspars. Highly oxidized upper breccia of lava flow. Pilotaxitic, hypocrystalline matrix. Mafic xenoliths up to 2 mm, resorbed plagioclase, plagioclase + clinopyroxene + oxide clots, orthopyroxene with clinopyroxene rims. Plagioclase up to 1 mm, clinopyroxene up to 0.25 mm, orthopyroxene up to 0.25 mm, oxides up to 0.5 mm. Apatite in groundmass. Mode: Plag 12.08%; oxides 0.91%; orthopyroxene 0.25%; clinopyroxene 0.16%; mafic xenolith 1.16%; clots 0.25%; matrix 85.16%.

Sample ID: MB01-46A Petrologic Type: Kg Map Type: Kg Northing: 476624 Easting: 4633793 Description: From highly eroded granitoid exposure cut by aplite dikes. Biotite hornblende granodiorite.

Sample ID: MB01-46B Petrologic Type: Kg Map Type: Kg Northing: 476624 Easting: 4633793 Description: Fine-grained aplite dike, quartz and feldspar bearing.

Sample ID: MB01-46C Petrologic Type: Kg Map Type: Kg Northing: 446624 Easting: 4633793 Description: Fine-grained aplite dike. Quartz, feldspar, and <2 mm garnets present.

Sample ID: MB01-47 Petrologic Type: Tad3 Map Type: Tcm Northing: 476410 Easting: 4633444 Description: Very fine grained gray/blue matrix. Sparse (up to 2 mm) feldspars and small (< 1mm) pyroxenes. Lava flow unconformably overlies granitoid basement. Pilotaxitic (feldspar), hypocrystalline, poikilitic oxides in orthopyroxene, resorbed plagioclase, plagioclase + oxide + pyroxene clots. Plagioclase up to 1.1 mm, oxides up to 0.25 mm, orthopyroxene up to 0.51 mm, apatite in matrix. Mode: Plagioclase 0.16%; oxides 1.08%; orthopyroxene 0.16%; clots 1.00%; matrix 97.58%.

354 Sample ID: MB01-48 Petrologic Type: Tad3 Map Type: Tcm Northing: 476280 Easting: 4633384 Description: From breccia of lava flow directly overlying MB01-47. Very fine grained blue gray glassy matrix with sparse 1 - 3 mm feldspars.

Sample ID: MB01-49 Petrologic Type: Tcm Map Type: Tcm Northing: 476164 Easting: 4633328 Description: Blue/dark gray matrix with abundant green and brown mafic xenoliths. Feldspars present also. Appears to be a highly welded ash flow.

Sample ID: MB01-50 Petrologic Type: Tcm Map Type: Tcm Northing: 476168 Easting: 4632858 Description: Highly welded, lava-like ash flow. Very fine-grained glassy matrix with abundant feldspars and mafic xenoliths (green/brown). Slight banding in places also within glassy zone. This grades upward into a more massive, but rubbly crystalline zone. Complex textures in thin section. Vitrophric, Pilotaxitic, hyalopilitic, mingling (dark/light vitrophyric zones)?. Resorbed plagioclase, pyroxene ± oxide ± plagioclase clots, mafic xenoliths (resemble Ta lava flows), cpx rimmed by opx. Plagioclase up to 2 mm, oxides up to 0.1 mm, orthopyroxene up to 0.9 mm, olivine up to 0.5 mm, clinopyroxene up to 1 mm, apatite in matrix. Mode: Plag 6.91%; oxides 1.33%; clinopyroxene 2.00%; orthopyroxene 0.66%; clots 2.33%; xenolith 0.75%; matrix 86.25%.

Sample ID: MB01-51 Petrologic Type: Tct Map Type: Tct Northing: 471151 Easting: 4614714 Description: Coyote Mountain ash flow. Pale gray to green altered matrix. Some quartz and orange mafics. Highly welded and lava-like.

Sample ID: MB01-53 Petrologic Type: Tct Map Type: Tct Northing: 470622 Easting: 4614161 Description: Black, very fine-grained glassy matrix with abundant feldspars and small 1-2 mm black pyroxenes. Underlies Coyote Mountain ash flow.

Sample ID: MB01-54A Petrologic Type: Tct Map Type: Tct Northing: 470826 Easting: 4614029 Description: Fall deposits that underlie Coyote Mountain ash flow. Frothy gray pumiceous matrix with abundant 1-3 mm feldspars.

Sample ID: MB01-54B Petrologic Type: Tct Map Type: Tct Northing: 470826 Easting: 4614029 Description: Basal vitrophyre of Coyote Mountain ash flow. Welded, with abundant banding. Extremely folded in places at the meter scale. Grades directly into underlying fall. Axiolitic matrix with banding in places. Resorbed and sieved plagioclase, embayed sanidine, and pyroxene + plagioclase clots present. Plagioclase up to 2 mm, oxides up to 0.25 mm, clinopyroxene (Ti- augite?) up to 0.3 mm, orthopyroxene up to 0.22 mm, sanidine up to 0.5 mm, zircon, and apatite. Mode: Plag 3.33%; oxides 0.83%; sanidine 1.83%; orthopyroxene 0.08%; clinopyroxene 0.16%; zircon 0.16%; clots 1.83%; void 1.75%; matrix 90.00%.

355 Sample ID: MB01-55 Petrologic Type: Tct Map Type: Tct Northing: 471604 Easting: 4614374 Description: Block and ash deposits. Frothy white ashy matrix with abundant 30-60+ cm blocks of highly vesiculated material.

Sample ID: MB01-56 Petrologic Type: Tct Map Type: Tct Northing: 471990 Easting: 4615070 Description: Coyote Mountain ash flow at the summit of Coyote Mountain. Flow banded with abundant feldspars. Purple/gray lava-like matrix that is devitrified in places. Sparse orange mafics.

Sample ID: MB01-57 Petrologic Type: Tct Map Type: Tct Northing: 472268 Easting: 4616925 Description: Very vesiculated and open-textured lava flow. Abundant stretched vesicles and 8-9 mm feldspars. Underlies Coyote Mountain ash flow.

Sample ID: MB01-58 Petrologic Type: Tct Map Type: Tct Northing: 472358 Easting: 4617079 Description: Very fine-grained massive blue/gray matrix. Platy weathering in places and slightly altered. Sparse 1-2 mm feldspars and small (<1 mm) pyroxenes. Felty, hypocrystalline matrix. Sieved/embayed/resorbed plagioclase, sieved sanidine, plag + pyroxene + oxide clots ± potassium feldspar. Plagioclase up to 0.7 mm, oxides up to 0.25 mm, potassium feldspar up to 1.25 mm, orthopyroxene up to 0.4 mm, clinopyroxene up to 0.7 mm, apatite in matrix. Mode: Plag 2.16%; oxides 3.08%; potassium feldspar 0.08%; orthopyroxene 0.33%; clinopyroxene 0.58%; clots 2.75%; matrix 91.00%.

Sample ID: MB01-59 Petrologic Type: Ta Map Type: Tct Northing: 472424 Easting: 4617277 Description: Sampled from Hill 6648 from the Coyote Mountain 1:24,000 quadrangle. Pervasively oxidized vesiculated blocks and smaller tephra abundant and indicate that this was a vent (shield volcano). Sample is from a highly oxidized bomb with a more vesiculated core. Frothy, open- textured red matrix abundant 1-4 mm feldspar microphenocrysts. Appear that flow-lobes emanate from high also.

Sample ID: MB01-60A Petrologic Type: Tct Map Type: Tct Northing: 471094 Easting: 4615936 Description: Coyote Mountain ash flow. Near base of flow, brecciated zone. Platy with gray, fine- grained matrix. Abundant feldspars and sparse orange mafics.

Sample ID: MB01-60B Petrologic Type: Tct Map Type: Tct Northing: 471094 Easting: 4615936 Description: Sampled from stratigraphically lower portion of outcrop. Oxidized, but also blue/gray matrix in places. Trachytic texture in some places evident by feldspar alignment. Further down- section the outcrop grades into vitrophyre and then less welded, ashy material.

Sample ID: MB01-62 Petrologic Type: Tcst Map Type: Tgl Northing: 469804 Easting: 4619910 Description: From an exposure that caps and drapes topography. Similar exposures are widespread nearby. Outcrop is ~2-3 m thick overall. Sampled from lower vitrophyric zone. Glassy matrix with abundant 1-3 mm feldspars.

356 Sample ID: MB01-63 Petrologic Type: Tad2 Map Type: Twsc Northing: 457511 Easting: 4624938 Description: Dense, dark gray/black matrix with conchoidal weathering. Sparse 1-2 mm feldspars. Lowermost exposed flow on Staunton Ridge. Pilotaxitic, hypocrystalline, and intersertal matrix. Sieved/embayed plagioclase. Plagioclase up to 1.55 mm, oxides up to 0.35 mm, clinopyroxene up to 0.55 mm, orthopyroxene up to 0.5 mm, olivine (partially replaced by opx?) up to 0.6 mm. Mode: Plag 7.50%; oxides 1.42%; orthopyroxene 2.41%; clinopyroxene 4.91%; olivine 1.66%; matrix 82.08%.

Sample ID: MB01-64 Petrologic Type: Tad2 Map Type: Twsc Northing: 457476 Easting: 4625002 Description: Dark gray/black sugary matrix. Orange alteration in spots. Platy weathering and abundant 1-2 mm feldspars. First flow above MB01-63, lava flows represented by MB01-65, 67, 68, and 69 all lie up-section.

Sample ID: MB01-65 Petrologic Type: Tad2 Map Type: Twsc Northing: 457374 Easting: 4625110 Description: Black dark/gray matrix with abundant stretched vesicles. Some sparse <1 mm feldspars. May lie within package of multiple thin flows (aside from those sampled).

Sample ID: MB01-67 Petrologic Type: Tad2 Map Type: Twsc Northing: 457261 Easting: 4625340 Description: Very fine-grained matrix, conchoidal fracture in places. Less feldspar than underlying flows, but occasional 1-2 mm crystals are present.

Sample ID: MB01-68 Petrologic Type: Tad2 Map Type: Twsc Northing: 457209 Easting: 4625562 Description: Very fine-grained matrix with sparse 1-3 mm feldspars. Groundmass appears to be very glassy. Upper zone is more oxidized and brecciated than underlying vesicle dominated flow-tops. Flow is at least 15 m thick.

Sample ID: MB01-69 Petrologic Type: Tad2 Map Type: Twsc Northing: 457176 Easting: 4625848 Description: From lava flow capping Staunton Ridge. Very fine-grained glassy matrix from upper vesiculated breccia. Some vesicles are stretched. Abundant 1-3 mm feldspars.

Sample ID: MB01-70 Petrologic Type: Tp1 Map Type: Tgl Northing: 457340 Easting: 4627145 Description: ~5 m thick exposure of welded tuff. White ashy matrix with abundant 1-2 mm feldspars and devitrified pumice. Lithic fragments up to 5 cm are present in some locations.

Sample ID: MB01-71 Petrologic Type: Tad2 Map Type: Twsc Northing: 457047 Easting: 4627211 Description: Directly underlies MB01-70. Dark gray/black glassy matrix with orange alteration in places. <1 mm feldspar microphenocrysts are present.

357 Sample ID: MB01-73 Petrologic Type: Tem Map Type: Tem Northing: 455793 Easting: 4626275 Description: From a fresh portion of a rhyolite lava flow that is highly altered in places (bleached, kaolinite? rich). Blue matrix with abundant, mostly clear 1-3 mm feldspars. Devitrified, hypocrystalline flow banded matrix. Hiatel porphyritic. Resorbed plagioclase, some plag + pyroxene clots. Oxides up to 0.65 mm, sanidine <0.2 mm, plagioclase up to 1 mm (porphyritic), orthopyroxene up to 0.25 mm, zircon up to 0.15 mm, apatite. Plagioclase composition is ~An14. Mode: Plag 3.50%; oxides 1.91%; sanidine 8.25%; orthopyroxene 0.17%; zircon 0.25%; apatite 0.08%; clots 0.833%; matrix 85.00%.

Sample ID: MB01-75 Petrologic Type: N/A Map Type: Smt Northing: 477976 Easting: 4617922 Description: “Swisher Mountain tuff.” Sample taken from highly welded ash flow near vitrophyric base. Pink/purple oxidized flow-banded matrix. Abundant 1-5 mm feldspars.

Sample ID: MB01-76 Petrologic Type: Tcm Map Type: Tcm Northing: 475462 Easting: 4624446 Description: Rhyolite lava flow in the southern Calico Mountains. Mahogany Pass-like rhyolite. Purple/pink matrix with orange alteration. Abundant 1-3 mm feldspars and quartz.

Sample ID: MB01-77 Petrologic Type: Tcm Map Type: Tcm Northing: 475751 Easting: 4623480 Description: Thick, platy weathering exposure of rhyolite; like MB01-76. Horizontal jointing in places that all seems to dip away from a highly columnar, vertically jointed zone. This could just be variations along a distal flow lobe, or it could be a potential vent region. Sample collected from very platy zone. Pink matrix with abundant 1-4 mm feldspars and quartz.

Sample ID: MB01-78 Petrologic Type: Ta Map Type: Tcm Northing: 475342 Easting: 4623425 Description: Ta lava flow that is directly underlying thick rhyolite. Upper portion of the flow is highly vesiculated and the flow appears to be at least 10 m thick. Very fine-grained dark gray matrix with slight open texture and abundant 1-3 mm feldspars. Flow dips to the northeast at ~10°. Below this lies 6-7 more similar lava flows that are highly vesiculated. Intersertal, diktytaxitic in spots, holocrystalline matrix. Subophitic plagioclase and clinopyroxene. Resorbed/sieved plagioclase, glomeroporphyritic plagioclase and olivine. Groundmass plagioclase ranges from An22 to An28. Plagioclase up to 2 mm, olivine (iddingsitized) up to 0.5 mm, clinopyroxene up to 0.3 mm, oxides up to 0.1 mm, apatite in matrix, and orthopyroxene with clinopyroxene rims. Mode: Plag 38.50%; oxides 1.83%; clinopyroxene 16.92%; olivine 2.42%; resorbed plag 0.83%; glomeroporphyritic clots 1.67%; void 5.50%; matrix 32.42.

Sample ID: MB01-79 Petrologic Type: Tcm Map Type: Tcm Northing: 474705 Easting: 4623436 Description: From Tcm lava flow down-dropped to the west along inferred normal fault. Fault lies between this location and the basal lava flow underlying MB01-78. Dark glassy matrix with abundant 1-3 mm feldspars. Vitrophyre.

358 Sample ID: MB01-81 Petrologic Type: Ta Map Type: Tgl Northing: 465040 Easting: 4624849 Description: Section of lava flows exposed where Black Ridge is cut by the North Fork of the Little Humboldt river. Samples MB01-81 to 85 are part of this section. Capping flow on Black Ridge. Very fine grained gray matrix. Abundant <1 mm feldspars. Flow is 1-3 m thick.

Sample ID: MB01-83 Petrologic Type: Ta Map Type: Tgl Northing: 465106 Easting: 4625037 Description: Very fine-grained gray, but slightly oxidized matrix. Slightly vesicular also. Abundant <2 mm feldspars.

Sample ID: MB01-84 Petrologic Type: Thc Map Type: Thc Northing: 465142 Easting: 4625118 Description: Very fine-grained reddish-oxidized matrix. Abundant 1-3 mm feldspars and pyroxenes? Flow is much thicker than those above and below and is close to 20 m thick.

Sample ID: MB01-85 Petrologic Type: Ta Map Type: Tgl Northing: 465130 Easting: 4625168 Description: Very fine-grained sugary matrix. Platy interior. Abundant feldspar microphenocrysts up to 2 mm. Basal flow exposed along Forest Service road 531. Diktytaxitic, intersertal, vesicular, subophitic plagioclase and clinopyroxene, resorbed/sieved plagioclase. Plag up to 1.25 mm (resorbed plag are phenocrysts), clinopyroxene up to 1.5 mm, orthopyroxene up to 0.1 mm, oxides up to 0.25 mm. Plagioclase + pyroxene + oxide in matrix. Mode: Plag 37.90%; oxides 4.01%; orthopyroxene 3.36%; clinopyroxene 10.48%; void 2.19%; matrix 42.04%.

Sample ID: MB01-86A Petrologic Type: Tpr Map Type: Tpr Northing: 458740 Easting: 4616852 Description: Tpr lava flow. Crystal-rich upper vitrophyre. Abundant 0.5-1 cm feldspars, quartz, and amphibole. Frothy flow top locally present.

Sample ID: MB01-87 Petrologic Type: Tba Map Type: Thc Northing: 458740 Easting: 4616852 Description: Light gray, vesicular matrix. Abundant feldspars up to 0.6 cm long. Appears to overlie Tpr lava flow and dips to the east at 10°.

Sample ID: MB01-88 Petrologic Type: Tpr Map Type: Tsi Northing: 455180 Easting: 4612097 Description: Tpr hypabyssal body southeast of Hinkey Summit. Very fine-grained gray-glassy matrix with abundant feldspars (up to 2 cm), quartz (1-5 mm), amphibole (up to 0.5 cm), and biotite (up to 4 mm).

Sample ID: MB01-90 Petrologic Type: N/A Map Type: Smt Northing: 479273 Easting: 4615469 Description: “Swisher Mountain tuff.” Thick cliff and canyon-forming unit exposed at Greeley Crossing along the North Fork of the Little Humboldt River. Abundant vertical jointing throughout entire exposure. Upper portion of the outcrop appears to have more flow-banding. Sample taken from massive, crystalline portion of flow. Banded dark pink/purple matrix. Extremely crystal rich, dominated by large (up to 5 mm) feldspars. Highly welded if it is an ash flow (lava-like).

359 Sample ID: MB01-91 Petrologic Type: N/A Map Type: Smt Northing: 479570 Easting: 4615104 Description: Exposure of vitrophyre. Radial pattern to exposure with vertical columnar jointing defining the radiating pattern. Looks like a plug based on the jointing and circular outcrop pattern. Glassy, black vitrophyre with abundant feldspars (<4 mm).

Sample ID: MB02-1 Petrologic Type: Tarc Map Type: Tarc Northing: 452783 Easting: 4613390 Description: Very fine grained gray matrix with green/yellow alteration. Stretched thin vesicles and sporadic 1-3 mm plagioclase laths are also present. Great ramp structures also present and suggest flow to the northeast (85º bearing)

Sample ID: MB02-2 Petrologic Type: Tarc Map Type: Tarc Northing: 452731 Easting: 4613020 Description: Sampled from lava flow sitting topographically above potential vent area. At vent area, tan matrix, lahar like with mafic/intermediate blocks lying in matrix. Also abundant pervasively oxidized Scoriaceous clasts in the vicinity of this spot. MB02-2 lava flow is platy and has a gray, very fine-grained matrix with sparse visible crystals (plagioclase). Green alteration also present.

Sample ID: MB02-3 Petrologic Type: Tba Map Type: Thc Northing: 452556 Easting: 4613154 Description: Dark blue-gray matrix with abundant orange alteration/mottling. Looks like aphyric to intermediate-plagioclase bearing Steens Basalt. This lava flow caps a small triangular knob and is separated from the underling Tarc package by a thin tuffaceous layer.

Sample ID: MB02-4 Petrologic Type: Tbr Map Type: Tpr Northing: 452567 Easting: 4613429 Description: Frothy, flow-banded rhyolite dike. Relatively aphyric in places but also zones of extremely open-textured rock. This dike is ~6 m wide and cuts through and bakes the Tarc package. The dike trends 23-24º. In close proximity to this dike are thinner mafic dikes that trend the same way and appear to reflect a zone of dike intrusion/faulting.

Sample ID: MB02-5 Petrologic Type: Tarc Map Type: Tarc Northing: 452911 Easting: 4613350 Description: Dark gray, aphyric, and very fresh matrix. Sampled from thicker, massive portion of flow.

Sample ID: MB02-6 Petrologic Type: Tarc Map Type: Tarc Northing: 453026 Easting: 4613645 Description: Fine-grained, gray-blue matrix with spars 1-2 mm feldspars. Some green-yellow alteration also present. Sampled from thick exposure of Tarc that extends towards FS road 084.

Sample ID: MB02-7 Petrologic Type: Tarc Map Type: Tarc Northing: 453285 Easting: 4611913 Description: Extremely platy outcrop with abundant ramp structures and vertical parting planes. Fine- grained gray-blue matrix with green alteration in places. Sparse 1-2 mm feldspars. Underlies exposure of numerous flows of more mafic material

360 Sample ID: MB02-8A Petrologic Type: Tba Map Type: Thc Northing: 452967 Easting: 4612048 Description: Fine-grained gray matrix with abundant plagioclase crystals (3-5 mm long). Alteration present within vesicles (white zeolite or calcite?). Basal flow/flow unit in package here, separated by upper flow(s) by tuffaceous material similar to that found in conjunction with Tba at Hinkey Summit.

Sample ID: MB02-9A Petrologic Type: Tba Map Type: Thc Northing: 452967 Easting: 4612048 Description: Fine-grained gray matrix with abundant plagioclase crystals (3-5 mm long). Alteration present within vesicles (white zeolite or calcite?). Fresher than MB02-8A.

Sample ID: MB02-9A Petrologic Type: Tb Map Type: Thc Northing: 452980 Easting: 4612818 Description: At pyramidal knob. Basal unit is fine-grained, crystalline mafic unit. Abundant 1-4 mm plagioclase crystals are present and occasionally, larger laths (> 5 mm) are also found. Splotchy weathering similar to mafic flows exposed along western SC margin north of Granite Peak (MB01-12). Overlain by MB02-9B.

Sample ID: MB02-11 Petrologic Type: Tba Map Type: Tmi Northing: 453083 Easting: 4616512 Description: Dike and vent-area of Tba. Very fine-grained dark gray matrix with sparse feldspars (similar to Chocolate Mountain). Pervasive oxidization in places away from central core and bombs also present. Other Chocolate Mountain-like (smaller) plugs also present in the vicinity. Dike trends 305-310º.

Sample ID: MB02-12 Petrologic Type: Tba Map Type: Thc Northing: 453383 Easting: 4616705 Description: Medium grained, orange-gray matrix with some <0.5 cm vesicles. Small laths of feldspar present, hard to see any mafics. Very jointed outcrops with rubbly base. Appears to be two thin (2-3 m) flows.

Sample ID: MB02-13 Petrologic Type: Tpr Map Type: Tsi Northing: 453475 Easting: 4616734 Description: Tpr dike that trends 335º. Amphibole and biotite present, but much finer grained than intrusive bodies exposed near/at Hinkey Summit. Sample taken from vitrophyre along west side of the dike. Extremely feldspar-rich (~4 mm euhedral crystals) and also quartz bearing (<2 mm crystals).

Sample ID: MB02-14 Petrologic Type: Tad1 Map Type: Thc Northing: 453846 Easting: 4616653 Description: Blocky, gray, very fine grained matrix. Spheroidally weathering present also. Abundant feldspar crystals up to 0.5 cm long, some appear to be sieved. Pyroxene (<2 mm) also present.

Sample ID: MB02-15 Petrologic Type: Thc Map Type: Thc Northing: 453857 Easting: 4616471 Description: At least 25 m thick flow of platy weathering silicic unit. Pink to gray/purple, very fine- grained matrix. Yellow/orange mottling present. Sparse (<3 mm) feldspars also evident. Appears to be stratigraphically below MB02-14, though there could be a fault separating the two units.

361 Sample ID: MB02-16 Petrologic Type: Twsc Map Type: Twsc Northing: 453755 Easting: 4615900 Description: Very fine-grained black matrix with light gray splotches. Sample taken from blocky float above darker brecciated zone that appears to be a basal flow breccia. Above this breccia, this material grades into a very platy more massive core. In some places, outcrop is vitrophyric. Diktytaxitic, hypocrystalline, felty matrix. Resorbed/sieved plagioclase. Plagioclase up to 2.4 mm (phenocrysts), clinopyroxene up to 0.1 mm, orthopyroxene up to 0.15 mm, oxides up to 0.25 mm. Mode: Plag 10.58%; oxides 0.75%; orthopyroxene 2.25%; clinopyroxene 5.83%; resorbed plag 0.83%; matrix 79.75%.

Sample ID: MB02-17A Petrologic Type: Tp2 Map Type: Ttuf Northing: 457280 Easting: 4618593 Description: Fall deposits underlying Tpr lava flow in the western Goosey Lake Depression. Entire outcrop of tephra is ~3 m thick and basal zone appears to be water affected. This sample was collected from near the base of the outcrop. Very fine-grained platy sheets.

Sample ID: MB02-18 Petrologic Type: Tpr Map Type: Tpr Northing: 457063 Easting: 4618642 Description: Topographically up along rhyolite flow front. Very fine-grained white-pink matrix with abundant 1-3 mm feldspar, quartz, and biotite crystals. Also reddish (altered?) mafic crystals (amphibole/biotite?). Much finer grained than the Tpr exposed at and around Hinkey Summit as well as the southern margin Tpr lava flow.

Sample ID: MB02-19 Petrologic Type: Tpr Map Type: Tpr Northing: 456953 Easting: 4618371 Description: Sample taken from basal vitrophyre of Tpr lava flow. More feldspar-rich and abundant perlitic fractures. Smoky quartz also present, feldspar is more altered. Vitrophyre is ~9-13 m thick. Vitrophyric matrix with trachytic feldspars, slight flow banding. Rounded quartz and amphibole that has undergone dehydration. Porphyritic plagioclase and quartz. Also appears to be clots of plagioclase and potassium feldspar up to 2.5 mm (granitoid?). Biotite up to 1 mm, sanidine up to 2 mm, quartz up to 2 mm, plagioclase up to 0.75 mm, amphibole up to 1.5 mm, zircon up to 0.15 mm, oxides, and apatite. Mode: Plag 8.79%; oxides 0.71%; sanidine 5.92%; quartz 6.37%; biotite 1.97%; amphibole 0.35%; zircon 0.44%; matrix 75.40%.

Sample ID: MB02-20 Petrologic Type: Tpr Map Type: Tpr Northing: 462031 Easting: 4610451 Description: Lower vitrophyre of Tpr lava flow. Gray, perlitic matrix with white to yellow feldspars and abundant amphiboles. Basal monolithologic breccia in places.

Sample ID: MB02-21 Petrologic Type: Tad1 Map Type: Thc Northing: 463122 Easting: 4610990 Description: Sampled from middle portion of at least three ~4 m thick Tad1 lava flows. Very fine-grained glassy black matrix, slight blue alteration. Abundant feldspar crystals.

Sample ID: MB02-22 Petrologic Type: Tad1 Map Type: Thc Northing: 462928 Easting: 4611883 Description: Very-fine grained glassy black matrix, abundant 1-3 mm feldspars. Same material as MB02- 22, sample taken from a stratigraphically lower part of this Tad1 section.

362 Sample ID: MB02-23 Petrologic Type: Tbr Map Type: Tpr Northing: 457684 Easting: 4614188 Description: Small ridge top covered with obsidian debris (true obsidian, not vitrophyre). Gray, subalkaline obsidian. Very fresh material with an occasional feldspar crystal. Extremely thin weathering rind. Unlike other locations across the SC where obsidian is found as rounded cobbles, the obsidian at this location is dominated by thin (~1 cm thick) and long (~5-6 cm) needles.

Sample ID: MB02-24 Petrologic Type: Tp1 Map Type: Tgl Northing: 457484 Easting: 4613942 Description: Welded ash flow with sparse ~1 cm long fiamme. Light gray/tan matrix with feldspars and opalescent quartz.

Sample ID: MB02-25 Petrologic Type: Tad1 Map Type: Thc Northing: 457414 Easting: 4613782 Description: Very fine-grained black glassy matrix with abundant 1-2 mm feldspars. Orange oxidation rind and yellow/green mottling in places.

Sample ID: MB02-26 Petrologic Type: Thc Map Type: Thc Northing: 457310 Easting: 4613672 Description: Medium-grained light gray matrix. Abundant 1-3 mm feldspars and small, 1-2 mm mafic minerals are also present. Slight trachytic texture, this silicic unit overlies intermediate package.

Sample ID: MB02-27 Petrologic Type: Thc Map Type: Thc Northing: 457041 Easting: 4613397 Description: Vesicular, very fine-grained gray/blue matrix with sparse 1-2 mm feldspars and some <1 mm mafic minerals. Sample from a platy zone of the Thc lava flow where it ramps up into the upper flow-top breccia.

Sample ID: MB02-28 Petrologic Type: Tad1 Map Type: Thc Northing: 456937 Easting: 4613084 Description: Medium to fine-grained gray matrix. Abundant large (0.5 x 0.5 cm) feldspars that exhibit disequilibrium textures. Stretched vesicles also present throughout the unit.

Sample ID: MB02-30 Petrologic Type: Tbr Map Type: Tpr Northing: 456443 Easting: 4613788 Description: Flow banded rhyolite with darker, vitrophyric base. Abundant green to red/brown rotated xenoliths also present within the outcrop. Sits stratigraphically above MB02-27. Crystal poor.

Sample ID: MB02-31A Petrologic Type: Tem Map Type: Tem Northing: 458677 Easting: 4632328 Description: Frothy, dark gray/pink pervasively oxidized and tack-welded agglutinate. Abundant 1-2 mm glassy zones and 1-3 mm feldspars. Cobble to boulder sized pervasively oxidized clasts of similar material in close proximity. Welded, devitrified matrix. Zones present where welding is more pronounced. Again, this is a near-vent pyroclastic deposit, not an ash flow. Embayed plagioclase. Plagioclase up to 1 mm, abundant 0.5 mm small laths also. Oxides <1mm (usually 0.1 mm and dendritic), sanidine <0.1 mm, clinopyroxene up to 0.75 mm. Mode: Plag 1.91%; oxides 9.59%; sanidine 0.25%; clinopyroxene 0.41%; resorbed plag 0.16%; vid 29.58%; matrix 67.16%.

363 Sample ID: MB02-31B Petrologic Type: Tem Map Type: Tem Northing: 458677 Easting: 4632328 Description: Highly oxidized breccia with large (1-5 cm) clasts of pervasively oxidized pumice and dark glassy vitrophyre. Near vent spatter/agglutinate.

Sample ID: MB02-32 Petrologic Type: Tad2 Map Type: Twsc Northing: 458646 Easting: 4632364 Description: Pink to light gray fine-grained matrix. White-green mottling present. Relatively aphyric with sparse <1 mm mafic minerals and some feldspar.

Sample ID: MB02-33 Petrologic Type: Tad2 Map Type: Twsc Northing: 458646 Easting: 4632364 Description: Gray, fine-grained matrix. Fresher rock than MB02-32, crystal poor.

Sample ID: MB02-34 Petrologic Type: Tem Map Type: Tem Northing: 456111 Easting: 4634694 Description: Light blue-pinkish gray matrix with reddish flow banding. Abundant orange mafic crystals and 2-3 mm white feldspars. Silicification and argillic alteration is present locally.

Sample ID: MB02-35 Petrologic Type: Tad2 Map Type: Twsc Northing: 456709 Easting: 4634488 Description: Very fine-grained gray to blue matrix with dark feldspars. Underlies MB02-34/36.

Sample ID: MB02-36 Petrologic Type: Tem Map Type: Tem Northing: 457064 Easting: 4634391 Description: Dark pink-purple fine grained matrix with orange mafic crystals and abundant 2-3 mm feldspars. Not as fresh as MB02-35.

Sample ID: MB02-37 Petrologic Type: Tem Map Type: Tem Northing: 457140 Easting: 4634282 Description: Glassy, black matrix with abundant feldspars. Likely upper vitrophyre of MB02-34/36 lava flow.

Sample ID: MB02-38A Petrologic Type: Tp1 Map Type: Tgl Northing: 457140 Easting: 4634282 Description: Basalt portion of Tp1 welded ash flow. Abundant blue pumice fragments, not all smeared out and ~ 2 mm thick.

Sample ID: MB02-38B Petrologic Type: Tp1 Map Type: Tgl Northing: 457140 Easting: 4634282 Description: Upper portion of Tp1 welded ash flow. Abundant ~4 cm long fiamme, slight green alteration, and abundant 2-3 mm clear feldspars. Entire outcrop is ~3 m thick and is the stratigraphically youngest unit that is locally exposed.

364 Sample ID: MB02-39 Petrologic Type: Tem Map Type: Tem Northing: 457957 Easting: 4634218 Description: Same material as MB02-34/36, but fresher. Gray-blue matrix with small 2-3 mm white to clear feldspars and abundant 1-2 mm orange mafic crystals.

Sample ID: MB02-40 Petrologic Type: Tem Map Type: Tem Northing: 458273 Easting: 4633901 Description: Sample taken from rounded cores of weathered outcrop. Very fine-grained white matrix, some pink flow banding. Sparse <1 mm feldspars and small mafic crystals also. From zone of Tem mapped as an intrusive body

Sample ID: MB02-41 Petrologic Type: Tem Map Type: Tem Northing: 459726 Easting: 4632826 Description: Pink-gray, fine-grained, flow-banded matrix. Abundant equant 2-3 mm white to clear feldspars and 2-3 mm orange mafic crystals.

Sample ID: MB02-42 Petrologic Type: Tem Map Type: Tem Northing: 459686 Easting: 4632782 Description: Sample taken from rounded cores of weathered outcrop. Abundant orange alteration throughout the white to purple fine grained matrix. Some sparse feldspars (<1 mm) and mafic crystals (<1 mm).

Sample ID: MB02-43 Petrologic Type: Tad1 Map Type: Thc Northing: 462886 Easting: 4607610 Description: From summit of Coal Pit peak. Very fine-grained black matrix with abundant, large crystals. Lots of vesicle fill also. Crystals are ~1-5 mm in size, appear to be feldspars. Intersertal, holocrystalline matrix. Sieved/resorbed plagioclase (some oscillatory zoned in clots), resorbed/rounded sanidine, resorbed microcline, clinopyroxene rimmed by orthopyroxene, orthopyroxene rimmed by clinopyroxene, clinopyroxene + orthopyroxene + plagioclase + oxide clots, quartzite? xenolith (~ 2mm wide). Plagioclase up to 2 mm (in clots and as phenocrysts), sanidine/k-spar up to 2 mm, olivine, up to 0.25 mm, oxides up to 0.1 mm, orthopyroxene (hy) up to 0.75 mm, clinopyroxene, clots up to 1.2 mm. Mode: Plag 8.92%; oxide 0.08% olivine 0.75%; orthopyroxene 1.08%; clinopyroxene 4.50%; resorbed plag/k-spar 4.25%; clots 13.67%.

Sample ID: MB02-44 Petrologic Type: Tpr Map Type: Tpr Northing: 462103 Easting: 4607365 Description: Gray, frothy matrix with abundant quartz, feldspar, and amphibole. Spotty orange alteration also present. Quartz = Feldspar > Amphibole. The amphibole has obvious red-brown oxidation rinds and overall, this material looks just like the Tpr exposed at .

Sample ID: MB02-45 Petrologic Type: Tad1 Map Type: Thc Northing: 461242 Easting: 4607284 Description: Very-fine grained gray matrix, fairly fresh. Some xenoliths that appear to be granitic/gneissic. Abundant ~ 3 mm feldspars, some appear resorbed.

365 Sample ID: MB02-46 Petrologic Type: Thc Map Type: Thc Northing: 459051 Easting: 4605818 Description: Blocky and very fresh with a pink, fine-grained flow-banded matrix. Abundant < 1 mm quartz and feldspar crystals, and occasional <5 mm green xenoliths.

Sample ID: MB02-47 Petrologic Type: Thc Map Type: Thc Northing: 459245 Easting: 4605978 Description: Devitrified, flow-banded, light pink frothy matrix. Slightly altered. Abundant < 1 mm quartz and larger 1-3 mm feldspars. Quartz > feldspars. Flow banding is very distinguishing.

Sample ID: MB02-50 Petrologic Type: Tbr Map Type: Tpr Northing: 464983 Easting: 4622524 Description: Obsidian clasts. Abundant cobble-sized clasts of subalkaline obsidian, very fresh. Collected from post-SC pavement overlying Tcst.

Sample ID: MB02-55 Petrologic Type: Tcst Map Type: Tgl Northing: 463995 Easting: 4621788 Description: Poorly welded ash flow. Sampled from basal portion of 20-30 m thick cliff. Basalt portion seems to be coarser grained than upper surface. Abundant large (4-5+ cm) tan/white pumices in zones and much smaller (<2 mm) pumice is also present. Very crystal-rich with abundant 2-3 mm feldspars. Lithic fragments appear to be 2-3 mm black vitrophyric glass and some pieces of flow banded rhyolite (Tem/Thc). Small, < 1mm brown glass shards also present. Plagioclase and sanidine up to 2.5 mm, pigeonite up to 1.2 mm, orthopyroxene (hypersthene?) up to 1.1 mm, oxides up to 0.38 mm, plagioclase + oxide + pigeonite + zircon clots, apatite, blocky glass shards up to 1mm and bubble wall shards up to 2 mm. Some plagioclase is resorbed, slight devitrification, poikilitic cpx in plagioclase, welded. Mode: Plag 1.50%; oxide 0.25%; sanidine 1.66%; pigeonite 0.16%; orthopyroxene 0.67%; blocky glass shards 4.67%; clots 20.25%; matrix 70.67%.

Sample ID: MB02-57A Petrologic Type: Tp2 Map Type: Tgl Northing: 463695 Easting: 4618521 Description: Gray/buff, very-fine grained ash. Abundant < 1mm glass shards, some bubble-wall type. Horizon is ~20-30 cm thick and overlies silt.

Sample ID: MB02-57B Petrologic Type: Tp2 Map Type: Tgl Northing: 463695 Easting: 4618521 Description: 0.9 to ~1.6 m thick bluish fine-grained ash. Glassier at the base of the horizon, bubble wall shards and pumice are present also. Coarser grained than MB02-57A, very fresh and pure. Overlies buff-colored lacustrine strata.

Sample ID: MB02-58A Petrologic Type: Tp2 Map Type: Tgl Northing: 463508 Easting: 4618595 Description: Gray, frothy, fine to medium-grained ash. Abundant bubble-wall shards, > 90% glass.

Sample ID: MB02-58B Petrologic Type: Tp2 Map Type: Tgl Northing: 463508 Easting: 4618595 Description: Gray, coarser grained lapilli-ash sized fall deposits. Abundant < 4 mm pumiceous shards.

366 Sample ID: MB02-58C Petrologic Type: Tp2 Map Type: Tgl Northing: 463508 Easting: 4618595 Description: Fine-grained, water affected (ripple-like textures) ash. More sand/lithic rich than MB02- 58A, B. Laminated.

Sample ID: MB02-59 Petrologic Type: Tcst Map Type: Tgl Northing: 461879 Easting: 4621227 Description: Vitrophyre from upper portion of welded ash-flow. Abundant 2 x 3 mm equant feldspars, some 1-2 mm vitrophyric lithic fragments, and pumice. Welding has masked individual glass shards.

Sample ID: MB02-60 Petrologic Type: Tcst Map Type: Tgl Northing: 461933 Easting: 4621158 Description: White and frothy, welded material that seems to underlie the black/glassy basal vitrophyre. Likely welded fall precursor to overlying Tcst ash flow. White, frothy matrix, with abundant 1-2 mm feldspars and <1 mm oxidized mafic crystals?

Sample ID: MB02-61 Petrologic Type: Tbr Map Type: Tpr Northing: 454363 Easting: 4610200 Description: Intrusive body of Tbr. Sample taken from ~1.5 m thick column. White/gray, aphyric matrix with some banding. Devitrification along banding and some spars mafic (< 1mm) crystals in places. Intruding phyllite.

Sample ID: MB02-62 Petrologic Type: Tba Map Type: Thc Northing: 453622 Easting: 4614814 Description: Highly altered mafic lava flow in rubbly exposure along the S side of Lye Creek. Extremely plagioclase-rich, with a massive, fine-grained matrix. Plagioclase laths are 0.8-1 cm long. Orange staining and alteration is present, likely water affected.

Sample ID: MB02-63A Petrologic Type: Tcst Map Type: Tgl Northing: 461855 Easting: 4622466 Description: Pumice clasts from Tcst ash flow.

Sample ID: MB02-63B Petrologic Type: Tcst Map Type: Tgl Northing: 461855 Easting: 4622466 Description: Black, basal vitrophyre. Highly welded, abundant 1-2 mm feldspars, lithic fragments, and some small 2-3 mm stretched pumice. Overlies welded-fall deposits.

Sample ID: MB02-64 Petrologic Type: Ta Map Type: Thc Northing: 459254 Easting: 4621572 Description: Very fine-grained black/tan mottled sugary matrix. Abundant >1 cm vesicles, some stretched. Sparse <1 mm feldspars, but fairly crystal poor.

Sample ID: MB02-65 Petrologic Type: Tpr Map Type: Tpr Northing: 453634 Easting: 4620910 Description: White/gray, very fine-grained matrix with slight open textured nature (vesicular flow top). 1-3 mm quartz crystals are abundant, in addition to fresh 3-5 mm feldspars. Appears to be also a population of <3 mm oxidized mafic crystals.

367 Sample ID: MB02-66 Petrologic Type: Ta Map Type: Tgl Northing: 453944 Easting: 4621698 Description: Very fine-grained gray/blue sugary matrix, with light yellow alteration. Sparse 1-2 mm feldspar laths and >0.5 cm stretched vesicles. Underlies MB02-65 material.

Sample ID: MB02-67 Petrologic Type: Tp2 Map Type: Tgl Northing: 453986 Easting: 4621844 Description: Gray, welded ash flow with 3-4 mm fiamme and sparse < 2mm feldspars. Outcrop appears to be youngest stratigraphically exposed material in the vicinity.

Sample ID: MB02-68 Petrologic Type: Tpr Map Type: Tpr Northing: 454690 Easting: 4622143 Description: White/blue open-textured matrix, with abundant quartz and feldspar crystals (~1-3 mm). Mafic mineral also present, likely biotite or amphibole. Fairly fresh overall

Sample ID: MB02-69 Petrologic Type: Tp1 Map Type: Ttuf Northing: 455291 Easting: 4621744 Description: 3-4 m thick fall deposit. Appears to stratigraphically underlie laharic deposits and vitrophyre of western margin Tpr lava flow. Ash sized bubble junction/wall shards, gray to dark gray. Large 2mm - 1 cm white/tan pumice also present.

Sample ID: MB02-71 Petrologic Type: Tbr Map Type: Tpr Northing: 456159 Easting: 4615605 Description: From massive block of flow-banded rhyolite. White-gray fine-grained matrix with <1 mm crystals of quartz and feldspar. Purple/reddish oxidized and embayed xenoliths are also present; appear to be more mafic. Spherulitic, devitrified matrix. Holocrystalline. Oxides up to 0.35 mm, zircon up to 0.25 mm, biotite up to 0.15 mm, plagioclase up to 0.2 mm, sanidine, apatite. Mode: Plag 3.58%; oxides 2.08%; sanidine 1.00%; zircon 0.50%; apatite 0.16%; biotite 0.16%; matrix 92.50%.

Sample ID: MB02-72 Petrologic Type: Tad1 Map Type: Thc Northing: 455938 Easting: 4615341 Description: Very fine-grained, conchoidally fracturing black matrix. Abundant vesicles and yellow- green alteration. Abundant <1.5 mm feldspar laths. Part of a package of thin Tad1 lava flows that lie below MB02-72 and above the Thc exposed at Lye Creek turn-off.

Sample ID: MB02-73 Petrologic Type: Tpr Map Type: Tsi Northing: 455824 Easting: 4615220 Description: Tpr dike, trends 10º. Gray, very fine-grained matrix with vitrophyre at margins. Abundant 1-3 mm quartz crystals and white feldspars. No apparent/obvious fresh mafic crystals.

Sample ID: MB02-74 Petrologic Type: Tpr Map Type: Tsi Northing: 458079 Easting: 4606549 Description: Tpr dike in Tom Basin. Trends 330º and is 7-8 m thick. Gray, slightly open-textured matrix with abundant feldspars (up to 0.4 mm), 1-3 mm quartz crystals, biotite books (1-3 mm), and amphibole (~2 mm).

368 Sample ID: MB02-75 Petrologic Type: Tarc Map Type: Tarc Northing: 458137 Easting: 4605998 Description: From lower flow of a series of at least ten (conservatively) 4-5 m thick flows. Very fine- grained gray/black sugary textured matrix with abundant yellow/green mottling. No apparent crystals. Below this lies a basal flow breccia that overlies Kg.

Sample ID: MB02-76 Petrologic Type: Tarc Map Type: Tarc Northing: 458016 Easting: 4605331 Description: May be a Tarc dike. Very fine-grained dark gray aphyric matrix. Extremely oxidized breccia present locally.

Sample ID: MB02-77 Petrologic Type: Kg Map Type: Kg Northing: 458050 Easting: 4605315 Description: Granitoid. Appears to be some slight lineation and N-S jointing in rounded, highly weathered outcrops. Feldspar and quartz also present.

Sample ID: MB03-1 Petrologic Type: Tem Map Type: Tem Northing: 465250 Easting: 4634949 Description: Gray to purple very fine-grained matrix. Abundant >2 mm mafic crystals and small feldspars (~2mm). Dark, glassy upper vitrophyre also locally exposed.

Sample ID: MB03-2A Petrologic Type: Tpr Map Type: Tpr Northing: 453933 Easting: 4617754 Description: Vitrophyre from Tpr lava flow. Glassy, perlitic matrix with abundant feldspars (up to 0.7 cm) and quartz.

Sample ID: MB03-3 Petrologic Type: Tpr Map Type: Tsi Northing: 452547 Easting: 4619048 Description: Tpr dike. ~4.5-6 m thick, trends N-S. Other dikes locally present. Abundant quartz, feldspar, and biotite. Green xenoliths also present.

Sample ID: MB03-5 Petrologic Type: Twsc Map Type: Twsc Northing: 452465 Easting: 4618630 Description: Vitrophyre with abundant 2-3 mm feldspars. Perlitic fracture in places. Twsc vent region.

Sample ID: MB03-7 Petrologic Type: Tba Map Type: Thc Northing: 452365 Easting: 4618393 Description: Very fine-grained platy dark gray matrix. Crystal poor. Abundant scoria float, scoria mounds, and bombs locally present also. Vent may/may not be related.

Sample ID: MB03-8 Petrologic Type: Ta Map Type: Thc Northing: 452202 Easting: 4617999 Description: Very fine-grained platy andesite. Sparse, <3 mm feldspars with a gray-blue matrix. Abundant stretched vesicles, platy massive interior, and ramp structures.

369 Sample ID: MB03-9 Petrologic Type: Twsc Map Type: Twsc Northing: 451770 Easting: 4617579 Description: Open textured gray matrix, with alteration. Sparse <0.7 mm feldspar laths. Overall, the sampled flow has an oxidized upper breccia and a more massive, platy interior that ramps into the breccia.

Sample ID: MB03-10A Petrologic Type: Tom Map Type: Tom Northing: 465783 Easting: 4636455 Description: Welded, frothy near vent tuffaceous material (near vent carapace). Abundant glass shards that are <1 mm.

Sample ID: MB03-10B Petrologic Type: Tom Map Type: Tom Northing: 465783 Easting: 4636455 Description: Banded devitrified rhyolite from within the carapace. Abundant 2-3 mm phenocrysts of feldspar and quartz. Massive core of vent (vertically dipping partially eroded plug). Devitrified spherulitic, hypocrystalline matrix. Microphenocrysts of quartz and altered pyroxene in matrix. Plagioclase up to 1.2 mm, quartz up to 2.3 mm, zircon and apatite <0.1 mm, sanidine (seriticized?), oxides, and brown glass shards. Mode: Plag 0.83%; oxides 0.50%; sanidine 1.75%; quartz 2.00%; zircon 0.16%; void 19.91%; matrix 74.83%.

Sample ID: MB03-11 Petrologic Type: Tp1 Map Type: Tgl Northing: 465667 Easting: 4635360 Description: Welded ash-flow. Gray to light green devitrified matrix, with large (>10 cm) flattened pumice throughout. Smaller flattened lithic fragments also present. Abundant 1-3 mm feldspar crystals. Outcrop is 12-15 m thick.

Sample ID: MB03-12A Petrologic Type: Tp1 Map Type: Tgl Northing: 465582 Easting: 4635216 Description: At base of welded ash-flow. Glassy, highly welded pumice fall. Perlitic fracture in places.

Sample ID: MB03-13 Petrologic Type: Tcst Map Type: Tgl Northing: 465752 Easting: 4631433 Description: From blocky portion of upper exposure. White open-textured/frothy matrix with abundant feldspars (up to 0.8 cm). Some feldspar crystals appear rounded and mafic crystals (pyroxene?) also present. Highly welded Tcst (lava-like).

Sample ID: MB03-15 Petrologic Type: Tem Map Type: Tem Northing: 462064 Easting: 4630928 Description: Pervasively oxidized purple, flow banded matrix. ~ 5-10% <3 mm feldspars and possibly ~2 mm pyroxenes (oxidized mafic minerals). Platy zone (joint) trends 230º. Tem lobe that flowed south into the Goosey Lake depression.

Sample ID: MB03-16 Petrologic Type: Ta Map Type: Tgl Northing: 462181 Easting: 4630576 Description: Uppermost Ta lava flow that underlies Tem. Aphyric, open textured gray matrix. Pervasive mottling, aphyric with some white (calcite or zeolite?) alteration in places.

370 Sample ID: MB03-17 Petrologic Type: Ta Map Type: Tgl Northing: 462272 Easting: 4630171 Description: Very fine-grained black matrix. Abundant dark pink/purple vesicles (>1 cm) and sparse (<1 %) feldspars that are <2 mm.

Sample ID: MB03-18 Petrologic Type: Tem Map Type: Tem Northing: 462218 Easting: 4629820 Description: From block in basal portion of lowest topographically exposed Tem lava flow (locally). Pink banded matrix with sheared vesicles. Sparse 1-3 mm feldspars and small 1-4 mm oxidized mafic minerals.

Sample ID: MB03-19 Petrologic Type: Tba Map Type: Tsb Northing: 449551 Easting: 4630256 Description: Very fine-grained sugary matrix. Highly jointed. Some sparse <2 mm feldspars.

Sample ID: MB03-20 Petrologic Type: Tba Map Type: Tmi Northing: 449105 Easting: 4630375 Description: Dike trending 266º and is ~3-4 m thick. Fine-grained sugary gray matrix. No visible phenocrysts.

Sample ID: MB03-21A Petrologic Type: Twsc Map Type: Twsc Northing: 448626 Easting: 4630508 Description: Upper frothy zone of lava flow. Pink massive matrix, locally flow-banded. Phenocrysts of quartz and feldspar (1-3 mm) and some biotite flakes. Highly altered (kaolinite).

Sample ID: MB03-21B Petrologic Type: Twsc Map Type: Twsc Northing: 448626 Easting: 4630565 Description: Upper vitrophyre of same material. Perlitic fracture, bluish devitrification of glass.

Sample ID: MB03-23 Petrologic Type: Tp1 Map Type: Tgl Northing: 464357 Easting: 4625037 Description: Welded ash-flow. Light gray-green matrix with >2 cm stretched pumice. Devitrified throughout the outcrop, which is ~4-5 m thick.

Sample ID: MB03-24 Petrologic Type: Tcst Map Type: Tgl Northing: 460195 Easting: 4626015 Description: Highly welded Tcst. Pink to grayish massive matrix with abundant 1-2 mm feldspar crystals. Seems to be upper welded zone of thicker exposure.

Sample ID: MB03-25 Petrologic Type: Ta Map Type: Tgl Northing: 462669 Easting: 4627440 Description: Dark gray, very fine-grained matrix that is open textured in places. Sparse, <2 mm feldspars.

371 Sample ID: MB03-26A Petrologic Type: Tcst Map Type: Tgl Northing: 462850 Easting: 4627544 Description: Tcst. Very fine grained, white highly welded matrix. Sparse stretched pumice and 1-3 mm feldspar crystals. Upper sample in a succession of samples from an ~15-17 m thick Tcst outcrop. Holyhyaline, slightly devitrified and welded. Pigeonite + clinopyroxene + oxide + zircon clots up to 1mm and 1mm and smaller blocky glass shards. Plagioclase up to 1.35 mm, pigeonite up to 0.35 mm, sanidine up to 0.65 mm, anorthoclase up to 1.75 mm, and oxides up to 0.25 mm. Trace apatite and zircon. Some plagioclase is resorbed. Mode: plag 1.16%; resorbed plag 2.00%; oxides 0.33%; sanidine 1.33%; anorthoclase 0.42%; zircon 0.41%; apatite 0.16%; clots 1.00%; glass 93.17%.

Sample ID: MB03-26B Petrologic Type: Tcst Map Type: Tgl Northing: 462850 Easting: 4627544 Description: From blocky upper part of glassy zone. Less pumice than below, abundant feldspar crystals, sparse lithic fragments. Tree mold aligned with pumice (northward directed flow).

Sample ID: MB03-26C Petrologic Type: Tcst Map Type: Tgl Northing: 462850 Easting: 4627544 Description: From spheroidal zone; highly welded and very glassy. Sparse lithic fragments. Abundant green 1-2 cm pumice clasts.

Sample ID: MB03-26D Petrologic Type: Tcst Map Type: Tgl Northing: 462850 Easting: 4627544 Description: Welded platy material, smaller/thinner plates (weather to 5-7 cm thick plates). Pumice present, ~1cm long and glass shards appear aligned.

Sample ID: MB03-26E Petrologic Type: Tcst Map Type: Tgl Northing: 462850 Easting: 4627544 Description: Lower, more massive platy material. Plate thickness increasing, abundant ~1 cm long pumices. Lithic fragments increasing.

Sample ID: MB03-26F Petrologic Type: Tcst Map Type: Tgl Northing: 462850 Easting: 4627544 Description: Lowermost exposure. Non-welded, massive lapilli tuff. Pumice from 0.5 to > 3 cm and abundant 2-3 mm feldspars and rounded glassy lithic fragments. No bedded; massive and pitted outcrop (case-hardened in places). This part of the outcrop is at least 6 m thick.

Sample ID: MB03-27 Petrologic Type: Tp1 Map Type: Tgl Northing: 462116 Easting: 4626623 Description: Welded ash-flow. Gray-green matrix with abundant stretched pumices (devitrified; up to ~2 cm). Abundant clear feldspar laths, from 1-3mm. Creates surface in this portion of the depression.

Sample ID: MB03-28A Petrologic Type: Tcst Map Type: Tgl Northing: 460582 Easting: 4625695 Description: Rounded frothy glassy clasts from Tcst at Table Mountain. Clasts are up to ~10-12 cm thick. Abundant feldspars from Tcst matrix, euhedral and ~3-5mm. Stretched pumice and up to 6 mm glassy black lithic fragments are also present. Poorly welded Tcst.

372 Sample ID: MB03-28B Petrologic Type: Tcst Map Type: Tgl Northing: 460582 Easting: 4625695 Description: Glassy and welded upper platy zone, capping Table Mountain. Abundant feldspar crystals (<4 mm), stretched small pumice, and <1 cm glassy lithic fragments also present.

Sample ID: MB03-30A Petrologic Type: Tcst Map Type: Tgl Northing: 468928 Easting: 4628166 Description: Extremely crystal rich Tcst, at upper-well exposed portion of at least 30 m thick outcrop. Feldspars are ~2-3 mm, large blocky vitrophyric lithic fragments are also present. Green-yellow devitrification throughout exposure. Welded.

Sample ID: MB03-30B Petrologic Type: Tcst Map Type: Tgl Northing: 468910 Easting: 4628166 Description: Welded, but not as welded as MB03-30A. Visible pumiceous and bubble-wall shards, sparse 2-3 mm feldspars. Up to 0.5 cm black vitrophyric lithic fragments also. Larger, frothy pumice is also present.

Sample ID: MB03-30C Petrologic Type: Tcst Map Type: Tgl Northing: 468910 Easting: 4628166 Description: Extremely glassy vitrophyric material, with flattened pumices (up to 1 cm; green). Spheroidally weathering also present. Feldspars (2-3 mm) and blocky vitrophyric lithic fragments present as well.

Sample ID: MB03-31 Petrologic Type: Tcst Map Type: Tgl Northing: 466804 Easting: 4629589 Description: From lower blocky zone underneath lithophysael zone in a Tcst outcrop. Welded, crystal- rich tuff; feldspars are up to 3.5 cm. >1 cm pumice clasts (some stretched, some not) also present. Grading into vitrophyre below.

Sample ID: MB03-32 Petrologic Type: Tcst Map Type: Tgl Northing: 466789 Easting: 4629521 Description: Welded clast-rich lapilli tuff. Abundant 1-3 mm feldspars present like other Tcst exposures.

Sample ID: MB03-33 Petrologic Type: Tcm Map Type: Tcm Northing: 475304 Easting: 4620116 Description: Very fine-grained, massive matrix with abundant 1-2 mm phenocrysts of feldspar and quartz. Also some small, <1 mm flecks of pyroxene. Devitrified matrix in places, whiter when fresh.

Sample ID: MB03-34 Petrologic Type: Ta Map Type: Tcm Northing: 474860 Easting: 4620195 Description: Purple, oxidized conchoidally fracturing matrix. Abundant stretched vesicles in places, some portions of the flow are more massive. Sparse 1-4 mm feldspars and slightly larger crystalline vesicle fill. Appears to be at least 30 m more of similar lava flows exposed downsection.

373 Sample ID: MB03-35 Petrologic Type: Tct Map Type: Tct Northing: 474722 Easting: 4619077 Description: Gray-purple matrix with some voids, highly welded (lava-like). Abundant, randomly oriented feldspars up to 0.5 cm. Distinctive columnar jointing present, is exposed from down at the N. Fork Little Humboldt river up to this location and appears to be present across the river. Approximately 60-90 m thick here. Open textured and no sign of welding (highly devitrified). Zoned and resorbed plagioclase, some appears structurally disturbed (kinked twins). Resorbed anorthoclase (microcline?). Oxide + plagioclase + clinopyroxene + zircon clots, some highly altered. Clinopyroxene up to 0.17 mm, orthopyroxene up to 1 mm, plagioclase up to 2.75 mm, sanidine up to 1 mm, anorthoclase up to 1.5 mm, apatite, and zircon up to 0.15 mm (associated with clots primarily). Mode: Plag 4.12%; oxides 0.88%; potassium feldspar 1.27%; orthopyroxene 0.58%; clinopyroxene 0.58%; apatite 0.09%; zircon 0.18%; voids 14.42%; matrix 77.82%. Opx, plag, and zircon primarily associated with clots.

Sample ID: MB03-36A Petrologic Type: Tcst Map Type: Tgl Northing: 470782 Easting: 4608204 Description: Tcst. Laminated and frothy, purple matrix. Abundant fiamme up to 2 mm. Some 1-2 mm dark vitrophyric clasts (lithic fragments?) as well as abundant 1-2 mm equant and lath-like feldspars. Overlies MB03-36B and overall, is the uppermost exposed ash-flow here.

Sample ID: MB03-36B Petrologic Type: Tcst Map Type: Tgl Northing: 470496 Easting: 4608413 Description: From block of lower vitrophyre below lithophysael zone. Glassy, welded dark matrix with abundant 1-3 mm feldspars. Some blocky vitrophyric clasts (lithic fragments?) and up to 2 mm fiamme. 1-2 mm equant feldspar crystals also present.

Sample ID: MB03-37A Petrologic Type: Tcst Map Type: Tgl Northing: 470424 Easting: 4608190 Description: From upper 3 m of ~9m thick Tcst package. Pumice-rich, with large ~5-6 cm glassy lithic fragments. Grades into welded and platy upper zone. Base coarser than upper portion. Overall, appears to be two separate Tcst flow units (with accompanying fall) exposed at this location.

Sample ID: MB03-37B Petrologic Type: Tcst Map Type: Tgl Northing: 470424 Easting: 4608190 Description: From what appears to be underlying basal surge deposit. ~3 m of low angle, bedded surge. Anti-dunal structures present and cross-bedding in places. Also scoured by overlying ash flow.

Sample ID: MB03-37C Petrologic Type: Tcst Map Type: Tgl Northing: 470424 Easting: 4608190 Description: Fall deposit. From fall deposit that lies directly below/is the base of underlying ash flow (#2; the lowermost exposed). Grayish fall deposit that overlies limonitic tuffaceous sand.

Sample ID: MB03-40 Petrologic Type: Tad1 Map Type: Thc Northing: 469281 Easting: 4608540 Description: From massive zone within a flow foliated/jointed lava flow. Abundant ramping in places. Very fine-grained conchoidal black matrix with gray-green mottling. Sparse 1-4 mm feldspars. Underlies sediment package exposed in Hardscrabble Basin.

374 Sample ID: MB03-41 Petrologic Type: Tcst Map Type: Tgl Northing: 471582 Easting: 4604139 Description: Cliff-forming Tcst flow unit that caps Hardscrabble basin section. Highly welded, crystal- rich ash flow. Massive purple matrix with fiamme up to 1 cm. Feldspar crystals range from 1-3 mm. Forms distinct cliff/bench.

Sample ID: MB03-42 Petrologic Type: Tct Map Type: Tct Northing: 473405 Easting: 4612328 Description: In Charlie Young goat corral region. Sedimentary package appears to overlie highly welded ash flows. From outcrop on the west side of Charlie Young canyon. Highly flow-banded, purple- gray matrix with 1-2 mm feldspars and small (~1 mm) oxidized mafic minerals. In places, 0.6 m long gas escape pipes are also present. Highly welded, lava-like ash flow.

Sample ID: MB03-43 Petrologic Type: Tct Map Type: Tct Northing: 473581 Easting: 4612058 Description: Flow-banded silicic lava flow. Abundant ~1 mm quartz and feldspar, as well as up to 2 mm oxidized mafic crystals. Appears to ramp up and onlap MB03-42 (Coyote Mt. derived units), likely source of origin is Zymns Butte.

Sample ID: MB03-44 Petrologic Type: Tcst Map Type: Tgl Northing: 473777 Easting: 4611129 Description: Tcst ash flow. Non-welded massive base (~3 m) that grades up into a welded and platy zone. Abundant pumiceous/bubble wall/junction gray shards that range from lapilli to ash size. Abundant <1 mm feldspars and 1-2 mm black lithic fragments. Up to 1 cm white/tan pumice clasts also present. Appears to have filled in a topographic low.

Sample ID: MB03-45 Petrologic Type: Tct Map Type: Tct Northing: 476386 Easting: 4612278 Description: From the summit of Zymns Butte. Monolithologic silicic unit. Appears to be a rhyolite dome that has been eroded; abundant vertical jointing throughout the hill and frothy, open- textured rock in places. Sample from upper, frothy material from near summit. Abundant 1-3 mm feldspars and small <1 mm quartz phenocrysts. Also 1-2 mm oxidized mafic crystals. Similar to MB03-43. Devitrified, hypocrystalline matrix. Inequigranular crystals. Rounded quartz, some with reaction rims. Resorbed and rounded plagioclase, altered mafic minerals (opx?). Plagioclase up to 2.1 mm, quartz up to 0.85 mm, oxides up to 0.25 mm, orthopyroxene (altered) up to 0.45 mm, anorthoclase up to 0.75 mm, apatite, and zircon up to 0.5mm (seems to be associated with opx). Mode: Plag 2.50%; oxides 1.00%; potassium feldspar 0.66%; orthopyroxene 0.25 %; zircon 0.33%; apatite 0.33%; quartz 0.41%; void 7.33%; matrix 87.16%.

Sample ID: MB03-46 Petrologic Type: Tct Map Type: Tct Northing: 467727 Easting: 4613957 Description: On high ridge of Black (Cloister), just north of summit. Eroded silicic dome at high, sample is from outflow. Massive blue-gray matrix. Abundant 1-2 mm feldspars and some <1.5 mm mafic crystals, possibly biotite. Banded and devitrified, holocrystalline matrix. Resorbed and sieved plagioclase, some highly zoned (oscillatory). Plagioclase + orthopyroxene + oxide clots also present. Plagioclase up to 2 mm, altered clinopyroxene up to 0.6 mm, oxides up to 0.35 mm, altered orthopyroxene up to 1.05 mm, zircon up to 0.6 mm, biotite (phlogopite?) up to 0.4 mm, and apatite. Mode: Plag 0.17%; oxides 0.96%; orthopyroxene o.26%; clinopyroxene 0.08%; zircon 0.08%; clots 5.98%; matrix 92.44%.

375 Sample ID: MB03-47 Petrologic Type: Tct Map Type: Tct Northing: 467540 Easting: 4614361 Description: Extremely jointed and platy silicic lava flow. Massive light blue-gray matrix with abundant feldspars, up to 3 mm (flow-aligned in places). Also some <1 mm mafic crystals also present. Outcrop is at least 60 m thick.

Sample ID: MB03-48 Petrologic Type: Ta Map Type: Thc Northing: 467517 Easting: 4614887 Description: ~12 m thick Ta lava flow that overlies a package of thinner intermediate lava flows. These overlie MB03-47. Platy jointing in places and large (~3 m) spheroidally weathering zones. Vesicular upper flow zone. Fine-grained, vesicular matrix, vesicles are stretched up to ~1 cm. Abundant feldspar crystals, ~2 mm.

Sample ID: MB03-49 Petrologic Type: Tad1 Map Type: Thc Northing: 467458 Easting: 4614834 Description: ~3 m thick lava flow that directly overlies MB03-47. Fine-grained vesicular matrix with abundant <1.5 mm feldspars.

Sample ID: MB03-50 Petrologic Type: Tad1 Map Type: Thc Northing: 468240 Easting: 4614355 Description: Platy weathering gray matrix. Some light colored mottles and sparse (<5%) feldspars. Appears to underlie MB03-47.

Sample ID: MB03-51 Petrologic Type: Tct Map Type: Tct Northing: 469220 Easting: 4614135 Description: Clast from thick (>60 m) zone of breccia. This appears to be the basal unit exposed here and also looks like Tct ash flows overlie/onlap this material. Clast is cobble sized and has a black, glassy very fine-grained matrix. Some small vesicles and sparse, <1.5 mm feldspars. Resembles heterologic breccia associated with Tarc lava flows in and around Hinkey Summit.

Sample ID: MB03-52 Petrologic Type: Tcst Map Type: Tgl Northing: 470424 Easting: 4608190 Description: Massive. Light blue matrix with some degree of banding. Abundant ~2 mm white to yellow feldspars an some sparse <1 mm mafic crystals.

Sample ID: MB03-53 Petrologic Type: Tarc Map Type: Tarc Northing: 449387 Easting: 4607346 Description: Dark and massive sugary matrix. Small <1 mm feldspars in matrix and >1 mm phenocrysts are present. Conchoidal fracture overall and parts of the flow are highly jointed. From Tarc lava flow that is near the contact with Trms.

Sample ID: MB03-54 Petrologic Type: Tpr Map Type: Tsi Northing: 455848 Easting: 4610821 Description: On top of hill just east of Indian Creek. Hill ringed by Tpr intrusive that intrude Trms and Tarc. Sample is from vitrophyre of Tpr intrusive body. Black perlitic matrix with abundant ~3 mm feldspars. Also >4 mm amphibole phenocrysts present. From afar, this hill resembles an eroded rhyolite dome but no evidence of local eruptive activity.

376 Sample ID: MB03-55 Petrologic Type: Tarc Map Type: Tarc Northing: 455693 Easting: 4610671 Description: From high point of hill. Heterologic Tarc breccia cut by Tpr intrusive bodies. Sampled breccia clasts. Very fine-grained black glassy matrix. Round cobble-sized clast, like MB03-51. Some small <1 mm feldspar crystals also present.

Sample ID: MB03-56 Petrologic Type: Tarc Map Type: Tarc Northing: 455261 Easting: 4610434 Description: From within Trms section, come across ~0.2 m blocks of mafic material that appear to weather out of hillside. Likely dike. White matrix with interlocking 304 mm laths of pyroxene/amphibole. Abundant feldspar also; apparent sub-ophitic texture.

Sample ID: MB03-57 Petrologic Type: Tpr Map Type: Tsi Northing: 454305 Easting: 4622512 Description: Along ridge south of Buckskin Mountain. Encounter shallow intrusive bodies of Tpr that trend roughly north/south. In some places they are definitely dikes, in others, they appear to be eroded necks. Massive blue-gray matrix with abundant and fresh 1-3 mm feldspar crystals. 1-3 mm quartz phenocrysts and <2 mm biotite crystals are also present. Feldspar = quartz > biotite.

Sample ID: JK04-2A Petrologic Type: Tcst Map Type: Tgl Northing: 463622 Easting: 4622630 Description: Tcst exposure that is at least 20 m thick. JK04-2A collected from plated of welded Tcst above cliff. Uppermost exposed material. Gray matrix with dark blocky vitrophyric lithics.

Sample ID: JK04-2B Petrologic Type: Tcst Map Type: Tgl Northing: 463588 Easting: 4622532 Description: From main body of Tcst exposure. Gray matrix, with dark blocky vitrophyric lithic fragments, unwelded.

Sample ID: JK04-2C Petrologic Type: Tcst Map Type: Tgl Northing: 463588 Easting: 4622532 Description: From platy, extremely welded zone underlying JK04-2C. Material is identical to MB03-26A

Sample ID: JK04-2D Petrologic Type: Tcst Map Type: Tgl Northing: 463588 Easting: 4622532 Description: From extremely welded zone underlying JK04-2C. Material is identical to MB03-26B

Sample ID: JK04-2E Petrologic Type: Tcst Map Type: Tgl Northing: 463588 Easting: 4622532 Description: From spheroidally weathering, highly welded zone that underlies JK04-2C. Material is identical to MB03-36C.

Sample ID: JK04-3A Petrologic Type: Tcst Map Type: Tgl Northing: 463421 Easting: 4623082 Description: Samples from Tcst ash flow directly overlying bedded fall and sediment. 3A is the platy, highly welded Tcst ash flow.

377 Sample ID: JK04-3B Petrologic Type: Tcst Map Type: Tgl Northing: 463421 Easting: 4623082 Description: This was sampled from ~0.6 m of crystal rich basal surge. Directly underlies Tcst flow unit. In places, incredible scours into underlying bedded fall deposits as well as load/flame structures indicating local effect of standing water.

Sample ID: JK04-4 Petrologic Type: Tcst Map Type: Tgl Northing: 469707 Easting: 4634534 Description: From upper plate of a welded, crystal-rich, ~12m thick Tcst outcrop. Abundant 2-3 mm white feldspars and altered pyroxenes. Fiamme present also in an altered purplish matrix.

Sample ID: JK04-5 Petrologic Type: Tcst Map Type: Tgl Northing: 469803 Easting: 4634504 Description: Vitrophyre of JK04-4 ash flow. Abundant 2-4 mm feldspars and perlitic fracturing glassy matrix. Near base of exposure.

Sample ID: JK04-6 Petrologic Type: Tcst Map Type: Tgl Northing: 470154 Easting: 4633522 Description: Further downsection from JK04-5 and through talus slope. Very fine-grained, highly- welded, platy Tcst. Some conchoidal weathering in places also. Associated with vitrophyre.

Sample ID: JK04-7B Petrologic Type: Tcst Map Type: Tgl Northing: 462422 Easting: 4624496 Description: Directly across (to the north) from Cold Springs Butte to ridge of Tcst. Outcrop is at least 15 m thick. 2-5 cm Tp1 lithic fragments are exposed nearby and some fiamme are imbricated indicating northward directed flow. Sampled poorly welded interior of outcrop from ~1 m up from base of ~30 m thick zone of similar material. Abundant dark 1-2 mm vitrophyric clasts, 1-3 mm feldspars and 1-10 mm white pumice that is compacted. This poorly welded zone is where the imbrication and lithic fragments are exposed.

378

APPENDIX 3: MAJOR, TRACE, AND ISOTOPE DATA

All newly collected and analyzed samples are listed below. Included within this appendix are new Kg and Tarc analyses, as well as the major and trace element characteristics of post-SC “Swisher Mountain tuff” samples that were collected along the eastern SC margin (see Fig. 6b for outcrop extent and Appendix 2 for sample locations and descriptions). Data is listed

according to petrologic unit and then sorted by wt. % SiO2 (within each unit). Following this section of the appendix (tabled geochemical and isotopic data) are two pages that (a) list the distribution coefficient values used in batch melting equations referred to in the text and (b) list the calculated melt compositions and Kg compositions used as the source of the melts.

379 Kg, "Swisher Mountain tuff" (Smt) and Tarc

Sample MB02-77 MB01-46A MB01-91 MB01-75 MB01-90 MB01-46C MB01-46B MB01-1 MB03-56 Unit Kg Kg ------Kg Kg Tarc Tarc Map Unit Kg Kg Smt Smt Smt Kg Kg Tarc Tarc Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 458050 476624 479570 477976 479273 476624 476624 449632 455261 Easting 4605315 4633793 4615104 4617922 4615469 4633793 4633793 4617566 4610434

SiO2 57.39 67.98 69.42 71.02 72.65 75.85 76.66 49.21 52.34

TiO2 0.92 0.38 0.55 0.56 0.55 0.03 0.05 2.18 1.26

Al2O3 16.99 16.24 13.47 13.29 12.79 13.45 13.69 14.98 18.33

Fe2O3 6.90 2.65 3.63 3.78 3.71 0.39 0.35 11.31 7.40 MnO 0.11 0.07 0.06 0.06 0.05 0.01 0.01 0.17 0.12 MgO 4.65 1.18 0.45 0.48 0.42 0.03 0.03 8.03 4.27 CaO 5.95 2.97 1.61 1.73 1.42 0.64 0.78 7.44 9.73

Na2O 3.66 4.59 3.24 3.44 3.33 4.17 4.37 2.95 4.00

K2O 2.53 2.45 5.47 5.41 5.02 4.43 4.15 1.83 0.58

P2O5 0.33 0.16 0.11 0.13 0.13 0.06 0.02 0.78 0.26 L.O.I. 0.75 1.02 2.86 0.94 0.23 0.45 0.44 2.24 1.49 Total 100.18 99.68 100.86 100.85 100.30 99.50 100.53 101.12 99.78

MB02-77 MB01-46A MB01-91 MB01-75 MB01-90 MB01-46C MB01-46B MB01-1 MB03-56 Ni 50 5 3 7 5 <1 <1 59 15 Cr 146 6 3 4 7 <1 <1 233 24 Sc 18 6.1 4.7 3.9 4.7 3.3 1.7 20 31 V 168 40 21 35 26 <1 <1 226 256 Ba 1826 1072 1633 1348 1280 29 61 770 265 Rb 84 67 173 186 177 172 120 36 20 Sr 1036 459 109 113 100 18 43 885 711 Zr 204 126 475 496 488 35 40 185 127 Y 18114550451262425 Nb 8.0 5.4 33 35 34 12 8.5 22 8.5 Ga 19.6 19.7 19.5 19.5 19.4 19.0 18.6 20.2 20.0 Cu 56 9 12 13 12 5 4 34 125 Co 19 5 5 5 4 2 2 37 16 Zn 90 62 63 64 58 16 17 116 59 Pb 13.6 17.1 24.3 26.3 24.6 37.5 39.7 5.8 11.0 U 2.00 3.00 <1 <1 <1 <1 1.10 2.80 1.80 Th 10.4 4.60 25 26 26 34 12.8 4.40 3.00 Cs ------Hf ------Ta ------La 25.6 13.1 67.7 68.9 68.3 5.40 1.40 33.4 12.0 Ce 54.7 37.1 114.0 110.1 108.9 21.9 19.9 65.2 28.0 Pr ------Nd ------Sm ------Eu ------Gd ------Tb ------Dy ------Ho ------Er ------Tm ------Yb ------Lu ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

380 Tarc

Sample MB01-2 MB01-18 MB01-4 MB02-75 MB01-5 MB01-19 MB02-76 MB03-53 MB01-16 Unit Tarc Tarc Tarc Tarc Tarc Tarc Tarc Tarc Tarc Map Unit Tarc Tarc Tarc Tarc Tarc Tarc Tarc Tarc Tarc Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 450151 450931 450510 458137 450604 450979 458016 449387 451256 Easting 4617302 4615665 4617135 4605998 4616873 4615829 4605331 4607346 4615493

SiO2 54.67 58.17 58.22 58.49 59.11 60.07 60.39 60.67 60.70

TiO2 1.09 0.93 0.90 1.00 0.90 0.81 0.94 0.81 0.90

Al2O3 16.39 16.62 16.55 16.27 16.56 16.61 16.41 16.42 16.75

Fe2O3 7.58 7.74 6.79 6.85 6.80 6.43 6.39 5.94 6.63 MnO 0.12 0.20 0.13 0.11 0.11 0.12 0.10 0.12 0.11 MgO 4.46 2.36 2.70 3.24 2.60 2.82 2.90 2.84 2.72 CaO 6.79 5.28 5.51 5.31 5.22 5.11 5.03 5.31 4.82

Na2O 3.60 3.57 4.09 3.66 4.01 3.63 3.93 3.83 3.77

K2O 2.50 3.07 2.70 2.95 2.74 3.24 3.18 2.76 3.24

P2O5 0.45 0.55 0.55 0.47 0.58 0.47 0.10 0.36 0.41 L.O.I. 3.19 2.01 2.02 0.76 0.24 1.45 0.37 1.46 0.93 Total 100.85 100.49 100.17 99.11 98.86 100.75 99.74 100.50 100.99

MB01-2 MB01-18 MB01-4 MB02-75 MB01-5 MB01-19 MB02-76 MB03-53 MB01-16 Ni 64 31 29 29 32 30 30 23 50 Cr 108 70 75 59 69 64 63 50 102 Sc 17 14 14 13 12 13 13 15 13 V 162 135 120 128 118 123 127 128 111 Ba 943 1379 1478 1184 1238 1309 1217 1140 1231 Rb 76 90 62 67 65 80 84 62 80 Sr 699 747 697 659 697 659 628 650 608 Zr 195 210 214 217 218 191 216 159 239 Y 192018191917191521 Nb 13 14 13 14 14 11 13 8.9 15 Ga 18.4 18.9 18.5 19.4 18.2 18.5 19.1 18.1 18.6 Cu 51 51 31 45 43 50 47 37 41 Co 24 18 17 18 16 16 17 16 17 Zn 79 112 102 92 112 100 91 85 91 Pb 9.00 10.6 11.6 11.3 11.4 11.2 12.3 11.0 12.6 U 1.40 2.70 1.90 1.70 2.60 1.40 1.80 2.00 2.10 Th 7.00 6.10 5.60 5.80 6.30 4.30 7.80 6.10 9.20 Cs ------Hf ------Ta ------La 31.3 40.5 36.6 38.4 40.2 31.8 35.5 28.0 42.7 Ce 57.7 79.2 78.7 70.8 77.1 67.1 66.4 62.0 72.0 Pr ------Nd ------Sm ------Eu ------Gd ------Tb ------Dy ------Ho ------Er ------Tm ------Yb ------Lu ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

381 Tarc

Sample MB01-20 MB03-51 MB01-21 MB03-55 MB02-5 MB02-2 MB02-1 MB02-7 MB02-6 Unit Tarc Tarc Tarc Tarc Tarc Tarc Tarc Tarc Tarc Map Unit Tarc Tarc Tarc Tarc Tarc Tarc Tarc Tarc Tarc Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 451089 468947 451336 455693 452911 452731 452783 453285 453026 Easting 4615776 4614200 4615481 4610671 4613350 4613020 4613390 4611913 4613645

SiO2 60.81 61.50 61.65 62.82 63.33 64.10 64.13 64.24 64.90

TiO2 0.76 0.89 0.83 0.71 0.69 0.67 0.67 0.71 0.69

Al2O3 16.55 16.15 16.35 16.01 16.37 16.03 16.07 16.55 16.42

Fe2O3 6.30 6.02 5.92 5.34 5.08 5.31 5.14 4.72 5.09 MnO 0.10 0.10 0.10 0.09 0.09 0.07 0.07 0.04 0.05 MgO 2.23 2.91 2.91 2.77 2.19 2.25 1.72 1.31 0.71 CaO 4.86 4.74 4.61 4.37 4.03 4.20 3.72 4.02 3.80

Na2O 3.70 3.97 4.02 3.92 4.14 3.95 3.97 4.14 4.00

K2O 3.22 3.23 3.28 3.32 3.59 3.42 3.61 3.51 3.66

P2O5 0.44 0.47 0.25 0.38 0.31 0.39 0.36 0.38 0.39 L.O.I. 1.23 0.02 0.29 0.22 0.29 0.39 0.51 0.63 0.80 Total 100.21 99.99 100.21 99.94 100.10 100.77 99.97 100.24 100.51

MB01-20 MB03-51 MB01-21 MB03-55 MB02-5 MB02-2 MB02-1 MB02-7 MB02-6 Ni 23 30 34 37 22 33 23 20 30 Cr 48 66 67 87 45 116 45 46 47 Sc 13 13 9.0 12 9.1 9.3 10 10 10 V 107 108 105 98 88 93 85 91 81 Ba 1160 1220 1286 1070 1391 1484 1440 1281 1478 Rb 75 93 90 96 100 100 116 102 110 Sr 652 595 601 544 557 539 545 560 553 Zr 193 263 242 246 253 247 252 254 254 Y 181918171718191623 Nb 11 15 15 13 13 12 13 13 13 Ga 18.4 18.4 18.5 18.7 18.7 18.5 18.5 18.8 18.9 Cu 43 22 43 27 39 38 39 42 35 Co 13 16 15 14 12 11 11 10 13 Zn 88 102 90 91 102 94 91 85 118 Pb 11.6 14.0 12.6 15.0 14.7 12.6 14.1 13.6 14.6 U 1.80 3.10 1.80 2.40 1.70 2.00 1.40 1.10 1.50 Th 5.00 9.10 10.0 10.2 10.3 10.6 12.5 11.4 12.4 Cs ------Hf ------Ta ------La 35.8 40.0 36.6 36.0 41.5 33.8 40.4 41.7 50.4 Ce 66.4 92.0 71.0 80.0 76.7 68.7 75.4 73.7 80.7 Pr ------Nd ------Sm ------Eu ------Gd ------Tb ------Dy ------Ho ------Er ------Tm ------Yb ------Lu ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

382 Tb and Tba

Sample MB01-24 MB02-9A MB01-17 MB01-13 MB01-12 MB02-62 MB01-25 MB03-20 MB03-7 Unit Tb Tb Tb Tb Tb Tba Tba Tba Tba Map Unit Thc Thc Thc Thc Thc Thc Tmi Tmi Thc Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 452668 452980 450868 451406 451323 453622 452204 449105 452365 Easting 4615722 4612818 4615494 4616233 4616329 4614814 4615387 4630375 4618393

SiO2 47.20 47.47 47.50 47.70 48.12 46.38 49.34 49.38 50.25

TiO2 1.29 1.33 1.38 1.34 1.37 1.87 1.98 2.17 2.21

Al2O3 17.02 17.17 17.16 16.97 16.81 17.33 14.70 15.22 14.76

Fe2O3 11.51 11.65 11.88 11.74 12.12 12.98 13.83 13.01 13.96 MnO 0.17 0.17 0.17 0.17 0.18 0.20 0.22 0.19 0.21 MgO 8.65 8.46 8.18 8.74 8.74 5.93 5.43 6.08 4.75 CaO 10.14 9.82 9.74 10.10 9.99 9.10 8.70 8.27 8.22

Na2O 2.68 2.70 2.78 2.70 2.66 2.92 3.20 3.17 3.38

K2O 0.32 0.27 0.30 0.34 0.33 0.69 1.33 1.04 1.41

P2O5 0.14 0.12 0.13 0.14 0.14 0.27 0.50 0.41 0.59 L.O.I. 0.50 0.62 0.46 0.61 0.65 2.44 0.26 1.58 0.26 Total 99.61 99.76 99.66 100.56 101.11 100.11 99.50 100.52 100.01

MB01-24 MB02-9A MB01-17 MB01-13 MB01-12 MB02-62 MB01-25 MB03-20 MB03-7 Ni 157 149 150 157 150 121 21 31 11 Cr 97 111 105 103 102 42 81 36 3 Sc 25 28 27 24 26 22 32 16 20 V 244 269 272 251 261 214 347 118 139 Ba 135 127 150 153 166 289 443 375 480 Rb 2.0 2.5 3.0 3.0 3.7 9.4 25 108 109 Sr 422 405 411 417 410 425 437 300 229 Zr 71 73 76 74 73 140 141 209 169 Y 212123212727352633 Nb 3.7 3.0 3.5 3.3 3.2 9.3 7.5 12 12 Ga 18.9 18.8 18.8 18.7 18.6 20.6 21.6 18.7 18.3 Cu 127 135 139 146 135 128 59 16 3 Co 53 53 53 54 54 55 44 16 15 Zn 80 81 83 87 82 96 120 79 87 Pb 4.10 4.80 8.00 4.00 <1 7.20 4.92 16.0 17.0 U 2.10 3.00 3.40 2.80 <1 3.20 <1 3.50 5.00 Th <2 <2 <2 <2 <2 <2 2.52 10.3 13.3 Cs ------0.30 --- 0.69 ------Hf ------1.96 --- 3.50 ------Ta ------0.27 --- 0.48 ------La 4.00 6.20 4.40 5.00 6.25 15.5 18.6 30.0 32.0 Ce 24.4 24.6 23.1 23.3 14.8 40.8 39.4 63.0 66.0 Pr ------2.16 --- 5.19 ------Nd ------11.2 --- 24.2 ------Sm ------3.52 --- 6.49 ------Eu ------1.31 --- 2.06 ------Gd ------4.07 --- 6.76 ------Tb ------0.69 --- 1.1 ------Dy ------4.26 --- 6.83 ------Ho ------0.87 --- 1.37 ------Er ------2.29 --- 3.71 ------Tm ------0.32 --- 0.52 ------Yb ------1.91 --- 3.18 ------Lu ------0.30 --- 0.5 ------87 86 Sr/ Srm ------0.70379 ------87 86 Sr/ Sri ------0.70378 ------143 144 Nd/ Ndm ------0.51290 ------143 144 Nd/ Ndi ------0.51288 ------

epsilon Ndi ------5.10 ------206Pb/204Pb ------18.941 ------207Pb/204Pb ------15.580 ------208Pb/204Pb ------38.541 ------

383 Tba

Sample MB02-12 MB00-47 MB02-11 MB00-30B MB03-19 MB00-13 MB01-22 MB01-8 MB00-12 Unit Tba Tba Tba Tba Tba Tba Tba Tba Tba Map Unit Thc Thc Tmi Tsb Tsb Thc Tmi Thc Thc Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 453383 453183 453083 453783 449551 454851 451336 451111 454850 Easting 4616705 4630664 4616512 4624034 4630256 4613247 4615481 4616443 4613207

SiO2 50.43 50.58 50.61 50.85 51.51 51.72 52.32 52.71 53.85

TiO2 2.22 1.21 2.41 3.30 2.11 1.96 1.73 1.85 1.64

Al2O3 14.51 15.70 14.65 13.85 14.76 15.94 15.68 15.68 15.91

Fe2O3 14.06 10.41 14.24 14.00 11.25 12.16 10.76 11.27 10.37 MnO 0.23 0.16 0.22 0.20 0.17 0.17 0.16 0.15 0.16 MgO 4.76 7.78 4.54 3.71 3.06 4.46 5.52 5.77 4.26 CaO 8.37 9.94 7.79 7.05 6.93 7.29 8.18 8.19 6.37

Na2O 3.34 2.49 3.21 3.41 3.62 3.66 2.95 2.99 3.55

K2O 1.30 0.75 1.66 1.69 2.10 1.63 1.71 1.32 2.37

P2O5 0.62 0.16 0.76 0.65 0.66 0.53 0.24 0.25 0.52 L.O.I. -0.06 0.30 -0.04 0.44 2.94 0.54 1.31 0.79 0.20 Total 99.79 99.48 100.05 99.16 99.11 100.05 100.57 100.96 99.20

MB02-12 MB00-47 MB02-11 MB00-30B MB03-19 MB00-13 MB01-22 MB01-8 MB00-12 Ni 12 132 13 28 4 67 79 87 50 Cr 49 319 45 27 <1 37 72 67 48 Sc 29 29 30 22 11 22 22 19 21 V 333 243 364 200 21 220 204 193 200 Ba 620 271 649 653 740 493 515 408 642 Rb 22 19 38 38 146 22 44 37 40 Sr 434 370 428 410 246 499 349 352 492 Zr 153 103 160 229 240 225 162 159 229 Y 392241333535282733 Nb 8.4 4.4 10 25 15 14 11 11 13 Ga 21.4 18.1 21.4 25.9 18.9 20.9 19.6 21.3 20.0 Cu 38 104 38 239 <2 128 79 80 99 Co 40 45 41 34 4 38 44 44 32 Zn 115 77 127 97 90 102 105 100 97 Pb 7.80 8.90 9.50 34.2 21.0 9.00 8.30 8.60 16.8 U 2.00 2.70 2.90 2.20 5.80 3.30 2.40 2.40 2.20 Th 3.10 <2 3.20 7.30 13.7 3.70 3.80 5.80 3.70 Cs ------Hf ------Ta ------La 20.3 10.0 24.6 33.0 35.0 22.6 15.3 19.5 26.1 Ce 47.2 31.6 51.9 64.3 79.0 50.7 44.2 40.8 57.8 Pr ------Nd ------Sm ------Eu ------Gd ------Tb ------Dy ------Ho ------Er ------Tm ------Yb ------Lu ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

384 Tba

Sample MB02-8A MB02-8B MB00-11 MB01-23 MB01-6A MB01-33 MB01-87 MB02-3 MB01-9 Unit Tba Tba Tba Tba Tba Tba Tba Tba Tba Map Unit Thc Thc Thc Thc Thc Tcm Thc Thc Thc Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 452967 452967 454865 452556 450844 474210 458740 452556 451159 Easting 4612048 4612048 4613197 4615837 4616690 4628834 4616852 4613154 4616397

SiO2 53.91 53.93 54.22 54.47 54.56 55.66 55.85 ------

TiO2 1.64 1.65 1.67 1.41 1.65 1.05 0.89 ------

Al2O3 16.12 16.23 16.26 16.19 15.57 14.32 15.67 ------

Fe2O3 10.22 10.58 10.38 9.83 10.32 8.47 8.56 ------MnO 0.16 0.16 0.16 0.16 0.16 0.13 0.15 ------MgO 3.88 4.18 4.28 4.19 5.53 6.31 4.79 ------CaO 6.42 6.43 6.59 6.77 7.55 7.50 8.16 ------

Na2O 3.51 3.49 3.80 3.18 2.82 2.54 2.81 ------

K2O 2.43 2.49 2.21 2.47 1.82 2.16 2.00 ------

P2O5 0.51 0.51 0.50 0.42 0.24 0.29 0.15 ------L.O.I. 1.78 0.45 -0.01 0.48 0.52 0.70 0.37 ------Total 100.57 100.10 100.07 99.58 100.74 99.13 99.40 ------

MB02-8A MB02-8B MB00-11 MB01-23 MB01-6A MB01-33 MB01-87 MB02-3 MB01-9 Ni 52 53 51 55 75 155 44 54 49 Cr 48 48 50 22 66 326 80 43 98 Sc 18 22 18 22 20 23 29 17 22 V 200 202 198 204 197 193 235 175 199 Ba 630 563 648 822 425 439 635 689 660 Rb 42 42 37 64 51 60 60 39 39 Sr 497 493 503 405 350 311 205 493 373 Zr 231 230 230 204 165 129 140 226 192 Y 333333342723323228 Nb 13 13 14 11 11.2 8.4 7.3 13 11 Ga 19.4 19.6 21.5 20.2 20.2 18.0 18.8 19.7 20.1 Cu 116 105 119 62 80 77 53 110 55 Co 30 33 31 31 39 36 38 33 35 Zn 97 100 95 110 95 86 93 92 92 Pb 10.0 10.2 72.1 11.4 6.20 10.5 10.7 9.10 9.00 U 3.00 2.60 3.40 1.50 1.47 2.26 <1 3.20 2.40 Th 3.70 4.70 3.80 5.20 4.41 5.68 5.60 2.90 4.80 Cs ------1.47 2.75 ------Hf ------4.04 3.48 ------Ta ------0.73 0.70 ------La 24.2 29.7 26.3 27.3 17.7 18.4 16.5 28.1 21.1 Ce 52.7 56.8 57.8 55.1 36.2 35.7 41.3 56.5 50.7 Pr ------4.47 4.35 ------Nd ------19.8 18.6 ------Sm ------5.32 4.84 ------Eu ------1.66 1.27 ------Gd ------5.37 4.73 ------Tb ------0.88 0.77 ------Dy ------5.25 4.56 ------Ho ------1.04 0.93 ------Er ------2.72 2.38 ------Tm ------0.39 0.34 ------Yb ------2.31 2.08 ------Lu ------0.36 0.33 ------87 86 Sr/ Srm ------0.70414 ------87 86 Sr/ Sri ------0.70401 ------143 144 Nd/ Ndm ------0.51265 ------143 144 Nd/ Ndi ------0.51275 ------

epsilon Ndi ------2.56 ------206Pb/204Pb ------19.044 ------207Pb/204Pb ------15.623 ------208Pb/204Pb ------38.762 ------

385 Ta

Sample MB01-10 MB02-9B MB03-25 MB03-16 MB01-81 MB00-7 MB02-64 MB03-8 MB02-53 Unit Ta Ta Ta Ta Ta Ta Ta Ta Ta Map Unit Thc Thc Tgl Tgl Tgl Thc Tgl Thc Tgl Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 451265 452980 462669 462181 465040 455548 459254 452202 464760 Easting 4616351 4612818 4627440 4630576 4624849 4612609 4621572 4617999 4622433

SiO2 54.98 56.30 56.52 56.55 56.81 57.17 57.27 57.44 57.63

TiO2 1.79 1.72 1.42 1.48 1.51 1.22 1.77 1.65 1.64

Al2O3 15.21 15.84 14.62 14.57 15.25 14.66 14.73 15.32 14.21

Fe2O3 10.85 10.03 9.90 9.79 10.65 9.56 9.98 9.23 9.96 MnO 0.17 0.15 0.16 0.16 0.15 0.17 0.17 0.15 0.17 MgO 3.21 2.52 3.39 3.28 3.05 3.45 2.76 2.41 2.68 CaO 6.35 5.65 6.30 6.33 6.26 5.99 6.14 5.03 5.62

Na2O 3.21 3.59 3.22 3.24 3.63 3.07 3.28 3.70 3.55

K2O 2.48 2.81 2.23 2.45 2.28 2.73 2.72 2.89 2.71

P2O5 0.43 0.50 0.47 0.51 0.52 0.33 0.73 0.53 0.63 L.O.I. 0.29 0.34 1.14 0.74 0.25 0.65 1.08 0.68 0.34 Total 98.98 99.44 99.37 99.09 100.38 98.99 100.63 99.03 99.14

MB01-10 MB02-9B MB03-25 MB03-16 MB01-81 MB00-7 MB02-64 MB03-8 MB02-53 Ni 9 9 16 19 14 20 14 17 14 Cr 12 10 63 11 16 12 6 4 7 Sc 21 20 36 22 25 25 23 23 21 V 228 194 357 251 219 202 187 197 180 Ba 1116 1070 1140 1220 1678 1796 1583 935 1499 Rb 67 71 31 52 56 66 67 68 71 Sr 367 375 413 539 331 277 390 357 349 Zr 232 253 143 243 198 225 241 238 247 Y 424337363839454344 Nb 14 15 10 19 10 10 11 13 12 Ga 22.0 22.7 21.2 22.5 19.0 18.8 19.8 19.9 20.3 Cu 32 33 26 19 26 25 30 12 28 Co 27 23 44 30 26 27 21 22 22 Zn 124 110 128 132 114 108 110 116 113 Pb 11.9 16.1 8.00 11.0 14.1 13.7 13.0 14.0 14.1 U 1.80 1.80 1.80 1.80 1.60 1.70 1.40 3.30 1.50 Th 6.50 7.70 4.40 5.00 5.20 8.20 7.90 7.60 9.90 Cs ------Hf ------Ta ------La 28.8 31.8 22.0 36.0 23.6 26.0 35.6 30.0 34.2 Ce 62.4 62.8 45.0 78.0 55.3 54.5 67.2 66.0 63.9 Pr ------Nd ------Sm ------Eu ------Gd ------Tb ------Dy ------Ho ------Er ------Tm ------Yb ------Lu ------87 86 Sr/ Srm --- 0.70462 ------87 86 Sr/ Sri --- 0.70450 ------143 144 Nd/ Ndm --- 0.51272 ------143 144 Nd/ Ndi --- 0.51273 ------

epsilon Ndi --- 2.22 ------206Pb/204Pb --- 19.072 ------207Pb/204Pb --- 15.637 ------208Pb/204Pb --- 38.745 ------

386 Ta and Tad1

Sample MB03-48 MB01-78 MB02-66 MB01-83 MB01-85 MB03-34 MB01-59 MB03-17 MB03-49 Unit Ta Ta Ta Ta Ta Ta Ta Ta Tad1 Map Unit Thc Tcm Tgl Tgl Tgl Tcm Tct Tgl Thc Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 467517 475342 453944 465106 465130 474860 472424 462272 467458 Easting 4614887 4623425 4621698 4625037 4625168 4620195 4617277 4630171 4614834

SiO2 57.88 58.08 58.15 58.16 58.44 59.89 ------59.26

TiO2 1.15 1.22 1.46 1.47 1.69 1.45 ------1.13

Al2O3 14.41 14.87 14.82 14.78 14.58 14.64 ------14.65

Fe2O3 9.50 9.09 8.98 9.99 9.85 8.89 ------8.56 MnO 0.16 0.14 0.16 0.17 0.18 0.13 ------0.14 MgO 3.53 4.05 3.10 2.94 2.76 1.98 ------3.05 CaO 6.19 6.57 5.99 6.11 5.87 4.68 ------5.59

Na2O 3.14 3.16 3.39 3.41 3.51 3.45 ------3.36

K2O 2.33 2.36 2.69 2.52 2.75 3.00 ------2.65

P2O5 0.26 0.27 0.54 0.53 0.70 0.48 ------0.31 L.O.I. 0.88 0.75 0.52 0.78 0.24 1.10 ------0.79 Total 99.43 100.56 99.80 100.85 100.56 99.70 ------99.48

MB03-48 MB01-78 MB02-66 MB01-83 MB01-85 MB03-34 MB01-59 MB03-17 MB03-49 Ni 18 38 14 14 15 8 5 81 17 Cr 96689643779 Sc 24 21 23 23 22 20 16 26 25 V 203 192 213 188 196 165 108 281 210 Ba 1050 1023 1439 1575 1724 925 1350 1310 1560 Rb 60 66 70 64 67 97 77 31 62 Sr 328 354 343 349 377 381 316 505 324 Zr 203 226 218 216 248 231 268 164 208 Y 383540414337453039 Nb 12 13 11 11 12 13 14 13 12 Ga 19.4 20.1 19.8 19.6 19.8 21.5 18.6 21.8 19.2 Cu 9 46 25 27 29 115 17 88 11 Co 25 28 25 25 20 18 13 47 25 Zn 113 107 110 105 123 115 95 122 109 Pb 13.0 11.7 13.0 13.6 13.3 15.0 16.1 7.00 12.0 U <1 2.18 <1 1.30 2.47 2.30 <1 <1 1.80 Th 6.80 7.37 9.80 8.60 8.22 9.50 10.0 2.00 7.50 Cs --- 2.22 ------1.44 ------Hf --- 5.90 ------6.22 ------Ta --- 0.79 ------0.69 ------La 27.0 29.2 28.6 31.6 32.7 32.0 34.3 20.0 27.0 Ce 59.0 57.4 55.6 62.9 65.2 70.0 67.7 42.0 59.0 Pr --- 6.93 ------7.99 ------Nd --- 29.4 ------34.7 ------Sm --- 7.34 ------8.74 ------Eu --- 1.89 ------2.39 ------Gd --- 7.07 ------8.43 ------Tb --- 1.12 ------1.36 ------Dy --- 6.73 ------8.19 ------Ho --- 1.34 ------1.65 ------Er --- 3.57 ------4.48 ------Tm --- 0.5 ------0.64 ------Yb --- 3.07 ------3.96 ------Lu --- 0.49 ------0.62 ------87 86 Sr/ Srm --- 0.70451 ------0.70507 ------87 86 Sr/ Sri --- 0.70440 ------0.70496 ------143 144 Nd/ Ndm --- 0.51272 ------0.51270 ------143 144 Nd/ Ndi --- 0.51271 ------0.51269 ------

epsilon Ndi --- 1.76 ------1.37 ------206Pb/204Pb --- 19.010 ------19.146 ------207Pb/204Pb --- 15.633 ------15.642 ------208Pb/204Pb --- 38.776 ------38.846 ------

387 Tad1

Sample MB00-16 MB00-6 MB02-51 MB02-72 MB02-14 MB00-1 MB02-28 MB02-25 MB03-50 Unit Tad1 Tad1 Tad1 Tad1 Tad1 Tad1 Tad1 Tad1 Tad1 Map Unit Thc Thc Thc Thc Thc Thc Thc Thc Thc Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 464148 455606 464983 455938 453846 456539 456937 457414 468240 Easting 4614200 4612602 4622524 4615341 4616653 4611169 4613084 4613782 4614355

SiO2 59.57 59.60 59.95 60.47 60.62 60.65 61.05 62.19 62.30

TiO2 1.13 0.95 1.08 0.97 0.98 0.95 0.94 0.89 0.84

Al2O3 14.68 14.28 14.81 14.36 14.73 14.57 14.58 14.55 14.94

Fe2O3 8.51 7.87 9.07 8.35 7.39 7.49 8.42 6.69 6.16 MnO 0.16 0.14 0.15 0.12 0.15 0.14 0.15 0.13 0.08 MgO 3.53 3.22 3.48 2.90 3.16 3.06 3.20 2.31 2.69 CaO 6.09 5.70 6.10 5.28 5.82 5.47 5.61 4.46 4.27

Na2O 3.08 3.09 3.30 2.89 3.13 3.10 3.08 3.04 3.43

K2O 2.79 2.88 2.29 2.78 2.97 3.06 3.07 3.53 3.51

P2O5 0.24 0.19 0.22 0.18 0.18 0.18 0.18 0.16 0.24 L.O.I. 0.61 0.69 0.69 1.37 0.80 0.69 0.96 1.69 1.28 Total 100.39 98.61 101.15 99.67 99.93 99.37 101.23 99.64 99.75

MB00-16 MB00-6 MB02-51 MB02-72 MB02-14 MB00-1 MB02-28 MB02-25 MB03-50 Ni 20 18 19 17 16 17 17 11 10 Cr 18 21 17 19 21 20 14 18 5 Sc 26 24 24 23 24 24 22 21 21 V 225 187 208 186 198 194 185 161 192 Ba 1168 897 1191 986 873 1099 779 1111 860 Rb 72 78 64 86 81 82 84 96 82 Sr 275 271 289 280 276 264 247 268 356 Zr 184 162 172 162 167 166 164 178 256 Y 353333343535353941 Nb 8.9 10 9.3 8.9 9.3 10 10 11 16 Ga 19.7 19.4 19.0 19.0 18.9 19.8 19.3 19.0 21.7 Cu 44 43 46 46 37 49 39 26 10 Co 29 25 28 21 23 26 26 18 20 Zn 100 101 98 92 94 94 93 92 122 Pb 14.6 15.1 13.6 15.3 14.9 35.4 14.8 16.9 14.0 U 1.20 <1 <1 <1 <1 <1 <1 <1 2.30 Th 8.40 10.6 7.80 9.40 11.3 12.4 11.1 13.7 7.80 Cs ------Hf ------Ta ------La 27.2 27.5 23.8 27.2 28.2 25.6 25.5 35.2 35.0 Ce 54.8 53.4 51.7 53.2 52.6 56.2 53.6 62.9 72.0 Pr ------Nd ------Sm ------Eu ------Gd ------Tb ------Dy ------Ho ------Er ------Tm ------Yb ------Lu ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

388 Tad1 and Tad2

Sample MB00-24 MB02-49 MB00-17 MB02-45 MB02-43 MB02-22 MB02-21 MB03-40 MB00-44 Unit Tad1 Tad1 Tad1 Tad1 Tad1 Tad1 Tad1 Tad1 Tad2 Map Unit Thc Thc Thc Thc Thc Thc Thc Thc Twsc Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 462424 459266 464148 461242 462886 462928 463122 469281 460814 Easting 4615126 4605990 4614200 4607284 4607610 4611883 4610990 4608540 4635614

SiO2 62.38 62.44 62.62 62.66 62.83 62.92 63.28 63.50 60.57

TiO2 0.90 0.92 0.90 0.92 0.86 0.91 0.92 0.78 1.20

Al2O3 14.81 14.92 14.63 14.73 14.17 14.37 14.54 14.10 15.15

Fe2O3 7.01 6.15 6.83 6.50 6.53 7.01 6.68 6.23 8.62 MnO 0.11 0.12 0.11 0.13 0.12 0.13 0.13 0.11 0.15 MgO 2.31 3.00 2.35 2.77 2.27 2.43 2.34 2.15 1.14 CaO 4.39 5.33 4.41 5.08 4.33 4.67 4.65 4.06 3.88

Na2O 3.09 3.12 3.32 3.10 3.36 3.27 3.16 3.05 4.14

K2O 3.30 3.07 3.10 3.20 3.16 3.14 3.32 3.51 3.31

P2O5 0.19 0.19 0.20 0.18 0.19 0.18 0.20 0.17 0.52 L.O.I. 1.30 0.85 0.90 0.93 0.84 0.98 1.20 1.46 0.33 Total 99.78 100.10 99.38 100.19 98.66 100.01 100.42 99.09 99.00

MB00-24 MB02-49 MB00-17 MB02-45 MB02-43 MB02-22 MB02-21 MB03-40 MB00-44 Ni 11 14 11 12 11 10 11 24 4 Cr 19 22 19 19 17 16 15 10 2 Sc 21 24 22 23 19 20 19 27 16 V 162 196 165 181 160 162 161 193 80 Ba 1060 1123 979 1049 1042 1030 1121 790 1312 Rb 94 80 93 86 94 91 91 63 94 Sr 271 288 259 266 264 260 267 267 356 Zr 178 165 180 167 180 177 179 185 255 Y 343234333333343645 Nb 10 9.4 11 10 11 11 11 11 16 Ga 18.7 18.9 18.8 19.0 18.8 18.9 18.5 19.3 21.5 Cu 27 38 26 32 26 26 26 26 22 Co 17 24 19 23 19 18 20 30 11 Zn 92 89 90 89 89 86 87 101 122 Pb 17.7 15.5 17.7 16.2 17.4 17.0 17.7 14.0 29.2 U <1 <1 <1 <1 4.19 <1 <1 2.10 <1 Th 12.9 10.0 11.7 11.4 12.2 12.5 12.7 8.30 10.3 Cs ------2.53 ------Hf ------4.97 ------Ta ------0.78 ------La 29.6 26.9 30.8 26.3 30.6 31.3 30.8 25.0 37.7 Ce 59.1 56.1 56.9 56.4 57.3 59.8 59.9 54.0 74.3 Pr ------6.53 ------Nd ------26.0 ------Sm ------6.25 ------Eu ------1.35 ------Gd ------5.96 ------Tb ------1 ------Dy ------6.16 ------Ho ------1.28 ------Er ------3.51 ------Tm ------0.52 ------Yb ------3.29 ------Lu ------0.52 ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

389 Tad2

Sample MB00-45 MB00-31 MB01-69 MB01-63 MB01-67 MB01-65 MB01-68 MB01-71 MB01-64 Unit Tad2 Tad2 Tad2 Tad2 Tad2 Tad2 Tad2 Tad2 Tad2 Map Unit Twsc Twsc Twsc Twsc Twsc Twsc Twsc Twsc Twsc Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 461000 453219 457176 457511 457261 457374 457209 457047 457476 Easting 4636921 4624512 4625848 4624938 4625340 4625110 4625562 4627211 4625002

SiO2 61.04 61.07 61.24 61.83 62.03 62.18 62.22 62.36 62.77

TiO2 1.23 1.06 1.10 1.00 1.06 1.04 1.07 1.01 1.02

Al2O3 15.38 16.11 15.13 15.10 15.36 15.20 15.23 15.22 15.29

Fe2O3 8.80 5.72 8.85 7.88 7.69 8.16 8.68 8.04 8.15 MnO 0.15 0.12 0.23 0.14 0.15 0.15 0.14 0.17 0.14 MgO 1.10 1.23 1.35 1.12 1.20 1.25 1.16 1.21 0.96 CaO 3.78 2.88 3.88 3.75 3.83 3.77 3.61 3.80 3.61

Na2O 4.09 4.82 4.16 3.70 3.71 3.63 4.05 3.73 4.10

K2O 3.32 4.33 3.34 3.88 4.02 4.01 3.40 3.95 3.45

P2O5 0.51 0.50 0.58 0.33 0.45 0.43 0.38 0.42 0.34 L.O.I. 0.45 1.07 -0.14 1.23 0.92 0.87 0.77 0.60 1.52 Total 99.86 98.90 99.72 99.96 100.42 100.68 100.72 100.50 101.35

MB00-45 MB00-31 MB01-69 MB01-63 MB01-67 MB01-65 MB01-68 MB01-71 MB01-64 Ni54 3343433 Cr <1 <1 3 <1 <1 <1 1 <1 <1 Sc 17 11 16 16 15 15 15 15 14 V 824553414643534039 Ba 1216 2472 1284 1262 1104 1210 1226 1273 1248 Rb 94 91 95 104 103 103 95 103 81 Sr 357 481 357 346 348 347 347 343 339 Zr 257 331 264 264 264 262 260 263 263 Y 453645444545454444 Nb 16 19 17 17 17 17 17 17 17 Ga 21.4 21.1 21.4 21.0 21.2 21.4 21.0 20.9 21.8 Cu 13 15 15 13 13 14 14 13 13 Co 12 7 11 10 9 11 11 11 8 Zn 115 97 128 122 120 117 126 124 120 Pb 16.1 17.7 16.3 17.4 16.7 16.8 17.4 17.5 16.6 U <1 1.30 1.30 3.61 <1 <1 <1 <1 <1 Th 9.90 8.60 9.40 10.1 10.3 8.80 10.3 10.4 10.4 Cs ------4.34 ------Hf ------6.87 ------Ta ------1.04 ------La 36.5 46.8 41.2 39.4 40.7 38.4 36.1 36.0 36.8 Ce 69.7 86.1 77.4 77.2 68.8 70.2 70.6 72.1 70.9 Pr ------9.31 ------Nd ------39.1 ------Sm ------9.39 ------Eu ------2.36 ------Gd ------8.85 ------Tb ------1.42 ------Dy ------8.6 ------Ho ------1.74 ------Er ------4.72 ------Tm ------0.69 ------Yb ------4.28 ------Lu ------0.69 ------87 86 Sr/ Srm ------0.70502 ------87 86 Sr/ Sri ------0.70484 ------143 144 Nd/ Ndm ------0.51270 ------143 144 Nd/ Ndi ------0.51269 ------

epsilon Ndi ------1.37 ------206Pb/204Pb ------19.098 ------207Pb/204Pb ------15.638 ------208Pb/204Pb ------38.786 ------

390 Tad2 and Tad3

Sample MB00-26A MB00-26B MB02-33 MB02-35 MB02-32 MB01-45 MB01-44 MB01-48 MB01-43 MB01-47 Unit Tad2 Tad2 Tad2 Tad2 Tad2 Tad3 Tad3 Tad3 Tad3 Tad3 Map Unit Twsc Twsc Twsc Twsc Twsc Tcm Tcm Tcm Tcm Tcm Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 454596 454596 458646 456709 458646 476030 475978 476280 475799 476410 Easting 4623796 4623796 4632364 4634488 4632364 4632892 4632952 4633384 4632976 4633444

SiO2 63.70 64.12 64.81 65.58 66.83 60.47 60.75 62.22 62.65 62.89

TiO2 0.84 0.82 0.74 0.62 0.80 1.41 1.38 1.27 1.15 1.17

Al2O3 15.33 15.29 15.02 14.77 16.16 14.70 14.80 15.39 14.49 15.44

Fe2O3 6.98 7.33 6.84 5.92 3.30 8.31 6.86 8.16 7.15 7.81 MnO 0.14 0.12 0.11 0.08 0.03 0.13 0.13 0.15 0.15 0.14 MgO 0.90 0.39 0.68 0.66 0.35 1.57 1.82 1.51 1.31 1.24 CaO 3.19 2.68 2.51 2.66 2.55 4.15 4.43 4.01 3.65 3.64

Na2O 4.19 4.06 4.13 4.06 4.35 3.88 3.59 4.13 3.53 4.14

K2O 3.67 3.78 3.86 4.00 4.13 3.15 3.57 3.43 3.79 3.52

P2O5 0.32 0.31 0.27 0.20 0.28 0.53 0.60 0.42 0.40 0.46 L.O.I. 0.39 0.73 0.55 0.67 0.75 1.07 1.15 0.08 2.04 0.54 Total 99.66 99.63 99.52 99.23 99.54 99.36 99.08 100.79 100.30 101.01

MB00-26A MB00-26B MB02-33 MB02-35 MB02-32 MB01-45 MB01-44 MB01-48 MB01-43 MB01-47 Ni2 3 3 3 354434 Cr 2 <1<1<1<19 9<1<1<1 Sc 10 15 14 11 16 17 16 12 14 11 V 18 24 22 12 21 114 122 83 73 71 Ba 1602 1189 1341 1330 1353 1136 1204 1244 1183 1179 Rb 101 106 111 118 119 90 96 100 105 102 Sr 300 310 292 267 305 435 444 436 415 427 Zr 268 269 276 283 297 278 285 314 284 309 Y 45444041383840403840 Nb 17 17 17 16 18 22 23 24 22 23 Ga 20.4 20.7 20.6 20.3 22.7 22.3 22.3 22.7 21.6 22.4 Cu 13 14 11 10 8 14 12 12 12 13 Co8 76651314121110 Zn 104 117 106 92 118 119 122 133 117 120 Pb 19.4 18.9 18.5 19.7 19.7 15.7 15.8 18.0 17.7 18.0 U <1 <1 <1 <1 <1 3.07 <1 <1 <1 3.54 Th 10.3 10.8 10.8 12.8 11.7 9.44 11.0 11.0 11.9 10.3 Cs ------2.87 ------2.78 Hf ------7.17 ------7.91 Ta ------1.33 ------1.41 La 38.2 41.6 42.6 37.3 43.7 43.3 44.4 48.0 42.2 46.0 Ce 80.1 72.1 70.4 70.8 73.8 84.7 80.2 84.9 78.3 90.7 Pr ------10.5 ------11.0 Nd ------44.5 ------45.4 Sm ------10.3 ------10.5 Eu ------2.67 ------2.63 Gd ------9.3 ------9.36 Tb ------1.38 ------1.4 Dy ------7.92 ------8.05 Ho ------1.54 ------1.58 Er ------3.92 ------4.11 Tm ------0.54 ------0.58 Yb ------3.24 ------3.55 Lu ------0.5 ------0.56 87 86 Sr/ Srm ------0.70507 ------0.70488 87 86 Sr/ Sri ------0.70494 ------0.70472 143 144 Nd/ Ndm ------0.51267 ------0.51267 143 144 Nd/ Ndi ------0.51265 ------0.51266

epsilon Ndi ------0.72 ------0.82 206Pb/204Pb ------19.010 ------19.024 207Pb/204Pb ------15.611 ------15.632 208Pb/204Pb ------38.712 ------38.767

391 Twsc and Thc

Sample MB00-10 MB02-16 MB00-29 MB01-15 MB03-9 MB01-14 MB03-21A MB03-21B MB03-5 Unit Twsc Twsc Twsc Twsc Twsc Twsc Twsc Twsc Twsc Map Unit Twsc Twsc Twsc Twsc Twsc Twsc Twsc Twsc Twsc Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 454910 453755 454184 451415 451770 451401 448626 448626 452465 Easting 4613135 4615900 4623801 4615976 4617579 4616036 4630508 4630565 4618630

SiO2 62.57 62.69 63.15 64.80 66.59 67.44 ------

TiO2 1.11 1.28 0.98 0.92 0.62 0.61 ------

Al2O3 14.71 14.91 15.88 14.81 14.45 14.28 ------

Fe2O3 6.27 6.82 6.31 6.16 5.25 5.37 ------MnO 0.12 0.13 0.09 0.12 0.09 0.10 ------MgO 1.32 1.58 1.36 1.10 0.58 0.59 ------CaO 3.79 4.19 3.50 2.90 2.48 2.27 ------

Na2O 3.40 3.42 3.86 3.88 3.27 3.85 ------

K2O 4.08 3.89 3.68 3.80 4.92 4.09 ------

P2O5 0.43 0.54 0.41 0.31 0.17 0.18 ------L.O.I. 0.89 0.67 0.32 0.38 1.67 0.32 ------Total 98.69 100.14 99.57 99.18 100.09 99.09 ------

MB00-10 MB02-16 MB00-29 MB01-15 MB03-9 MB01-14 MB03-21A MB03-21B MB03-5 Ni 3 3 3 3 19 3 2 2 3 Cr 42 5 2 7<1<1<12 Sc 16 15 11 11 24 10 3.0 5.0 12 V 103 123 99 55 180 13 12 12 19 Ba 1036 1095 1449 1181 1135 1163 835 920 1405 Rb 118 111 112 123 66 138 185 181 141 Sr 338 352 316 300 267 255 98 96 230 Zr 227 222 251 224 212 237 191 191 244 Y 383834353635262836 Nb 15 15 14 15 11 15 12 11 15 Ga 20.1 19.6 20.0 19.1 19.0 19.3 17.3 17.2 18.4 Cu 15 13 20 12 15 11 <2 <2 97 Co 9 11 12 8 21 6 2 2 5 Zn 100 101 90 95 102 94 43 52 89 Pb 18.2 16.9 16.9 21.3 15.0 21.2 23.0 25.0 21.0 U <1 4.75 <1 <1 3.10 <1 6.20 7.50 6.70 Th 12.4 10.7 11.0 13.9 8.30 15.2 17.4 17.0 14.4 Cs --- 4.05 ------Hf --- 5.78 ------Ta --- 0.98 ------La 34.6 36.2 37.0 37.9 26.0 35.3 36.0 36.0 36.0 Ce 63.8 71.5 77.0 67.8 57.0 64.9 79.0 83.0 82.0 Pr --- 8.66 ------Nd --- 36.2 ------Sm --- 8.77 ------Eu --- 2.06 ------Gd --- 8.12 ------Tb --- 1.29 ------Dy --- 7.64 ------Ho --- 1.5 ------Er --- 4.04 ------Tm --- 0.58 ------Yb --- 3.5 ------Lu --- 0.55 ------87 86 Sr/ Srm --- 0.70508 ------87 86 Sr/ Sri --- 0.70488 ------143 144 Nd/ Ndm --- 0.51268 ------143 144 Nd/ Ndi --- 0.51266 ------

epsilon Ndi --- 0.91 ------206Pb/204Pb --- 19.143 ------207Pb/204Pb --- 15.608 ------208Pb/204Pb --- 38.833 ------

392 Thc

Sample MB02-27 MB00-8 MB01-84 MB02-26 MB02-15 MB00-2 MB00-3 MB00-9 MB02-47 Unit Thc Thc Thc Thc Thc Thc Thc Thc Thc Map Unit Thc Thc Thc Thc Thc Thc Thc Thc Thc Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 457041 454927 465142 457310 453857 456372 456372 454983 459245 Easting 4613397 4612880 4625118 4613672 4616471 4611475 4611475 4613020 4605978

SiO2 66.65 67.05 67.41 67.54 69.96 70.48 70.77 71.43 74.02

TiO2 0.85 0.69 0.81 0.82 0.58 0.44 0.49 0.42 0.28

Al2O3 14.34 14.16 14.35 14.59 13.98 13.83 14.09 13.64 12.96

Fe2O3 4.96 4.52 4.85 4.74 3.24 2.88 2.74 2.41 2.13 MnO 0.06 0.09 0.08 0.06 0.04 0.05 0.04 0.06 0.07 MgO 0.88 1.40 0.85 0.86 0.42 0.34 0.31 0.34 0.21 CaO 2.34 2.51 2.39 2.28 1.43 0.99 1.10 0.99 0.69

Na2O 3.84 3.85 3.98 3.95 3.97 3.95 4.06 3.95 3.75

K2O 4.20 4.29 3.98 4.25 4.75 5.17 5.13 5.26 5.19

P2O5 0.26 0.23 0.29 0.22 0.17 0.08 0.11 0.09 0.04 L.O.I. 1.32 0.60 0.69 1.24 0.58 0.29 0.45 0.52 0.40 Total 99.70 99.40 99.68 100.55 99.12 98.51 99.28 99.11 99.72

MB02-27 MB00-8 MB01-84 MB02-26 MB02-15 MB00-2 MB00-3 MB00-9 MB02-47 Ni1116111187765 Cr <1 2 <1 2 <1 49 <1 2 <1 Sc 12 12 13 13 8.3 8.4 7.7 7.2 6.4 V 60495152301617117 Ba 4194 3792 5369 5090 3528 1376 2184 1601 559 Rb 111 114 110 109 117 130 127 136 156 Sr 170 104 173 153 52 24 29 23 16 Zr 377 378 365 416 574 561 606 561 360 Y 414444454145413549 Nb 16 17 17 17 18 19 19 19 20 Ga 17.5 16.8 17.7 17.2 17.5 18.2 19.0 16.8 17.8 Cu 16 25 20 23 15 12 10 25 10 Co8766534 33 Zn 91 87 90 89 90 97 90 83 83 Pb 17.7 18.6 18.1 19.6 19.8 23.4 23.1 23.6 27.2 U <1 <1 <1 <1 <1 <1 4.95 <1 <1 Th 14.4 15.6 15.4 15.1 14.7 16.1 14.9 16.6 18.3 Cs ------2.04 ------Hf ------13.9 ------Ta ------1.05 ------La 40.0 42.9 41.2 42.1 41.7 41.8 42.0 42.4 49.7 Ce 75.6 76.6 76.9 78.0 73.5 57.3 70.7 65.3 75.7 Pr ------9.44 ------Nd ------38.1 ------Sm ------8.84 ------Eu ------3.02 ------Gd ------7.71 ------Tb ------1.27 ------Dy ------7.71 ------Ho ------1.61 ------Er ------4.51 ------Tm ------0.67 ------Yb ------4.33 ------Lu ------0.71 ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

393 Thc and Tem

Sample MB02-46 MB02-31B MB03-1 MB03-18 MB00-43 MB02-37 MB03-15 MB02-36 MB02-31A Unit Thc Tem Tem Tem Tem Tem Tem Tem Tem Map Unit Thc Tem Tem Tem Tem Tem Tem Tem Tem Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 459051 458677 465250 462218 461356 457140 462064 457064 458677 Easting 4605818 4632328 4634949 4629820 4632709 4634282 4630928 4634391 4632328

SiO2 75.45 64.21 66.30 67.09 67.44 67.85 67.88 67.98 68.75

TiO2 0.18 0.38 0.50 0.44 0.45 0.38 0.40 0.39 0.40

Al2O3 12.56 14.36 14.88 14.92 14.98 14.63 15.00 14.79 14.80

Fe2O3 1.10 4.36 5.32 4.68 5.01 4.55 4.58 4.38 4.60 MnO 0.02 0.11 0.08 0.03 0.10 0.09 0.07 0.03 0.09 MgO 0.15 0.43 0.46 0.16 0.37 0.33 0.21 0.12 0.36 CaO 0.54 1.95 2.09 1.73 1.66 1.88 1.61 1.56 1.89

Na2O 3.61 3.45 4.15 4.25 4.07 4.29 4.26 4.29 4.23

K2O 5.19 4.51 4.17 4.40 4.41 4.53 4.52 4.63 4.53

P2O5 0.04 0.20 0.15 0.13 0.13 0.09 0.11 0.10 0.11 L.O.I. 0.33 --- 0.96 1.03 0.72 0.19 0.76 0.53 0.62 Total 99.16 93.96 99.05 98.86 99.36 98.80 99.40 98.80 100.39

MB02-46 MB02-31B MB03-1 MB03-18 MB00-43 MB02-37 MB03-15 MB02-36 MB02-31A Ni3 53434444 Cr <1 <1 <1 <1 <1 1 <1 <1 <1 Sc 4.0 10 11 11 7.9 7.0 8.0 8.2 8.6 V <11326246 620125 Ba 240 1875 1240 1320 1480 1575 1685 1066 1428 Rb 186 131 127 134 134 144 137 148 137 Sr 9 229 239 218 228 206 204 209 211 Zr 212 305 298 307 300 308 309 302 303 Y 424333304640384742 Nb 21 16 16 16 18 18 17 17 18 Ga 17.8 18.4 19.9 20.0 19.7 19.6 19.1 19.7 19.5 Cu6 <2<2<21111<29 11 Co 1 3 4 <1 6 3 3 3 4 Zn 57 93 91 76 86 84 87 112 87 Pb 25.6 20.3 20.0 22.0 20.4 21.3 21.0 24.1 21.9 U <1 5.14 5.10 4.90 <1 <1 5.40 <1 <1 Th 22.6 13.7 12.5 12.9 13.5 14.6 13.2 14.5 13.3 Cs --- 5.21 ------Hf --- 7.86 ------Ta --- 1.09 ------La 55.2 44.5 35.0 35.0 46.7 42.6 41.0 50.7 41.4 Ce 80.2 79.3 71.0 70.0 80.7 80.5 89.0 71.7 75.7 Pr --- 9.81 ------Nd --- 38.9 ------Sm --- 8.99 ------Eu --- 1.79 ------Gd --- 8.24 ------Tb --- 1.34 ------Dy --- 8.22 ------Ho --- 1.66 ------Er --- 4.6 ------Tm --- 0.68 ------Yb --- 4.32 ------Lu --- 0.69 ------87 86 Sr/ Srm --- 0.70569 ------87 86 Sr/ Sri --- 0.70535 ------143 144 Nd/ Ndm --- 0.51268 ------143 144 Nd/ Ndi --- 0.51267 ------

epsilon Ndi --- 1.00 ------206Pb/204Pb --- 19.104 ------207Pb/204Pb --- 15.651 ------208Pb/204Pb --- 38.837 ------

394 Tem

Sample MB02-40 MB02-42 MB02-39 MB02-41 MB01-73 MB02-34 MB02-52 Unit Tem Tem Tem Tem Tem Tem Tem Map Unit Tem Tem Tem Tem Tem Twsc Thc Utm Zone 11T 11T 11T 11T 11T 11T 11T Northing 458273 459686 457957 459726 455793 456111 464823 Easting 4633901 4632782 4634218 4632826 4626275 4634694 4622498

SiO2 69.16 69.34 69.46 69.81 72.99 ------

TiO2 0.54 0.46 0.42 0.44 0.28 ------

Al2O3 15.83 15.35 15.22 15.62 14.87 ------

Fe2O3 2.25 2.64 3.36 1.52 1.09 ------MnO 0.03 0.02 0.02 0.01 0.01 ------MgO 0.18 0.16 0.16 0.07 0.03 ------CaO 2.09 1.81 1.70 1.75 0.95 ------

Na2O 4.51 4.37 4.23 4.52 4.20 ------

K2O 4.58 4.68 4.67 4.74 5.27 ------

P2O5 0.17 0.13 0.10 0.13 0.05 ------L.O.I. 0.39 0.55 0.65 0.36 0.08 ------Total 99.73 99.52 99.98 98.95 99.83 ------

MB02-40 MB02-42 MB02-39 MB02-41 MB01-73 MB02-34 MB02-52 Ni34 22222 Cr 1<1<1<1<1<1<1 Sc 12 8.9 7.1 9.1 8.9 9.1 7.2 V 79 88455 Ba 1105 1374 1325 1173 1918 1565 1496 Rb 146 150 148 155 173 173 167 Sr 261 233 219 234 142 117 70 Zr 307 316 306 307 333 333 322 Y 40312932333134 Nb 18 18 18 18 18 17 20 Ga 22.1 21.4 20.7 20.9 21.2 19.0 17.7 Cu89 885106 Co53 23222 Zn 70 75 85 63 107 70 66 Pb 21.6 20.7 20.8 23.9 23.4 22.4 25.7 U <1 <1 <1 <1 5.56 <1 <1 Th 13.0 15.3 14.7 14.7 15.1 15.6 19.4 Cs ------3.86 ------Hf ------8.61 ------Ta ------1.15 ------La 46.9 48.2 41.7 48.0 48.9 42.3 37.6 Ce 69.1 72.6 67.2 67.9 88.8 66.3 64.3 Pr ------10.4 ------Nd ------41.5 ------Sm ------9.5 ------Eu ------1.71 ------Gd ------8.53 ------Tb ------1.37 ------Dy ------7.95 ------Ho ------1.53 ------Er ------4.01 ------Tm ------0.58 ------Yb ------3.56 ------Lu ------0.53 ------87 86 Sr/ Srm ------0.70571 ------87 86 Sr/ Sri ------0.70501 ------143 144 Nd/ Ndm ------0.51267 ------143 144 Nd/ Ndi ------0.51266 ------

epsilon Ndi ------0.77 ------206Pb/204Pb ------19.077 ------207Pb/204Pb ------15.623 ------208Pb/204Pb ------38.738 ------

395 Tbr and Tpr

Sample MB02-30 MB02-61 MB02-71 MB02-50 MB02-23 MB02-4 MB00-22 MB02-20 MB01-86A Unit Tbr Tbr Tbr Tbr Tbr Tbr Tpr Tpr Tpr Map Unit Tpr Tpr Tpr Tpr Tpr Tpr Tpr Tpr Tpr Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 456443 454363 456159 464983 457684 452567 462412 462031 458740 Easting 4613788 4610200 4615605 4622524 4614188 4613429 4612636 4610451 4616852

SiO2 75.55 75.61 75.99 76.12 76.80 77.21 69.31 69.88 69.93

TiO2 0.10 0.08 0.08 0.07 0.09 0.07 0.56 0.47 0.54

Al2O3 12.46 12.61 12.30 12.70 12.34 12.61 14.09 13.37 14.16

Fe2O3 1.15 1.22 1.28 1.10 1.33 0.45 3.47 3.16 3.13 MnO 0.01 0.02 0.02 0.02 0.02 0.01 0.08 0.06 0.06 MgO 0.10 0.13 0.10 0.05 0.04 0.06 0.55 0.68 0.70 CaO 0.56 0.46 0.64 0.48 0.62 0.50 2.15 1.94 2.05

Na2O 3.59 3.22 3.81 3.57 3.68 3.34 3.29 3.11 3.28

K2O 4.94 4.92 4.81 4.92 4.92 4.99 4.47 4.57 4.42

P2O5 0.02 0.03 0.04 0.03 0.14 0.06 0.22 0.15 0.16 L.O.I. 0.37 0.52 0.56 0.38 0.34 0.43 1.93 2.12 2.16 Total 98.85 98.82 99.62 99.42 100.31 99.71 100.13 99.52 100.61

MB02-30 MB02-61 MB02-71 MB02-50 MB02-23 MB02-4 MB00-22 MB02-20 MB01-86A Ni 2 <1 2 <1 <1 <1 5 4 3 Cr <1 <1 <1 <1 <1 <1 4 5 9 Sc 5.6 4.4 3.5 3.6 3.9 3.1 9.0 6.6 9.1 V <1<1<1<1<1<1404240 Ba 35 24 19 12 31 36 1260 1192 1455 Rb 269 364 355 371 273 363 153 166 153 Sr 3512 <2 <2 3 200 178 202 Zr 169 83 149 84 178 80 218 207 216 Y 707673778580313630 Nb 22 16 24 17 23 16 13 13 12 Ga 24.6 23.1 25.9 23.7 25.5 20.7 17.2 17.4 18.0 Cu 10 11 8 9 8 7 12 13 10 Co 3 3 3 3 3 2 6 7 5 Zn 85 62 87 65 107 36 64 63 73 Pb 35.2 28.7 36.8 29.0 36.7 28.1 23.6 23.9 23.7 U <1 <1 10.2 <1 <1 <1 <1 <1 <1 Th 29.7 18.6 29.8 18.6 31.5 18.2 19.3 21.3 18.8 Cs ------9.14 ------Hf ------7.01 ------Ta ------2.05 ------La 41.2 21.1 23.2 22.6 41.0 21.7 36.1 40.2 37.3 Ce 71.0 46.1 49.2 50.5 77.9 47.4 68.2 69.1 71.6 Pr ------6.96 ------Nd ------30.2 ------Sm ------10.2 ------Eu ------0.03 ------Gd ------10.64 ------Tb ------2.01 ------Dy ------12.7 ------Ho ------2.59 ------Er ------7.22 ------Tm ------1.11 ------Yb ------7.12 ------Lu ------1.08 ------87 86 Sr/ Srm ------0.70697 ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------0.51261 ------143 144 Nd/ Ndi ------0.51259 ------

epsilon Ndi ------0.56 ------206Pb/204Pb ------19.142 ------207Pb/204Pb ------15.646 ------208Pb/204Pb ------38.852 ------

396 Tpr

Sample MB00-18 MB00-5 MB03-54 MB00-15A MB03-2A MB02-13 MB02-44 MB01-88 MB02-19 Unit Tpr Tpr Tpr Tpr Tpr Tpr Tpr Tpr Tpr Map Unit Tpr Tpr Tsi Tsi Tsi Tsi Tpr Tsi Tpr Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 462074 456173 455848 454679 453933 453475 462103 455180 456953 Easting 4613975 4611864 4610821 4612908 4617754 4616734 4607365 4612097 4618371

SiO2 69.96 70.79 71.97 74.13 74.31 74.59 74.80 74.96 75.89

TiO2 0.53 0.45 0.30 0.16 0.10 0.11 0.30 0.30 0.10

Al2O3 13.90 13.94 13.06 11.75 12.00 12.02 13.27 12.89 12.03

Fe2O3 3.33 3.19 2.00 1.52 1.03 1.03 2.12 2.05 1.08 MnO 0.06 0.05 0.04 0.03 0.02 0.02 0.03 0.03 0.01 MgO 0.72 0.55 0.37 0.29 0.06 0.12 0.24 0.53 0.10 CaO 2.11 1.95 1.50 0.95 0.63 0.61 1.48 1.40 0.67

Na2O 3.34 3.21 3.07 2.72 3.00 2.36 3.34 3.08 3.16

K2O 4.51 4.57 4.66 5.03 4.94 5.77 4.42 4.40 5.00

P2O5 0.18 0.15 0.07 0.04 0.03 0.04 0.11 0.09 0.15 L.O.I. 1.55 1.03 2.75 2.53 2.38 3.35 0.41 0.57 2.19 Total 100.19 99.87 99.80 99.14 98.50 100.02 100.51 100.30 100.40

MB00-18 MB00-5 MB03-54 MB00-15A MB03-2A MB02-13 MB02-44 MB01-88 MB02-19 Ni 4 5 3 2 <1 <1 2 3 <1 Cr 3 6 2 7 <1 <1 2 3 2 Sc 7.7 7.2 5.0 3.3 4.0 2.6 4.3 2.4 2.7 V 43 31 24 10 9 <1 17 19 <1 Ba 1278 959 645 236 87 95 669 681 83 Rb 154 167 179 229 263 289 194 192 295 Sr 196 163 116 30 17 12 130 129 14 Zr 217 210 154 126 111 114 155 155 110 Y 303026505861222564 Nb 13 13 12 13 16 15 11 11 15 Ga 17.4 17.8 15.5 19.8 19.6 19.8 17.5 16.1 21.5 Cu811229<279 96 Co4534223 43 Zn 63 65 49 53 54 54 47 47 55 Pb 23.7 25.7 27.0 26.5 31.0 29.4 27.0 25.2 31.0 U 6.30 <1 9.40 <1 12.0 <1 <1 <1 11.16 Th 17.2 24.8 24.3 24.3 29.2 32.5 27.7 27.3 31.4 Cs 4.34 ------13.70 Hf 6.27 ------4.68 Ta 1.01 ------1.59 La 37.0 39.8 36.0 42.0 46.0 44.5 39.9 41.0 54.6 Ce 66.5 69.6 82.0 78.7 109.0 84.0 70.1 70.6 108.0 Pr 7.67 ------12.4 Nd 29.7 ------46.6 Sm 6.76 ------11.3 Eu 1.23 ------0.12 Gd 6.12 ------10.21 Tb 0.98 ------1.78 Dy 5.98 ------11.1 Ho 1.22 ------2.29 Er 3.35 ------6.36 Tm 0.51 ------0.96 Yb 3.18 ------5.98 Lu 0.53 ------0.9 87 86 Sr/ Srm ------0.70639 87 86 Sr/ Sri ------143 144 Nd/ Ndm ------0.51258 143 144 Nd/ Ndi ------0.51256

epsilon Ndi ------1.03 206Pb/204Pb ------19.135 207Pb/204Pb ------15.631 208Pb/204Pb ------38.821

397 Tpr

Sample MB02-68 MB02-74 MB02-65 MB02-18 MB03-3 MB03-57 MB02-73 MB00-28 Unit Tpr Tpr Tpr Tpr Tpr Tpr Tpr Tpr Map Unit Tpr Tsi Tpr Tpr Tsi Tsi Tsi Tsi Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T Northing 454690 458079 453634 457063 452547 454305 455824 454534 Easting 4622143 4606549 4620910 4618642 4619048 4622512 4615220 4623632

SiO2 76.49 76.62 76.74 77.01 77.14 77.58 77.69 78.50

TiO2 0.09 0.14 0.13 0.09 0.08 0.08 0.12 0.08

Al2O3 12.33 12.49 12.10 12.74 11.85 11.76 11.06 11.46

Fe2O3 0.99 1.23 1.18 1.07 0.99 1.00 1.13 1.02 MnO 0.02 0.03 0.02 0.02 0.02 0.02 0.01 0.02 MgO 0.07 0.17 0.07 0.07 0.07 0.05 0.12 0.07 CaO 0.48 0.78 0.68 0.49 0.57 0.56 0.71 0.41

Na2O 3.52 3.25 3.27 3.30 3.20 3.27 2.94 3.16

K2O 4.88 4.99 4.80 5.19 4.65 4.54 4.39 4.69

P2O5 0.03 0.05 0.04 0.03 0.03 0.02 0.05 0.03 L.O.I. 0.28 0.40 0.22 0.46 0.82 0.81 0.54 0.54 Total 99.16 100.14 99.25 100.49 99.43 99.68 98.78 99.99

MB02-68 MB02-74 MB02-65 MB02-18 MB03-3 MB03-57 MB02-73 MB00-28 Ni 1 2 <1 2 2 <1 2 1 Cr <1 <1 3 <1 <1 <1 <1 <1 Sc 1.5 2.8 2.5 2.1 4.0 4.0 2.8 3.2 V<154<1862<1 Ba 50 223 99 62 70 55 129 47 Rb 363 246 260 321 289 310 233 313 Sr 4 39 20 6 16 16 23 3 Zr 107 128 123 114 101 99 116 102 Y 5034526134814374 Nb 21 12 15 18 17 19 14 18 Ga 23.1 16.0 21.5 21.5 19.6 20.5 18.4 21.4 Cu 7 6 9 8 <2 <2 8 8 Co2 323<1222 Zn 68 39 50 52 56 66 49 65 Pb 36.6 30.9 29.9 33.5 32.0 33.0 26.5 30.5 U <1 <1 <1 <1 8.80 12.6 <1 <1 Th 41.3 38.5 28.1 37.8 31.4 33.2 27.0 36.8 Cs ------Hf ------Ta ------La 40.6 37.3 45.0 45.1 35.0 40.0 37.0 37.0 Ce 76.9 68.0 68.5 81.5 82.0 95.0 68.9 73.4 Pr ------Nd ------Sm ------Eu ------Gd ------Tb ------Dy ------Ho ------Er ------Tm ------Yb ------Lu ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

398 Tcm

Sample MB01-32 MB01-50 MB01-31 MB01-38 MB01-40 MB01-49 MB01-36 MB01-79 MB01-28B Unit Tcm Tcm Tcm Tcm Tcm Tcm Tcm Tcm Tcm Map Unit Tcm Tcm Tcm Tcm Tcm Tcm Tcm Tcm Tcm Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 474210 476168 474210 475254 475572 476164 474210 474705 474014 Easting 4628834 4632858 4628834 4633209 4633075 4633328 4628834 4623436 4631060

SiO2 63.29 66.09 66.30 67.27 67.96 68.34 68.48 70.63 71.02

TiO2 1.00 0.78 0.79 0.86 0.65 0.56 0.70 0.28 0.48

Al2O3 14.29 14.54 13.76 14.73 13.90 13.94 13.84 13.09 13.42

Fe2O3 8.14 5.85 5.85 6.05 4.50 4.63 4.88 2.82 4.68 MnO 0.17 0.10 0.07 0.12 0.03 0.05 0.06 0.05 0.07 MgO 0.63 1.10 1.88 0.83 0.34 0.58 1.43 0.14 0.24 CaO 3.09 2.89 3.72 2.70 1.80 2.02 2.87 1.02 1.21

Na2O 4.11 3.73 3.39 4.22 3.98 3.86 3.48 3.14 3.83

K2O 4.21 4.24 3.70 4.02 4.33 4.41 4.12 5.66 4.86

P2O5 0.35 0.22 0.28 0.24 0.13 0.22 0.12 0.07 0.13 L.O.I. 0.34 1.47 0.63 0.17 1.51 0.44 0.46 2.33 0.80 Total 99.62 101.01 100.38 101.19 99.15 99.05 100.43 99.23 100.72

MB01-32 MB01-50 MB01-31 MB01-38 MB01-40 MB01-49 MB01-36 MB01-79 MB01-28B Ni 5 5 24 3 4 5 35 3 6 Cr <1 12 51 7 <1 9 31 2 1 Sc 20 11 14 11 7.7 7.4 10 3.6 9.1 V 24488426272966514 Ba 3215 1344 497 1270 1339 1471 666 1614 1676 Rb 105 127 126 131 142 143 134 170 155 Sr 262 292 181 327 249 259 138 86 139 Zr 346 315 200 314 337 335 251 330 377 Y 443737383836393538 Nb 22 23 16 24 23 24 16 17 20 Ga 22.2 20.7 19.3 21.1 20.9 21.1 19.9 19.5 20.7 Cu 17 11 21 14 13 10 39 9 13 Co 8 9 15 9 6 6 12 3 4 Zn 143 105 89 107 89 98 86 66 95 Pb 17.5 19.7 18.9 19.8 20.7 21.6 22.0 25.3 24.1 U 3.44 <1 <1 <1 <1 <1 <1 <1 <1 Th 10.7 13.2 15.2 16.6 16.7 15.5 15.8 17.5 17.2 Cs 2.52 ------Hf 8.76 ------Ta 1.34 ------La 44.2 43.6 40.7 50.0 49.0 49.6 41.7 46.1 42.9 Ce 86.6 91.3 72.9 88.3 88.2 88.2 72.3 79.5 76.2 Pr 10.7 11.3 ------Nd 45.1 43.3 ------Sm 10.8 8.76 ------Eu 3.98 2.02 ------Gd 9.73 8.28 ------Tb 1.5 1.18 ------Dy 8.91 6.65 ------Ho 1.75 1.28 ------Er 4.68 3.77 ------Tm 0.66 ------Yb 4.04 3.38 ------Lu 0.65 0.5 ------87 86 Sr/ Srm --- 0.70495 ------87 86 Sr/ Sri --- 0.70467 ------143 144 Nd/ Ndm --- 0.51264 ------143 144 Nd/ Ndi --- 0.51263 ------

epsilon Ndi --- 0.24 ------206Pb/204Pb --- 19.022 ------207Pb/204Pb --- 15.616 ------208Pb/204Pb --- 38.703 ------

399 Tcm and Tct

Sample MB00-32B MB01-35 MB01-28A MB03-33 MB01-34 MB01-39 MB01-77 MB01-76 MB03-42 Unit Tcm Tcm Tcm Tcm Tcm Tcm Tcm Tcm Tct Map Unit Tcm Tcm Tom Tcm Tcm Tcm Tcm Tcm Tct Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 475284 474210 474014 475304 474210 475373 475751 475462 473405 Easting 4626584 4628834 4631060 4620116 4628834 4633178 4623480 4624446 4612328

SiO2 71.42 71.78 72.01 73.98 74.22 74.33 74.51 75.37 63.36

TiO2 0.27 0.26 0.40 0.27 0.26 0.28 0.27 0.26 0.79

Al2O3 12.87 12.50 13.47 13.81 12.78 13.33 13.47 13.36 14.42

Fe2O3 2.95 2.79 2.84 0.83 2.22 1.20 0.98 1.01 6.32 MnO 0.05 0.05 0.02 0.01 0.01 0.01 0.00 0.01 0.11 MgO 0.18 0.16 0.17 0.03 0.06 0.05 0.03 0.05 2.18 CaO 1.08 0.98 1.08 0.87 0.78 0.77 0.87 0.78 4.14

Na2O 3.32 3.23 3.77 3.84 3.53 3.69 3.79 3.68 3.08

K2O 5.15 4.92 4.98 5.17 4.85 5.04 5.24 5.19 3.49

P2O5 0.06 0.06 0.11 0.06 0.06 0.07 0.10 0.07 0.16 L.O.I. 2.63 2.58 0.79 0.38 0.43 0.32 0.50 0.31 0.56 Total 99.99 99.30 99.65 99.25 99.20 99.10 99.76 100.07 98.61

MB00-32B MB01-35 MB01-28A MB03-33 MB01-34 MB01-39 MB01-77 MB01-76 MB03-42 Ni2333332 33 Cr 2 <1 <1 <1 3 <1 <1 <1 2 Sc 3.7 3.2 7.0 7.0 4.5 4.4 6.3 5.2 6 V 6 2 8 11 18 9 4 4 12 Ba 1663 1496 1669 1535 1418 1365 1475 1419 1790 Rb 162 166 168 161 169 177 175 177 160 Sr 88 80 135 91 84 85 95 84 69 Zr 342 333 374 360 334 381 346 343 288 Y 353623383032433639 Nb 18 17 19 17 17 19 18 18 18 Ga 19.7 19.7 21.6 19.2 20.3 22.1 20.3 21.5 16.7 Cu98101379886<2 Co2232342 32 Zn 70 68 78 50 53 92 57 49 62 Pb 25.9 25.5 25.0 26.0 23.9 27.9 24.9 26.1 25.0 U 5.58 <1 <1 5.30 <1 <1 <1 <1 5.90 Th 16.8 17.8 18.1 16.8 19.1 19.9 18.6 18.0 19.4 Cs 5.36 ------Hf 9.48 ------Ta 1.22 ------La 46.1 47.4 50.9 43.0 48.1 52.0 52.4 59.5 42.0 Ce 86.6 79.9 77.0 93.0 78.4 85.4 80.0 86.9 96.0 Pr 9.92 ------Nd 39.0 ------Sm 8.85 ------Eu 1.28 ------Gd 7.75 ------Tb 1.25 ------Dy 7.54 ------Ho 1.49 ------Er 4.05 ------Tm 0.59 ------Yb 3.72 ------Lu 0.58 ------87 86 Sr/ Srm 0.70537 ------87 86 Sr/ Sri ------143 144 Nd/ Ndm 0.51264 ------143 144 Nd/ Ndi 0.51262 ------epsilon Ndi 0.14 ------206Pb/204Pb 19.061 ------207Pb/204Pb 15.623 ------208Pb/204Pb 38.765 ------

400 Tct

Sample MB01-57 MB01-58 MB03-47 MB01-60B MB01-53 MB01-60A MB01-54B MB03-52 MB03-46 Unit Tct Tct Tct Tct Tct Tct Tct Tct Tct Map Unit Tct Tct Tct Tct Tct Tct Tct Tct Tct Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 472268 472358 467540 471094 470622 471094 470826 469220 467727 Easting 4616925 4617079 4614361 4615936 4614161 4615936 4614029 4614135 4613957

SiO2 66.71 66.77 68.12 69.16 69.48 69.85 70.91 71.42 71.92

TiO2 0.81 0.85 0.66 0.64 0.69 0.61 0.39 0.42 0.35

Al2O3 14.02 14.33 14.46 14.88 14.49 14.37 13.76 14.01 13.89

Fe2O3 4.94 5.27 3.84 3.68 4.26 3.52 2.21 2.46 2.08 MnO 0.11 0.08 0.05 0.10 0.11 0.06 0.08 0.06 0.08 MgO 0.77 0.73 0.52 0.39 0.86 0.43 0.31 0.30 0.27 CaO 2.68 2.75 2.21 2.07 2.44 1.99 1.20 1.34 1.14

Na2O 3.81 4.08 4.14 4.32 4.13 4.19 3.64 4.11 4.08

K2O 3.99 3.69 3.91 4.27 4.02 4.20 5.02 4.58 4.65

P2O5 0.34 0.35 0.21 0.20 0.23 0.18 0.09 0.10 0.08 L.O.I. 0.60 0.57 0.96 0.88 0.29 0.54 3.21 0.50 0.24 Total 98.78 99.47 99.07 100.58 100.99 99.94 100.82 99.31 98.78

MB01-57 MB01-58 MB03-47 MB01-60B MB01-53 MB01-60A MB01-54B MB03-52 MB03-46 Ni3 42333133 Cr <1 <1 <1 <1 <1 2 2 <1 <1 Sc 10 11 10 7 9.1 8 7.6 4 7 V 70723224392121412 Ba 1913 1153 1480 1429 1125 1486 1660 1445 1575 Rb 99 123 119 123 131 122 149 169 144 Sr 218 206 211 214 187 208 124 19 11 Zr 266 256 347 363 283 364 412 395 409 Y 434744444543423244 Nb 17 17 17 18 18 18 19 21 18 Ga 18.5 19.0 18.3 18.0 18.5 18.6 17.7 23.1 17.4 Cu 10 20 <2 7 9 7 6 <2 <2 Co8 824522<12 Zn 84 94 87 82 85 89 70 108 77 Pb 19.5 21.0 21.0 20.9 21.8 21.7 24.8 20.0 25.2 U <1 5.50 5.90 <1 <1 <1 6.77 2.80 6.72 Th 14.5 14.1 14.4 17.2 16.8 17.4 17.6 15.8 17.7 Cs --- 3.01 ------3.94 --- 2.74 Hf --- 6.88 ------10.4 --- 9.94 Ta --- 1.15 ------1.25 --- 1.27 La 35.6 38.7 38.0 41.5 39.9 38.8 42.1 35.0 42.4 Ce 71.2 67.0 82.0 70.8 72.3 69.7 79.5 65.0 78.7 Pr --- 9.1 ------9.07 --- 9.1 Nd --- 37.6 ------35.9 --- 35.7 Sm --- 9.32 ------8.31 --- 8.21 Eu --- 1.78 ------1.57 --- 1.59 Gd --- 8.8 ------7.73 --- 7.5 Tb --- 1.46 ------1.27 --- 1.26 Dy --- 8.91 ------7.93 --- 7.77 Ho --- 1.83 ------1.64 --- 1.62 Er --- 4.92 ------4.69 --- 4.55 Tm --- 0.72 ------0.71 --- 0.71 Yb --- 4.47 ------4.56 --- 4.57 Lu --- 0.69 ------0.75 --- 0.74 87 86 Sr/ Srm --- 0.70550 ------0.70604 ------87 86 Sr/ Sri --- 0.70512 ------0.70533 ------143 144 Nd/ Ndm --- 0.51269 ------0.51269 ------143 144 Nd/ Ndi --- 0.51268 ------0.51267 ------

epsilon Ndi --- 1.20 ------1.08 ------206Pb/204Pb --- 19.167 ------19.213 ------207Pb/204Pb --- 15.625 ------15.657 ------208Pb/204Pb --- 38.793 ------38.914 ------

401 Tct and Tom

Sample MB01-54A MB03-35 MB01-55 MB03-45 MB03-43 MB01-51 MB01-56 MB00-37 MB00-35 Unit Tct Tct Tct Tct Tct Tct Tct Tom Tom Map Unit Tct Tct Tct Tct Tct Tct Tct Tom Tom Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 470826 474722 471604 476386 473581 471151 471990 468472 467954 Easting 4614029 4619077 4614374 4612278 4612058 4614714 4615070 4636691 4638094

SiO2 71.97 72.09 72.51 73.94 74.10 74.17 74.42 72.81 73.62

TiO2 0.20 0.34 0.15 0.13 0.12 0.25 0.20 0.17 0.21

Al2O3 12.99 12.73 12.87 13.26 13.32 13.73 13.25 12.35 13.05

Fe2O3 1.45 3.02 1.37 1.49 1.41 1.72 1.60 2.00 2.68 MnO 0.06 0.05 0.06 0.05 0.03 0.04 0.04 0.02 0.04 MgO 0.16 0.15 0.18 0.05 0.09 0.11 0.12 0.09 0.43 CaO 0.74 0.81 0.67 0.74 0.69 0.71 0.87 0.68 1.28

Na2O 3.51 3.42 3.01 3.59 3.50 3.91 3.78 2.76 3.53

K2O 5.32 5.43 5.68 5.03 5.12 5.20 5.22 5.79 4.81

P2O5 0.06 0.10 0.09 0.02 0.02 0.04 0.06 0.02 0.04 L.O.I. 1.08 0.45 0.64 0.42 0.60 3.01 0.66 Total 96.46 99.23 96.59 98.75 99.04 100.31 100.14 99.72 100.34

MB01-54A MB03-35 MB01-55 MB03-45 MB03-43 MB01-51 MB01-56 MB00-37 MB00-35 Ni <1 3 <1 3 2 2 2 <1 7 Cr --- <1 --- <1 <1 <1 <1 3 12 Sc 5.4 9 5.0 6 7 4.3 4.8 4 6 V <116<19962613 Ba 1601 1230 1169 585 550 1425 1357 210 484 Rb 157 138 181 170 175 172 172 235 190 Sr 58 137 39 46 40 63 60 23 49 Zr 244 436 205 178 168 287 255 248 223 Y 474249314853464952 Nb 9.1 18 9.1 15 16 20 19 27 19 Ga --- 17.7 --- 16.2 16.3 17.9 18.1 20.7 20.1 Cu --- <2 --- <2 128 6 6 13 13 Co --- <1 --- <1 3 3 2 3 6 Zn 93 78 67 53 50 61 61 75 76 Pb --- 25.9 --- 25.1 27.0 25.3 24.9 27.7 27.1 U --- 4.85 --- 5.24 8.10 <1 <1 <1 <1 Th --- 22.1 --- 19.3 21.0 21.1 19.5 29.0 21.5 Cs --- 3.18 --- 3.86 ------Hf --- 15.2 --- 5.95 ------Ta --- 2.65 --- 1.16 ------La --- 79.1 --- 52.7 61.0 48.8 41.3 61.0 57.7 Ce --- 153.4 --- 82.6 133.0 78.3 73.8 103.0 89.5 Pr --- 16.6 --- 11.1 ------Nd --- 63.4 --- 41.7 ------Sm --- 14.0 --- 8.44 ------Eu --- 1.61 --- 1.02 ------Gd --- 12.73 --- 6.51 ------Tb --- 2.21 --- 1.02 ------Dy --- 13.8 --- 5.99 ------Ho --- 2.79 --- 1.18 ------Er --- 7.73 --- 3.3 ------Tm --- 1.16 --- 0.5 ------Yb --- 7.28 --- 3.23 ------Lu --- 1.11 --- 0.5 ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

402 Tom

Sample MB01-27 MB01-26 MB03-10B MB03-10A MB00-41 MB00-33 MB00-36B Unit Tom Tom Tom Tom Tom Tom Tom Map Unit Tom Tcm Tom Tom Tom Tom Tom Utm Zone 11T 11T 11T 11T 11T 11T 11T Northing 474243 473596 465783 465783 468822 469083 468450 Easting 4631219 4630042 4636455 4636455 4635586 4638444 4637062

SiO2 73.65 75.04 75.17 75.74 76.24 76.27 76.62

TiO2 0.18 0.21 0.11 0.12 0.16 0.16 0.14

Al2O3 11.90 12.23 12.15 11.95 12.61 11.84 12.25

Fe2O3 2.15 2.56 1.64 1.72 1.93 2.04 1.83 MnO 0.03 0.03 0.02 0.02 0.01 0.02 0.03 MgO 0.07 0.06 0.03 0.18 0.07 0.02 0.07 CaO 0.54 0.31 0.41 0.36 0.49 0.11 0.42

Na2O 2.89 3.71 3.75 3.67 3.74 3.76 3.63

K2O 5.94 5.26 4.90 4.79 5.15 4.82 5.09

P2O5 0.03 0.05 0.02 0.05 0.06 0.05 0.03 L.O.I. 2.43 0.48 0.93 0.93 0.59 0.52 0.24 Total 99.81 99.94 99.14 99.54 101.05 99.62 100.34

MB01-27 MB01-26 MB03-10B MB03-10A MB00-41 MB00-33 MB00-36B Ni22 3<1422 Cr 5 1 <1 <1 2 <1 <1 Sc <1 1 2 <1 2 <1 1 V <13565141 Ba 55 74 65 61 276 49 194 Rb 219 211 237 218 230 241 239 Sr 5 6 8 10 24 6 15 Zr 455 538 274 289 296 404 251 Y 60315645355342 Nb 32 32 33 35 29 49 27 Ga 24.1 24.5 22.1 22.6 21.8 24.6 21.8 Cu710<2<2131813 Co22 22212 Zn 107 119 93 110 77 100 83 Pb 32.2 26.6 47.0 46.0 26.2 21.0 59.8 U 8.12 <1 9.34 9.90 <1 7.28 <1 Th 22.4 23.5 29.1 28.9 28.1 26.2 29.3 Cs 7.33 --- 6.95 ------6.28 --- Hf 12.5 --- 10.2 ------12.9 --- Ta 2.17 --- 2.96 ------2.88 --- La 62.6 48.7 66.5 61.0 50.4 66.5 56.8 Ce 120.6 59.8 129.4 142.0 73.0 137.0 91.3 Pr 13.8 --- 14.2 ------14.8 --- Nd 54.1 --- 54.0 ------56.1 --- Sm 12.2 --- 11.9 ------12.0 --- Eu 0.28 --- 0.2 ------0.25 --- Gd 10.8 --- 9.93 ------9.82 --- Tb 1.75 --- 1.64 ------1.61 --- Dy 10.6 --- 9.67 ------9.56 --- Ho 2.13 --- 1.88 ------1.9 --- Er 5.78 --- 5.08 ------5.24 --- Tm 0.85 --- 0.76 ------0.77 --- Yb 5.32 --- 4.72 ------4.85 --- Lu 0.83 --- 0.71 ------0.76 --- 87 86 Sr/ Srm 0.70511 --- 0.70583 ------0.70594 --- 87 86 Sr/ Sri ------143 144 Nd/ Ndm 0.51276 --- 0.51269 ------0.51268 --- 143 144 Nd/ Ndi 0.51263 --- 0.51267 ------0.51266 ---

epsilon Ndi 0.37 --- 1.13 ------0.94 --- 206Pb/204Pb 19.024 --- 19.021 ------19.030 --- 207Pb/204Pb 15.616 --- 15.624 ------15.619 --- 208Pb/204Pb 38.710 --- 38.727 ------38.774 ---

403 Tcst

Sample MB03-32 MB03-36B JK04-5 MB03-37A MB02-55 MB01-62 MB03-28B MB02-63A MB03-28A Unit Tcst Tcst Tcst Tcst Tcst Tcst Tcst Tcst Tcst Map Unit Tgl Tgl Tcst Tgl Tgl Tgl Tgl Tgl Tgl Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 466789 470496 469803 470424 463995 469804 460582 461855 460582 Easting 4629521 4608413 4634504 4608190 4621788 4619910 4625695 4622466 4625695

SiO2 66.98 67.22 67.75 68.02 68.08 68.85 69.15 69.17 69.49

TiO2 0.51 0.52 0.53 0.49 0.45 0.49 0.44 0.40 0.42

Al2O3 13.64 13.61 14.07 12.68 13.32 13.69 13.18 13.11 13.23

Fe2O3 4.20 4.19 3.92 5.82 3.73 3.80 3.41 3.40 3.27 MnO 0.07 0.07 0.08 0.13 0.07 0.06 0.05 0.06 0.05 MgO 0.44 0.46 0.38 0.28 0.45 0.35 0.28 0.56 0.28 CaO 1.83 1.75 1.86 1.72 2.24 1.53 1.42 1.33 1.47

Na2O 3.32 3.27 3.51 2.71 3.24 3.48 3.21 2.66 3.18

K2O 5.29 5.35 5.59 5.36 5.35 5.15 5.60 5.99 5.68

P2O5 0.11 0.05 0.10 0.09 0.10 0.09 0.08 0.09 0.09 L.O.I. 2.21 2.40 1.77 2.63 2.70 1.98 2.46 ------Total 98.58 98.89 99.55 99.95 99.73 99.46 99.29 96.77 97.16

MB03-32 MB03-36B JK04-5 MB03-37A MB02-55 MB01-62 MB03-28B MB02-63A MB03-28A Ni 5 5 5 4 5 4 4 ------Cr 2 4 <1 4 5 2 2 ------Sc 7 7 150 10 4 6 6 ------V 27252219161721------Ba 2480 2460 2840 1820 2188 2233 2110 ------Rb 143 144 141 144 156 163 160 ------Sr 142 133 6 125 119 121 105 ------Zr 685 690 675 650 600 644 628 ------Y 62656689707167------Nb 34 35 35 43 41 42 37 ------Ga 21.3 21.5 21.7 23.4 21.0 22.2 20.2 ------Cu 44 95 12 <2 16 13 <2 ------Co 5 4 4 3 4 4 3 ------Zn 101 98 101 158 87 93 90 ------Pb 23.0 23.0 19.0 25.0 24.4 24.2 25.0 ------U 4.40 4.20 4.70 4.50 4.64 <1 5.20 ------Th 17.4 17.4 18.3 17.1 18.6 20.9 19.3 ------Cs ------3.41 ------Hf ------16.6 ------Ta ------2.28 ------La 65.0 71.0 70.0 77.0 70.9 75.1 74.0 ------Ce 134.0 145.0 198.0 167.0 133.7 116.1 155.0 ------Pr ------15.0 ------Nd ------58.5 ------Sm ------13.1 ------Eu ------2.57 ------Gd ------12.1 ------Tb ------2.02 ------Dy ------12.6 ------Ho ------2.55 ------Er ------7.04 ------Tm ------1.06 ------Yb ------6.56 ------Lu ------1.02 ------87 86 Sr/ Srm ------0.70692 ------87 86 Sr/ Sri ------0.70608 ------143 144 Nd/ Ndm ------0.51243 ------143 144 Nd/ Ndi ------0.51241 ------

epsilon Ndi ------3.93 ------206Pb/204Pb ------18.716 ------207Pb/204Pb ------15.640 ------208Pb/204Pb ------39.104 ------

404 Tcst

Sample MB03-44 MB02-59 MB03-36A JK04-7B MB03-31 MB03-26F MB03-37C MB03-26D MB03-30B Unit Tcst Tcst Tcst Tcst Tcst Tcst Tcst Tcst Tcst Map Unit Tgl Tgl Tgl Tcst Tgl Tgl Tgl Tgl Tgl Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 473777 461879 470782 462422 466804 462850 470424 462850 468910 Easting 4611129 4621227 4608204 4624496 4629589 4627544 4608190 4627544 4628135

SiO2 69.69 69.91 70.09 70.77 70.97 70.98 71.27 71.84 71.89

TiO2 0.43 0.42 0.35 0.36 0.46 0.33 0.29 0.32 0.30

Al2O3 12.11 13.18 12.53 12.36 13.86 12.06 11.79 11.98 12.25

Fe2O3 4.77 3.57 2.98 3.77 2.30 3.24 3.39 3.20 2.37 MnO 0.11 0.06 0.06 0.06 0.02 0.06 0.08 0.04 0.03 MgO 0.23 0.23 0.44 0.12 0.10 0.25 0.07 0.12 0.09 CaO 1.48 1.27 1.22 0.77 1.09 0.90 0.95 0.93 0.67

Na2O 2.96 3.14 2.39 2.84 3.76 2.65 3.04 2.77 2.48

K2O 5.02 5.73 5.73 5.85 5.33 5.66 5.30 5.54 5.83

P2O5 0.06 0.08 0.23 0.05 0.08 0.06 0.07 0.04 0.04 L.O.I. 2.75 2.54 3.44 3.06 0.81 2.88 --- 2.70 3.22 Total 99.59 100.12 99.46 100.01 98.77 99.07 96.25 99.46 99.16

MB03-44 MB02-59 MB03-36A JK04-7B MB03-31 MB03-26F MB03-37C MB03-26D MB03-30B Ni 4 3 4 --- 4 3 --- 3 3 Cr 8 6 <1 <1 <1 <1 --- 2 2 Sc 6 5 6 55 8 4 --- 5 5 V 20 13 18 9 24 13 --- 8 9 Ba 1585 1756 1095 1330 2250 1185 --- 1297 1155 Rb 154 177 172 168 162 181 --- 175 182 Sr 79 86 62 5 112 50 --- 55 47 Zr 685 597 531 665 647 664 --- 654 608 Y 947274937295---8695 Nb 45 44 42 49 39 49 --- 48 49 Ga 22.7 21.2 20.0 21.8 21.8 21.3 --- 21.5 21.4 Cu <2 14 62 7 <2 <2 --- <2 <2 Co243232---22 Zn 140 92 84 125 93 116 --- 114 107 Pb 27.0 24.8 29.0 29.0 25.0 29.0 --- 27.0 29.0 U 5.90 <1 6.80 7.50 6.70 5.70 --- 6.10 7.00 Th 18.7 22.1 23.8 24.9 20.3 22.5 --- 22.6 23.0 Cs ------Hf ------Ta ------La 84.0 78.6 86.0 89.0 70.0 89.0 --- 88.0 93.0 Ce 181.0 119.8 182.0 212.0 146.0 191.0 --- 192.0 200.0 Pr ------Nd ------Sm ------Eu ------Gd ------Tb ------Dy ------Ho ------Er ------Tm ------Yb ------Lu ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

405 Tcst

Sample MB03-30C MB03-37B JK04-6 JK04-4 MB03-26C MB02-63B MB02-60 MB03-41 MB03-26B Unit Tcst Tcst Tcst Tcst Tcst Tcst Tcst Tcst Tcst Map Unit Tgl Tgl Tcst Tcst Tgl Tgl Tgl Tgl Tgl Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 468910 470424 470154 469707 462850 461855 461933 471582 462850 Easting 4628135 4608190 4633522 4634534 4627544 4622466 4621158 4604139 4627544

SiO2 72.14 72.34 72.47 72.57 72.79 72.92 72.93 73.42 74.52

TiO2 0.30 0.30 0.30 0.33 0.32 0.32 0.29 0.31 0.31

Al2O3 12.00 12.01 11.82 12.55 12.10 12.23 12.07 11.89 11.65

Fe2O3 3.09 3.50 2.72 2.92 2.46 2.61 2.38 2.79 3.10 MnO 0.05 0.08 0.04 0.05 0.04 0.05 0.04 0.05 0.02 MgO 0.09 0.06 0.05 0.11 0.09 0.06 0.14 0.26 0.11 CaO 0.85 1.01 0.69 0.65 0.90 0.83 0.70 1.19 0.56

Na2O 2.59 3.19 2.26 3.49 2.79 2.77 2.41 3.14 3.15

K2O 5.87 5.24 6.42 5.46 5.56 6.08 6.13 4.98 5.10

P2O5 0.04 0.09 0.04 0.06 0.05 0.04 0.04 0.05 0.05 L.O.I. 2.85 --- 2.67 0.61 2.19 2.71 3.09 1.44 0.92 Total 99.86 97.82 99.48 98.81 99.29 100.62 100.21 99.53 99.47

MB03-30C MB03-37B JK04-6 JK04-4 MB03-26C MB02-63B MB02-60 MB03-41 MB03-26B Ni 3 --- 3 6 2 3 3 6 3 Cr <1 --- <1 <1 <1 <1 4 <1 <1 Sc 5 --- 45 61 4 3 3 6 4 V 12---819122 52515 Ba 1145 --- 1450 1330 1840 1148 888 1235 1150 Rb 182 --- 179 184 173 192 199 168 165 Sr 40 --- 4 4 64 44 37 60 51 Zr 629 --- 633 535 655 632 573 520 622 Y 89 --- 97 76 88 98 95 63 73 Nb 48 --- 50 43 48 62 65 38 47 Ga 21.0 --- 21.2 20.8 21.0 23.3 22.9 19.1 20.5 Cu <2 --- 10 12 <2 8 9 63 <2 Co 2 --- <1 <1 3 2 2 3 2 Zn 107 --- 126 84 116 117 100 77 96 Pb 29.0 --- 26.0 26.0 28.0 27.6 29.1 24.0 24.0 U 5.80 --- 5.50 5.00 6.90 <1 <1 5.10 5.70 Th 23.1 --- 25.7 24.4 22.0 25.0 26.6 19.8 21.7 Cs ------Hf ------Ta ------La 90.0 --- 94.0 85.0 95.0 100.0 96.8 72.0 80.0 Ce 189.0 --- 210.0 175.0 212.0 149.5 148.8 153.0 158.0 Pr ------Nd ------Sm ------Eu ------Gd ------Tb ------Dy ------Ho ------Er ------Tm ------Yb ------Lu ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

406 Tcst

Sample MB03-26A MB03-24 JK04-2E JK04-2C MB03-26E JK04-3A JK04-2D JK04-3B JK04-2B Unit Tcst Tcst Tcst Tcst Tcst Tcst Tcst Tcst Tcst Map Unit Tgl Tgl Tcst Tcst Tgl Tcst Tcst Tcst Tcst Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 462850 460195 463588 463588 462850 463421 463588 463421 463588 Easting 4627544 4626015 4622532 4622532 4627544 4623082 4622532 4623082 4622532

SiO2 75.24 76.11 ------

TiO2 0.32 0.30 ------

Al2O3 12.47 12.61 ------

Fe2O3 1.28 0.40 ------MnO 0.01 0.01 ------MgO 0.08 0.06 ------CaO 0.45 0.33 ------

Na2O 3.53 3.58 ------

K2O 5.51 5.64 ------

P2O5 0.05 0.04 ------L.O.I. 0.44 0.46 ------Total 99.38 99.55 ------

MB03-26A MB03-24 JK04-2E JK04-2C MB03-26E JK04-3A JK04-2D JK04-3B JK04-2B Ni <1 2 3 5 3 4 5 4 6 Cr <1 <1 <1 <1 2 <1 <1 7 <1 Sc54 4356456 V 11 13 9 10 10 10 11 18 18 Ba 1235 1005 1160 1040 1357 1180 1170 1525 1595 Rb 185 194 184 184 184 176 182 157 170 Sr 45 37 46 50 49 51 64 110 84 Zr 668 663 625 637 659 645 612 610 633 Y 939890859295879268 Nb 51 52 50 54 48 50 51 46 41 Ga 22.4 22.9 20.8 22.7 21.0 21.2 21.2 21.7 19.9 Cu <2 <2 12 10 <2 <2 10 4 13 Co 2 3 2 <1 <1 2 2 3 3 Zn 97 129 107 107 119 119 114 125 96 Pb 29.9 27.0 29.0 32.0 27.0 28.0 28.0 28.0 22.0 U 5.67 7.90 6.60 6.00 6.50 5.70 6.70 7.90 6.40 Th 24.5 24.4 25.6 27.4 22.4 25.4 26.6 21.9 24.5 Cs 2.83 ------Hf 18.4 ------Ta 3.38 ------La 100.7 87.0 88.0 93.0 91.0 92.0 90.0 94.0 80.0 Ce 178.0 170.0 201.0 190.0 201.0 200.0 206.0 206.0 183.0 Pr 21.8 ------Nd 85.1 ------Sm 19.3 ------Eu 1.9 ------Gd 17.7 ------Tb 3 ------Dy 18.5 ------Ho 3.71 ------Er 10.2 ------Tm 1.49 ------Yb 9.14 ------Lu 1.38 ------87 86 Sr/ Srm 0.70642 ------87 86 Sr/ Sri ------143 144 Nd/ Ndm 0.51248 ------143 144 Nd/ Ndi 0.51246 ------

epsilon Ndi -3.01 ------206Pb/204Pb 18.710 ------207Pb/204Pb 15.614 ------208Pb/204Pb 38.968 ------

407 Tcst

Sample MB03-13 MB03-30A JK04-2A Unit Tcst Tcst Tcst Map Unit Tgl Tgl Tcst Utm Zone 11T 11T 11T Northing 465752 468928 463622 Easting 4631433 4628166 4622630

SiO2 ------

TiO2 ------

Al2O3 ------

Fe2O3 ------MnO ------MgO ------CaO ------

Na2O ------

K2O ------

P2O5 ------L.O.I. ------Total ------

MB03-13 MB03-30A JK04-2A Ni 3 4 8 Cr <1 2 3 Sc 6 9 6 V222326 Ba 2010 2960 2380 Rb 173 145 142 Sr 113 140 144 Zr 679 672 689 Y676267 Nb 40 33 37 Ga 22.8 20.8 21.1 Cu 64 5 29 Co 4 2 5 Zn 67 100 97 Pb 26.0 23.0 22.0 U 6.70 5.00 4.20 Th 21.9 15.5 20.2 Cs ------Hf ------Ta ------La 73.0 69.0 73.0 Ce 149.0 145.0 200.0 Pr ------Nd ------Sm ------Eu ------Gd ------Tb ------Dy ------Ho ------Er ------Tm ------Yb ------Lu ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

408 Tp1

Sample MB03-11 MB00-42 MB03-27 MB03-12A MB03-23 MB00-20 MB01-70 MB02-67 MB02-38B Unit Tp1 Tp1 Tp1 Tp1 Tp1 Tp1 Tp1 Tp1 Tp1 Map Unit Tgl Tgl Tgl Tgl Tgl Tgl Tgl Tgl Tgl Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 465667 462879 462116 465582 464357 462775 457340 453986 457140 Easting 4635360 4632110 4626623 4635216 4625037 4614605 4627145 4621844 4634282

SiO2 72.86 73.41 73.61 73.63 73.67 74.34 74.55 74.63 74.66

TiO2 0.31 0.33 0.34 0.25 0.32 0.34 0.33 0.33 0.38

Al2O3 11.84 11.94 12.51 10.90 12.01 12.40 12.02 12.33 12.62

Fe2O3 3.25 3.74 2.45 3.23 2.70 2.49 3.13 2.51 2.39 MnO 0.07 0.08 0.03 0.05 0.03 0.02 0.02 0.04 0.04 MgO 0.09 0.07 0.15 0.03 0.19 0.14 0.15 0.05 0.07 CaO 0.43 0.22 0.25 0.33 0.26 0.22 0.30 0.26 0.40

Na2O 4.10 4.08 4.25 2.75 4.11 4.22 4.08 4.31 4.39

K2O 4.95 4.90 5.23 6.04 5.01 5.18 5.01 5.19 5.21

P2O5 0.04 0.04 0.05 0.02 0.03 0.06 0.05 0.08 0.05 L.O.I. 0.85 0.91 1.05 2.89 1.12 0.77 1.06 0.37 0.56 Total 98.79 99.71 99.93 100.13 99.47 100.19 100.69 100.10 100.76

MB03-11 MB00-42 MB03-27 MB03-12A MB03-23 MB00-20 MB01-70 MB02-67 MB02-38B Ni 4 2 3 2 2 2 <1 2 2 Cr <1 <1 <1 <1 <1 <1 <1 <1 <1 Sc 4 <1 3 3 <1 3 2 2 3 V 27 6 11 13 11 7 6 10 4 Ba 365 310 440 55 370 409 232 315 510 Rb 180 182 170 167 199 176 182 184 178 Sr 71 9 18 20 6 27 14 9 18 Zr 568 395 403 377 491 351 403 388 372 Y 733257318254666341 Nb 42 24 22 20 27 22 24 23 23 Ga 20.1 24.7 23.6 23.2 24.0 24.4 25.0 25.3 25.5 Cu 28 8 <2 <2 <2 8 8 8 7 Co 2 2 <1 <1 2 2 3 3 2 Zn 85 143 125 90 171 89 119 125 103 Pb 26.0 24.2 27.0 20.0 29.0 22.3 16.7 24.2 21.8 U 4.40 1.30 3.20 4.40 9.00 1.20 <1 <1 1.20 Th 22.1 18.8 16.9 16.5 18.7 15.7 17.5 16.8 15.5 Cs ------Hf ------Ta ------La 79.0 34.1 50.0 36.0 56.0 34.8 50.1 44.0 36.2 Ce 169.0 56.9 101.0 56.0 114.0 60.2 71.2 73.0 64.5 Pr ------Nd ------Sm ------Eu ------Gd ------Tb ------Dy ------Ho ------Er ------Tm ------Yb ------Lu ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

409 Tp1 and Tp2

Sample MB00-46 MB00-23 MB02-24 MB00-27 MB02-38A MB02-58C MB00-38D MB02-57A1 MB02-58B Unit Tp1 Tp1 Tp1 Tp1 Tp1 Tp2 Tp2 Tp2 Tp2 Map Unit Tgl Tgl Tgl Tgl Tgl Tsed Tgl Tsed Tsed Utm Zone 11T 11T 11T 11T 11T 11T 11T 11T 11T Northing 452842 462452 457484 456235 457140 463515 468546 463695 463515 Easting 4631002 4615107 4613942 4622454 4634282 4618630 4636597 4618521 4618630

SiO2 74.76 75.16 75.17 75.27 77.08 67.20 67.73 68.08 69.07

TiO2 0.34 0.31 0.33 0.36 0.24 0.34 0.48 0.33 0.33

Al2O3 12.06 11.84 12.30 12.67 10.93 11.03 11.93 11.86 11.22

Fe2O3 3.81 2.95 2.48 2.04 2.72 3.91 4.37 3.10 3.57 MnO 0.07 0.02 0.03 0.02 0.04 0.08 0.06 0.06 0.07 MgO 0.11 0.14 0.08 0.04 0.05 0.11 0.24 0.21 0.14 CaO 0.42 0.30 0.23 0.17 0.22 1.16 1.55 1.18 1.04

Na2O 4.26 4.14 4.28 4.32 4.13 2.93 2.58 2.08 2.73

K2O 5.08 5.02 5.13 5.20 4.78 4.54 4.60 4.75 4.76

P2O5 0.04 0.06 0.03 0.04 0.04 0.01 0.08 0.01 0.08 L.O.I. 0.56 0.82 0.89 0.44 0.32 ------Total 101.50 100.75 100.94 100.56 100.56 91.32 93.62 91.66 92.99

MB00-46 MB00-23 MB02-24 MB00-27 MB02-38A MB02-58C MB00-38D MB02-57A1 MB02-58B Ni3222164 32 Cr <1 <1 <1 4 1 4 --- 7 5 Sc1222<187 77 V 6 6 3 15 <1 --- 12 ------Ba 329 256 231 263 67 1405 1204 1658 1350 Rb 179 185 186 185 205 136 150 133 140 Sr 13 14 14 9 <2 90 135 120 83 Zr 300 418 405 398 496 484 376 373 422 Y 465757698969495259 Nb 23 25 24 24 30 27 11 13 26 Ga 24.4 24.9 25.1 25.4 26.4 ------Cu 8 8 8 7 9 ------Co 4 2 2 3 3 ------Zn 86 129 118 110 167 114 98 98 98 Pb 21.9 17.3 21.5 24.1 29.3 ------U <1 <1 <1 <1 <1 ------Th 13.5 18.6 19.9 18.1 22.5 ------Cs ------Hf ------Ta ------La 44.3 44.6 40.5 52.9 60.2 ------Ce 71.0 71.1 68.0 73.3 92.1 ------Pr ------Nd ------Sm ------Eu ------Gd ------Tb ------Dy ------Ho ------Er ------Tm ------Yb ------Lu ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

410 Tp2

Sample MB02-58A MB02-69 MB02-57B MB00-21B MB02-17A Unit Tp2 Tp2 Tp2 Tp2 Tp2 Map Unit Tsed Tsed Tsed Tsed Tsed Utm Zone 11T 11T 11T 11T 11T Northing 463508 455291 463695 462704 457280 Easting 4618595 4621744 4618521 4614424 4618593

SiO2 69.59 69.95 70.58 70.90 72.07

TiO2 0.19 0.35 0.20 0.29 0.23

Al2O3 10.86 11.48 11.05 11.53 11.06

Fe2O3 2.36 3.39 2.37 2.83 1.95 MnO 0.04 0.06 0.04 0.05 0.10 MgO 0.01 0.09 0.01 0.05 0.09 CaO 0.45 1.01 0.45 0.76 0.26

Na2O 2.78 2.71 2.73 2.96 1.87

K2O 5.52 4.90 5.82 4.87 6.27

P2O5 0.04 0.20 0.02 0.03 0.15 L.O.I. ------Total 91.85 94.14 93.27 94.26 94.04

MB02-58A MB02-69 MB02-57B MB00-21B MB02-17A Ni 29 <1 3 3 <1 Cr 41 --- 9 8 --- Sc <1 6 <1 5 6 V --- <1 ------<1 Ba 709 1283 727 1373 107 Rb 126 148 104 145 217 Sr 8 81 8 55 5 Zr 564 318 581 396 445 Y 104 48 106 51 78 Nb 35 6.4 34 17 16 Ga ------Cu ------Co ------Zn 154 93 169 99 117 Pb ------U ------Th ------Cs ------Hf ------Ta ------La ------Ce ------Pr ------Nd ------Sm ------Eu ------Gd ------Tb ------Dy ------Ho ------Er ------Tm ------Yb ------Lu ------87 86 Sr/ Srm ------87 86 Sr/ Sri ------143 144 Nd/ Ndm ------143 144 Nd/ Ndi ------

epsilon Ndi ------206Pb/204Pb ------207Pb/204Pb ------208Pb/204Pb ------

411 APPENDIX 4: 40AR/39AR GEOCHRONOLOGY

40Ar/39Ar method geochronology was obtained through the New Mexico Geochronological Research Laboratory (NMGRL) at New Mexico Tech, Socorro NM (http://www.ees.nmt.edu/Geol/labs/Argon_Lab/NMGRL_homepage.html). A detailed discussion of the techniques used during this study was compiled by Matthew T. Heizler (NMGRL) and is presented below:

ANALYTICAL METHODS AND RESULTS Twelve groundmass and one plagioclase were degassed by the incremental step-heating method using a heating schedule that varied between 7 and 12 steps. The 5 samples irradiated in package

NM-172 were heated with a defocused CO2 laser beam, whereas the 7 samples from irradiation package NM-156 were heated within a double vacuum Mo resistance furnace. A synopsis of the analytical methods along with a summary of the age results is provided in Table 1 with complete isotopic results given in Table 2. More complete information about the general operational procedures of the NMGRL can be found at Internet site http://geoinfo.nmt.edu/publications/openfile/argon/home.html. All of the age spectra and associated isochron diagrams are presented in Figures 1-4. The samples yield apparent ages that range between about 13 and 35 Ma with significantly varying levels of precision that can be linked to age spectrum complexity and K-content. Groundmass age spectra display 2 basic shapes that are characterized by either overall increasing ages from low to high temperature (Fig. 1; Figs. 3a, 3b) or overall decreasing ages from low to high temperature (Figs. 3f, g). Only two samples, MB01-12 and MB01-24, have completely flat spectra with MSWD values that fall within the 95% confidence window for n-1 degrees of freedom (Figs. 3d, 3e). Most age spectra display segments with at least 50% of the total 39Ar released giving acceptable MSWD values, but some samples like MB01-1, MB01-47 and MB00-13 have weighted mean ages that are associated with slightly elevated MSWD values suggestive of scatter than cannot be solely explained by analytical error (Table 1). K/Ca spectra are typical of mafic groundmass and have initial steps that begin at about 1 and decrease across the spectra as Ca-bearing phases contribute

412 more to the gas release (Fig. 1, 3). MB01-12 and MB01-24 have overall lower K/Ca and also much lower K contents compared to the other samples. Radiogenic yields either increase across the entire spectrum or reach a maximum prior to descending at intermediate to high temperatures (Figs. 1, 3). The plagioclase from MB00-13 has a well-defined (MSWD=1.26) plateau age of 16.45±0.17 Ma for nearly 75% of the total 39Ar released and a fusion step that gives a slightly older apparent age (Fig. 3h; Table 2). This sample has a flat K/Ca spectrum and a hump-shaped radiogenic yield distribution. All of the samples except MB00-16, MB02-5, and MB01-1 have isochron arrays with acceptable MSWD values for the chosen steps (Figs. 2, 4; Table 1). MB00- 16 is characterized by highly scattered isochron data (Fig. 2a) and therefore no regression data are provided. MB02-5 displays a slightly elevated MSWD of 2.9 that is likely related to the highly radiogenic nature of this sample that causes the isochron data to cluster near the X-axis (Fig. 2c). MB01-1 is essentially a two-point isochron as steps G through H are clustered with step F plotting at only a slightly less radiogenic location (Fig. 4a). Most of the isochron ages are indistinguishable from the weighted mean plateau ages except for sample MB00-13 groundmass that has an isochron age (17.25±0.14 Ma) that is older than the plateau age (16.34±0.15 Ma), however this latter sample has an anomalously low 40Ar/36Ari value of 272±4 (Fig. 4g; Table 1). DISCUSSION A preferred eruption age is assigned to each sample except for the highly disturbed sample MB00-16 (Table 1). As discussed below, eruption ages are determined from either plateau, isochron or integrated ages depending upon the preferred interpretation of each result. Eruption ages are determined from plateau ages for 8 of the 13 age spectra, however because overall climbing spectra can be recording the effects of radiogenic argon loss associated with either alteration and/or glass hydration some care should be exercized with some plateau ages. A good example of an age gradient that is probably related to 40Ar* loss is given by MB01-2 (Fig. 3b). The substantial age gradient may be indicating that the assigned plateau age is underestimating the true eruption age and perhaps other better-behaved samples can strengthen the interpretation of samples such as these. MB02-43 and MB02-53 also have climbing spectra but, seem best explained by contamination with excess argon as the isochron arrays for these samples are well- 40 36 defined and yield Ar/ Ari values that are measurably higher than the atmospheric value of 295.5 (Figs. 1d, 1e, 2d, 2e; Table 1). The isochron ages of these two samples are interpreted as

413 eruption ages. The overall decreasing groundmass age spectra (MB01-47, MB00-13; Figs. 3f, g) are somewhat difficult to interpret and may be caused by excess argon contamination that is more dominant in the low temperature heating steps and/or 39Ar recoil problems. Typically these spectra types are characterized by highly radiogenic and precise apparent ages and perhaps the best estimate of the eruption age is the integrated age under the assumption that the complexity stems for 39Ar redistribution. This situation would have relative depletion of 39Ar from early gas steps that was implanted into parts of the sample that degas at relatively high temperature steps. The coexisting plagioclase for groundmass sample MB00-13 provides some insight into the interpretation of the groundmass age spectrum. The plagioclase has a plateau age of 16.45±0.17 Ma that is consistent with the groundmass integrated age and the weighted mean calculated for the final 3 heating steps (16.64±0.11 Ma; 16.34±0.15 Ma, respectively). The relatively large uncertainty for all of the age assignments for the plagioclase and groundmass do not allow a distinction between the integrated or plateau age of the groundmass, however the plagioclase does indicate that the isochron age of the groundmass (17.25±0.14 Ma) is not accurate despite the reasonable regression statistics. The isochron for the groundmass is likely affected by 39Ar recoil redistribution as neither the age nor the initial 40Ar/36Ar is reliable. Thus, the preferred eruption ages for both of the groundmass samples with steadily decreasing age spectra are the integrated ages (Table 1).

References Cited Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., and DePaolo, D.J., 1998, Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating: Chemical Geology, v. 145, p. 117-152. Steiger, R.H., and Jäger, E., 1977, Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, p. 359-362. Taylor, J.R., 1982, An Introduction to Error Analysis: The Study of Uncertainties in Physical Measurements: University Scientific Books, Mill Valley, 270 p. York, D., 1969, Least squares fitting of a straight line with correlated errors: Earth and Planetary Science Letters, v.5, p. 320-324.

414 60 (b) MB01-65 Ar* (a) MB00-16 40

% 0 100

1 K/Ca

30 0.01 Plat=15.76±0.14 Ma (MSWD=1.98) no plateau 20

L E FG D E F G H I JK 10 D C C B

Apparent Age (Ma) B Integrated Age=13.50±0.04 Ma Integrated Age=15.31±0.26 Ma 0

(d) MB02-43 60 (c) MB02-5 Ar* 40

% 0 100

1 K/Ca

30 0.01

Iso=13.90± 0.30 Ma (MSWD=1.3) 20 D E F G H I J K L C F H I J 10 B Plat=22.48±0.04 Ma (MSWD=1.93) C D E G KL

Apparent Age (Ma) Integrated Age=22.38±0.04 Ma B Integrated Age=15.65±0.33 Ma 0 0 20 40 60 80 100 (e) MB02-53 39 60 Cumulative % Ar Released Ar* 40

% 0 100

1 K/Ca

30 0.01

Iso=14.94± 0.12 Ma (MSWD=2.3) 20

D E F G H I J KL 10 B C

Apparent Age (Ma) Integrated Age=15.25±0.13 Ma 0 0 20 40 60 80 100 Cumulative %39Ar Released

Figure 1. Age spectra, K/Ca and radiogenic yield diagrams for samples irradiated in package NM-172. Plat = plateau age and Iso = Isochron age for selected steps. 415 0.004 (a) MB00-16 (b) MB01-65 Steps F-K ± A No isochron B Age= 15.6 0.6 Ma A 40 36 ± 0.003 C Ar/ Ari= 297 5 D MSWD=2.4, n=6 B H JK

Ar I L G

40 E F 0.002 Ar/ 36

0.001 C

E D G F 0 0.004 (c) MB02-5 Steps D-L (d) MB02-43 Steps D-L Age= 13.9±0.3 Ma Age= 22.47±0.03 Ma B 40 36 40 36 A Ar/ Ar = 304.6±2.0 0.003 Ar/ Ar = 341±19 i i IG MSWD=1.3, n=9 MSWD=2.9, n=9 H C J K L F Ar B E

40 D 0.002 Ar/ 36

0.001

K A-L C 0 0.004 0 0.02 0.04 0.06 0.08 0.10 (e) MB02-53 Steps D-L 39Ar/40Ar A Age= 14.94±0.12 Ma B 40 36 ± 0.003 Ar/ Ari= 304.4 1.8 L K MSWD=2.3, n=9 Ar J I 40 H 0.002 Ar/ G C

36 F E D

0.001

0 0 0.02 0.04 0.06 0.08 0.10 39Ar/40Ar Figure 2. Isochron diagrams for samples irradiated in package NM-172. 416 100

Ar* MB01-2

40 MB01-1 % 0 1 K/Ca 40 0.01 23.50±0.12 Ma (MSWD=0.96) F G H I 30 E ± 20 D 35.52 0.17 Ma (MSWD=3.77) G H E F I C D 10 C Integrated Age=35.13±0.62 Ma a) B Integrated Age=21.60±0.61 Ma b) Apparent Age (Ma) 0

100 MB01-6a Ar* MB01-12 40

% 0 1 K/Ca 40 0.01 16.07±0.46 Ma (MSWD=0.43) 30 14.35±0.19 Ma (MSWD=1.66)

20

10 D E F H I F G H I C G B Integrated Age=12.93±0.22 Ma c) D E Integrated Age=15.2± 3.2 Ma d) Apparent Age (Ma) 0 100

Ar* MB01-24 MB01-47 40

% 0 1 K/Ca 40 0.01 16.95±0.65 Ma (MSWD=2.21) 30 16.54±0.08 Ma (MSWD=3.36)

20 H F I B C D E F G I 10 G H E Integrated Age=14.9±1.8 Ma e) Integrated Age=16.89±0.07 Ma f) Apparent Age (Ma) 0 100 Ar*

40 MB00-13 MB00-13 plagioclase % 0 1 K/Ca 40 0.01 ± 30 16.34±0.15 Ma (MSWD=3.13) 16.45 0.17 Ma (MSWD=1.26)

20 D E F G G H I 10 H I B C D E F Integrated Age=16.64±0.11 Ma g) Integrated Age=16.56±0.18 Ma h) Apparent Age (Ma) 0 0 20 40 60 80 100 0 20 40 60 80 100 39 Cumulative %39Ar Released Cumulative % Ar Released Figure 3. Age spectra, K/Ca and radiogenic yield 417diagrams for samples irradiated in package NM-156. 0.004 B MB01-1 MB01-2 C A Steps F-I Steps G-I 0.003 D B Age = 35.4±0.2 Ma Age = 23.5±0.3 Ma Ar E 40 36 ± Ar/ Ari = 301 11 40Ar/36Ar = 295±6 40 C i MSWD = 5.5, n = 4 MSWD = 1.8, n = 3 0.002 Ar/ D

36 I E H 0.001 F F G H G a) I b) 0 0.004 A MB01-6A MB01-12 Steps E-I A-E Age = 14.9±0.5 Ma Steps A-I 0.003 40 36 ± G F Age = 16.2±0.5 Ma B Ar/ Ari = 285 9 H Ar 40 36 ± Ar/ Ari = 294.8 1.2 H MSWD = 1.7, n = 5 I 40 I MSWD = 0.44, n = 9 0.002 G C Ar/ F D 36 E 0.001 c) d) 0 0.004 MB01-24 MB01-47 BD C Steps B-I Steps F-I ± A 0.003 E Age = 17.7 0.5 Ma Age = 16.62±0.06 Ma F 40 36 Ar/ Ar = 292.3±1.3 40 36 ± Ar H i Ar/ Ari = 282 5 MSWD = 1.7, n = 8 MSWD = 2, n = 4 40 G 0.002 I Ar/ H 36 I 0.001

e) f) B G C D E F 0 0.004 MB00-13 MB00-13 plagioclase

Steps F-I A Steps D-I 0.003 ± B Age = 17.25±0.14 Ma Age = 16.3 0.3 Ma 40 36 ± 40Ar/36Ar = 272±4 Ar/ Ari = 370 30 Ar i MSWD = 0.67, n = 4 MSWD = 2.0, n = 6 40 0.002 C I H Ar/ B 36 D G C 0.001 E I F D g) h) H EF G 0 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 39Ar/40Ar 39Ar/40Ar

Figure 4.Isochron diagrams for samples irradiated in package NM-156. 418 Table 1. Summary of assigned ages and analytical methods.

Summary Weight Plateau data Isochron data TGA Sample L# Irrad Min (mg) Age ±1σ n %39Ar MSWD Age ±1σ n MSWD Age ±1σ MB00-16 54415 NM-172 GM 26.39 NA NA 13.50 0.04 MB01-65 54414 NM-172 GM 26.77 15.76 0.14 6 48.8 1.98 15.60 0.60 6 2.40 15.31 0.26 MB02-5 54413 NM-172 GM 24.43 22.48 0.04 7 81.5 1.93 22.47 0.03 9 2.90 22.38 0.04 MB02-43 54416 NM-172 GM 23.22 13.90 0.30 9 1.30 17.1 1.4 MB02-53 54412 NM-172 GM 20.89 14.94 0.12 9 2.30 15.25 0.13 MB01-1 53425 NM-156 GM 22.99 35.52 0.17 4 83.9 3.77 35.40 0.20 4 5.50 35.13 0.17 MB01-2 53421 NM-156 GM 23.77 23.50 0.12 3 46.9 0.96 23.50 0.30 3 1.80 21.6 0.61 MB01-6A 53422 NM-156 GM 24.39 14.35 0.19 5 66.0 1.66 14.90 0.50 5 1.70 12.93 0.22 MB01-12 53420 NM-156 GM 22.74 16.07 0.46 9 100 0.43 16.20 0.50 9 0.44 15.2 3.2 MB01-24 53423 NM-156 GM 22.04 16.95 0.65 8 100 2.21 17.70 0.50 8 1.70 14.9 1.8 MB01-47 53424 NM-156 GM 23.48 16.54 0.08 4 54.5 3.36 16.62 0.06 4 2.00 16.89 0.07 MB00-13 53510 NM-156 GM 29.23 16.34 0.15 3 64.0 3.13 17.25 0.14 4 0.67 16.64 0.11 MB00-13 53509 NM-156 P 31.36 16.45 0.17 8 75.2 1.26 16.30 0.30 6 2.00 16.56 0.18

L# = Lab number Irrad = Irradiation number Min = Material dated: GM=Groundmass concentrate, P=Plagioclase n = number of steps for plateau or isochron Age result shown in box is preferred eruption age. %39Ar = percentage of total 39Ar comprising the plateau steps. TGA = Integrated age All errors are 1σ. NA = No assigned age

Methods

Sample preparation and irradiation: Groundmass concentrates and plagioclase separate provided by Matt Brueseke. Samples irradiated for 7 hours (NM-172) or 15.3 hours (NM-156) in D-3 position, Nuclear Science Center, College Station, TX Neutron flux monitor Fish Canyon Tuff sanidine (FC-2). Assigned age = 28.02 Ma (Renne et al., 1998)

Instrumentation: Mass Analyzer Products 215-50 mass spectrometer on line with automated all-metal extraction system.

Groundmass concentrates step-heated by a 50 watt Synrad CO2 laser (NM-172) or within a double vacuum Mo furnace. Laser step-heating analysis: Samples heated for 1 minute. Reactive gases removed during a 8 minute reaction with 2 SAES GP-50 getters. 1 operated at ~450°C and 1 at 20°C. Gas also exposed to a W filament operated at ~2000°C and a cold finger operated at -140°C. Furnace step-heating analysis: Samples heated for 8 minutes. Reactive gases removed during heating with a SAES GP-50 operated at ~450°C followed by a second stage cleanup using 2 SAES GP-50 getters with 1 operated at ~450°C and 1 at 20°C. Gas also exposed to a W filament operated at ~2000°C.

Analytical parameters: Electron multiplier sensitivity averaged 1.61x10-16 moles/pA for laser and 2.8x10-16 moles/pA for furnace. Total system blank and background: Laser step-heat = 39, 0.3, 0.1, 0.2, 0.18 x 10-17 moles for masses 40, 39, 38, 37, 36, respectively. Total system blank and background: Furnace = 250, 1.3, 0.2, 1.9, 0.88 x 10-17 moles for masses 40, 39, 38, 37, 36, respectively.

J-factors determined to a precision of ± 0.1% by CO2 laser-fusion of 6 single crystals from each of 4 radial positions around the irradiation tray.

Correction factors for interfering nuclear reactions were determined using K-glass and CaF2 and are as follows: 40 39 36 37 39 37 NM-172: ( Ar/ Ar)K = 0.0±0.0004; ( Ar/ Ar)Ca = 0.00028±0.00001; and ( Ar/ Ar)Ca = 0.00070±0.00005. 40 39 36 37 39 37 NM-156: ( Ar/ Ar)K = 0.0±0.0004; ( Ar/ Ar)Ca = 0.000280±0.000005; and ( Ar/ Ar)Ca = 0.00070±0.00002.

419 Table 2. Argon isotopic data for groundmass concentrates.

40 39 37 39 36 39 39 40 39 ID Power/Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ (Watts/°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

MB00-16, B4:172, 26.39 mg groundmass, J=0.000737, D=1.007±0.001, NM-172, Lab#=54415-01 # A 2 2241 0.1180 7371.0 0.044 4.3 2.8 0.0 82 23 # B 4 28.86 0.1916 77.60 1.44 2.7 20.6 1.4 7.89 0.36 # C 7 11.48 0.3521 10.55 30.5 1.4 73.1 30.3 11.130 0.046 # D 10 12.61 0.4914 7.044 44.8 1.0 83.8 72.8 13.998 0.043 # E 12 13.13 0.7746 5.342 18.2 0.66 88.5 90.0 15.385 0.046 # F 15 13.24 1.223 4.620 7.4 0.42 90.5 97.0 15.864 0.069 # G 20 12.41 2.654 5.645 3.12 0.19 88.3 100.0 14.54 0.13 Integrated age ± 1σ n=7 105.5 K2O=2.08 % 13.502 0.041

MB01-65, B3:172, 26.77 mg groundmass, J=0.0007361, D=1.007±0.001, NM-172, Lab#=54414-01 # A 2 2738 0.0418 9077.9 0.054 12.2 2.0 0.0 72 24 # B 4 269.4 0.1005 871.0 1.35 5.1 4.5 0.9 15.9 2.1 # C 7 113.7 0.1551 349.4 19.2 3.3 9.3 12.6 13.93 0.72 # D 10 62.33 0.1889 172.5 32.4 2.7 18.2 32.3 15.03 0.33 # E 12 38.95 0.2495 93.21 28.6 2.0 29.3 49.7 15.11 0.18 F 15 34.94 0.3283 77.74 27.9 1.6 34.3 66.7 15.87 0.18 G 20 39.00 0.4637 92.28 23.6 1.1 30.2 81.1 15.57 0.19 H 25 47.36 0.8136 120.9 13.8 0.63 24.7 89.5 15.50 0.30 I 30 46.75 0.9533 116.7 6.4 0.54 26.4 93.4 16.31 0.28 J 40 44.47 0.9322 109.4 4.18 0.55 27.5 96.0 16.17 0.34 K 45 42.94 0.8519 106.5 4.04 0.60 26.9 98.4 15.29 0.30 # L 50 42.67 0.8302 101.5 2.58 0.61 29.9 100.0 16.86 0.36 Integrated age ± 1σ n=12 164.1 K2O=3.20 % 15.31 0.26 Plateau ± 1σ steps F-K n=6 MSWD=1.98 80.0 1.1 48.8 15.76 0.14 Isochron±1σ steps F-K n=6 MSWD=2.4 40Ar/36Ar=297±5 15.60 0.60

MB02-5, B2:172, 24.43 mg groundmass, J=0.0007353, D=1.007±0.001, NM-172, Lab#=54413-01 # B 4 34.51 0.3343 82.43 0.264 1.5 29.5 0.3 13.4 1.2 # C 7 16.40 0.3030 3.601 4.19 1.7 93.7 4.7 20.262 0.083 D 10 17.17 0.3827 0.7590 6.6 1.3 98.9 11.6 22.393 0.067 E 12 17.26 0.4510 0.6559 5.4 1.1 99.1 17.2 22.557 0.074 F 15 17.08 0.4739 0.4126 5.7 1.1 99.5 23.3 22.421 0.086 G 20 17.01 0.5455 0.4364 8.2 0.94 99.5 31.9 22.319 0.069 H 25 17.11 0.5074 0.2278 16.9 1.0 99.9 49.7 22.537 0.043 I 30 17.08 0.4968 0.3181 21.3 1.0 99.7 72.0 22.450 0.051 J 40 17.36 0.5244 1.082 13.4 0.97 98.4 86.1 22.525 0.048 # K 45 18.19 0.5216 3.509 8.2 0.98 94.5 94.8 22.677 0.073 # L 50 17.29 0.5181 0.3698 5.0 0.98 99.6 100.0 22.712 0.075 Integrated age ± 1σ n=11 95.3 K2O=2.04 % 22.38 0.04 Plateau ± 1σ steps D-J n=7 MSWD=1.93 77.6 1.0 81.5 22.48 0.04 Isochron±1σ steps D-L n=9 MSWD=2.9 40Ar/36Ar=341±19 22.47 0.03

420 Table 2. Argon isotopic data for groundmass concentrates.

40 39 37 39 36 39 39 40 39 ID Power/Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ (Watts/°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

MB02-43, B5:172, 23.22 mg groundmass, J=0.0007371, D=1.007±0.001, NM-172, Lab#=54416-01 A 2 24790 0.0610 83154.5 0.028 8.4 0.9 0.0 269.2 171.4 B 4 610.1 0.1787 2023.8 1.24 2.9 2.0 1.0 16.0 4.7 C 7 56.68 0.2522 156.5 9.6 2.0 18.4 8.8 13.85 0.45 D 10 34.97 0.3407 80.29 18.9 1.5 32.2 23.9 14.93 0.24 E 12 35.42 0.4269 82.50 16.0 1.2 31.3 36.7 14.68 0.26 F 15 42.03 0.5436 103.1 13.6 0.94 27.6 47.7 15.39 0.32 G 20 65.17 0.4808 182.0 21.1 1.1 17.5 64.7 15.13 0.40 H 25 78.94 0.5804 224.0 29.2 0.88 16.2 88.1 16.93 0.44 I 30 72.06 0.8142 202.0 6.9 0.63 17.2 93.6 16.46 0.60 J 40 62.68 0.9644 169.5 3.85 0.53 20.2 96.7 16.80 0.81 K 45 63.31 1.132 171.9 1.99 0.45 19.9 98.3 16.7 1.4 L 50 61.41 1.219 164.6 2.10 0.42 21.0 100.0 17.1 1.4 Integrated age ± 1σ n=12 124.6 K2O=2.80 % 15.65 0.33 Isochron±1σ steps D-L n=9 MSWD=1.3 40Ar/36Ar=304.6±2.0 13.90 0.30

MB02-53, B1:172, 20.89 mg groundmass, J=0.0007354, D=1.007±0.001, NM-172, Lab#=54412-01 A 2 2651 0.1377 8870.8 0.055 3.7 1.1 0.1 39 24 B 4 102.6 0.2797 312.7 2.24 1.8 9.9 2.5 13.48 0.83 C 7 23.81 0.4116 46.24 13.3 1.2 42.8 17.1 13.46 0.15 D 10 19.93 0.5620 28.80 18.4 0.91 57.5 37.4 15.158 0.091 E 12 21.45 0.7085 32.88 11.4 0.72 55.0 49.9 15.58 0.12 F 15 23.09 0.8745 39.16 8.5 0.58 50.2 59.2 15.32 0.14 G 20 27.59 1.009 53.16 10.3 0.51 43.4 70.5 15.81 0.19 H 25 32.81 1.186 71.68 11.3 0.43 35.7 82.8 15.50 0.19 I 30 42.14 1.141 102.6 8.6 0.45 28.3 92.3 15.74 0.24 J 40 46.25 1.991 113.8 4.31 0.26 27.6 97.1 16.89 0.31 K 45 49.71 1.765 126.5 1.85 0.29 25.1 99.1 16.50 0.66 L 50 52.49 2.215 135.7 0.83 0.23 24.0 100.0 16.63 0.73 Integrated age ± 1σ n=12 91.1 K2O=2.28 % 15.25 0.13 Isochron±1σ steps D-L n=9 MSWD=2.3 40Ar/36Ar=304.4±1.8 14.94 0.12

MB01-1, B5:156, 22.99 mg groundmass, J=0.0016428, D=1.0052±0.00172, NM-156, Lab#=53425-01 # B 700 580.4 1.157 1931.1 0.778 0.44 1.7 0.4 29 13 # C 750 420.8 1.638 1385.2 7.35 0.31 2.8 4.4 34.3 8.6 # D 800 108.0 0.9939 332.3 4.79 0.51 9.2 7.0 29.1 2.4 # E 875 55.16 0.7401 148.3 16.9 0.69 20.7 16.1 33.5 1.1 F 975 15.81 0.6397 12.63 56.1 0.80 76.7 46.3 35.60 0.18 G 1075 13.90 0.7298 5.951 50.0 0.70 87.8 73.2 35.83 0.14 H 1250 13.72 3.723 7.064 32.8 0.14 87.0 90.8 35.14 0.15 I 1650 13.71 5.363 7.236 17.1 0.095 87.6 100.0 35.40 0.26 Integrated age ± 1σ n=8 185.9 K2O=1.89 % 35.13 0.62 Plateau ± 1σ steps F-I n=4 MSWD=3.77 156.0 0.55 83.9 35.52 0.17 Isochron±1σ steps F-I n=4 MSWD=5.5 40Ar/36Ar=301±11 35.40 0.20

421 Table 2. Argon isotopic data for groundmass concentrates.

40 39 37 39 36 39 39 40 39 ID Power/Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ (Watts/°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

MB01-2, B1:156, 23.77 mg groundmass, J=0.0016399, D=1.0052±0.00172, NM-156, Lab#=53421-01 # A 625 3956 0.5410 13349.2 1.04 0.94 0.3 0.5 35 82 # B 700 35.25 0.5496 105.5 9.0 0.93 11.7 5.3 12.19 0.91 # C 750 15.23 0.5818 35.62 11.9 0.88 31.2 11.5 14.01 0.41 # D 800 13.23 0.7122 23.50 13.6 0.72 48.0 18.6 18.69 0.33 # E 875 12.40 0.8831 18.22 26.8 0.58 57.2 32.6 20.85 0.21 # F 975 11.30 0.7056 12.09 39.4 0.72 68.9 53.1 22.91 0.18 G 1075 10.82 0.9373 9.857 40.4 0.54 73.8 74.1 23.48 0.15 H 1250 14.35 2.165 21.89 23.2 0.24 56.2 86.2 23.74 0.24 I 1650 15.49 4.751 27.06 26.4 0.11 50.9 100.0 23.24 0.26 Integrated age ± 1σ n=9 191.9 K2O=1.89 % 21.60 0.61 Plateau ± 1σ steps G-I n=3 MSWD=0.96 90.0 0.34 46.9 23.50 0.12 Isochron±1σ steps G-I n=3 MSWD=1.8 40Ar/36Ar=295±6 23.50 0.30

MB01-6A, B2:156, 24.39 mg groundmass, J=0.0016378, D=1.0052±0.00172, NM-156, Lab#=53422-01 # A 625 3627 -0.2605 12810.8 0.042 - -4.4 0.0 -543 211 # B 700 13.64 0.4044 37.32 7.19 1.3 19.4 8.4 7.80 0.61 # C 750 7.672 0.5726 13.97 9.4 0.89 46.8 19.3 10.59 0.41 # D 800 8.538 0.8770 13.97 12.6 0.58 52.5 34.0 13.20 0.28 E 875 8.975 1.733 14.21 16.9 0.29 54.8 53.6 14.49 0.24 F 975 9.845 2.879 17.41 17.9 0.18 50.2 74.4 14.56 0.24 G 1075 10.51 3.906 21.46 7.30 0.13 42.7 82.9 13.26 0.50 H 1250 13.58 11.01 33.11 10.5 0.046 34.6 95.2 13.95 0.44 I 1650 15.07 16.81 39.32 4.16 0.030 32.1 100.0 14.42 0.88 Integrated age ± 1σ n=9 86.0 K2O=0.83 % 12.93 0.22 Plateau ± 1σ steps E-I n=5 MSWD=1.66 56.8 0.17 66.0 14.35 0.19 Isochron±1σ steps E-I n=5 MSWD=1.7 40Ar/36Ar=285±9 14.90 0.50

MB01-12, A6:156, 22.74 mg groundmass, J=0.0016434, D=1.0052±0.00172, NM-156, Lab#=53420-01 A 625 8882 8.126 29998.8 0.137 0.063 0.2 0.5 53 201 B 700 1863 11.25 6360.7 0.279 0.045 -0.9 1.6 -48 52 C 750 1316 12.13 4461.1 0.603 0.042 -0.1 4.0 -5 33 D 800 433.8 9.424 1442.3 1.09 0.054 1.9 8.3 25 10 E 875 118.1 10.86 386.7 3.47 0.047 4.0 22.0 14.1 2.9 F 975 27.03 8.601 75.61 6.48 0.059 20.0 47.5 16.03 0.81 G 1075 30.11 8.357 85.47 5.41 0.061 18.4 68.9 16.47 0.92 H 1250 25.69 9.132 71.57 3.56 0.056 20.6 82.9 15.7 1.2 I 1650 17.29 40.07 51.75 4.35 0.013 30.7 100.0 16.12 0.96 Integrated age ± 1σ n=9 25.4 K2O=0.26 % 15.2 3.2 Plateau ± 1σ steps A-I n=9 MSWD=0.43 25.4 0.049 100.0 16.07 0.46 Isochron±1σ steps A-I n=9 MSWD=0.44 40Ar/36Ar=294.8±1.2 16.20 0.50

422 Table 2. Argon isotopic data for groundmass concentrates.

40 39 37 39 36 39 39 40 39 ID Power/Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ (Watts/°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

MB01-24, B3:156, 22.04 mg groundmass, J=0.0016383, D=1.0052±0.00172, NM-156, Lab#=53423-01 B 700 1605 8.077 5478.5 0.186 0.063 -0.8 0.7 -39 46 C 750 1332 11.11 4566.8 0.411 0.046 -1.2 2.2 -49 36 D 800 449.8 13.16 1518.2 0.91 0.039 0.5 5.5 7 11 E 875 125.3 12.92 410.6 4.54 0.039 4.0 22.1 15.0 3.0 F 975 30.69 9.425 86.78 9.4 0.054 19.0 56.4 17.24 0.70 G 1075 23.73 7.379 62.98 4.82 0.069 24.1 74.0 16.94 0.93 H 1250 19.56 8.651 52.92 2.91 0.059 23.7 84.7 13.7 1.2 I 1650 18.40 37.95 52.38 4.19 0.013 33.0 100.0 18.32 0.86 Integrated age ± 1σ n=8 27.3 K2O=0.29 % 14.9 1.8 Plateau ± 1σ steps B-I n=8 MSWD=2.21 27.3 0.048 100.0 16.95 0.65 Isochron±1σ steps B-I n=8 MSWD=1.7 40Ar/36Ar=292.3±1.3 17.70 0.50

MB01-47, B4:156, 23.48 mg groundmass, J=0.0016408, D=1.0052±0.00172, NM-156, Lab#=53424-01 # A 625 275.1 1.668 921.4 1.20 0.31 1.1 0.4 8.7 7.1 # B 700 6.893 0.4267 3.709 12.5 1.2 84.6 4.3 17.19 0.26 # C 750 6.403 0.3298 1.003 23.0 1.5 95.8 11.4 18.07 0.14 # D 800 6.155 0.2800 0.4664 37.5 1.8 98.1 23.1 17.794 0.091 # E 875 5.930 0.2457 0.4739 72.3 2.1 98.0 45.5 17.119 0.061 F 975 5.797 0.2987 0.6176 87.8 1.7 97.3 72.8 16.618 0.054 G 1075 6.141 0.6542 2.081 36.9 0.78 90.9 84.3 16.45 0.10 H 1250 8.268 1.660 11.55 3.51 0.31 60.4 85.4 14.73 0.88 I 1650 7.930 1.060 8.371 47.0 0.48 69.9 100.0 16.35 0.11 Integrated age ± 1σ n=9 321.7 K2O=3.21 % 16.89 0.07 Plateau ± 1σ steps F-I n=4 MSWD=3.36 175.2 1.2 54.5 16.54 0.08 Isochron±1σ steps F-I n=4 MSWD=2 40Ar/36Ar=282±5 16.62 0.06

MB00-13 L4:156, 29.23 mg, groundmass, J=0.0016494, D=1.00743±0.00124, NM-156, Lab#=53510-02 # B 700 67.86 1.144 210.9 3.19 0.45 8.3 1.9 16.7 1.8 # C 750 14.55 1.200 30.73 3.20 0.43 38.3 3.7 16.50 0.69 # D 800 10.27 1.465 14.78 11.3 0.35 58.7 10.3 17.85 0.26 # E 875 9.946 1.478 13.94 18.8 0.35 59.8 21.2 17.63 0.18 # F 975 7.275 1.391 5.666 25.5 0.37 78.6 36.0 16.944 0.100 G 1075 7.622 1.476 7.205 18.3 0.35 73.7 46.6 16.65 0.15 H 1250 9.675 2.375 14.88 43.5 0.21 56.6 71.8 16.25 0.14 I 1650 10.33 3.920 17.66 48.7 0.13 52.6 100.0 16.15 0.15 Integrated age ± 1σ n=8 172.5 K2O=1.37 % 16.64 0.11 Plateau ± 1σ steps G-I n=3 MSWD=3.13 110.5 0.20 64.0 16.34 0.15 Isochron±1σ steps F-I n=4 MSWD=0.67 40Ar/36Ar=272±4 17.25 0.14

423 Table 2. Argon isotopic data for groundmass concentrates.

40 39 37 39 36 39 39 40 39 ID Power/Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1σ (Watts/°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

MB00-13, L3:156, 31.36 mg plagioclase, J=0.001651, D=1.0052±0.00172, NM-156, Lab#=53509-01 A 625 395.9 15.20 1345.2 0.219 0.034 -0.1 0.4 -1 21 B 700 8.968 13.24 16.83 4.85 0.039 56.8 8.6 15.24 0.64 C 750 6.825 12.34 9.485 2.29 0.041 73.9 12.5 15.1 1.0 D 800 6.322 11.65 6.684 4.02 0.044 84.0 19.3 15.88 0.65 E 875 5.952 11.92 5.096 6.02 0.043 91.3 29.5 16.24 0.48 F 975 5.863 12.38 4.662 7.95 0.041 94.0 43.0 16.48 0.33 G 1075 5.809 12.42 4.275 9.72 0.041 95.9 59.5 16.67 0.28 H 1250 7.199 11.68 8.591 9.25 0.044 78.2 75.2 16.82 0.32 # I 1650 7.216 12.03 7.841 14.6 0.042 81.7 100.0 17.62 0.22 Integrated age ± 1σ n=9 58.9 K2O=0.44 % 16.56 0.18 Plateau ± 1σ steps A-H n=8 MSWD=1.26 44.3 0.042 75.2 16.45 0.17 Isochron±1σ steps D-I n=6 MSWD=2.0 40Ar/36Ar=370±30 16.30 0.30

Notes: Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions. Errors quoted for individual analyses include analytical error only, without interfering reaction or J uncertainties. Integrated age calculated by recombining isotopic measurements of all steps. Integrated age error calculated by recombining errors of isotopic measurements of all steps. Plateau age is inverse-variance-weighted mean of selected steps. Plateau age error is inverse-variance-weighted mean error (Taylor, 1982) times square root MSWD where MSWD>1. Isochron age and error from York (1969) regression methods. Decay constants and isotopic abundances after Steiger and Jager (1977). # symbol preceding sample ID denotes analyses excluded from plateau age calculations. Ages calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory standard at 28.02 Ma. Decay Constant (LambdaK (total)) = 5.543e-10/a. D= 1 AMU Discrimination in favor of light isotopes. Correction factors: 40 39 36 37 39 37 NM-172: ( Ar/ Ar)K = 0.0±0.0004; ( Ar/ Ar)Ca = 0.00028±0.00001; and ( Ar/ Ar)Ca = 0.00070±0.00005. 40 39 36 37 39 37 NM-156: ( Ar/ Ar)K = 0.0±0.0004; ( Ar/ Ar)Ca = 0.000280±0.000005; and ( Ar/ Ar)Ca = 0.00070±0.00002.

424 CHAPTER 5: SUMMARY AND SUGGESTIONS FOR FUTURE RESEARCH

Summary This study provides: [1] the first comprehensive view into the development of a mid- Miocene Oregon Plateau volcanic field and its relationship with regional flood basalt volcanism and [2] conclusively demonstrates that flood basalt volcanism occurred on the Oregon Plateau over at least a 2 million year duration. It also provides new age constraints on the well-known geomagnetic reversal found in the Steens type section exposed at Steens Mountain. Furthermore, this research is the most detailed and complete examination of a volcanic field associated with the Yellowstone-Newberry melt anomaly (Yellowstone Hotspot), besides the Yellowstone volcanic field. Accordingly, new questions have been generated during the course of this study. Detailed stratigraphic examination of the geochemical and chronologic characteristics of flood basalt lava flows in the vicinity of Steens Mountain illustrates a number of important observations. First, new 40Ar/39Ar ages and recalculated literature ages from the Steens Basalt type section at Steens Mountain illustrate that the upper portion of the flood basalt sequence is 16.35 ± 0.07 (2σ) Ma, not 16.6 Ma as commonly quoted. While the lower portion of the sequence may be older, these new constraints suggest that the geomagnetic reversal is younger than previously thought. Major and trace element data and field characteristics from stratigraphic sections in the vicinity of Steens Mountain indicate that multiple magmatic centers were present locally. This interpretation is supported by new 40Ar/39Ar ages from these same locations that define a 1.05 ± 0.46 Ma duration and indicate that local flood basalt activity was coeval with the main phase of Columbia River Basalt Group volcanism (Imnaha and Grande Ronde basalts), which disputes models of northward-directed flood basalt propagation through time. Understanding how this flood basalt event initiated and affected the lithosphere of western North America is central to understanding the apparent differences between coeval silicic-dominated volcanic fields and their younger Snake River Plain-Yellowstone and Oregon High Lava Plains counterparts. By unraveling the complex volcanic history of the Santa Rosa- Calico volcanic field (SC), this research demonstrates that continued mafic magma input into the

425 lithosphere was necessary to drive Oregon Plateau silicic-dominated volcanism and confirms at least an ~ 2 Ma duration of mafic magma input into the upper crust. The igneous products exposed in the SC record a complex set of magmatic processes and the spatial distribution of vents and styles of volcanism illustrate an intimate relationship with coeval extension. SC mafic lava flows and shallow intrusive bodies form two broad chemical groups: a more primitive type that is chemically similar to the dominant basaltic magma erupted across the Oregon Plateau during the last <11 Ma (low-K, high-alumina olivine tholeiite, HAOT) and locally erupted Steens Basalt. HAOT-like lava flows only erupted at the onset of SC volcanism; however, Steens Basalt eruptive loci are as young as 14.4 Ma. Silicic volcanism was diverse and characterized by multiple, often temporally and spatially unrelated magmatic systems. It is also characterized by effusively erupted lava flows that were sourced from domes and fissures, explosively erupted ash flows that were sourced from fissures and a caldera, and numerous tephra horizons exposed within sedimentary deposits and between SC lava/ash flows that likely represent both proximal and distal eruptive events. Unlike other mid-Miocene Oregon Plateau volcanic fields, the SC silicic record was not dominated by caldera-forming events. Physical, chemical, and isotopic characteristics suggest that these silicic magmas were generated through upper crustal melting of local Cretaceous granitoid, interacted with less evolved magmas, and at least in the eastern SC, were the products of a larger, possibly batholith-sized magmatic system. An abundance of locally erupted intermediate products also distinguish the SC from other coeval, regional volcanic fields. These intermediate lava flows record the presence of at least four chemically and spatially distinct eruptive systems. Major element and isotopic data also indicate that these magmatic systems were characterized by complex open-system processes including magma mixing (Calico Mountains and Staunton Ridge region) and assimilation- fractional crystallization (Hinkey-Coal Pit Peak). Accordingly, the presence of these intermediate products provides an opportunity to further study the generation of intermediate magmas and ultimately, how they can be related to more mafic and/or more silicic magma. At ~14 Ma, SC volcanism ceased while silicic eruptive products continued to erupt from regional eruptive centers, including those intermittently exposed on the Owyhee Plateau. At ~11-10 Ma, basin and range style block-faulting initiated, which combined with glaciation and other erosive processes, led to the landscape present today.

426 Suggestions for Future Research The Santa Rosa-Calico volcanic field, other coeval volcanic fields, and the Oregon Plateau itself exist as a consequence of a complex set of tectonic and magmatic processes that are ultimately related to the assembly of the North American continent. Questions to answer with future research address both this regional framework as well as broader issues related to magma generation and physical controls on the styles and volumes of continental volcanism. In this context and within the framework of the work discussed and presented in this dissertation, the most outstanding questions to address with future work are: 1) The information presented in this study indicates that Steens Basalt volcanism in the vicinity of Steens Mountain was characterized by multiple eruptive centers and provides more evidence that Steens lava flows are physically dissimilar than the thicker and areally extensive Columbia River Basalt Group flows. Are these regional differences in the style of flood basalt volcanism a function of varying magma properties (e.g. composition, viscosity, etc.) or tectonic controls (e.g. eruption rate, number and type of vents, lithospheric properties)? Furthermore, do these differences shed light on how flood basalt provinces can differ architecturally? 2) Pre-SC Cenozoic volcanic products overlie Mesozoic igneous and metamorphic basement in the Santa Rosa-Calico volcanic field. Evidence of local eruption appears present and geochemical and chronologic data indicate that these eruptive products are temporally and compositionally more diverse than previously thought. What is their relationship to other regionally exposed Eocene through early-Miocene volcanic products (e.g. across northern Nevada, southeastern Oregon, and southwestern Idaho)? Did, or how did, SC magmas interact with these products and their subvolcanic intrusive bodies? 3) The HAOT-like lava flows and shallow intrusive bodies exposed in the SC are the oldest occurrence of this magma type on the Oregon Plateau. Their simultaneous eruption with more evolved Steens magmas and their isotopic similarity to regionally erupted Steens Basalt suggests a possible genetic link. Is there a link and if so, what is the tectonomagmatic significance of this link? Also, do these mid-Miocene HAOT-like magmas differ from HAOT magmas that erupted across the Oregon Plateau after 11 Ma? 4) The complex textural, chemical, and isotopic characteristics of SC eruptive products and shallow intrusive bodies record the effects of complex magmatic processes. The first-

427 order models presented in Chapter 4 explain some of this variability; however, they also point to the need for more detailed investigation. More detailed study should focus on better characterizing the xenoliths and xenocrysts found in SC units. This includes addressing the genesis of the nearly ubiquitous feldspar ± pyroxene ± oxide crystal clots through detailed analytical approaches (e.g. electron microprobe and crystal isotope stratigraphy). Additionally, the geochemical and isotopic characteristics of the local Triassic metasedimentary package (Trms) are virtually unknown. Petrographic evidence

illustrates the interaction of SC magmas (e.g. Tad1; Fig. 8, Chapter 4) with this metamorphic basement, thus examining this interaction will also help determine whether assimilation (and/or melting) of Trms was an important open-system process affecting the genesis of SC magmas. 5) SC silicic magmas appear to be the result of upper crustal melting of granitoid country rock. Is upper crustal melting a process that generated silicic magmas in other coeval and younger Oregon Plateau volcanic systems? 6) Is the relationship between faulting and vent location that is observed in the SC also present in other Oregon Plateau volcanic fields? Caldera forming events and products have been identified in the McDermitt, Lake Owyhee, and Northwest Nevada volcanic fields, however, no conclusive evidence exists that indicates similar events occurred on the Owyhee Plateau. Did the silicic units (e.g. the Swisher Mountain tuff and associated units) exposed across the Owyhee Plateau erupt in a similar manner to those in the SC? 7) This study demonstrates that the SC is a potential source for many of the subalkaline tephra horizons that are locally and regionally exposed. Can these be conclusively linked into the SC volcanic record? If so, positive correlations could yield excellent chronostratigraphic control and also provide new constraints on the pyroclastic record of “greater” Snake River Plain-Yellowstone silicic volcanism. 8) How does the timing of SC volcanism and basin development correspond to local mid- Miocene epithermal precious metal mineralization? Are there any common characteristics exhibited by this relationship that can be applied toward other volcanic hosted epithermal deposits?

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