MIAMI UNIVERSITY The Graduate School

Certificate for Approving the Dissertation

We hereby approve the Dissertation

of

Kurt A. Shoemaker

Candidate for the Degree:

Doctor of Philosophy

William K. Hart, Director

Elisabeth Widom, Reader

Craig M. White, Reader

John M. Hughes

Michael J. Pechan, Graduate School Representative

ABSTRACT

THE TECTONOMAGMATIC EVOLUTION OF THE LATE CENOZOIC OWYHEE PLATEAU, NORTHWESTERN

by Kurt A. Shoemaker

The Owyhee Plateau (OWP) is an intermontane plateau located at the nexus of the Plain (SRP) and Plateau/High Lava Plains (OP/HLP) volcanic provinces, which has been the locus of nearly continuous basaltic magmatism since 17 Ma. Between 17-11 Ma, generally evolved basaltic lavas related to the Steens flood basalt event and Oregon- Graben (OIG) volcanism were erupted around the extending margins of the OWP. Beginning 11 Ma, less differentiated olivine tholeiites were erupted throughout the OWP proper, from low shield vents with alignments consistent with regional stress fields. After 5 Ma, volcanism retreated to the margins of the OWP, ultimately becoming limited to the northern transition region between the OWP and the OIG. The youngest in the OWP region are <0.25 Ma mildly alkaline basalts erupted in this transition region. The OWP is the only location in the northwestern US where basalt types characteristic of the OP/HLP (high-alumina olivine tholeiite, HAOT) and the SRP (SRP- type olivine tholeiite, SROT) occur together in significant quantity, in close spatial and temporal association, and with a full spectrum of compositions intermediate between the two. Sr, Nd, and Pb isotopic characteristics are decoupled from bulk chemistry, and reflect time-dependent variations in contributions from different lithospheric and sublithospheric mantle reservoirs. I propose that the OWP is a discrete tectonomagmatic entity within the North American Cordillera resulting from Sevier-style thrusting of accreted lithosphere over a westward-projecting shelf of Precambrian cratonic lithospheric mantle. Low-angle subduction during Laramide time trapped a layer of asthenospheric mantle below the OWP region, which was subsequently modified by fluids and melts from the subducting Farallon slab. Foundering of the Farallon slab caused upwelling of hot, fertile asthenosphere that mixed with this volatile-enriched layer, triggering Steens volcanism. Subsequent melt production from fluid- and melt-metasomatized cratonic and accreted mantle reservoirs beneath the OWP produced the post-11 Ma HAOT-SROT association. The retreat of volcanism to the margins of the OWP, the isotopic character of these lavas, and the absence of endmember SROT on the OWP after 5 Ma reflect the exhaustion of fusible components from the Precambrian lithospheric mantle shelf.

THE TECTONOMAGMATIC EVOLUTION OF THE

LATE CENOZOIC OWYHEE PLATEAU,

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

Kurt A. Shoemaker

Miami University

Oxford, Ohio

2004

Dissertation Director: William K. Hart, Ph.D. TABLE OF CONTENTS

CHAPTER 1: The Owyhee Plateau: Tectonomagmatic Context and 1 Regional Significance Introduction 1 Regional spatial and temporal observations 5 Summary 15

CHAPTER 2: Geochemical, Isotopic, and Chronostratigraphic 17 Characterization of Late Cenozoic Basaltic Volcanism in the Owyhee Plateau Region: Constraints on Lithospheric and Sublithospheric Mantle Architecture and Magma Genesis Introduction 17 Data 25 Analytical methods 25 Previous geochemical observations 26 Chronostratigraphy 26 Petrography 39 Major and trace element geochemistry 41 Isotope geochemistry 51 Discussion 58 Constraints on magma source characteristics 58 Constraints on basalt petrogenesis 86 Synthesis 94 Conclusions 102

CHAPTER 3: The Owyhee Plateau: A Tectonic and Magmatic Link 105 Between the Snake River Plain—Yellowstone and High Lava Plains— Newberry Volcanic Trends Introduction 105 Geology of the Owyhee Plateau 110 Sample suite 115 Data 116 Geochronology and elemental and isotope geochemistry 116 Spatial and temporal patterns of volcanism 128 Discussion 136 Conclusions 142

CHAPTER 4: The Tectonomagmatic Evolution of the Late Cenozoic 144 Owyhee Plateau, Northwestern United States: Summary and Suggestions for Future Research Summary 144 Suggestions for future research 146

ii REFERENCES 150

APPENDIX 1: Analytical Methods 162 Sample Preparation 163 Loss on Ignition (LOI) 164 Major and Trace Element Analysis 164 Isotope Analysis 165 Sr Isotopes 165 Pb and Nd Isotopes 166 40Ar/39Ar Geochronology 169

APPENDIX 2: Data 171 Appendix 2A: Sample Locations and Descriptions 172 Appendix 2B: Summarized Geochemical, Isotopic, and Geochronologic 205 Data Appendix 2C: Normative Mineralogies 232 Appendix 2D: New 40Ar/39Ar Geochronology 251 Appendix 2E: Previously Reported K-Ar and 40Ar/39Ar Geochronology 258

APPENDIX 3: preprint of Shoemaker, K.A. and Hart, W.K., 2002, Temporal 260 controls on basalt genesis and evolution on the Owyhee Plateau, Idaho and Oregon, in Bonnichsen, B., White, C.M., and McCurry, M. (eds.), Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, in press.

iii LIST OF TABLES

2-1. Location index for map numbers in Figure 2-1b. 21

2-2. Summarized geochemical, isotopic, and geochronologic data for samples 28 with isotopic data, analytically determined ages, or both.

3-1. New 40Ar/39Ar method geochronology. 118

3-2. Summary of isotopic and average geochemical characteristics of aligned 135 basaltic vents.

iv LIST OF FIGURES

1-1. Tectonomagmatic development of the late Cenozoic northwestern United 3 States volcanic provinces.

1-2. Digital elevation model and sketch map of the Owyhee Plateau study area 7 and surrounding tectonic features.

1-3. Illustration of the regional change in basalt geochemistry throughout the 9 Oregon and Owyhee Plateaus ca. 11 Ma.

1-4. Illustration of the longitudinal variation in Sr and Nd isotope compositions 12 of late Cenozoic basaltic rocks.

1-5. Compilation of Sr isotope data for northwestern United States basaltic 14 rocks.

2-1. Digital elevation model of the Owyhee Plateau and surrounding features, 19 and sample index map.

2-2. MgO-TiO2-K2O ternary diagram depicting Owyhee Plateau region basalt 34 chemical types.

2-3. Variations in K2O/MgO and TiO2 with eruptive age. 36

2-4. Total alkalies vs. silica diagram showing classification of Owyhee Plateau 43 basaltic rocks.

2-5. Relationship between differentiation and alkalinity in Owyhee Plateau 45 basalts.

2-6. Major element variations with MgO content. 48

2-7. Selected trace element variations with MgO content. 50

2-8. NMORB-normalized spider diagrams of all basalts in this study. 53

2-9. NMORB-normalized spider diagrams of (a.) average and (b.) relatively 55 little fractionated SB & OIG, HAOT, SROT, and AB.

2-10. Variations in Sr and Nd isotope compositions of Owyhee Plateau basalts 57 with age of eruption.

2-11. Variations in Pb isotope compositions of Owyhee Plateau basalts with age 60 of eruption.

v 2-12. Pb isotope characteristics of ocean island basalt and northwestern United 63 States mantle components, in the context of MORB, OIB, Pacific sediments, and Wyoming Craton SCLM.

2-13. Plot of 143Nd/144Nd vs. 87Sr/86Sr for Owyhee Plateau basalts. 66

2-14. Plots of 207Pb/204Pb and 208Pb/209Pb vs. 206Pb/204Pb for Owyhee Plateau 69 basalts.

2-15. Plots of 87Sr/86Sr and 143Nd/144Nd vs. 206Pb/204Pb for Owyhee Plateau 71 basalts.

2-16. Variations in 87Sr/86Sr and 206Pb/204Pb with Mg#. 74

2-17. 87Sr/86Sr and 206Pb/204Pb of Owyhee Plateau basalts relative to observed 76 Archean “deep crustal” granulite xenoliths from the Snake River Plain.

2-18. Variations in Sr concentration and Rb/Sr with 87Sr/86Sr. 79

2-19. Variations in Rb/Sr with K/P. 82

2-20. Variations in 87Sr/86Sr, 206Pb/204Pb, and Zr/Nb with Rb/Y. 85

2-21. Expanded basalt tetrahedron with experimentally-derived possible primary 88 mantle melt compositions and differentiation paths.

2-22. ALFE diagram with differentiation paths depicting high pressure, 91 clinopyroxene-dominated crystallization, low pressure crystallization, and olivine extraction.

2-23. CaO/Al2O3 vs. Fe-factor companion plots for the ALFE diagram in Figure 93 2-22.

2-24. Summarized geochemical characteristics of Owyhee Plateau basalts as a 96 function of eruptive age.

2-25. Conceptual cartoon of sub-Owyhee Plateau mantle development during 100 the Cenozoic.

3-1. Late Cenozoic volcanic provinces and selected important 107 tectonomagmatic features of the northwestern United States.

vi 3-2. Digital elevation model of the Owyhee Plateau and surrounding features, 112 and relationship of Owyhee Plateau to pre-11 Ma basaltic and silicic eruptive centers.

3-3. MgO-TiO2-K2O ternary diagram depicting Owyhee Plateau region basalt 120 chemical types.

3-4. Variations in TiO2 content versus age of eruption. 122

3-5. NMORB-normalized spider diagram of representative SB, HAOT, and 125 SROT samples.

3-6. Variations in 87Sr/86Sr and K/P versus age of eruption. 127

3-7. Temporal summary of eruptive style, eruptive loci, and eruptive products 130 found in the Owyhee Plateau region.

3-8. Owyhee Plateau basaltic vent alignments. 133

vii ACKNOWLEDGEMENTS This work was nine years in the making, so this may take a while… First, I gratefully acknowledge the sources of funding for this research: the Geological Society of America Student Research Grants Program (Lipman Research Award 5929-96 and Research Grant 6326-98); the Miami University Graduate School Dissertation Research Support Fund; the Miami University Geology Department; the Miami University Committee on Faculty Research (to Bill Hart); the National Science Foundation (EAR-9204780, to Bill Hart); and the Saint Joseph’s College Faculty Development Fund. I would like to thank John Morton for providing direction and assistance in the labs (not to mention his patience in battling the Nuclide) at Miami over my many years working there; Stan Mertzman at Franklin & Marshall for a couple hundred XRF trace element analyses; Rick Carlson and Steve Shirey for accommodating me at DTM; Mary Horan and Tim Mock, also at DTM, for all of their help in the chemistry labs and with the ICP-MS, respectively; and Matt Heizler at the New Mexico Geochronological Research Laboratory for the 40Ar/39Ar analyses. Thanks to Barry and Terri Cupp for opening their home to us in Jordan Valley. Thanks, too, to the proprietors of Scott’s Shady Court Motel (“For the Rest of Your Life”) in Winnemucca, City of Paved Streets; the Rome Station Café (“Quite Possibly the Best Food on Earth”); and the Rome Cabin Motel (“If you see any spiders, just step on ’em!”) – their establishments provided a welcome relief from tents and camp stoves. And thanks to whoever watches out for wayward geologists in rented four-wheel-drives with poor ground clearance (the Butteland Highway is no place for a ruptured gas tank). I would like to thank all of the people who served on my committees over the years: John Hughes, Liz Widom, Yildirim Dilek, Glenn Julian, Michael Pechan, and Craig White. I extend special thanks to Liz and Craig for their careful reviews of my manuscript (and to Craig for making the trip to Ohio for my defense). Claire McKee collected, prepared, and did most of the analysis of the CM97- series samples as part of her senior thesis; in addition to providing our first real look at the southern part of the Owyhee Plateau, it saved me a little work.

viii Darin Snyder, my fellow long-suffering grad student, ran a few Sr isotope analyses for me but more importantly shared the experience of interminable schooling. I appreciate all of his help and friendship over the years. Mom and Dad never questioned my decisions to pursue what I wanted. Thank you for supporting me and never trying to force me into being a lawyer or accountant or some other thing I would have hated doing (and for that matter, for not talking me out of spending over a third of my life in college). Others that I drew support or inspiration from: the rest of the geology faculty and staff and my fellow grad students over the last decade at Miami; Wiese, Gray, and all the Dougs from Mount Union (Gowganda!); Matt Barnett (you can stop calling me Hudson now); Mike (“How’s your paper coming?”) Davis, Roger Olson, Bob Brodman, and Suzanne Zurn-Birkhimer at Saint Joe’s; and 8,700 large black coffees. And the people I would have been helpless without: I am indebted to Matt Brueseke – apart from being the best field partner ever, apart from our frequent extended (and usually rambling) discussions about the geology of the northwestern U.S., flood basalts, mantle plumes and so forth, apart from being an all around swell guy. Matt opened up his apartment to me and gave me a place to stay during my endless trips to Oxford – typically in exchange for nothing more than a few cans of cheap beer. Matt, your collegiality, generosity and friendship over the years are more than just appreciated. You made it logistically possible for me to finish this work. Bill Hart accepted me into his research group way back in 1995 – I don’t know why. Maybe he liked my application, or maybe he just liked that I had “Helter Skelter” on my answering machine. Whatever the reason, I could not have wished for a better advisor. His guidance, patience, trust, enthusiasm, and sense of humor were perfectly tempered by his ability to give a well-timed and well-placed kick when needed. Bill, thanks for taking a chance on a know-it-all kid who hadn’t realized there was desert in Oregon, who’d never done a lick of research, and who wouldn’t have known an isotope if you showed it to him. Thanks for never giving up on me. And finally, my wife Beth and my son Gus. Gus, now two years old, provided no logistical help whatsoever but was a source of joy in contrast to the frustration and despair I often felt during the latter stages of this work. Beth has been a constant source

ix of support and strength through her love and her persistent belief in me. She has been with me for my successes but more importantly has stood by me through disappointments, postponed deadlines, countless weeks and weekends spent apart, days when I worked all day, all evening and into the wee hours of the morning, holidays I spent writing…I could go on and on. Beth, you believed in me when I no longer believed in myself; many times, it was the only thing that kept me going. This work represents a promise, at long last fulfilled.

x CHAPTER 1:

THE OWYHEE PLATEAU: TECTONOMAGMATIC CONTEXT AND REGIONAL SIGNIFICANCE

INTRODUCTION For >200 million years the North American Cordillera has been a complexly evolving tectonomagmatic system, involving terrane accretion (Coney et al., 1980; Saleeby, 1983; Coney, 1989) and lithosphere-scale thrusting (Leeman et al., 1992); subhorizontal subduction (Engebretson et al., 1985; Hamilton, 1988), subsequent slab removal (Humphreys, 1995), and migrating arc magmatism (Snyder et al., 1976; Lipman, 1980); ridge-trench collisions (Engebretson et al., 1985; Bohannon and Parsons, 1995) and strike-slip terrane transport (Oldow et al., 1989; Stewart and Crowell, 1992); diffuse lithospheric extension (Wernicke, 1992; Anderson, 1989; Sonder and Jones, 1999; Dilek and Moores, 1999) and rift initiation (Zoback et al., 1994); and continental intraplate basaltic volcanism (Hooper and Hawkesworth, 1993; Carlson and Hart, 1987; Pierce and Morgan, 1992; and references therein). While the causes, effects, and processes of most of these episodes are generally understood and reconciled within the greater framework of plate tectonics, the processes and sources responsible for widespread late Cenozoic northwestern United States basaltic magmatism have remained enigmatic. Prior to ca. 19 Ma, magmatism in the western United States was largely related to active subduction zone processes as North America overrode the Farallon Plate and the various microplates into which it fragmented. Following a brief magmatic hiatus, flood basalt volcanism in the northwestern United States initiated ca. 17 Ma (Figure 1-1) with the simultaneous eruption of the Clarkston subgroup of the Columbia River Basalt from a narrowly focused array of fissures near the Washington-Oregon-Idaho border (Hooper and Hawkesworth, 1993); the eruption of the Steens Basalt on the Oregon Plateau (Hart and Carlson, 1985; Hart et al., 1989; Brueseke and Hart, in preparation); and the emplacement of basaltic dikes in the Northern Nevada Rift (Zoback et al., 1994). While the Clarkston subgroup is volumetrically the most significant (approximately 175,000

1

Figure 1-1. Generalized maps illustrating the tectonomagmatic development of the late Cenozoic northwestern United States volcanic provinces and locations of features referred to in the text. BR, Basin and Range; C, Cascade arc; CP, ; CRB, Columbia River Basalt; HLP, High Lava Plains; MTJ, Mendocino Triple Junction; NNR, Northern Nevada Rift; OIG, Oregon-Idaho Graben; OP, Oregon Plateau; OWP, Owyhee Plateau; SRP, Snake River Plain; WSRP, Western Snake River Plain Graben. The western limit of Precambrian North America is inferred from isotopic compositions of Cenozoic and older materials (Armstrong et al., 1977; Farmer and DePaolo, 1983; Bennett and DePaolo, 1987; Elison et al., 1990; Wright and Wooden, 1991; Leeman et al., 1992; Wooden et al., 1998; Tosdal et al., 2000). Locations of tectonic and magmatic features are taken from Bohannon and Parsons (1995), Zoback et al (1994), Carlson and Hart (1987), and Cummings et al (2000).

2 o o o 125oW 120oW 115oW 125oW 120oW 115oW 125 W 120 W 115 W

Western edA ge of R

PrecambrC ian Waning stages North America of CRB volcanism CP (Saddle Mtns.) CP Main CRB dike swarms o 45oN 45 N C C OP OIG and WSRP OP SRP HLP SRP

3 Steens Basalt dikes Incipient Owyhee Plateau

NNR Diffuse extension and continuing volcanism across o 40oN southern Oregon & Idaho 40 N MTJ OWP

A R C C BR BR MTJ o 35oN 35 N MTJ N 0 Ma 17 - 14 Ma ca. 10 Ma 0 250 500 (MTJ and ARC ca. 17 Ma) kilometers km3; Tolan et al., 1989), and the Steens Basalt is estimated to be between 50,000 km3 (Camp and Ross, 2000) and 65,000 km3 (Carlson and Hart, 1988), the volume of contemporaneous basalt erupted along the Northern Nevada Rift was comparatively minor. Immediately subsequent to these eruptions a series of northerly-striking grabens formed along the Washington-Idaho and Oregon-Idaho borders (Hooper and Conrey, 1989; Cummings et al., 2000). These fissures, dike swarms, and grabens together form a 1,000 km long lithospheric rift (Zoback et al., 1994). By 14 Ma the bulk (>97%) of the basaltic lavas had been erupted, and eruptive activity began to contract in a north-south sense along the rift zone, while expanding westward throughout the Oregon Plateau and eastward along the developing Snake River Plain (Figure 1-1). As a consequence, only in the Oregon-Nevada-Idaho tristate region – that is, the Owyhee Plateau – have basalts been erupted, essentially continuously, from the middle Miocene inception of flood basalt volcanism through the Quaternary (Shoemaker and Hart, 2003). Over the last three decades, two dominant categories of models have emerged to explain northwestern United States intraplate volcanism: some researchers have attributed the initial, large volume outpourings of basalt to the impingement of the proposed Yellowstone mantle plume on the base of the lithosphere, which caused a major melting event in the mantle (Hooper and Hawkesworth, 1993; Geist and Richards, 1993; Camp, 1995), while others have proposed that the volcanism is related to back-arc spreading behind the Cascade magmatic arc (Christiansen and McKee, 1978; Carlson and Hart, 1987). Strong arguments can be made in support of both models. For example, the eastward-younging trend of silicic eruptive centers on the Snake River Plain (Pierce and Morgan, 1992) and the 3He/4He isotopic characteristics of some Snake River Plain (Reid, 1997) and Columbia Plateau (Dodson et al., 1997) basalts have been cited as evidence of plume processes and sources. On the other hand, the presence of a long lithospheric rift parallel to the middle Miocene volcanic arc argues strongly that lithospheric pull-apart due to back-arc extension along a previously established zone of weakness – the suture zone between cratonic North America and younger accreted terranes (Leeman et al., 1992) – controlled the location of (at least) the early, large- volume flood basalt eruptions (Carlson and Hart, 1987; King and Anderson, 1995).

4 However, other tectonomagmatic features of the northwestern United States have been more difficult to explain with either simple plume or rift models: for example, the westward-younging trend of silicic eruptive centers of the Oregon High Lava Plains, which forms an approximate “mirror image” of the Snake River Plain (MacLeod et al., 1975; Grunder et al., 1999). More recently proposed models have attempted to resolve such conundrums by placing greater emphasis on the interactions between the North American and Farallon plates (Humphreys et al., 2000) and the importance of pre- existing zones of structural weakness in the lithosphere (Christiansen et al., 2002). The Owyhee Plateau is an intermontane basalt plateau of some 6,000 km2, located at the intersection of several tectonomagmatic provinces and features: the Snake River Plain, Oregon High Lava Plains, Oregon Plateau, Basin and Range, Oregon-Idaho Graben, Northern Nevada Rift, and Midas Trough (Figures 1-1 and 1-2). Authors have variously considered the Owyhee Plateau to be a part of the Snake River Plain (Rodgers et al., 1990; Pierce and Morgan, 1992), the Oregon Plateau (Carlson and Hart, 1987, 1988; Hart and Carlson, 1987), and the Basin and Range (Lawrence, 1976; Draper, 1991) or northwestern Great Basin (Hart et al., 1984; Hart, 1985). While the Owyhee Plateau shares characteristics with each of these provinces, I propose that the Owyhee Plateau is a discrete tectonomagmatic entity, and, because of its critical location, a tectonomagmatic “missing link” in our understanding of the evolution of the late Cenozoic northwestern United States volcanic provinces.

REGIONAL SPATIAL AND TEMPORAL OBSERVATIONS Previous investigations of late Cenozoic basaltic volcanism documented a significant change in the geochemical character of the dominant basalts being erupted throughout the Oregon and Owyhee Plateaus ca. 11 Ma (Carlson and Hart, 1987, 1988; Hart and Carlson, 1987, 1992; Draper, 1991; Figure 1-3). The older eruptions produced strongly fractionated basalts and basaltic (Steens Basalt). After 11 Ma, more primitive low K, low Ti, high alumina olivine tholeiites (HAOT) became dominant throughout the Oregon Plateau (Hart et al., 1984). On the Owyhee Plateau, HAOT is found in close association with higher K, higher Ti Snake River Plain-type olivine tholeiites (SROT), as well as a full spectrum of compositions between the two. This

5

Figure 1-2. (a.) Digital elevation model of the Owyhee Plateau study area and surrounding tectonic features, modified from Sterner (1997). Plateau boundary is dashed where its location is uncertain. (b.) Sketch map of the same area, emphasizing the locations and orientations of significant regional extensional features mentioned in the text. BR-T, Bull Run and Tuscarora Mountains; OIG, Oregon-Idaho Graben; OM, ; MT, Midas Trough; NNR, Northern Nevada Rift; SM, Steens Mountain; SR, Santa Rosa Mountains; WSRP, Western Snake River Plain Graben.

6 a. OIG WSRP

SM OM

OR ID NV NV SR

BR-T 50 km NNR MT

OIG b. WSRP

SM

OR ID NV NV

NNR MT 50 km

7

Figure 1-3. Illustration of the regional change in basalt geochemistry throughout the Oregon and Owyhee Plateaus, after Hart and Carlson (1987). Note the change from strongly fractionated basalts and basaltic andesites prior to ca. 11 Ma to relatively unfractionated basalts after ca. 11 Ma. Additional data are from M. Brueseke (unpublished) and this study.

8 3

>300 Oregon Plateau & Owyhee Plateau basalts 5

x 2

) O g M / O

2 1 K (

0 0 4 8 12 16 Age (Ma)

9 change in basalt types, from strongly fractionated to less fractionated varieties, is temporally coincident with the regional shift from large volume, fissure-fed eruptions to small volume eruptions from discrete eruptive centers, and with the onset of diffuse regional extension and lithospheric thinning throughout the entire Oregon Plateau (Hart and Carlson, 1987; Hooper, 1990; Draper, 1991). An isotopic study of late Cenozoic volcanic rocks by Leeman et al. (1992) demonstrated that the maximum diversity in both 87Sr/86Sr and 143Nd/144Nd in northwestern United States basalts occurs in a “band” corresponding to approximately 117o to 118o west longitude (Figure 1-4). This region of maximum diversity coincides with the transition between the Proterozoic-Archean age cratonic lithosphere to the east and Mesozoic-Cenozoic accreted terranes to the west, as inferred from the Sr, Nd, and Pb isotopic compositions of Cenozoic and older materials (Armstrong et al., 1977; Farmer and DePaolo, 1983; Bennett and DePaolo, 1987; Elison et al., 1990; Wright and Wooden, 1991; Leeman et al., 1992; Wooden et al., 1998; Tosdal et al., 2000). It is significant that the Owyhee Plateau overlies this region of transitional lithosphere. The observations of Leeman et al. (1992) were substantiated and expanded upon by Hart (1997) in an examination of eruptive location, age, and isotopic composition of northwestern United States basalts. Figure 1-5 shows that not only does a wide range in basalt 87Sr/86Sr values occur between approximately 117o and 118o west longitude, but also that Owyhee Plateau basalts span nearly this entire range, from less than 0.704 to greater than 0.707. Additional trends emerge when the Sr isotope data are plotted versus the age of eruption. Those basalts erupted during the earliest pulses of flood basalt activity (Clarkston, Picture Gorge, and Steens basalts) have lower 87Sr/86Sr values than many of the basalts erupted later (Snake River Plain, Saddle Mountains, and Owyhee Plateau basalts). In general, 87Sr/86Sr values increase with decreasing age from ca. 17 to 11 Ma, at which point a divergence in the data is noted. With the exception of the young Yellowstone area basalts, which span nearly the entire observed range of 87Sr/86Sr, Snake River Plain basalts retain their radiogenic signatures, while Owyhee Plateau basalts trend toward lower 87Sr/86Sr values between 11 to 0 Ma. Shoemaker and Hart (2003) further explored these systematic time-dependent variations in Owyhee Plateau basalts, proposing a preliminary model in which the observed variations in Sr isotope

10

Figure 1-4. Illustration of the longitudinal variation in Sr and Nd isotope compositions of late Cenozoic basaltic rocks, after Leeman et al (1992); additional data is from this study. The Owyhee Plateau study area coincides with the inferred edge of cratonic North America, where the maximum diversity in both 87Sr/86Sr and 143Nd/144Nd occurs.

11 0.708 r S 6 8

/ 0.706

r S 7 8

0.704 inferred edge of craton

inferred edge 0.5130 of craton

d 0.5128 N 4 4 1

/

0.5126 d N 3 4

1 0.5124

0.5122

120 118 116 114

Degrees W longitude

12

Figure 1-5. Compilation of Sr isotope data for northwestern United States basaltic rocks, after Hart (1997). Wherever possible, the longitude of eruption has been plotted. Data are from Leeman and Manton (1971), Noble et al. (1973), Mark et al. (1975), Carlson et al. (1981), Leeman (1982), Carlson (1984), Hart (1985), Carlson and Hart (1987), Hart et al. (1989), Leeman et al. (1992), Hooper and Hawkesworth (1993), Lambert et al. (1995), and this study.

13 0.709

0.708

r 0.707 S 6 8

/ 0.706

r S 7

8 0.705

14 0.704

0.703

121 119 117 115 113 111 0 4 8 12 16 20 Degrees W Longitude Age (Ma)

Columbia Plateau Oregon Plateau Snake River Plain Picture Gorge Steens/Pueblo Mtns Misc. SRP Clarkston Misc. Oregon Plateau Yellowstone area Saddle Mtns Owyhee Plateau composition were controlled by temporal variations in magma production, the structure of the lithosphere, and relative contributions from lithospheric and sublithospheric source reservoirs.

SUMMARY The development of models for late Cenozoic intraplate basaltic volcanism in the northwestern United States has been largely hindered by the complications of regional variations in the age, thickness, structure, and composition of the lithosphere through which the basalts were erupted (Carlson and Hart, 1987; Leeman et al., 1992). Because its location eliminates many of these complications, the Owyhee Plateau is unique among the northwestern United States basalt provinces, and thus understanding the tectonomagmatic evolution of the Owyhee Plateau is a critical step in understanding the late Cenozoic evolution of these provinces as a whole. To summarize the regional significance of the Owyhee Plateau: 1) The Owyhee Plateau overlies the boundary between the ancient, cratonic North American lithosphere to the east and the younger accreted lithosphere to the west, and preserves an isotopically diverse suite of basalts (Leeman et al., 1992). 2) The Owyhee Plateau contains significant volumes of the dominant basaltic lava types characteristic of both the Oregon Plateau (HAOT) and the Snake River Plain (SROT), as well as a full spectrum of compositions transitional between the two (Hart et al., 1984) 3) The Owyhee Plateau region preserves the most complete temporal record of middle Miocene to Recent basaltic volcanism in the northwestern United States (Shoemaker and Hart, 2003). 4) The Owyhee Plateau lies at the intersection of the Basin and Range, the Oregon Plateau, and the Snake River Plain tectonomagmatic provinces, as well as lying at the geographic and temporal origin of both the eastward- migrating Snake River Plain and the westward-migrating Oregon High Lava Plains silicic volcanic trends (MacLeod et al., 1975; Pierce and Morgan, 1992)

15 5) The Owyhee Plateau is the central segment of the rift zone defined by the Oregon-Idaho Graben and dike swarms to the north and the Northern Nevada Rift to the south, and marks the transition region between the large volume flood basalt eruptions to the north and the suppressed activity to the south (Zoback et al., 1994; Cummings et al., 2000; Carlson and Hart, 1987). This dissertation constitutes the first in-depth study of the Owyhee Plateau as a discrete tectonomagmatic entity within the northwestern United States, rather than as a constituent of another volcanic province. As such, it is not my intention to propose any sort of unique, unified model to account for all late Cenozoic intraplate magmatism in the northwestern United States. Rather, it is my objective that this detailed treatment of the Owyhee Plateau, which has been a focal point of magmatic and tectonic activity since the middle Miocene, will provide an integrated foundation of field, spatial, geochronological, geochemical, and isotopic data for future proposed geophysical research on the structure of the crust and lithospheric mantle in this complex but important region, which in turn will clarify the magmatic and tectonic relationships among these provinces and ultimately contribute to the resolution of the debate.

16 CHAPTER 2:

GEOCHEMICAL, ISOTOPIC, AND CHRONOSTRATIGRAPHIC CHARACTERIZATION OF LATE CENOZOIC BASALTIC VOLCANISM IN THE OWYHEE PLATEAU REGION: CONSTRAINTS ON LITHOSPHERIC AND SUBLITHOSPHERIC MANTLE ARCHITECTURE AND MAGMA GENESIS

INTRODUCTION Since approximately 17 Ma, the northwestern United States has been the site of significant intraplate magmatism which has variously been ascribed to the impingement of the hypothesized Yellowstone mantle plume on the base of the lithosphere (Hooper and Hawkesworth, 1993; Geist and Richards, 1993; Camp, 1995), spreading in the back- arc region of the Cascade magmatic arc (Christiansen and McKee, 1978; Carlson and Hart, 1987), complex plate interactions resulting in the focusing of mantle flow (Humphreys et al., 2000), and magmatic propagation along ancient zones of structural weakness (Christiansen et al., 2002). Regional variations in the age, thickness, structure, and composition of the lithosphere through which the basalts were erupted have imparted a variety of contrasting elemental and isotopic signatures to these basalts (Carlson and Hart, 1987; Leeman et al., 1992), complicating interpretations of the ultimate causes of this magmatism. Bounded by the Owyhee, Bull Run, and Tuscarora Ranges to the east and Steens Mountain and the Santa Rosa Range to the west, the Owyhee Plateau is a geographically restricted basalt plateau of some 6,000 km2 located in the Oregon-Nevada-Idaho tristate region (Figure 2-1). The plateau is deeply incised by the and its tributaries, providing excellent flow-on-flow exposures of basalt stratigraphy. Small fault scarps, with apparent displacements of less than a few meters, provide additional exposures of basalt stratigraphy, especially in the northern part of the plateau. Throughout the entire Owyhee Plateau, small basaltic shield vents occur along apparent alignments consistent with regional strain directions (Zoback et al., 1994; Rodgers et al., 1990; Lawrence, 1976; Cummings et al., 2000). As a consequence, the Owyhee Plateau

17

Figure 2-1. (a.) Digital elevation model of the Owyhee Plateau and surrounding features, modified from Sterner (1997). Note the deep incision of the Owyhee River and its tributaries. Owyhee Plateau outline is solid where well defined, and dashed where uncertain or inferred. Triangles mark sampled basalt vent areas, and circles mark other sample locations. BR-T, Bull Run and Tuscarora Mountains; MT, Midas Trough; NNR, Northern Nevada Rift; OIG, Oregon-Idaho Graben; OM, Owyhee Mountains; SM, Steens Mountain; SR, Santa Rosa Mountains; WSRP, Western Snake River Plain Graben. (b.) Index map for samples in this study; locations and samples identified by the map numbers are given in Table 2-1.

18 a. OIG WSRP OM SM

OR ID NV NV

SR BR-T 50 km NNR MT

OIG 8 2 3 6 b. 1 4 5 7 10 11 9 12 16 13 14 WSRP 20 21 15 17 19 24 28 30 18 2223 25 29 31 32 33 2627 34 39 35 OM 3637 40 38 41 42 44 SM 43 45 46

48 49 47 50 51 52 OR 53 54 55 56 ID 57 NV 58 NV 62 59 63 SR 60 61 6564 66 68 67 69

BR-T 50 km NNR 70 MT

19

Table 2-1. Location index for map numbers in Figure 2-1b.

20 Map ID # Sample ID#s Locality

1 KS96-5 along road between Riley Horn Reservoir and Mud Creek

2 H8-42, H8-43, H8-45, H8-47 Owyhee Canyon at The Hole in the Ground

3 KS96-1, KS96-2, KS96-3, KS96- Birch Creek Canyon Road 4

4 JC-4, JC-5, H8-63

5 H9-44 older flow exposed in wall of canyon now occupied by Jordan Craters lava field, east of Coffeepot Crater

6 JV96-3, JV96-4, JV96-5 Spring Mountain

7 JV96-6, JV96-7 Sheaville area

8 CH82-32 south of Squaw Butte, Owyhee Mountains

9 H9-42 Jordan Craters Road, 0.25 mile southeast of Upper Cow Lake

10 85-1 Table Mountain

11 H8-70 Clarks Butte

12 H8-57

13 H9-49 Threemile Hill

14 H8-60A Skinner Hill

15 H8-18, SM75-12A Jordan Creek, south of Threemile Hill

16 JV96-1, JV96-2 Jordan Valley quarry area

17 H9-46, H9-47 southeast of Jordan Valley on road to Triangle, between Trout Creek and Jordan Creek

18 H9-48 ~12 miles southeast of Jordan Valley, on road to Triangle

19 H85-6, H85-26A Silver City Range, Owyhee Mountains

20 H8-73, H8-74, H9-32, H9-34 Oregon Highway 78, 13 to 17 miles northwest of Burns Junction

21 KS96-8A, KS96-8B, KS96-8C, small side canyon on west side of Owyhee Canyon, KS96-8D ~8 miles northwest of Rome

21 Map ID # Sample ID#s Locality

22 KS98-1 Scott Butte

23 H9-36A, H9-36B, H9-36C Crooked Creek at U.S. 95, northeast of Burns Junction

24 H8-50A, H8-50B, H8-50C, H8- Jordan Creek, southwest of Arock 50D, H8-50E

25 H8-56A, H8-56B, H8-56C, H8- Owyhee River at U.S. 95, Rome 56D

26 H9-37A, H9-37B, H9-37C, H9- Owyhee Canyon, 7 miles upstream from Rome 37D

27 KS98-32A, KS98-32B, KS98- Sand Hollow 32C, KS98-32D, KS98-32E

28 H8-28A, H8-28B, H8-28C, H8- Jordan Creek, southwest of Threemile Hill 28D, H8-28E

29 H8-29 Three Forks Road, ~2.5 miles north of Little Grassy Reservoir

30 H85-11A, H85-11B, H85-11C Antelope Rim

31 H85-5A, H85-5B south of Antelope Reservoir

32 H85-12B Juniper Ridge, east of Antelope Reservoir

33 H8-98 U.S. 95, 1 mile south of Burns Junction

34 H8-30A, H8-30B, H8-30C, H8- Soldier Creek, ~0.8 miles east of Owyhee Canyon 30D

35 H8-39, H85-10A, H85-10B Soldier Creek, ~17 miles south of Jordan Valley, near Oregon-Idaho border

36 H9-28A Crooked Creek at U.S. 95, south of Burns Junction

37 H9-30 Rattlesnake Creek Road at north end of Bowden Hills, ~3.5 miles from U.S. 95

38 H9-29 Rattlesnake Creek Road at north end of Bowden Hills, ~7.5 miles from U.S. 95

39 KS98-31 small valley on west side Deadhorse Butte

22 Map ID # Sample ID#s Locality

40 H8-69D, H8-69E, H8-69F, H8- Soldier Creek at the Owyhee Canyon 69G 41 H9-38, H8-39 Grassy Mountain

42 KS98-29, KS98-30 Mousetrap Butte

43 H8-34, H8-35, H8-36 Three Forks

44 KS96-10 crossing, near Oregon- Idaho border

45 KS96-7 rim of Middle Fork Owyhee River canyon, ~5 miles upstream from Three Forks

46 KS96-6B, KS96-6C Middle Fork Owyhee River at BLM crossing, ~10 miles upstream from Three Forks

47 H9-27 West Little Owyhee River at Louse (LaRosa) Canyon

48 CM97-25

49 CM97-1 Toppin Creek Canyon

50 CM97-2A, CM97-2B Barking Butte

51 CM97-3, CM97-4, CM97-5 and Lookout Buttes

52 CM97-12, CM97-13, CM97-14, Twin Buttes CM97-15

53 CM97-16, CM97-17, CM97-18, Oregon Butte CM97-19, CM97-20, CM97-21

54 CM97-22, CM97-23, CM97-24 Mahogany Butte

55 CM97-8, CM97-9, CM97-10, Willow Creek Butte CM97-11

56 CM97-6, CM97-7 Star Valley Knoll

57 KS98-9 Rubber Hill

58 H9-21A, H9-21B, H9-21C, H9- South Fork Owyhee River at Devil's Corral, 21E southeast of Rubber Hill

59 KS98-2, KS98-3, KS98-4, KS98- Maiden Butte 5

60 KS98-6A, KS98-6B, KS98-6C Little Owyhee River at Twin Valley Spring

23 Map ID # Sample ID#s Locality

61 KS98-27, KS98-28 Hill 5751

62 KS98-7, KS98-10 Pipeline Butte

63 KS98-22, KS98-23, KS98-24, Ant Hill KS98-25, KS98-26

64 KS98-16, KS98-17, KS98-18, Bartome Knoll KS98-19, KS98-20, KS98-21

65 KS98-15 Corral Lake Butte

66 KS98-13, KS98-14 Guzzler Butte

67 KS98-11, KS98-12 Wolf Butte

68 H9-20C, H9-20D, H9-20E Owyhee River at Poole Valley

69 H9-19A, H9-19B, H9-19C, H9- Deep Creek, near South Fork Owyhee River 19D

70 H8-95 Midas Trough

24 region preserves, and provides access to, an exceptional record of post-17 Ma basaltic volcanism in the northwestern United States. The Owyhee Plateau basalts presented in this study constitute a geochemically and isotopically diverse suite, erupted from small shield vents and isolated fissures within this geographically restricted area over the past 17 Ma. This area is underlain by lithosphere transitional between that of the Proterozoic-Archean Wyoming Craton to the east and accreted terranes younger than 200 Ma to the west (Leeman et al., 1992). Basaltic magmas erupted on the Owyhee Plateau, having passed through and having had the opportunity to interact with this heterogeneous transitional lithosphere, preserve the greatest diversity of isotopic signatures in the northwestern United States. Furthermore, since the Owyhee Plateau contains the most complete record of middle Miocene to Recent basaltic volcanism in the northwestern United States (Shoemaker and Hart, 2003), it is uniquely suited to a study of the temporal variations in lithospheric and sublithospheric contributions to this volcanism. The purposes of this chapter are to: (1) describe the characteristic major and trace element geochemical and isotopic variations in Owyhee Plateau basalts, and document their variations through time; (2) provide constraints on mantle source materials and regions for Owyhee Plateau basaltic volcanism; and (3) suggest probable melt- generation and magma evolution scenarios for Owyhee Plateau basaltic volcanism.

DATA Analytical methods Owyhee Plateau basalts in this study include newly collected samples as well as previously collected and prepared samples from prior investigations by W. K. Hart and colleagues. The field sampling strategy was twofold: to represent , as best as possible, the full age spectrum and the complete geographic distribution of basaltic eruptive materials on the Owyhee Plateau, with particular attention to basaltic vents and eruptive loci. One hundred seventy (170) samples were analyzed for major and trace elements by X-ray fluorescence (XRF) at Franklin & Marshall College or direct current argon plasma spectrometry (DCP) at Miami University. A subset of these were selected for Sr, Nd, and Pb isotope analysis at Miami University (Sr isotopes by thermal ionization mass

25 spectrometry), and at the Department of Terrestrial Magnetism, Carnegie Institution of Washington (Sr isotopes by thermal ionization mass spectrometry, and Nd and Pb isotopes by inductively coupled plasma mass spectrometry). Geochronology by the 40Ar/39Ar method was performed at the New Mexico Geochronological Research Laboratory. A summary of geochemical, isotopic, and geochronologic data for samples with isotopic data or analytically determined ages, or both, is presented in Table 2-2; complete data are presented in Appendix 2. A detailed discussion of the analytical procedures and methods used can be found in Appendix 1.

Previous geochemical observations Prior investigations (e.g. Hart and Mertzman, 1983; Carlson and Hart, 1987, 1988; Hart and Carlson, 1987, 1992; Russell et al., 1988; Draper, 1991; Hart, 1996) of basaltic volcanism throughout the Oregon Plateau region, inclusive of the Owyhee Plateau, have documented the presence of several distinct basalt geochemical types in the Owyhee Plateau region (Figure 2-2). Of great regional significance is a change ca. 11 Ma in the dominant basalt geochemical types erupted (Carlson and Hart, 1987, 1988; Hart and Carlson, 1987, 1992; Draper, 1991), illustrated in Figure 2-3a. From 17-11 Ma, eruptions were dominated by strongly fractionated flood basalts related to the Steens Mountain flood basalt events; later in this interval, basalts related to the opening of the Oregon-Idaho Graben (Cummings et al., 2000) were also erupted. After 11 Ma, a range of less differentiated tholeiites were erupted (Figure 2-3b), which Hart et al. (1984) subdivided into low-Ti, low-K, high-alumina olivine tholeiites (HAOT), high-Ti, high-K Snake River Plain-type olivine tholeiites (SROT), and compositions intermediate between the two (transitional basalts, TB). Additionally, small volumes of mildly alkaline basalt were erupted after 0.25 Ma from vents located near the northernmost edge of the Owyhee Plateau (Hart and Mertzman, 1983; Russell et al., 1988; Hart, 1996).

Chronostratigraphy Careful sampling of basalt flow sequences exposed in the canyons of the Owyhee River and its tributaries and in fault scarps, combined with new and previously reported 40Ar/39Ar (Shoemaker and Hart, 2003) and K-Ar (Hart and Mertzman, 1982, 1983; Hart

26

Table 2-2. Summarized geochemical, isotopic, and geochronologic data. Only samples with isotopic data or analytically determined ages, or both, are presented here. Ages reported as whole numbers are estimated based on stratigraphic relationships as described in the text. For a complete discussion of analytical methods, see Appendix 1; for a summary of data for all samples in this study, see Appendix 2B; for details of analytical geochronology, see Appendices 2D and 2E.

27 GROUP 1: Steens Basalts and Oregon-Idaho Graben basalts (17-11 Ma)

H85-10A CH82-32 H85-6 H85-12B H85-10B H9-47 JV96-4 JV96-7 JV96-2 H8-74 H9-32 SiO2 53.82 48.46 49.95 49.26 48.72 49.41 51.89 55.38 48.00 46.13 49.80 TiO2 2.00 2.20 1.97 2.54 2.07 2.32 1.21 1.04 2.53 2.86 3.00 Al2O3 14.72 15.83 14.73 16.45 15.76 15.18 16.71 16.19 14.20 14.80 15.40 Fe2O3 3.45 12.19 2.93 9.83 7.61 4.85 9.58 8.00 14.05 15.47 7.59 FeO 7.09 - - 10.41 3.02 5.33 7.56 ------6.40 MnO 0.15 0.15 0.25 0.14 0.18 0.19 0.17 0.15 0.20 0.20 0.23 MgO 5.01 5.72 2.78 2.77 5.67 4.69 4.88 4.36 6.62 5.01 3.51 CaO 7.08 8.17 7.28 8.66 9.98 9.05 8.23 7.09 10.24 8.75 7.24 Na2O 3.52 3.00 3.15 3.69 3.09 3.19 3.57 3.36 2.51 3.25 3.50 K2O 1.41 0.93 1.60 1.06 0.48 0.92 1.36 1.86 0.48 1.02 1.60 P2O5 0.47 0.41 0.43 0.42 0.26 0.43 0.67 0.62 0.44 0.98 1.37 L.O.I. 1.33 3.16 4.75 2.61 1.49 2.37 0.80 1.37 0.11 0.08 0.73 TOTAL 100.05 100.22 100.23 100.45 100.64 100.16 99.06 99.42 99.37 98.55 100.37

Rb 28 20 44 17 6.9 15 15 40 6.2 16 23 Sr 455 440 482 561 459 466 678 595 307 424 468 28 Y 3632343829363128404864 Zr 263 189 206 216 146 218 206 213 153 270 350 Nb 11.9 13.8 13.2 14.9 9.5 15.0 14.7 14.5 10.8 23.2 30.5 Ni 94 95 63 62 93 49 58 47 86 59 19 Ga 21.6 22.5 22.0 24.3 22.4 23.1 19.4 18.8 20.7 22.2 22.8 Cu 76 74 55 72 59 52 55 45 79 46 31 Zn 113 124 119 116 105 132 111 103 133 138 167 U 0.8 1.4 0.3 0.9 0.3 1.7 0.6 1.4 0.1 0.6 1.4 Th 2.6 3.0 5.3 2.5 0.6 4.9 1.8 5.4

87Sr / 86Sr 0.70432 0.70409 0.70418 0.70436 0.70397 0.70426 0.70442 0.70460 0.70638 0.70521 0.70557 143Nd / 144Nd - - 0.512810 ------0.512687 0.512632 0.512369 0.512479 0.512526 206Pb / 204Pb 18.984 18.990 - - - - 18.831 19.017 18.858 18.921 18.336 - - 18.722 207Pb / 204Pb 15.609 15.590 - - - - 15.565 15.612 15.604 15.616 15.605 - - 15.593 208Pb / 204Pb 38.629 38.650 - - - - 38.440 38.710 38.556 38.656 39.064 - - 38.780

Age (Ma) 16.27 16.23 16 16 16 15.01 14.61 13.87 12.37 11.70 11.20 GROUP 2: HAOT-TB-SROT (<11 Ma)

KS96-1 KS98-23 H9-37A H8-69D H8-29 KS98-19 H9-37C H9-20C H8-69E H9-20E KS98-28 SM75-12A H8-69G SiO2 46.49 49.34 45.77 46.35 49.11 47.12 45.48 47.15 45.08 46.51 47.50 47.65 48.34 TiO2 2.66 1.46 2.40 1.52 1.57 2.28 2.80 2.10 1.36 2.08 2.03 1.25 1.94 Al2O3 13.87 16.09 15.28 16.84 15.89 15.02 16.25 15.67 16.57 16.26 15.55 16.26 15.53 Fe2O3 14.69 11.62 6.11 7.46 4.77 14.26 5.58 4.32 12.80 4.11 13.08 2.74 3.36 FeO - - - - 7.44 5.20 6.64 - - 8.48 8.64 - - 8.48 - - 7.33 9.28 MnO 0.22 0.18 0.19 0.18 0.19 0.21 0.20 0.19 0.18 0.17 0.19 0.17 0.20 MgO 8.38 8.57 8.36 7.40 7.57 7.38 7.50 7.62 7.79 7.15 8.30 9.89 8.93 CaO 9.50 11.11 10.92 10.80 11.67 9.85 9.77 10.21 10.41 10.48 10.67 11.49 10.24 Na2O 2.21 2.39 2.20 2.34 2.38 2.48 2.53 2.38 2.36 2.49 2.31 2.28 2.23 K2O 0.54 0.45 0.48 0.25 0.34 0.53 0.53 0.45 0.29 0.46 0.34 0.34 0.42 P2O5 0.50 0.28 0.49 0.30 0.21 0.60 0.50 0.35 0.42 0.43 0.44 0.15 0.28 L.O.I. 0.10 -0.22 1.42 1.13 0.93 -0.28 0.88 0.92 2.71 1.16 -0.16 0.72 1.03 TOTAL 99.15 101.27 101.06 99.77 101.27 99.44 100.50 100.00 99.97 99.78 100.23 100.27 101.78

Rb 7.5 6.8 9.9 3.4 7.4 6.4 8.9 6.9 4.4 5.1 5.1 6.5 7.5 Sr 274 210 242 241 235 311 252 249 210 276 232 201 260 29 Y 38 27 36 30 32 36 36 29 27 31 31 24 35 Zr 187 126 167 111 115 176 159 131 98 142 138 77 123 Nb 13.3 10.3 14.8 9.4 9.7 15.8 14.0 12.6 7.7 13.8 13.5 5.6 9.5 Ni 176 133 130 100 118 127 94 118 117 143 142 166 145 Ga 20.5 20.5 18.4 18.9 18.5 22.4 19.6 22.3 18.3 21.3 21.2 16.5 18.3 Cu 58 83 61 58 78 65 57 58 49 82 52 89 63 Zn 128 96 117 92 96 123 119 107 95 110 106 77 105 U 0.6 1.6 0.6 0.0 0.7 1.0

87Sr / 86Sr - - 0.70733 0.70688 - - 0.70693 0.70786 0.70723 - - 0.70656 - - 0.70699 0.70667 0.70668 143Nd / 144Nd ------0.512387 - - 0.512363 ------0.512445 0.512543 206Pb / 204Pb ------18.702 - - 18.585 ------18.679 18.732 207Pb / 204Pb ------15.660 - - 15.654 ------15.653 15.662 208Pb / 204Pb ------39.242 - - 39.290 ------39.021 39.289

Age (Ma) 10.44 10.39 10 9.94 9.87 9.71 9.57 9.34 9 8.93 8.90 8.51 8.42 GROUP 2 (continued)

KS96-7 KS96-6C H8-34 CM97-22 H8-28C H9-21C KS98-4 H8-36 KS98-30 H9-39 H8-28E CM97-7 H9-21E SiO2 47.62 48.14 48.42 47.74 48.27 48.24 48.55 47.55 49.19 46.34 47.58 47.40 46.89 TiO2 1.29 1.61 1.82 2.14 1.64 1.66 1.06 1.75 1.58 2.15 1.58 1.27 1.52 Al2O3 15.39 15.66 15.78 14.92 15.54 16.27 16.26 15.07 14.94 14.84 15.55 15.76 16.64 Fe2O3 11.29 11.96 4.09 13.84 12.26 4.99 11.14 2.25 12.14 2.81 4.17 11.92 2.93 FeO - - - - 7.80 - - - - 6.68 - - 8.72 - - 10.00 6.96 - - 8.24 MnO 0.18 0.18 0.19 0.19 0.19 0.18 0.18 0.18 0.19 0.20 0.18 0.19 0.17 MgO 9.19 8.08 7.35 8.59 7.96 6.61 8.39 8.53 8.39 8.87 8.01 9.52 8.11 CaO 11.17 10.49 11.82 9.94 11.04 10.74 11.69 11.57 11.25 10.20 12.15 11.39 11.18 Na2O 2.16 2.39 2.06 2.50 2.29 2.51 2.45 2.26 2.22 2.15 2.48 2.24 2.28 K2O 0.28 0.45 0.31 0.36 0.42 0.54 0.16 0.28 0.35 0.40 0.36 0.21 0.25 P2O5 0.21 0.28 0.27 0.29 0.32 0.43 0.14 0.24 0.38 0.42 0.21 0.16 0.22 L.O.I. 0.34 0.16 1.05 -0.22 0.26 1.36 0.21 0.97 -0.28 1.53 1.44 -0.25 1.02 TOTAL 99.10 99.40 100.96 100.28 100.19 100.21 100.23 99.37 100.33 99.91 100.67 99.81 99.45

Rb 5.5 9.1 5.8 11 7.9 7.6 1.9 5.6 5.5 6.7 8.2 3.9 3.0 Sr 193 226 222 212 232 235 225 213 195 292 215 175 205 30 Y 26 31 35 34 31 30 21 33 30 40 31 25 25 Zr 87 134 156 137 116 134 63 144 104 152 114 74 105 Nb 6.6 9.3 10.5 12.7 9.9 11.8 5.6 9.6 10.0 14.9 9.5 7.0 8.9 Ni 135 127 98 165 128 127 130 88 93 165 130 166 146 Ga 17.6 18.8 18.0 19.0 18.8 20.7 19.4 18.5 19.5 18.7 17.8 18.2 20.4 Cu 78 58 69 68 86 90 109 54 66 74 73 72 56 Zn 81 93 102 113 104 100 85 98 101 119 93 96 96 U 0.4 0.5

87Sr / 86Sr 0.70735 0.70654 0.70675 0.70599 0.70682 - - 0.70669 0.70658 0.70642 0.70713 0.70676 0.70663 - - 143Nd / 144Nd - - - - 0.512434 ------0.512437 ------206Pb / 204Pb - - - - 18.730 ------18.733 ------207Pb / 204Pb - - - - 15.664 ------15.650 ------208Pb / 204Pb - - - - 39.241 ------39.145 ------

Age (Ma) 8.36 8.34 8.21 8.15 8.14 8.11 7.61 7.58 7.56 7.20 7.05 6.54 6.01 GROUP 2 (continued)

KS98-9 H9-27 KS98-15 KS98-13 KS98-7 KS98-12 H8-42 H8-95 H8-45 H8-47 H9-44 H9-49 H8-50E SiO2 47.55 47.71 46.15 47.67 48.39 48.29 47.90 48.08 47.48 47.79 46.96 46.97 47.87 TiO2 1.48 0.81 2.07 1.80 1.61 1.25 1.46 1.30 1.20 1.30 1.57 1.92 1.58 Al2O3 15.86 16.54 15.50 15.50 15.69 16.17 16.81 16.46 16.82 15.75 16.26 15.89 16.13 Fe2O3 11.70 1.63 14.23 12.46 11.61 11.39 2.74 2.27 8.34 2.52 3.98 4.81 1.35 FeO - - 8.72 ------8.16 8.24 2.86 8.00 7.20 7.28 10.00 MnO 0.18 0.17 0.20 0.18 0.17 0.18 0.18 0.19 0.17 0.18 0.17 0.17 0.19 MgO 9.19 9.22 8.51 8.11 7.84 8.76 8.76 8.65 8.90 9.52 8.94 7.88 8.60 CaO 11.36 12.19 9.97 11.22 11.26 11.61 10.95 11.60 11.14 11.03 10.86 10.51 10.91 Na2O 2.31 2.29 2.59 2.35 2.51 2.33 2.44 2.14 2.65 2.60 2.43 2.63 2.93 K2O 0.23 0.11 0.34 0.35 0.44 0.18 0.44 0.23 0.33 0.32 0.39 0.72 0.42 P2O5 0.30 0.11 0.47 0.43 0.21 0.25 0.33 0.17 0.16 0.26 0.31 0.34 0.22 L.O.I. -0.15 0.89 -0.55 0.16 0.09 -0.09 0.70 0.59 0.60 0.59 0.44 0.85 0.49 TOTAL 99.99 100.39 99.48 100.21 99.83 100.31 100.87 99.92 100.65 99.86 99.51 99.97 100.69

Rb 3.4 2.1 2.5 4.5 8.4 1.5 7.8 2.3 5.9 5.0 7.9 15 6.3 Sr 198 159 224 243 221 193 246 200 273 229 275 305 261 31 Y 25 22 34 30 28 25 28 23 23 27 28 27 28 Zr 104 45 148 137 128 85 110 59 96 103 123 131 105 Nb 8.9 1.8 14.5 13.4 9.5 7.3 10.7 5.9 8.4 10.4 11.7 16.0 9.8 Ni 158 152 133 92 104 144 131 137 149 138 141 124 125 Ga 20.1 16.3 21.7 21.3 21.2 19.2 18.3 18.7 16.3 17.4 18.4 18.3 19.0 Cu 84 79 73 63 92 97 70 102 64 61 80 60 73 Zn 99 62 121 107 93 86 81 75 75 76 83 95 89 U 0.6

87Sr / 86Sr 0.70631 0.70591 0.70719 0.70664 0.70569 0.70614 0.70522 - - 0.70457 0.70528 0.70502 0.70549 0.70533 143Nd / 144Nd - - 0.512622 ------0.512610 0.512599 - - 206Pb / 204Pb - - 18.715 ------18.693 18.638 18.774 207Pb / 204Pb - - 15.631 ------15.638 15.621 15.643 208Pb / 204Pb - - 38.838 ------38.940 38.730 39.006

Age (Ma) 6 5.04 11-5 11-5 11-5 11-5 4.49 4.40 4.09 4.06 3.84 1.86 1.55 GROUP 2 (continued) GROUP 3: AB (<0.25 Ma)

H9-37D H9-36C H9-36A KS96-8D H9-42 H8-73 H9-29 H8-70 H8-57 JC-4 JC-5 H8-60A H8-63 SiO2 46.07 47.48 47.42 46.92 46.77 48.17 47.08 48.79 48.47 47.53 48.34 48.08 47.66 TiO2 1.88 1.25 0.94 0.92 1.30 0.92 1.80 1.94 2.11 2.16 1.78 2.08 2.11 Al2O3 15.98 16.03 16.52 15.79 16.58 16.12 16.16 16.75 16.35 15.98 16.25 16.20 15.98 Fe2O3 1.91 2.76 4.44 10.92 1.68 1.23 1.28 6.41 2.74 2.31 1.75 4.83 2.92 FeO 10.08 8.00 6.24 - - 8.80 8.80 9.36 4.61 9.04 8.55 7.84 5.52 7.47 MnO 0.18 0.18 0.17 0.17 0.17 0.18 0.17 0.17 0.18 0.17 0.16 0.16 0.15 MgO 8.61 8.73 8.81 9.62 8.68 9.28 9.08 6.69 7.18 9.29 8.62 7.71 8.53 CaO 11.14 10.45 11.59 11.71 11.19 11.77 11.00 8.82 10.09 9.93 9.98 9.24 9.26 Na2O 2.79 2.69 2.48 2.40 2.55 2.47 2.54 3.08 2.93 3.08 2.67 3.33 2.85 K2O 0.43 0.38 0.17 0.16 0.26 0.28 0.50 1.79 1.23 0.69 1.00 1.14 0.71 P2O5 0.41 0.19 0.10 0.20 0.17 0.08 0.23 0.54 0.40 0.29 0.41 0.37 0.37 L.O.I. 0.86 1.46 0.65 0.39 0.80 0.58 0.82 0.61 0.87 0.15 0.80 0.57 1.19 TOTAL 100.34 99.60 99.53 99.19 98.95 99.88 100.02 100.20 101.59 100.13 99.60 99.23 99.20

Rb 8.1 5.7 2.7 2.9 4.4 4.4 8.7 41 27 12 17 22 12 Sr 257 241 191 185 242 237 282 476 460 655 486 541 586 32 Y 33 22 20 19 23 19 22 26 30 23 26 22 24 Zr 156 86 53 53 88 67 94 198 174 117 162 144 96 Nb 15.6 10.5 3.3 3.4 7.3 6.0 13.7 43.6 32.3 17.9 - - 36.9 - - Ni 130 127 147 190 137 112 140 86 85 156 118 138 166 Ga 20.2 17.5 18.4 17.0 18.7 18.0 18.3 17.9 18.5 18.1 - - 19.4 - - Cu 79 85 84 106 79 79 74 51 63 65 - - 57 - - Zn 92 72 71 74 75 67 77 83 95 84 - - 95 - - U

87Sr / 86Sr 0.70541 0.70476 0.70523 0.70534 0.70435 0.70498 0.70479 0.70390 0.70486 0.70382 0.70398 0.70439 0.70385 143Nd / 144Nd 0.512605 ------0.512841 0.512699 0.512850 0.512804 0.512711 0.512836 206Pb / 204Pb 18.709 - - 18.884 ------18.644 18.765 18.957 18.838 18.829 18.794 207Pb / 204Pb 15.634 - - 15.618 ------15.579 15.581 15.578 15.567 15.596 15.600 208Pb / 204Pb 39.009 - - 38.758 ------38.640 38.710 38.545 38.561 38.673 38.545

Age (Ma) 1.49 1.25 0.91 0.65 0.44 0.43 0.36 0.25 <0.03 <0.15 0 0 0

Figure 2-2. MgO-TiO2-K2O ternary diagram depicting Owyhee Plateau region basalt chemical types. SB & OIG, Steens Basalt and Oregon-Idaho Graben basalts (17-11 Ma); HAOT, high-alumina olivine tholeiite (<11 Ma); TB, transitional basalt (<11 Ma); SROT, Snake River Plain-type olivine tholeiite (<11 Ma); AB, alkaline basalt (<0.25 Ma).

33 MgO x 0.5

HAOT

TB

SROT AB

SB & OIG

TiO2 x 1.5 KO2 x 5

34

Figure 2-3. (a.) Variations in K2O/MgO with eruptive age, illustrating the strongly differentiated nature of basalts in the 17-11 Ma age group, and the more alkaline nature of the <0.25 Ma AB. (b.) Variations in TiO2 with eruptive age. The dashed lines delineate the limits of HAOT, TB, and SROT, and only apply to the 11-0 Ma tholeiites (i.e., Group 2). Symbols are age groups defined in the text.

35 3 a. 5

2 x

) O g M

/ 1

O 2 K ( G 1 I

17 - 13 Ma p 0 O

u & o

r B

13 - 11 Ma G S T

11 - 5 Ma O R S 2

-

5 - 0 Ma p B u T o 4 r -

G variable age T O

(< 11 Ma) A H 3 b. p B u A <0.25 Ma o r G 3

SROT 2

O 2 i

T TB

1 HAOT

0 0 3 6 9 12 15 18 Age (Ma)

36 and Carlson, 1983, 1985; Hart et al., 1984) age determinations has allowed the construction of a composite chronostratigraphic section representing the history of basaltic volcanism on the Owyhee Plateau over the last approximately 17 million years. Eleven new 40Ar/39Ar method age determinations (see Appendix 2D) were obtained in order to help clarify ambiguous stratigraphic relationships and better constrain the timing of eruptive activity; this was especially necessary on the southern Owyhee Plateau, where exposures of flow-on-flow basalt are limited. Age estimates were interpolated for samples with relatively tight (approximately 2 m.y. or less) stratigraphic constraints, based on 40Ar/39Ar or K-Ar data; those estimated ages are indicated in Appendix 2B. The sample suite is divided into three primary groups on the basis of basalt geochemical types. Group 1 consists of Steens Basalt (SB) and Oregon-Idaho Graben basalts (OIG), which were erupted between 17-11 Ma. Group 2 consists of HAOT, TB, and SROT lavas erupted after 11 Ma. Group 3 consists of the young, <0.25 Ma alkaline basalts (AB). The sample suite is further divided based on eruptive ages; these subgroups are described below and keyed to Figure 2-3. Group 1 (17-13 Ma): The initiation of flood basalt activity throughout the Oregon Plateau is represented by the Steens Basalt, which is temporally correlative with the Clarkston subgroup of the Columbia River Basalt (Hart and Carlson, 1985; Hart et al., 1989; Hooper and Hawkesworth, 1993; Brueseke and Hart, in prep.). These magmas, erupted both from isolated fissures and large fissure systems, are strongly fractionated tholeiitic basalts to basaltic andesites. Group 1 (13-11 Ma): This interval represents the waning stages of the Steens Basalt eruptions. These basalts are compositionally similar to those of the older group, but the volumes erupted are significantly smaller and individual flows are less areally extensive (Hart and Mertzman, 1982; Hart and Carlson, 1985). Also during this interval, basalts were erupted as extension in the Oregon-Idaho Graben progressed (Cummings et al., 2000). Group 2 (11-5 Ma): This interval marks the regional change from the strongly fractionated basalts and basaltic andesites of the Steens Basalt to relatively unfractionated olivine tholeiites. This change is accompanied by a change in eruptive style, from primarily fissure-fed eruptions to a combination of eruptions

37 from small shield cones and perhaps small fissures associated with local extensional features. Eruptions occurred throughout the entire Owyhee Plateau. Compositions of basalts erupted at this time range from primitive low-K, low-Ti, high alumina olivine tholeiites (HAOT) to high-K, high-Ti Snake River Plain- type tholeiites (SROT), with a full spectrum of compositions intermediate between these endmembers (Hart et al., 1984; Hart, 1985). Given the plateau- wide distribution of eruptive centers and the thick flow units exposed in the canyons of the Owyhee River and its tributaries, this interval is interpreted to be the period of greatest output of HAOT through SROT lavas on the Owyhee Plateau. Group 2 (5-0 Ma): This interval is marked by a widespread suppression of eruptive activity throughout the Owyhee Plateau. Between ca. 4.5 Ma and 3.5 Ma, small volume HAOT to TB eruptions were limited to the margins of the plateau; after an apparent magmatic hiatus, HAOT to TB eruptions on the Owyhee Plateau resumed ca. 2 Ma, but were limited to only the extreme northernmost regions of the plateau. Group 2 (variable age, <11 Ma): Numerous HAOT through SROT samples are included in this study for which eruptive ages cannot be tightly constrained. However, field relationships, such as stratigraphic position of flows above materials of known age (e.g., other basalts in this study, and rhyolites of the Juniper Mountain volcanic center; Manley and McIntosh, 2002), and the close proximity and similar geomorphic expression of low shield vents to other shields of known age, point to a younger age for these basalts. Additionally, many of these lavas appear to have been erupted from low shield vents, and the lack of any recognized constructional shield vents on the Owyhee Plateau older than 11 Ma indicates that the basalts of this group must have been erupted after the regional change from the large volume, fissural eruptions of Steens Basalt to the smaller volume tholeiite eruptions, ca. 11 Ma. Group 3 (<0.25 Ma): The most recent interval of Owyhee Plateau volcanism consists of small volume eruptions of mildly alkaline to mildly subalkaline basalt, from low shields and tephra cones near the northernmost margin of the Owyhee Plateau

38 (Hart and Mertzman, 1983; Russell et al., 1988; Hart, 1996). This group includes basalts representing the only true alkali olivine basalts observed on the Oregon and Owyhee Plateaus. Related lavas range to mildly subalkaline compositions; taken as a group, these mildly alkaline to mildly subalkaline basalts are compositionally distinct from the earlier erupted tholeiites. Previous authors (e.g., Otto and Hutchinson, 1977; Hart and Mertzman, 1983; Russell et al., 1988; Hart, 1996) have referred to all lavas of this group as alkali olivine basalts; however, to avoid any potential confusion with regard to petrogenetic interpretations, as well as to simply distinguish them from the older subalkaline tholeiites, I will refer to them simply as alkaline basalts (AB).

Petrography Newly collected and previously undescribed samples were examined petrographically; detailed descriptions are presented in Appendix 2A. Nearly all of the samples in this study are characterized by assemblages of plagioclase, clinopyroxene, olivine, and oxides. In the following, the basalts are discussed in the context of the three primary chronostratigraphic groups described above. Group 1 (17-11 Ma Steens Basalt and Oregon-Idaho Graben basalts): This group comprises only 16 samples, but exhibits great textural diversity. Most are holocrystalline, although some are extraordinarily fine-grained (grain size <<0.25 mm) and a few appear to be hypocrystalline. They range in color from medium gray to black and typically have dense, subophitic, intergranular to intersertal matrices containing plagioclase, clinopyroxene, olivine and oxides; pervasive clay alteration is common, and secondary carbonate and chalcedony are observed. Clinopyroxene compositions range from augite to titanaugite. The most frequently observed phenocrystic phase is plagioclase (typically 2-5 mm, rarely 10 mm), which is occasionally glomeroporphyritic. Plagioclase phenocrysts are often sieved, zoned, resorbed, or some combination thereof. Olivine, if present as a phenocrystic phase, is subordinate to plagioclase, is subhedral to anhedral, and is not observed to exceed 2 mm; often it is completely altered to clay. In some cases mafic phases are so completely altered as to be unidentifiable.

39 Group 2 (11-0 Ma HAOT-TB-SROT): This group is by far the largest with 147 samples, and, given the spectrum of compositions from low-K, low-Ti HAOT through high-K, high-Ti SROT, exhibits some diversity; however, each chemical type does exhibit a set of distinctive characteristics. They range in color from light/medium gray (HAOT) to dark gray/black (SROT). All are holocrystalline or nearly so. Endmember HAOT have distinctive subophitic and diktytaxitic textures resulting in what can be described as an “open texture” and a somewhat “sugary” appearance in hand sample. Though commonly equigranular and non- porphyritic, HAOT do occasionally exhibit subhedral to anhedral olivine phenocrysts up to 3 mm and plagioclase phenocrysts 3 mm in length; in some cases plagioclase ± olivine occurs in glomeroporphyritic clumps. Normal zoning is observed in some of the larger plagioclase phenocrysts. Clinopyroxene is augitic and occurs only in the groundmass as an intergrowth with plagioclase. Oxides occur as intergranular grains and as inclusions in olivine. Clay alteration and secondary carbonate and chalcedony are observed. Olivine is frequently iddingsitized, and in some cases has been completely oxidized. TB, true to their transitional nature, share some characteristics with HAOT. However, they are more commonly porphyritic (or glomeroporphyritic) than HAOT; they tend to have more clinopyroxene, which may occur as larger grains (up to 2 mm) in the groundmass and which may ophitically enclose plagioclase; and the clinopyroxene ranges to Ti-rich compositions. SROT are typically subophitic/ophitic with more massive, non-diktytaxitic textures; they range from equigranular to porphyritic with olivine (up to 3 mm) and plagioclase (5 mm or more) phenocrysts. Clinopyroxene is Ti-rich. Clay alteration and secondary carbonate and chalcedony are observed, and olivine may be altered to iddingsite or oxides. Group 3 (<0.25 Ma AB): This is the smallest group, represented by only seven samples. Most are hypocrystalline, containing small amounts of interstitial glass. They display a range of textures from ophitic/subophitic and diktytaxitic to intergranular or intersertal. The groundmass constituents are plagioclase, olivine, clinopyroxene, oxides, and glass, with secondary carbonate, chalcedony, and clay.

40 Clinopyroxene is Ti-rich and occasionally is observed as microphenocrysts up to 1 mm. Plagioclase phenocrysts are up to 5 mm in length; in some samples, they are resorbed. Olivine phenocrysts are up to 5 mm in size, are euhedral to anhedral, and often contain abundant inclusions of oxides. The olivine ranges from fresh to strongly altered to iddingsite or oxides.

Major and trace element geochemistry The samples presented in this study are dominantly basalts as defined by Le Bas et al. (1986), although a few of the Group 1 samples representing the older (pre-11 Ma) pulses of eruptive activity plot in the basaltic field (Figure 2-4). The samples are also dominantly subalkaline as defined by Irvine and Baragar (1971), with the exception of a few Group 1 and Group 3 samples. The higher proportion of alkalis in the older lavas can be explained as a consequence of their more evolved compositions, whereas the higher proportion of alkalis in the younger lavas is a true reflection of their mildly alkaline nature. CIPW method normative mineralogies have been calculated for the samples in this study, with total Fe divided according to the method described by LeMaitre (1976); these norms are presented in Appendix 2C. Figure 2-5 illustrates the relationship between the degree of differentiation as indicated by Mg# [Mg# = Mg2+/(Mg2++Fe2+)] and the normative composition. While the method used to apportion Fe will obviously have some bearing on the calculated norms (e.g., LeMaitre’s method tends to favor hypersthene and quartz normative compositions), the figure nonetheless illustrates the strongly tholeiitic nature of Owyhee Plateau basaltic eruptive products: few samples have normative nepheline (and in all cases it is <1%), while nearly the entire sample suite has >5% normative hypersthene. Further, the figure underscores the relatively undifferentiated nature of the Group 2 and 3 (i.e. post-11 Ma) lavas, nearly all of which have Mg# >50, and many of which have Mg# >60 over a wide range of normative hypersthene contents. In contrast, the older Group 1 lavas show a general trend toward somewhat less hypersthene-normative compositions with increasing degree of differentiation.

41

Figure 2-4. Total alkalies vs. silica diagram showing classification of Owyhee Plateau basaltic rocks in this study, after Le Bas et al (1986). Alkaline-subalkaline division is from Irvine and Baragar (1971). In this and in all plots of major element data presented in this study, analyses have been normalized to 100% anhydrous, with total Fe split according to the method described by Le Maitre (1976).

42 8

Basaltic e trachy- lin ka ne andesite Al li lka ba Trachy- Su 6 basalt O

2 Andesite K

+ 4 Basaltic G 1 I

O andesite

17 - 13 Ma p O

u 2 & o

r B

13 - 11 Ma G S a N T

11 - 5 Ma O R S 2

2 -

5 - 0 Ma p B u T o

r -

Basalt G variable age T O

(< 11 Ma) A H 3

p B u A <0.25 Ma o r 0 G 44 48 52 56 60

SiO2

43

Figure 2-5. Relationship between differentiation and alkalinity in Owyhee Plateau basalts. Normative mineralogies are CIPW norms calculated with total Fe split according to the method of Le Maitre (1976). Samples are identified by (a.) age groups as in Figure 2-3 and (b.) chemical types as in Figure 2-2.

44 a. 60

50 # g M G 1 I

17 - 13 Ma p O

40 u & o

r B

13 - 11 Ma G S T

11 - 5 Ma O R S 2

-

5 - 0 Ma p B u T o

r -

30 G variable age T O

(< 11 Ma) A H 3

p B u A <0.25 Ma o r G

b. 60

50 # g

M 40

SB & OIG 30 SROT TB HAOT AB 5 0 5 10 15 20 25 Ne Hy

45 Figures 2-6 and 2-7 illustrate the relationships between selected major and trace elements and MgO. Like Mg# in Figure 2-5, MgO is used in these figures as an indicator of the overall degree of differentiation. Given the wide distribution of vent locations and range in eruptive ages of these basalts (i.e., Figures 2-1 and 2-3), there is no reason to consider any apparent trends between age groups to be indicative of differentiation from a common parental magma.

Major element variations, such as decreasing CaO and increasing TiO2, Na2O,

K2O, and P2O5 with decreasing MgO clearly indicate the more evolved nature of the Group 1 (pre-11 Ma) basalts. Also, the less differentiated nature of nearly all members of the variable age basalt group lends additional support to the idea that these basalts are

younger than 11 Ma. The wide ranges in Al2O3, TiO2, FeO*, CaO, Na2O, K2O, and P2O5 at higher MgO contents suggest that variable depths and degrees of partial melting, heterogeneous source lithologies, or both have exerted a strong control on these basalt geochemical signatures. Note, the single sample containing >12 wt % MgO appears to be an olivine cumulate, and is excluded from subsequent discussion. The trace element characteristics indicate the importance of fractional crystallization in the evolution of these basalts. Notably, the correlated decreases in Ni and MgO suggest that olivine fractionation was common, while decreases in Sc and MgO in the Group 1 samples imply that clinopyroxene fractionation was an important process during the prolonged crystallization experienced by pre-11 Ma basalts. Variations in the large ion lithophile (Ba, Sr), high field strength (Zr, Nb), and light rare earth (La) elements are also consistent with variable degrees of crystallization, although the scatter in Ba and Zr above approximately 8 weight percent MgO indicates that multiple geochemical reservoirs likely exerted some control on the overall geochemical patterns. The Sr versus MgO plot reveals that the sample suite is divided into both high (>400 ppm) and low (<400 ppm) Sr groups. While the presence of high Sr in the pre-11 Ma Group 1 basalts is consistent with their strongly differentiated character, the presence of high Sr (approximately 650 ppm) and low Sr (approximately 200 ppm) lavas at identical high MgO (approximately 9 weight percent) contents in the post-11 Ma Group 2 and 3 basalts is more intriguing, and suggests an important role for partial melting or

46

Figure 2-6. Major element variations with MgO content; all oxides reported as wt%.

47 58 13

54 11 2 O O a i C S 50 9

46 7

4 3 2 O 2 O 2 3 i a T N 1 2

17 2

3 16 O O 2 2 1 l K

A 15

14 0

14 1.0 12 * 5 O 0.5 O 2 e P F 10 0.0 8

3 6 9 12 3 6 9 12 MgO MgO

Group 2 Group 1 HAOT - TB - SROT Group 3 SB & OIG AB 11 - 5 Ma 17 - 13 Ma 5 - 0 Ma <0.25 Ma 13 - 11 Ma variable age (< 11 Ma)

48

Figure 2-7. Selected trace element variations with MgO content; all trace elements reported in ppm.

49 300 40 i 200 30 c N S

100 20

700 2000

500 r a 1000 S B 300

0 100

60 60

40 a 40 Y L 20

20 0

40 300 r 200 20 b Z N

100 0

3 6 9 12 3 6 9 12 MgO MgO

Group 2 Group 1 HAOT - TB - SROT Group 3 SB & OIG AB 11 - 5 Ma 17 - 13 Ma 5 - 0 Ma <0.25 Ma 13 - 11 Ma variable age (< 11 Ma)

50 source composition complexities. The younger high Sr suite consists of the young Group 3 (AB) lavas identified previously. The NMORB-normalized spider diagrams in Figures 2-8 and 2-9 depict the combined incompatible element characteristics of Owyhee Plateau basalts. Figure 2-8 depicts all of the samples in this study. Figure 2-9a depicts the average incompatible element characteristics of each chemical type, with the exception of TB. Figure 2-9b depicts the incompatible element characteristics of relatively unfractionated

representatives (that is, samples which display high Ni, Cr, and Mg#, and low SiO2) of each chemical type, again with the exception of TB. Despite differences in major element chemistry (i.e. Figures 2-2 and 2-3), the samples have similar overall trace element patterns characterized by strong enrichments in large ion lithophile elements (LILE), especially Ba, and with the exception of the Group 3 AB lavas, negative Nb anomalies. In Figure 2-9, the chemical type with the most MORB-like affinities from La through Y (HAOT) has the lowest concentrations of all elements shown, but still has Sr, K, Rb, and most notably Ba well in excess of NMORB. The Group 1 and Group 3 lavas are more enriched in LILE, especially Sr, K, and Rb, than the Group 2 basalts. Although similar patterns are observed between groups, the absolute abundances, as well as many incompatible element ratios, appear to vary throughout the history of the Owyhee Plateau. Such features suggest that similar heterogeneous magma sources and differentiation processes have been involved in the evolution of Owyhee Plateau basaltic magmatism, but to varying degrees as a function of time.

Isotope geochemistry A cursory examination of initial isotopic ratios of Owyhee Plateau basalts reveals an intriguing relationship: the basalts exhibit a wide range in initial 87Sr/86Sr values, which clearly varies as a function of eruptive age (Figure 2-10). The most radiogenic Sr (>0.7065) occurs in those Group 2 basalts erupted during the interval 11-5 Ma; this is significant, as it coincides with (1) the change in character of the dominant basalt chemical type erupted, from strongly fractionated to less fractionated, (2) the change in eruptive style from large volume, fissure fed eruptions to small volume eruptions from

51

Figure 2-8. NMORB-normalized spider diagrams of trace element characteristics of all basalts in this study. Abbreviations are as in Figure 2-2.

52 HAOT SB & OIG 100 100 B B R R O O

M 10 10 M N N

/ / k k c c o 1 1 o R R

0.1 0.1 Sr K Rb Ba Nb La Ce P Zr Ti Y Sr K Rb Ba Nb La Ce P Zr Ti Y

TB AB 100 100 B B R R O O

M 10 10 M N N

/ / k k c c o 1 1 o R R

0.1 0.1 Sr K Rb Ba Nb La Ce P Zr Ti Y Sr K Rb Ba Nb La Ce P Zr Ti Y

SROT 100 B R O

M 10 N

/ k c o 1 R

0.1 Sr K Rb Ba Nb La Ce P Zr Ti Y

53

Figure 2-9. NMORB-normalized spider diagrams showing trace element characteristics of (a.) average SB & OIG, HAOT, SROT, and AB; and (b.) relatively little fractionated SB & OIG, HAOT, SROT, and AB samples.

54 SB & OIG a. HAOT 100 SROT AB B R O

M 10 N

/ k c o

R 1

average compositions

0.1 Sr K Rb Ba Nb La Ce P Zr Ti Y

b. 100 B R O

M 10 N

/ k c o

R 1

“least fractionated”

0.1 Sr K Rb Ba Nb La Ce P Zr Ti Y

55

Figure 2-10. Variations in Sr and Nd isotope compositions of Owyhee Plateau basalts with age of eruption. 2σ uncertainties based on long-term reproducibility of standards. Samples are plotted by chemical types to emphasize that the isotopic composition is decoupled from the bulk chemistry. Note that the maximum in 87Sr/86Sr and the minimum in 143Nd/144Nd coincide at ca. 11 Ma, coincident with the regional change to small volume, less differentiated basalt eruptions and the onset of major lithospheric extension.

56 2s SB & OIG SROT 0.707 TB HAOT AB

r 0.706 S 6 8

/

r S

7 0.705 8

0.704

0.5128

d 0.5127 N 4 4 1

/ 0.5126

d N 3 4

1 0.5125

0.5124 2s

0 3 6 9 12 15 18 Age (Ma)

57 discrete vents, and (3) the onset of diffuse extension and lithospheric thinning throughout the entire Oregon Plateau-northwestern Great Basin region, all of which occurred beginning ca. 11 Ma (Carlson and Hart, 1987, 1988; Hart and Carlson, 1987, 1992; Draper, 1991; Shoemaker and Hart, 2003). Sr isotope ratios systematically decrease from this maximum to values less than or equal to 0.704 in both the youngest and oldest basalts. Furthermore, the peak in 87Sr/86Sr values appears to be decoupled from bulk geochemistry, as HAOT, TB, and SROT all exhibit equally radiogenic signatures. The pattern in the 143Nd/144Nd vs. age (Figure 2-10) complements the 87Sr/86Sr vs. age pattern; that is, the highest 87Sr/86Sr occurs in rocks with low 143Nd/144Nd, and vice versa. An interesting contrast is observed in the Pb isotope systematics of Owyhee Plateau basalts (Figure 2-11). While the patterns observed in 207Pb/204Pb and 208Pb/204Pb vs. age generally mimic that of 87Sr/86Sr vs. age, 206Pb/204Pb exhibits a strikingly different pattern: the 206Pb/204Pb values of the Group 2 tholeiites are lower than those of most of the the Group 1 or 3 lavas, and in contrast to the observed decreases in 207Pb/204Pb and 208Pb/204Pb between 11-0 Ma in the Group 1 and 3 lavas, the Group 2 lavas maintain relatively constant 206Pb/204Pb values. Regardless of any specific cause, the decoupling of bulk geochemical characteristics from isotopic characteristics clearly suggests a complex interplay of heterogeneous magma sources which has varied through time.

DISCUSSION Constraints on magma source characteristics Central to understanding the origin and evolution of any continental intraplate basalt province are a number of questions: What mantle materials are melting, and to what degree? In what mantle reservoirs (lithosphere? asthenosphere? plume?) do these materials reside? In what proportion are these melts or mantle sources mixing, and how does this vary through time? Does the upper or lower continental crust contribute geochemically or isotopically to these basalts? To what extent have shallow-level differentiation processes modified the composition of the primary basaltic magmas? The data presented so far reveal time-dependent but often decoupled variations in Owyhee Plateau basalt geochemical and isotopic parameters; the following discussion will

58

Figure 2-11. Variations in Pb isotope compositions of Owyhee Plateau basalts with age of eruption. 2σ uncertainties based on long-term reproducibility of standards. Note that the overall patterns in 207Pb/204Pb and 208Pb/204Pb generally agree with the Sr and Nd isotope patterns in Figure 2-10, but that 206Pb/204Pb appears decoupled from these trends. Symbols are chemical types as defined in Figure 2-10.

59 19.0

b 18.8 P 4 0 2

/

b P

6 18.6 0 2

18.4 2s

15.66 2s 15.64 b P 4 0

2 15.62

/

b P 7

0 15.60 2

15.58

2s 39.2

b 39.0 P 4 0 2

/

b 38.8 P 8 0 2 38.6

38.4

0 3 6 9 12 15 18 Age (Ma)

60 attempt to reconcile these variations and provide qualitative constraints on the above questions. Late Cenozoic basaltic volcanism in the northwestern United States has been attributed by different workers to both deep mantle (plume) and upper mantle sources and processes. A review of the proposed mantle components, then, provides a necessary context for the discussion of Owyhee Plateau basaltic magmatism to follow. Figure 2-12 depicts the ocean-island basalt mantle components EM1, EM2, and HIMU (Zindler and Hart, 1986, and references therein), and the northwestern United States mantle components C1, C2, and C3 (Carlson, 1984) in the context of mid-ocean ridge basalt, ocean-island basalt, Pacific Ocean sediment, and Wyoming Craton subcontinental lithospheric mantle compositions. The ocean-island basalt mantle components are assumed to represent long-term recycling of lithospheric materials through the mantle via convection: ancient subducted oceanic crust (HIMU; Hofmann and White, 1982), upper continental crust cycled into the mantle in the form of pelagic and terrigenous sediments (EM1 and EM2, respectively; Weaver, 1991), and possibly delaminated subcontinental lithospheric mantle (EM1 and EM2; McKenzie and O’Nions, 1983 and others). These sources are well established and widely accepted in the current body of literature on basaltic volcanism, and the involvement of these sources in basaltic volcanism has traditionally been taken as evidence for deep mantle plumes (Carlson, 1995; Hofmann, 1997). The northwestern US mantle components were first defined by Carlson (1984), and elaborated by Carlson and Hart (1987, 1988). Rather than invoke plumes, they call on source materials and processes in the upper mantle. C1 is an incompatible element- depleted source similar to the MORB source in the northern Pacific, and C2 is interpreted to be mantle originally similar to C1, which experienced sediment-derived contamination as a consequence of Farallon-Juan de Fuca plate subduction during the Mesozoic and Cenozoic. Such a mechanism of enrichment is consistent with the strong LILE enrichments observed in Owyhee Plateau lavas (Figures 2-8 and 2-9), a characteristic which has been suggested to indicate a back-arc basin eruptive setting (Hart et al., 1984; Hart, 1985). The location of the Owyhee Plateau inboard of an active subduction margin since the Mesozoic and the presence of pre-17 Ma arc-related volcanics in both the Santa

61

Figure 2-12. Pb isotope characteristics of the ocean island basalt mantle components EM1, EM2, and HIMU (Zindler and Hart, 1986) and the northwestern United States mantle components C1, C2, and C3 (Carlson, 1984), in the context of observed MORB, ocean island basalt (OIB), Pacific Ocean trench sediments (SED), and Wyoming Craton subcontinental lithospheric mantle (SCLM) compositions. NHRL: Northern Hemisphere Reference Line (Hart, 1984). MORB and OIB compositions are from Zindler and Hart (1986). Sediment compositions are calculated bulk sediment averages for Pacific Ocean trenches from Plank and Langmuir (1998). SCLM compositions are from spinel peridotite, pyroxenite, and glimmerite xenoliths from Montana, reported in Carlson and Irving (1994).

62 EM2 15.80 HIMU SED 15.75 RL NH

b 15.70 SCLM P 4

0 C3

2 15.65

/

C2

b 15.60 P 7 0

2 15.55 C1 OIB 15.50 EM1 15.45 MORB

L HIMU 40.5 R NH 40.0

b 39.5 EM2 P

4 C3 0 SCLM 2

/

39.0 C2 OIB b P

8 38.5 0 2 EM1 C1 SED 38.0

37.5 MORB

17.5 18.0 18.5 19.0 19.5 20.0 20.5 21.0 206Pb / 204Pb

63 Rosa and Owyhee Ranges, adjacent to the Owyhee Plateau, lends support to this interpretation of C2. The similarity of C1 and C2 to observed Pacific MORB and sediment compositions, respectively, further supports these interpretations and justifies their consideration here. C3 is interpreted to be a source in the subcontinental lithospheric mantle which became incompatible element-enriched through the addition of incompatible element-rich fluids or melts during the late Archean (Carlson, 1984; Carlson and Hart, 1987, 1988). This interpretation is similar to those reached by Leeman (1975, 1982a), Menzies (1983), and Reid (1997) regarding the source of SROT. While a paucity of mantle xenoliths in the Owyhee Plateau region precludes any definitive conclusions about the nature of the subcontinental lithospheric mantle beneath the Owyhee Plateau, the wide range of observed Wyoming Craton mantle xenolith isotopic compositions for locations to the east and the presence of basaltic lavas with true SROT compositions on the Owyhee Plateau justifies consideration of the C3 component as a potential source here. The combined isotopic characteristics of Owyhee Plateau basalts are shown in Figures 2-13, 2-14, and 2-15, in the context of other northwestern United States basalts, northern Pacific Ocean normal mid-ocean ridge basalt (NMORB), Pacific Ocean sediments, the ocean-island basalt components (EM1, EM2, and HIMU), and the northwestern US basalt mantle components (C1, C2, and C3). On the plot of 143Nd/144Nd versus 87Sr/86Sr (Figure 2-13), Owyhee Plateau basalts, as well as Columbia River Basalts (CRB), western Oregon Plateau HAOTs, and most Snake River Plain basalts (SRP), fall on an apparent mixing curve between NMORB and Pacific Ocean sediment. The oldest (Group 1) and youngest (Group 3) Owyhee Plateau basalts trend toward more NMORB-like isotopic compositions, while the Group 2 basalts erupted during the interval 11-5 Ma have Sr- and Nd-isotope characteristics more similar to sediment. These 11-5 Ma Group 2 basalts plot between C3 and the ocean island basalt endmembers EM1 and EM2, potentially suggesting some manner of sediment-derived enrichment of a subcontinental lithospheric mantle source for these basalts, although on the basis of these data alone it is not clear if the enrichment was related to upper mantle (i.e., flux from the subducting slab) or deeper mantle (i.e., plume metasomatism of the subcontinental lithospheric mantle) processes. Nearly all of the Group 1 and Group 2

64

Figure 2-13. Plot of 143Nd/144Nd vs. 87Sr/86Sr for Owyhee Plateau basalts. Other fields provided for reference. NMORB: Juan De Fuca Ridge, East Pacific Rise, and Galapagos Ridge basalts (Hegner and Tatsumoto, 1987; Ito et al, 1987); SED: Pacific Ocean trench sediments (Plank and Langmuir, 1998); CRB: Columbia River Basalt (Carlson et al, 1981; Carlson, 1984; Hooper and Hawkesworth, 1993; Lambert et al, 1995); SRP: Snake River Plain basalts (Leeman and Manton, 1971; Noble et al, 1973; Mark et al, 1975; Leeman, 1982; Leeman et al, 1992; and Leeman, unpublished data); HAOT: western Oregon Plateau high alumina olivine tholeiites (Hart, 1985). HIMU, EM1, and EM2 ocean island basalt mantle components from Zindler and Hart (1986). C1, C2, and C3 northwestern United States mantle components from Carlson (1984).

65 0.5135 G 1 I

17 - 13 Ma p O

u & o

r B

13 - 11 Ma G NMORB S T

11 - 5 Ma O R S 2

-

5 - 0 Ma p B u T o

r -

G CRB variable age T C1 O (< 11 Ma) A HIMU H

d 0.5130 3

p B u A N <0.25 Ma o r 4 G 4 1

/

HAOT C2 d

N EM2 3

4 SED

1 0.5125

SRP EM1 C3

0.5120 0.700 0.705 0.710 0.715

87Sr / 86Sr

66 basalts define an array between the C2 and C3 mantle components, while the young Group 3 basalts, although still plotting near C2, mostly fall between C2 and C1. Clearly different mantle sources have been involved in the generation of Owyhee Plateau region basaltic magmas, and the observed relationships suggest that these sources have interacted variably as a function of time. The Pb isotope systematics (Figure 2-14) provide additional indications of the nature of the mantle sources for Owyhee Plateau basalts. With a single exception, all of the data fall within a triangular field defined by the C1, C2, and C3 components. As in Figure 2-13, the 11-5 Ma Group 2 basalts plot near C3, while the older 17-13 Ma Group 1 basalts have Pb isotope characteristics most similar to C2 and the very young Group 3 basalts are most similar to C1. The scatter among the remaining samples, and even within the groups just mentioned, again indicate that melts from these sources interacted variably throughout the history of Owyhee Plateau volcanism as a function of eruptive age. Plots of 87Sr/86Sr and 143Nd/144Nd versus 206Pb/204Pb (Figure 2-15) further substantiate these observations and underscore the time-dependent interactions of these sources. While it is true that nearly all Owyhee Plateau basalts fall within the oceanic array, and could therefore be attributed to a deep mantle plume, the close similarity of Owyhee Plateau basalt isotope characteristics to the northwestern US mantle components defined by Carlson (1984) are consistent with sources in the upper mantle: a source in the subcontinental lithospheric mantle (C3) for the 11-5 Ma Group 2 basalts; a source similar to the MORB source (C1) for the Group 3 AB; and a subduction-modified MORB-like source (C2) for the initial 17-13 Ma Group 1 flood basalts. While a deep- mantle plume cannot be irrefutably excluded, it does not appear to be required in the Owyhee Plateau region. In Figures 2-14 and 2-15, the oldest (Group 1, 17-13 Ma) and some of the youngest (Groups 2 and 3, <5 Ma) basalts plot within or near the field for western Oregon Plateau HAOT. Western Oregon Plateau HAOT are specified here to represent those HAOT unaffected by passage through or melting of the ancient (Wyoming Craton) subcontinental lithosphere. Hart et al. (1997), on the basis of Os isotopic data, interpreted primitive HAOT compositions from throughout the northwestern United States to represent MORB-like primary melts of depleted mantle, contaminated with minor

67

Figure 2-14. Plots of 207Pb/204Pb and 208Pb/209Pb vs. 206Pb/204Pb for Owyhee Plateau basalts. Other fields, mantle components, and Owyhee Plateau basalt symbols are as described in Figure 2-13.

68 15.8 EM2 CRB U IM 15.7 SED H C3 b P

4 SRP 0 C2 2

/ 15.6

b P 7

0 C1 2

15.5 HAOT EM1 NMORB

15.4 40.0

C3 EM2 U CRB IM H 39.0 b

P SRP C2 4 0 2

/

b C1 P EM1 8 0

2 38.0 SED HAOT NMORB

37.0 17.0 17.5 18.0 18.5 19.0 19.5 206Pb / 204Pb

69

Figure 2-15. Plots of 87Sr/86Sr and 143Nd/144Nd vs. 206Pb/204Pb for Owyhee Plateau basalts. Other fields, mantle components, and Owyhee Plateau basalt symbols as described in Figure 2-13.

70 0.715 SED

0.710 r SRP S 6 8

EM2

/ C3

r S 7

8 EM1 0.705 C2 CRB HIMU C1 NMORB HAOT 0.700 0.5135 NMORB HAOT

d 0.5130 CRB C1 HIMU N 4 4

1 SED

/ C2

d SRP N 3 4

1 0.5125 EM2

EM1 C3

0.5120 17.0 17.5 18.0 18.5 19.0 19.5 206Pb / 204Pb

71 amounts of lithospheric mantle materials less mafic than peridotite. The isotopic affinity of the oldest and youngest Owyhee Plateau basalts with the C1 and C2 (depleted mantle) components, and the general similarity between the oldest and youngest Owyhee Plateau basalts and western Oregon Plateau HAOT isotopic characteristics again are consistent with the sources for Owyhee Plateau basaltic volcanism residing in the upper mantle. The higher 87Sr/86Sr, 207Pb/204Pb, and 208Pb/204Pb, and lower 143Nd/144Nd of the 11-5 Ma Group 2 Owyhee Plateau basalts, which include both depleted mantle (HAOT) and enriched mantle (SROT) products, likely reflect a complex interplay between shallow sublithospheric and subcontinental lithospheric mantle. Combined elemental and isotopic data further elucidate the sources and processes contributing to Owyhee Plateau volcanism. Figure 2-16 further emphasizes the decoupling between whole-rock isotopic and elemental geochemical characteristics. The plot of 87Sr/86Sr versus Mg# [Mg# = Mg2+/(Mg2++Fe2+)] illustrates two important features of the data set: first, that the widest range in Sr-isotopic compositions is observed in the least fractionated basalts, and second, that the lowest 87Sr/86Sr signatures occur both in relatively unfractionated (Group 3) and in strongly fractionated (Group 1) basalts. As was previously noted, 143Nd/144Nd, 207Pb/204Pb, and 208Pb/204Pb characteristics generally agree with the Sr-isotope systematics, while 206Pb/204Pb appears “decoupled” from the other isotopic systems. While there is still moderate spread in 206Pb/204Pb at Mg# >50, the more fractionated samples have generally higher 206Pb/204Pb than is observed in less fractionated samples. One possibility is that the lower Mg# of Group 1 basalts could be caused by assimilation of chemically evolved but isotopically primitive lower crustal rocks; this is examined in Figure 2-17. While extreme isotopic compositions are observed in the Archean intermediate to felsic granulite lower crustal xenoliths from the Snake River Plain reported by Leeman et al. (1985), the Owyhee Plateau basalts plot completely outside the xenolith field, to much higher 206Pb/204Pb values than are observed for the lower 87Sr/86Sr xenoliths. Furthermore, among the Owyhee Plateau lavas the Group 1 basalts plot the furthest from the xenolith field. If, on the other hand, this isotopic behavior is related to subducted sediment-derived fluid metasomatism of mantle source materials, as suggested for the C2 source, then the implications of the other isotopic variations become clearer: assimilation of lower crustal materials, such as those

72

Figure 2-16. Variations in 87Sr/86Sr and 206Pb/204Pb with Mg#.

73 0.707

r 0.706 S 6 8

/

r S

7 0.705 8

Group 1: SB & OIG 0.704 17 - 13 Ma 13 - 11 Ma

Group 2: HAOT - TB - SROT 11 - 5 Ma 5 - 0 Ma variable age 19.0 (< 11 Ma) Group 3: AB <0.25 Ma

b 18.8 P 4 0 2

/

b P

6 18.6 0 2

18.4

20 30 40 50 60 70 Mg#

74

Figure 2-17. 87Sr/86Sr and 206Pb/204Pb of Owyhee Plateau basalts relative to range of observed 87Sr/86Sr and 206Pb/204Pb of Archean intermediate to felsic “deep crustal” granulite xenoliths from the Snake River Plain (Leeman et al., 1985).

75 0.730

SRP

r intermediate to felsic S

6 0.720 “deep crust” 8

/

r S 7 8 0.710

0.700 13 14 15 16 17 18 19 20

206Pb / 204Pb

76 discussed above, with high 87Sr/86Sr, 207Pb/204Pb, and 208Pb/204Pb, and low 143Nd/144Nd during prolonged magma chamber residence times had minimal influence on the isotopic compositions of these basalts. Rather, the observed variations are more likely related to heterogeneous sampling of different mantle source reservoirs throughout the history of Owyhee Plateau volcanism. The potential for involvement of upper crustal materials can be tested by examining variations in Sr concentration and Rb/Sr with 87Sr/86Sr (Figure 2-18). The high (>400 ppm) and low (<400 ppm) Sr groups noted in Figure 2-7 are plainly evident. The low-Sr samples have the widest range in 87Sr/86Sr, from 0.7043 to 0.7079, and have Rb/Sr ratios between approximately 0.01 and 0.05. By contrast, the high-Sr suite, consisting of Group 1 (Steens and Oregon-Idaho Graben basalts) and Group 3 (AB) lavas, has a more limited range in 87Sr/86Sr and includes the least radiogenic Sr observed in Owyhee Plateau basalts, but has the widest range in Rb/Sr, from approximately 0.01 to 0.09. Nearly all of the 17-13 Ma Group 1 basalts and <0.25 Ma Group 3 AB have Sr isotopic compositions less than 0.7045 and exhibit little variation in 87Sr/86Sr with increasing Rb/Sr within their respective age groups. The negative slope of the trend of the 13-11 Ma Group 1 basalts is suggestive of source mixing between endmembers with low 87Sr/86Sr and high Sr and Rb/Sr (i.e., high-Sr sample suite) and high 87Sr/86Sr and low Sr and Rb/Sr (i.e., 11-5 Ma Group 2 basalts). The characteristics of the 11-5 Ma Group 2 basalts help to clarify the role of the felsic upper continental crust in the evolution of these basaltic magmas. The heterogeneous crust in the Owyhee Plateau region does contain lithologies with 87Sr/86Sr >0.710 (Leeman et al., 1992), and assimilation of such materials could certainly contribute to the high 87Sr/86Sr observed in the 11-5 Ma basalts. However, such a contaminant would presumably have a high Rb/Sr ratio (average upper crustal Rb/Sr = 0.32; Taylor and McLennan, 1995); the low Rb/Sr (<0.05) observed in the 11-5 Ma basalts is then in disagreement with a significant input of felsic upper crust. Given the lack of correlation between increasing 87Sr/86Sr and Rb/Sr in all age groups this conclusion can be extended to the remainder of the sample suite. While the nature of the lower crust in the Owyhee Plateau region proper is unconstrained due to a lack of observed crustal xenoliths and an absence of detailed geophysical data, the consistent ranges observed in both Sr concentrations and Rb/Sr

77

Figure 2-18. Variations in Sr concentration and Rb/Sr with 87Sr/86Sr. Symbols are age groups as in Figure 2-16.

78 600

500

r 400 S

300

200

0.08

r 0.06 S

/

b

R 0.04

0.02

0.703 0.704 0.705 0.706 0.707 0.708 87Sr / 86Sr

79 ratios in the entire suite of Group 2 basalts could potentially be reconcilable with a mafic lower crustal contaminant. However, while the lower crust in this region almost certainly contains large amounts of underplated Steens flood basalt, the lower 87Sr/86Sr (<0.7045) and higher Sr concentrations associated with the main phase (17-13 Ma) of Steens Basalt eruptions (Carlson and Hart, 1987; Hart et al., 1989; and Group 1 basalts, this study) is contrary to the more radiogenic 87Sr/86Sr (>0.7045) and lower Sr concentrations observed in nearly all of the Group 2 basalts. If Steens basalt is an unlikely contaminant, could the wide range of Sr-isotope compositions observed in the Group 2 lavas be attributed to variable degrees of contamination by a much older mafic lower crustal composition? The lower 87Sr/86Sr values observed in the younger (5-0 Ma) Group 2 basalts alone do not preclude, but certainly limit, the influence that an older mafic lower crustal composition could have exerted on the observed compositions. This possibility is examined in Figure 2-19. While considerable scatter is observed, the 5-0 Ma basalts define a general trend toward higher K/P with little variation in Rb/Sr, as might be expected if a mafic lower crustal contaminant was involved (Rudnick and Fountain, 1995). However, given the less radiogenic 87Sr/86Sr of the 5-0 Ma basalts, and the lack of any discernable trend toward higher K/P with little variation in Rb/Sr in the 11-5 Ma basalts, an older mafic lower crustal contaminant seems unlikely. While some degree of crustal interaction is certainly expected in continental volcanic systems, the main point of the preceding discussion is that the dominant cause of isotopic heterogeneity in Owyhee Plateau basalts likely resides in the mantle source regions and from variable interactions between melts (± solids) from these sources. Moreover, the absolute lack of crustal xenoliths or xenocrysts (see Petrography section) in Owyhee Plateau basalts supports the contention that crustal input was minimal and, if present, likely occurred via interactions between mafic magmas and small volume crustal melts. In this regard, it is noteworthy that silicic magmas in the Owyhee Plateau region illustrate a similar range in Sr and Nd isotopic characteristics (for example, see Leeman et al. 1992) as the basalts presented in this study. Other textural features, such as sieved or highly resorbed feldspar phenocrysts, which are commonly cited as evidence for open system processes also are lacking in all the observed basalts except for some of the Group 1 samples.

80

Figure 2-19. Variations in Rb/Sr with K/P. Upper continental crust (UCC) and lower continental crust (LCC) compositions are from Rudnick and Fountain (1995).

81 Group 1: SB & OIG 17 - 13 Ma C 13 - 11 Ma

C Group 2: 0.08 U HAOT - TB - SROT 11 - 5 Ma 5 - 0 Ma variable age (< 11 Ma) r 0.06 Group 3: AB S

<0.25 Ma / b

R 0.04 L C C 0.02

0 3 6 9 12 K / P

82 Figure 2-20 displays variations in the ratio Rb/Y with 87Sr/86Sr, 206Pb/204Pb, and Zr/Nb. Rb is a fluid mobile element; Y is essentially non-mobile. An elevated Rb/Y ratio may be indicative of several things: assimilation of upper continental crust, clinopyroxene removal during fractionation, small percent mantle melts, fluid metasomatism, or a mantle source deep enough to have garnet in its melting residue. The greatest range in 87Sr/86Sr occurs in those basalts with the lowest Rb/Y values (Group 2 basalts), while the oldest (Group 1, 17-13 Ma) and youngest (Group 3) basalts exhibit Rb/Y values as high as 1.5. The majority of Owyhee Plateau basalts fall below the average crustal value of Zr/Nb = 16.2 (Weaver, 1991). Zr and Nb are both incompatible, non-fluid mobile, high field strength elements. The elevation of Rb/Y values in Group 1 and 3 basalts without corresponding elevations in Zr/Nb or 87Sr/86Sr again argue against bulk crustal assimilation and hint at the role of fluid addition to the mantle. Inspection of the elemental data (Table 2-2) reveals that the elevated Rb/Y of the Group 1 and 3 lavas is a consequence of the Group 1 and 3 lavas having greater Rb than the Group 2 lavas, rather than having less Y; thus, a deeper, garnet-bearing source can be excluded. The wide range in Rb/Y values for the 17-13 Ma Group 1 basalts and the <0.25 Ma Group 3 AB then must reflect (1) fractionation, (2) fluid metasomatism of the source, or (3) small percent melting of the source rocks. The first possibility is quite reasonable for the 17-13 Ma lavas given their low Mg#, evolved major element compositions, and petrographic characterisitics, and the second possibility is consistent with their inferred relationship to the subduction modified C2 mantle source. The third possibility is less likely given the voluminous nature of the initial flood basalt episode versus the relatively small volumes of lava which were subsequently erupted. The young, small volume AB lavas could potentially reflect any of the three scenarios, or possibly some combination of them. Alkalic magmas are conventionally believed to originate due to lower extents of melting at higher pressures than tholeiites (Green and Ringwood, 1967; Frey et al., 1979), although it has recently been shown that they may also originate by fractionation of augite-rich assemblages from tholeiitic parent magmas at pressures of ca. 4 kbar (Naumann and Geist, 1999). Regardless, their relatively unradiogenic isotopic signatures are suggestive of a predominantly sublithospheric source.

83

Figure 2-20. Variations in 87Sr/86Sr, 206Pb/204Pb, and Zr/Nb with Rb/Y.

84 0.707

r 0.706 S 6 8

/

r

S 0.705 7 8

0.704

19.0 Group 1: SB & OIG 17 - 13 Ma 13 - 11 Ma b 18.8 Group 2: P

4 HAOT - TB - SROT 0 2

/ 11 - 5 Ma

b 5 - 0 Ma P

6 18.6 variable age 0

2 (< 11 Ma)

Group 3: AB <0.25 Ma 18.4

25

20 b N

/ 15

r Z 10

5

0.0 0.5 1.0 1.5 2.0 Rb / Y

85 Constraints on basalt petrogenesis Potential melt-generation and differentiation scenarios can be qualitatively evaluated using diagrams such as the expanded basalt tetrahedron (Figure 2-21) and the ALFE (Al-factor versus Fe-factor; Figure 2-22) projection. Normative mineral compositions of the samples in this study are shown on the expanded basalt tetrahedron in Figure 2-21. Superimposed on the expanded basalt tetrahedron are possible primary melt compositions and post-generation differentiation paths determined through experimental petrology (Thompson et al., 1983), which allow some inferences to be made regarding depth of melting and subsequent differentiation. Most of the Group 2 and Group 3 basalts plot between the 9 ± 1.5 kbar and the 1 atm olivine + plagioclase + clinopyroxene + liquid cotectics, with most of the younger (5-0 Ma) Group 2 tholeiites (HAOT and TB) and Group 3 AB plotting in or near the region of primary anhydrous mantle melts. Most of the older (11-5 Ma) Group 2 tholeiites (HAOT through SROT) plot outside, but near, this region of primary melts. Within the Group 2 basalts, the fields for TB and SROT extend toward more diopside- and hypersthene- normative compositions than HAOT, which is consistent with the interpretation of SROT as having an enriched subcontinental lithospheric mantle source (Leeman, 1982b; Menzies et al., 1984). The generally less differentiated character of most of these lavas (e.g., Mg# > 50, Ni > 100 ppm, and Cr > 150 ppm) suggests that the parental liquids experienced minimal crustal-level differentiation, an interpretation consistent with conclusions reached based on the trace element and isotope characteristics. Thus, if these bulk chemical characteristics are little modified from those of the primary mantle derived melts, a relatively shallow mantle source may be implied for the Group 2 tholeiites with a slightly, though not dramatically, deeper origin for the Group 3 AB. A lack of geophysical data (e.g., Dueker et al., 2001) leaves the thickness and structure of the lithosphere unconstrained in this region, but such an interpretation is in agreement with results of experimental studies by Bartels et al. (1991) which indicate that HAOT primary liquids are in equilibrium with an olivine + augite + orthopyroxene + spinel + plagioclase lherzolite source at 11 kbar, as well as with conventional interpretations of the origins of alkaline basalts which invoke lower degrees of melting at higher pressures than tholeiites (Green and Ringwood, 1967; Frey et al., 1979). In contrast to the Group 2 and Group 3

86

Figure 2-21. Expanded basalt tetrahedron with experimentally-derived possible primary mantle melt compositions and differentiation paths, after Thompson et al. (1983). For this figure only, normative compositions have been calculated with Fe2O3 fixed at 1.5 wt %. Insets depict range of compositions exhibited by each chemical type (left inset) and within age groups (right inset).

87 Ne Di Q G 1 I

17 - 13 Ma p O

u & o

r B

13 - 11 Ma G S region approximating 8 kbar (Di-rich) T

to 35 kbar (Ol-rich) primary melts 11 - 5 Ma O R S 2

-

from anhydrous fertile (Ne-rich) approx. 1 atm 5 - 0 Ma p B u T o

r to depleted (Hy-rich) mantle - ol + plag + cpx + liquid G variable age T cotectic O (< 11 Ma) A H 3

p B u A <0.25 Ma o r G 88

approx. 9 +/- 1.5 kbar ol + plag + cpx + liquid cotectic approx. “deep crust” plag + cpx +/-ol +/-mt + thol. andesite liquid cotectic

(Fe O fixed at 1.5 wt.%) Ol 23 Hy

SROT Group 2 (11-5 Ma) TB Group 2 (5-0 Ma) HAOT Group 3 (<0.25 Ma)

AB

SB & OIG Group 1 (17-11 Ma) basalts, the Group 1 basalts exhibit considerable scatter but define a general trend extending along the lower pressure cotectics, suggesting more extensive differentiation in magma chambers at various levels in the crust. Although part of this trend does overlap with the fields for HAOT, TB, and SROT, given their more evolved nature it is not clear where magmas parental to the Group 1 basalts may have originated.

The ALFE projection of Reid et al. (1989) and companion plots of CaO/Al2O3 versus Fe-factor (Figures 2-22 and 2-23, respectively) can be used to further evaluate the nature of differentiation of mafic magmas. The ALFE projection is an X-Y plot depicting Fe-factor (defined as molar [FeO*/(FeO*+MgO)], where FeO* has been

reduced by the amount of FeO in ilmenite) versus Al-factor (molar [(Al2O3-CaO*-Na2O-

K2O)/(FeO*+MgO)], where CaO* has been reduced by the amount of CaO in apatite), which can eliminate the effects of additions or subtractions of feldspar of any

composition. The CaO/Al2O3 versus Fe-factor companion plots can discriminate the

nature of fractionation: decreasing CaO/Al2O3 with increasing Fe-factor indicates

clinopyroxene fractionation at either high or low pressure, while increasing CaO/Al2O3 with increasing Fe-factor indicates olivine + plagioclase fractionation. Similar to what is observed on the expanded basalt tetrahedron, most of the data cluster between the 10 kbar and 1 atm cotectics on the ALFE diagram. The companion plots clearly distinguish the more differentiated nature of the Group 1 basalts and the less differentiated nature of the Group 2 and Group 3 basalts. Though widely scattered, general trends in the Group 1 basalts hint at the importance of clinopyroxene as well as olivine + plagioclase fractionation in the evolution of the older lavas. In sharp contrast, the tight clustering of the Group 2 tholeiites, especially HAOT, underscores the interpretation that they are little modified from their parental liquids, although weakly defined trends of decreasing

CaO/Al2O3 with increasing Fe-factor may point to a minor role for clinopyroxene fractionation in the TB and SROT. Though the data for the Group 3 AB are limited,

apparent decreases in CaO/Al2O3 with increasing Fe-factor may point to a minor role for clinopyroxene fractionation as well.

89

Figure 2-22. ALFE (Al-factor vs. Fe-factor) diagram, after Reid et al. (1989), with differentiation paths depicting high pressure, clinopyroxene-dominated crystallization (10 kbar cotectic), low pressure crystallization (1 atm cotectic region from Tormey et al., 1987), and olivine extraction. Al-factor and Fe-factor are defined in the text. Insets depict range of compositions exhibited by each chemical type (left inset) and within age groups (right inset).

90 -0.02 Group 1: SB & OIG 17 - 13 Ma 13 - 11 Ma

10 kbar Group 2: HAOT - TB - SROT -0.10 cotectic 11 - 5 Ma 5 - 0 Ma

r variable age o -0.18 (< 11 Ma) t Group 3: AB c a <0.25 Ma f

-

l -0.26

A o lv e x t ra c t io -0.34 n 1 atm olv + plag + cpx cotectic region

-0.42 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Fe - factor

AB SROT SB & OIG Group 3 (<0.25 Ma) Group 1 (17-11 Ma) TB Group 2 HAOT (5-0 Ma) r r o o t t c c a a f f

- -

l Group 2 l

A (11-5 Ma) A

Fe - factor Fe - factor

91

Figure 2-23. CaO/Al2O3 vs. Fe-factor companion plots for the ALFE diagram in Figure 2-22.

92 G 1 I

17 - 13 Ma p O

0.8 u & o

3 r B

13 - 11 Ma G S O 2 T l

11 - 5 Ma O R A S 2

-

p

/ 0.6 5 - 0 Ma B u

T o

r -

G variable age T O O

(< 11 Ma) A H a C 3

p

0.4 B u A <0.25 Ma o Group 2 - HAOT r G

0.8 0.8 3 3 O O 2 2 l l A A

/ 0.6 0.6 /

O O a a

C 0.4 0.4 C Group 2 - TB Group 1 - SB & OIG

0.8 0.8 3 3 O O 2 2 l l A A

/ 0.6 0.6 /

O O a a

C 0.4 0.4 C Group 2 - SROT Group 3 - AB

0.3 0.5 0.7 0.3 0.5 0.7 Fe - factor Fe - factor

93 Synthesis The unique isotopic signatures exhibited by the older (17-13 Ma) Group 1 basalts, the very young (<0.25 Ma) Group 3 basalts, and 11-5 Ma Group 2 basalts must reflect primary contributions from distinct mantle source reservoirs: an isotopically more evolved source or sources (subcontinental lithospheric mantle?) for the 11-5 Ma basalts and an isotopically less evolved source or sources (sublithospheric mantle?) for the 17-13 Ma basalts and the <0.25 Ma basalts. The characteristics of the remaining basalts are suggestive of interactions between these sources, or between melts derived from them. Qualitative constraints on depth of melting suggest a slightly shallower source for the Group 2 basalts, consistent with the subcontinental lithospheric mantle inference, while incompatible trace element and especially Pb isotope characteristics suggest an important role for fluid metasomatism of the source for the older Group 1 basalts. The inferences are consistent with basalt derivation from the northwestern US mantle components C1 (Group 3 basalts), C2 (17-13 Ma Group 1 basalts), and C3 (11-5 Ma Group 2 basalts) proposed by Carlson (1984). A summary of important time-dependent variations in Owyhee Plateau basaltic volcanism is presented below, and is keyed to Figure 2-24. The initiation of basalt eruptive activity in the Owyhee Plateau region (Stage 1a, 17-13 Ma) is marked by the eruption of basalts temporally and compositionally equivalent to the Steens flood basaltsthat are characterized by lower 87Sr/86Sr, 207Pb/204Pb, and 208Pb/204Pb, and higher 143Nd/144Nd and 206Pb/204Pb than subsequently erupted Owyhee Plateau basalts; however, these basalts are still more isotopically evolved than MORB. Highly elevated Rb/Y ratios in these basalts can be attributed to fractionation; however, the less fractionated members of this group still have Rb/Y exceeding that observed in younger lavas, suggesting a more important role for fluid metasomatism than in subsequently erupted lavas. Likewise, the higher 206Pb/204Pb in these basalts compared to subsequently erupted lavas indicates that the mantle source for the initial Steens Basalt eruptions experienced a greater degree of fluid enrichment than the shallower subcontinental lithospheric mantle. The generally evolved bulk compositions of these basalts suggests significant residence times in crustal magma chambers, but because of

94

Figure 2-24. Summarized geochemical characteristics of Owyhee Plateau basalts as a function of eruptive age. Circled numbers indicate magmatic stages described in the text. Symbols are as in Figure 2- 23.

95 3 2b 2a 1b 1a C3 0.707

r 0.706 S 6 8

/

r S

7 0.705 8

0.704 C2

C1

C2 19.0

b 18.8 P 4 0 2

/

C1 b P

6 18.6 0 C3 2

18.4

1.6

1.2 Y

/

0.8 b R

0.4

0.0

0 3 6 9 12 15 18 Age (Ma)

96 the large volumes of magma involved, the effects of contamination through interaction with the crust and the subcontinental lithospheric mantle are minimal. The decline in volume of Steens Basalt eruptions (Stage 1b, 13-11 Ma) is accompanied by increases in 87Sr/86Sr, 207Pb/204Pb, and 208Pb/204Pb ratios, and decreases in 143Nd/144Nd and 206Pb/204Pb ratios. Lower Rb/Y coupled with lower 206Pb/204Pb observed at this time indicates an increased role for the shallower, less fluid metasomatized subcontinental lithospheric mantle. Despite the smaller volumes of melt being produced at this time, the crust continues to exert minimal influence on the compositions of these basalts. Stage 2a (11-5 Ma) is marked by the regional change to small volume HAOT to SROT eruptions at ca. 11 Ma. Lavas produced during this time are characterized by the highest 87Sr/86Sr, 207Pb/204Pb, and 208Pb/204Pb ratios observed on the Owyhee Plateau, coupled with low 143Nd/144Nd. The low Rb/Y values and the low 206Pb/204Pb ratios of these basalts indicate that melts were separating from a source in the shallow mantle that had been relatively unaffected by fluid metasomatism. Given the apparent depth of melt generation (ca. 30 km) and the isotopic character of these lavas, a subcontinental lithospheric mantle source is inferred, which would be able to melt by decompression due to the thinning of the lithosphere (Harry and Leeman, 1995) associated with diffuse extension throughout the northern Great Basin region. Following a brief magmatic hiatus, HAOT to TB eruptions continued during Stage 2b (5-0 Ma), but were limited to the margins of the Owyhee Plateau. This interval is marked by a range of less radiogenic 87Sr/86Sr and more radiogenic 206Pb/204Pb characteristics, coupled with low Rb/Y similar to that observed during Stage 2a (11-5 Ma). The shift in eruptive loci from the interior regions of the Owyhee Plateau to the margins at this time suggests that the lithospheric mantle directly beneath the Owyhee Plateau had become melt-depleted. The change in isotopic character may suggest a decreased role for the sublithospheric mantle, or possibly the involvement of a less isotopically evolved region of the subcontinental lithospheric mantle that had played little or no role in the earlier periods of Owyhee Plateau basalt production. Stage 3 (post-0.25 Ma) consists of the small volume AB eruptions, which trend to low 87Sr/86Sr and higher 206Pb/204Pb and Rb/Y. Their isotopic similarity to MORB

97 indicates a sublithospheric source, with melts separating from their source at a slightly greater depth than the previously erupted tholeiites. Any discussion of magma genesis in the northwestern United States requires some consideration of the physical and chemical state of the mantle as a result of the complex regional tectonic evolution (Figure 2-25). In the Owyhee Plateau region, Leeman et al. (1992), on the basis of isotopic mapping, inferred the existence of a mantle- level decollement, most likely developed due to Sevier thrusting, which juxtaposes Mesozoic or younger accreted lithospheric mantle over a “shelf” of Precambrian cratonic lithospheric mantle. Consequently, the edge of the thick, cratonic mantle keel is displaced westward of the (surface) locus of the Idaho suture zone, contributing to the relative stability of the Owyhee Plateau through the latest Cenozoic, as well as explaining why the bulk of the Steens Mountain flood basalts appear to have been erupted to the west of the Owyhee Plateau. With this lithosphere geometry in place by the late Mesozoic, low-angle subduction of the rigid Farallon Plate during the Laramide Orogeny would have trapped a layer of ambient asthenosphere beneath the Owyhee Plateau and adjacent regions of the northwestern US (Feeley, 2003). Isolated from upper mantle convection, this layer of trapped asthenosphere would have evolved isotopically, while becoming enriched in LILE due to fluid flux accompanying dewatering of the slab. This fluid enriched, trapped asthenospheric layer may have also experienced melt metasomatism as a consequence of the low-angle subduction (Gutscher et al., 2000). Given the geometry of the base of the lithosphere, the thickness of this layer would have been variable, being thinner under the Owyhee Plateau proper and much thicker beneath the accreted lithosphere to the west. Accompanying slab roll-back or foundering following the end of the Laramide event (Humphreys, 1995), the influx of hot asthenosphere from below the slab and into the mantle wedge would cause heating of the fluid-rich layer which, if accompanied by extension, would predictably lead to large scale melting (Gallagher and Hawkesworth, 1992; Harry and Leeman, 1995). The generation of the Steens flood basalts (Group 1, 17-13 Ma) in this fashion would explain their LILE-enriched compositions as well as their slightly evolved isotopic compositions relative to MORB (e.g. Steens 87Sr/86Sr = 0.704 versus MORB 87Sr/86Sr = 0.703). The trend toward somewhat more evolved

98

Figure 2-25. Conceptual cartoon of sub-Owyhee Plateau mantle development during the Cenozoic. No vertical or horizontal scales are implied. CC, continental crust; SCLM, subcontinental lithospheric mantle; OWP, Owyhee Plateau. (a.) Sevier thrusting resulted in the emplacement of younger (Mesozoic-Cenozoic) lithospheric mantle over a “shelf” of Precambrian lithospheric mantle (Leeman et al., 1992), while low-angle subduction of the Farallon Plate during the Laramide Orogeny would have “trapped” a layer of asthenosphere at the base of the lithosphere, allowing it to become fluid metasomatized and isotopically evolved (Feeley, 2003). (b.) Foundering of the Farallon slab following the Laramide event (Humphreys, 1995) would have forced hot asthenosphere to ascend into the mantle wedge, where interaction with the fluid-rich trapped layer would lead to large-scale melting (Harry and Leeman, 1995). With the Owyhee Plateau stabilized by a thick mantle keel, the majority of these magmas would have ascended along the western edge of the Owyhee Plateau to be erupted as the Steens Mountain flood basalts.

99 OR ID a. “Laramide time”

Idaho Suture accreted Zone C terranes C (Mesozoic-Cenozoic) Wyoming thrust (Sevier) lithosphere-scale Craton (Precambrian) M L

zone of trapped C asthenosphere S fluid

flux e r e h

F p arallon Pla s te o n e h t s a

OR ID b. Early to Middle Miocene

incipient OWP “block”

fluid-enriched trapped asthenospheric layer

slab removal forced ascent of hot asthenosphere

100 isotope compositions in the younger (13-11 Ma) Group 1 basalts reflects a decreased role for upwelling asthenosphere, an increased role for the subcontinental lithospheric mantle, or both. While the isotopic characteristics of the 11-5 Ma Group 2 basalts presented in this chapter are suggestive of derivation from sources in the subcontinental lithospheric mantle, the spectrum of chemical characteristics observed cannot be reconciled with simple melting of bulk subcontinental lithospheric mantle. Studies of SROT (Leeman, 1982b; Menzies et al., 1984; Hart 1985) indicate an enriched subcontinental lithospheric mantle source; however, HAOT genesis requires a depleted mantle source (Hart et al., 1984; Hart, 1985). On the basis of Os isotope data, Hart et al. (1997) proposed that HAOT result from the contamination of MORB-like magmas with small amounts of material, less mafic than peridotite, residing in the subcontinental lithospheric mantle as a result of prior melting events. In this context, one way to generate the HAOT-TB-SROT association, so that the full range of chemical types is produced concomitantly while exhibiting no systematic variation in isotopes, is if these veins or pods of mafic material in the lithosphere are involved in the generation of SROT as well as HAOT, and have isotopic compositions extreme enough to overprint those of the primary magmas. In other words, extreme endmember HAOT magmas result from contamination of depleted mantle melts with these mafic lithospheric materials; endmember SROT magmas result from melting of “bulk” enriched mantle lithosphere, including these mafic lithospheric materials; and the spectrum of TB compositions result from varying degrees of interaction of depleted mantle melts with “bulk” enriched mantle melts, which include these mafic lithospheric materials. If this is the case, the similar LILE enrichments observed in HAOT, TB, and SROT suggest that it is these mafic materials in the lithosphere which bear the LILE enrichment. In short, it is the proportions of the materials melting out of the sublithospheric or lithospheric mantle which control the bulk chemistry of the post-11 Ma lavas, but because of the extreme composition of the contaminant shared by all, similar isotopic signatures and LILE enrichments appear. The change in distribution of eruptive loci, ca. 5 Ma, from vents located throughout the Owyhee Plateau to vents around the extended margins of the Owyhee Plateau – and with it, a shift toward less evolved isotopic compositions as well as the end

101 of true SROT eruptions in the Owyhee Plateau region and an overall decrease in eruptive output – must reflect the near exhaustion of fusible materials from the “shelf” of Precambrian subcontinental lithospheric mantle underlying the Owyhee Plateau. Instead, the younger, overthrust layer of subcontinental lithospheric mantle was tapped as extension around the Owyhee Plateau’s margins thinned the lithosphere and allowed the sublithospheric mantle to rise to shallower depths and melt. The very young, <0.25 Ma Group 3 AB are unusual, given their small volume, their limited geographic distribution, and their very recent age. Their isotopic characteristics, of all the basalts in this study, are most similar to MORB, and suggest little or no role for the subcontinental lithospheric mantle in their genesis; yet they exhibit LILE enrichments similar to the Group 1 and Group 2 basalts. Given these observations, and the interpretation that alkaline magmas originate at higher pressures than tholeiites, the most reasonable explanation is that they represent fertile, fluid metasomatized asthenosphere delivered from greater depths in the wedge by convection to replace the thinned and melt-depleted subcontinental lithospheric mantle.

CONCLUSIONS The first-order processes responsible for the generation and evolution of late Cenozoic Owyhee Plateau basaltic magmatism were controlled by the structure and nature of the lithospheric and sublithospheric mantle, and temporal variations in magmatic volume and relative contributions from lithospheric and sublithospheric mantle reservoirs. Lithosphere-scale thrusting during Sevier time resulted in a layer of younger accreted lithospheric mantle being thrust over a “shelf” of Precambrian lithospheric mantle jutting west of the observed trace of the Idaho Suture Zone (approximately 117O west longitude). These regional-scale processes created a peninsula of thick-keeled lithosphere surrounded on three sides by thinner lithosphere; this lithospheric “block” would become the Owyhee Plateau. Subhorizontal subduction of the Farallon plate during Laramide time trapped a layer of asthenosphere between the subducting plate and the lithosphere, allowing it to become more isotopically evolved than ambient asthenospheric mantle. As a consequence of the difference in the thickness of the

102 lithosphere beneath the Owyhee Plateau and the lithosphere to the west, this trapped layer would have been thinner beneath the Owyhee Plateau than to the west of the plateau; thus the subcontinental lithospheric mantle beneath the Owyhee Plateau became enriched in LILE, and perhaps mafic melts, as a consequence of fluid flux from, and possibly melt generation related to, the slab. With the foundering of the Farallon slab, hot, fertile asthenosphere rose into the mantle wedge, where it mixed with the formerly trapped, fluid-rich layer, leading to a massive melting event culminating in the outpouring of the Steens Mountain flood basalts to the west of the Owyhee Plateau; minor volumes of this magma ascended and were erupted around the margins of the Owyhee Plateau. As the magma source potential of this mantle became exhausted, Steens basalt eruptions waned, and the subcontinental lithospheric mantle began to exert a greater influence on the compositions of the magmas produced. The onset of diffuse regional extension ca. 11 Ma set in place a new melting regime which tapped the subcontinental lithospheric mantle beneath the Owyhee Plateau, where melts of depleted mantle and enriched mantle mixed with trapped melts from previous episodes of melting to generate the isotopically indistinguishable HAOT-TB- SROT association. As the source potential of the sub-Owyhee Plateau lithospheric mantle was exhausted, magma generation migrated from the central region of the plateau to its extended margins, where ultimately the lithosphere became so thinned as to provide little interaction with melts from fertile, ascending asthenosphere, yielding the very young mildly alkaline basalts. The Owyhee Plateau preserves a 17 m.y. record of continental intraplate volcanism documenting time-dependent geochemical and isotopic variations which cannot be attributed solely to lateral lithospheric heterogeneities or to crustal contamination accompanying upper-level differentiation. Rather, they reveal a complexly evolving tectonomagmatic regime in which multiple basalt mantle sources and petrogenetic processes have been involved as a function of the age of magma generation, evolution, and eruption. Consideration of temporal variations in magmatic volume and relative contributions from lithospheric and sublithospheric mantle reservoirs allows interpretation of the architecture of the lithospheric and sublithospheric mantle beneath

103 the Owyhee Plateau, which has previously been difficult to define. The results of this study underscore the importance of time as a factor in the evolution of continental basalt provinces.

104 CHAPTER 3:

THE OWYHEE PLATEAU: A TECTONIC AND MAGMATIC LINK BETWEEN THE SNAKE RIVER PLAIN—YELLOWSTONE AND HIGH LAVA PLAINS— NEWBERRY VOLCANIC TRENDS

INTRODUCTION The Owyhee Plateau of the Oregon-Nevada-Idaho tristate region is a tectonically and magmatically enigmatic subprovince of the late Cenozoic northwestern United States volcanic provinces, which appears to have behaved as a unique and coherent lithospheric block since at least the middle Miocene. The goals of this contribution are to formally define the Owyhee Plateau as a discrete tectonomagmatic entity in the northwestern United States; to describe the fundamental spatial, temporal, and geochemical characteristics of Owyhee Plateau basaltic volcanism; and to establish the significance of the Owyhee Plateau as a tectonic and magmatic link between the Snake River Plain— Yellowstone and High Lava Plains—Newberry volcanic trends. While this contribution will address general constraints on the geochemical architecture of the sub-Owyhee Plateau mantle, the petrogenesis of Owyhee Plateau basalts is beyond the scope of the discussion presented herein, and is presented in a separate chapter. The Owyhee Plateau (Figure 3-1) is located at the intersection of the Snake River Plain, the High Lava Plains, the Oregon Plateau, and the Basin and Range. It also lies between the Northern Nevada Rift (Zoback and Thompson, 1978; Zoback at al., 1994) and the Oregon-Idaho Graben (Cummings et al., 1994; Ferns, 1997; Cummings et al., 2000), which, together with the dike swarms of the Columbia River Basalt Group and Steens Basalt (Hooper et al., 2002; Hooper and Hawkesworth, 1993; Carlson and Hart, 1987, 1988; Hart and Carlson, 1987) form an apparent ca. 1000 km long lithospheric "rift" initiated in the middle Miocene (Zoback et al., 1994; Carlson and Hart, 1987; Christiansen and McKee, 1978). Much of this rift coincides with the boundary between the Precambrian Wyoming Craton and Mesozoic-Cenozoic accreted terranes to the west (Leeman et al., 1992). It was along the northern half of this rift that the largest volumes of flood basalts (Clarkston Basalt on the Columbia Plateau and Steens Basalt on the

105

Figure 3-1. (a.) Late Cenozoic volcanic provinces of the northwestern United States. C, Cascade arc; CP, Columbia Plateau; HLP, High Lava Plains; ORP, Oregon Plateau; OWP, Owyhee Plateau; SRP, Snake River Plain. (b.) Selected important tectonomagmatic features of the northwestern United States. CRB, Columbia River Basalt dikes; M, McDermitt Caldera; N, Newberry ; NNR, Northern Nevada Rift; OIG, Oregon-Idaho Graben; OWP, Owyhee Plateau, SB, Steens Basalt dikes; WSRP, Western Snake River Plain Graben; Y, Yellowstone Caldera. Arrows indicate directions of propagation of the Snake River Plain—Yellowstone and High Lava Plains—Newberry volcanic trends. Locations of tectonic and magmatic features are taken from Carlson and Hart (1987), Cummings et al. (2000), Hooper and Hawkesworth (1993), MacLeod et al. (1975), Pierce and Morgan (1992), Rytuba and McKee (1984), and Zoback et al. (1994).

106 a.

CP

C

ORP

HLP SRP

OWP

122 120 118 116 114 Degrees W Longitude b.

CRB Y WSRP N OIG SB M OWP

NNR

122 120 118 116 114 Degrees W Longitude

107 Oregon Plateau) were erupted beginning approximately 17 Ma. Following the cessation of large volume basalt eruptions on the Columbia and Oregon plateaus by approximately 14 Ma (Carlson and Hart, 1988; Hooper and Hawkesworth, 1993), eruptive activity began to contract along the rift in a north-south sense, while expanding eastward across southern Idaho and westward across central Oregon, forming the Snake River Plain and High Lava Plains volcanic trends, respectively. Consequently, only in the central segment of the rift, and at the origin of the Snake River Plain and High Lava Plains – that is, in the Owyhee Plateau region – is there a continuous record of 17-0 Ma basaltic volcanism (Hart et al., 1984; Hart and Mertzman, 1983; Russell et al., 1988; Hart, 1996). Post-11 Ma basalt eruptions across central and eastern Oregon and southern Idaho are dominated by two distinct chemical types. Throughout the Oregon Plateau and High Lava Plains, basalt eruptions are dominated by a distinctive low-K, low-Ti, high alumina olivine tholeiite (HAOT; Hart et al., 1984; Draper, 1991). On the Snake River Plain, eruptions are dominated by a high-K, high-Ti olivine tholeiite (SROT; Leeman, 1982b). While isolated, small volume flows of HAOT are present on the Snake River Plain, SROT is completely absent from the High Lava Plains. Only on the Owyhee Plateau do significant volumes of both HAOT and SROT occur, in close spatial and temporal association; also present are a complete spectrum of basalts transitional between the HAOT-SROT endmembers (Hart et al., 1984). Central to understanding the origin and evolution of the northwestern United States late Cenozoic volcanic provinces is the vigorous debate over the cause of the initial outburst of flood basalt volcanism and the sustained eruptive activity across central and eastern Oregon and southern Idaho. Many researchers have attributed it to the impingement of the hypothesized Yellowstone mantle plume at the base of the lithosphere (Camp, 1995; Geist and Richards, 1993; Hooper and Hawkesworth, 1993), while others have called on rifting in the back-arc region of the middle Miocene volcanic arc and differences in the thickness and physical properties of the lithosphere (Christiansen and McKee, 1978; Carlson and Hart, 1987, 1988; Hart and Carlson, 1987). Some observations of the northwestern United States volcanic provinces are apparently consistent with at least some aspects of a mantle plume model. The northeast-

108 younging progression of Snake River Plain silicic eruptive centers, from the McDermitt volcanic field (ca. 17 Ma) to Yellowstone (O Ma; Pierce and Morgan, 1992), and the presence of high 3He/4He in some Columbia Plateau (Dodson et al., 1997) and Snake River Plain (Reid, 1997) basalt flows are commonly cited as evidence of a plume. Alternatively, the existence of a long lithospheric rift developed parallel to the middle Miocene volcanic arc, coincident with the edge of the craton and along the northern half of which the great majority of basalts erupted, has been used to argue that lithospheric pull-apart due to back-arc extension along a previously established zone of weakness (the suture zone between the Precambrian craton and the younger accreted terranes) focused mantle flow and controlled the location of (at least) the early, large volume flood basalt eruptions (Carlson and Hart, 1987; King and Anderson, 1995). Complicating simple plume or rift interpretations are features and observations that are difficult to reconcile with either scenario. Notable among these is the Oregon High Lava Plains subprovince, which is defined by a northwest-younging progression of silicic volcanic centers originating at McDermitt (ca. 17 Ma) and ending at Newberry (O Ma; MacLeod et al., 1975). The shared geographic and temporal origin of the High Lava Plains and Snake River Plain trends suggests that the two subprovinces are related, but explaining them in the context of a traditional plume model is paradoxical. Likewise, the divergence of both trends from the axis of the middle Miocene rift is perplexing in terms of a simple rift model. Two recently proposed models by Humphreys et al. (2000) and Christiansen et al. (2002) have sought to simultaneously explain both the Snake River Plain and High Lava Plains trends with specific attention to regional tectonic considerations, rather than to localized zones of lithospheric weakness or deep mantle processes alone. The model of Humphreys et al. (2000) calls for the southwesterly motion of the North American plate to have induced a northeast-directed shear in the asthenosphere away from the subduction zone, while near the subduction zone, the upper mantle was forced to flow northwest due to corner flow above the descending Farallon Plate. Consequently, hot, fertile mantle was forced to ascend to the base of the lithosphere, in the vicinity of the cratonic boundary. Partial melting generated the initial large volume flood basalt episode and created a body of melt-depleted residuum above the axis of mantle upwelling. The

109 northeast-propagating Snake River Plain trend and the northwest-propagating High Lava Plains trend were subsequently generated by the flow of hot asthenosphere around the residuum body, lateral growth of the residuum body due to partial melting of the asthenosphere, and thermal erosion of the lithosphere as a consequence of this heat flux. The model of Christiansen et al. (2002) also calls for mantle upwelling to be focused at the cratonic boundary, but calls for a greater role for the regional structure of the lithosphere. As extension progressively widened the central and eastern Oregon— southern Idaho region, magmatism was focused where the extending zones intersected cooler, thicker lithosphere to the north. To the west, crustal melting propagated with extensional widening to form the High Lava Plains trend, while to the east, magmatic propagation along an ancient structural zone, parallel to the direction of plate motion and possibly enhanced by shear melting, gave rise to the Snake River Plain trend. The Owyhee Plateau may hold the keys to the ultimate understanding of late Cenozoic northwestern United States magmatism. Its location at the nexus of the Snake River Plain and the High Lava Plains, its position in the middle Miocene rift between the Northern Nevada Rift and the Oregon-Idaho Graben, as well as its location in the transition zone between the Basin and Range and the volcanic plateaus to the north, makes the Owyhee Plateau a "missing link" in our knowledge of the tectonomagmatic evolution of the northwestern United States. Furthermore, the Owyhee Plateau region is the only location in the northwestern United States that contains a complete temporal record of late Cenozoic basaltic magmatism; contains significant volumes of both HAOT and SROT lavas in close spatial and temporal association; and is inferred to directly overlie the edge of the Precambrian Wyoming Craton. Thus, defining and understanding the temporal and spatial patterns of volcanism on the Owyhee Plateau is seen as a necessary step toward ultimately resolving the nature of late Cenozoic magmatism in the northwestern United States.

GEOLOGY OF THE OWYHEE PLATEAU The Owyhee Plateau is an intermontane basalt plateau with an approximate area of 6,000 km2 (Figure 3-2). It is bounded by the Owyhee, Bull Run, and Tuscarora ranges to the east, Steens Mountain and the Santa Rosa Range to the west, and the Midas Trough

110

Figure 3-2. (a.) Digital elevation model of the Owyhee Plateau and surrounding features, modified from Sterner (1997). Owyhee Plateau outline is solid where clearly defined, and dashed where uncertain or inferred. Triangles mark sampled basalt vent areas, and circles mark other sample locations. BR-T, Bull Run and Tuscarora Mountains; MT, Midas Trough; NNR, Northern Nevada Rift; OIG, Oregon-Idaho Graben; OM, Owyhee Mountains; SM, Steens Mountain; SR, Santa Rosa Mountains; WSRP, Western Snake River Plain Graben. (b.) Relationship of Owyhee Plateau to pre-11 Ma basaltic and silicic eruptive centers. Diamonds mark Steens Basalt dikes or eruptive loci. Outlined areas mark silicic volcanic fields and eruptive centers (MD and LO fields include areal limits of outflow units). JM, Juniper Mountain; LO, Lake Owyhee; MD, McDermitt; SC, Santa Rosa-Calico; SI, Silver City-Delamar. Locations from Hart and Carlson, 1985; Carlson and Hart, 1987; Mellott, 1987; Shoemaker and Hart, 2003; Rytuba and McKee, 1984; Ferns, 1997; Manley and McIntosh, 2003; Ekren et al., 1982; Cupp, 1989; and M. Brueseke, personal communication).

111 a. OIG WSRP OM SM

OR ID NV NV

SR BR-T 50 km NNR MT

LO b. SI

JM MD

OR ID NV NV

SC

50 km

112 to the south; the plateau’s boundaries with the Oregon-Idaho Graben and the Snake River Plain are not as clearly defined. Typical elevations on the plateau are between 4000-5000 feet, with isolated buttes having elevations of 5500 feet or more. In general, elevations are higher on the southern part of the plateau than the northern part of the plateau. The plateau is deeply incised by the Owyhee River and its tributaries, providing an excellent stratigraphic record of post-17 Ma continental basaltic volcanism. The northern part of the Owyhee Plateau is host to a complex array of fault orientations and apparent basaltic vent alignments which may have developed in response to the initiation of rifting ca. 17 Ma (Zoback et al., 1994; Carlson and Hart, 1987; Christiansen and McKee, 1978), the subsequent development of the Snake River Plain to the east (Rodgers et al., 1990), and the northern limit of Basin and Range extension (Lawrence, 1976). The northern Owyhee Plateau region preserves the only continuous record of middle Miocene to Recent basaltic volcanism in the northwestern United States. The southern Owyhee Plateau is characterized by small basaltic shield volcanoes with apparent alignments consistent with the trend of the Northern Nevada Rift (Zoback et al., 1994). All but the largest and most recent faults are obscured by younger basalt flows. Access to the southern part of the Owyhee Plateau is limited; the few roads that do exist are extremely rugged. Consequently, the southern Owyhee Plateau is a relatively "unexplored" region geologically; to the best of this author's knowledge, the only geologic maps and reports that exist are of a very general nature (i.e., Walker, 1977; Luedke and Smith, 1981, 1982; Ekren et al., 1981) or lack detail with respect to basalt stratigraphy (i.e., Walker and Repenning, 1966; Coats, 1985; Ach, 1986) and only a handful of published geochemical analyses exist (Hart et al., 1984; Coats, 1985; Hart et al., 1997). The Owyhee Plateau is surrounded by extensional features. Between the Owyhee Plateau and Steens Mountain lies an extensional corridor, extending south from Burns Junction to the Oregon-Nevada border (and continuing into Nevada as the Quinn River valley), occupied by Oregon Canyon Creek and Crooked Creek (Walker and Repenning, 1966; Walker, 1977). Further west, the Steens Mountain fault scarp exposes some 1000 meters of flow-on-flow basalt from the initial flood basalt event (Gunn and Watkins, 1970), implying that this extensional corridor may mark a significant lithospheric

113 boundary. Steens-age volcanic rocks unconformably overlie granitic basement rocks in the Owyhee (Ekren et al., 1982; Cupp, 1989) and Santa Rosa (Mellott, 1987; M. Brueseke, personal communication) ranges, indicating that these regions were undergoing extension and uplift prior to the initiation of flood basalt volcanism. To the north, the precise location of the plateau boundary is obscured where Owyhee Plateau basalts overlap the sediment fill of the Oregon-Idaho Graben (Cummings et al., 2000; Hart and Mertzman, 1983); however, Antelope Rim, a major east-west trending escarpment, likely defines part of the plateau’s northern boundary. To the south, the Midas Trough forms the boundary between the Owyhee Plateau and the Basin and Range. However, while the lithosphere around the Owyhee Plateau has been extended significantly, the plateau itself appears to be little extended. Few faults on the Owyhee Plateau have observable displacements in excess of a few meters, and through the central and southern parts of the plateau few faults are observed (Walker and Repenning, 1966; Walker, 1977; Ekren et al., 1981; Ach, 1986). Dikes and plugs of 17-11 Ma material are found marginal to the plateau, at Steens Mountain, in the Owyhee Mountains, in the Santa Rosa Range, in the Oregon-Idaho Graben, and in the Northern Nevada Rift (Hart and Carlson, 1985; Carlson and Hart, 1987; Mellott, 1987; Zoback et al., 1994; Cummings et al., 2000; Shoemaker and Hart, 2003; Brueseke and Hart, 2000). However, eruptive loci of this age have not been observed within the Owyhee Plateau proper. Likewise, large silicic systems such as the McDermitt (Rytuba and McKee, 1984), Lake Owyhee (Ferns, 1997), Juniper Mountain (Manley and McIntosh, 2003), and Santa Rosa-Calico (M. Brueseke, personal communication) volcanic fields occur around the edges of the plateau, but no such eruptive centers are recognized within the plateau itself. This is not to say that they are absent from the interior of the plateau; however, if they are present, they are buried beneath younger basalt flows. While the canyons of the Owyhee River and its tributaries expose silicic volcanic products beneath the younger basalt flows, it cannot be demonstrated that these silicic materials were erupted within the boundaries of the plateau. Post-11 Ma volcanism in the Owyhee Plateau region is largely limited to small- volume basalt eruptions from discrete shield volcanoes and perhaps isolated fissures.

114 The apparent alignment of many of these shields with important regional extensional features (i.e., the Northern Nevada Rift and Oregon-Idaho Graben) suggests that they may be rooted on deep fractures resulting from dike injection into the lithosphere, a process that has also been proposed for the eastern Snake River Plain (Kuntz, 1992). Significantly hindering interpretations of the lithospheric structure of the Owyhee Plateau is the lack of detailed geophysical data for southeastern Oregon and adjacent Nevada and Idaho (e.g., see Appendix A, Figure 1 of Simpson and Jachens, 1989, and Figure 2 of Dueker et al., 2001). However, the presence of highly extended features and both basaltic and silicic pre-11 Ma eruptive loci around the margins of the plateau, coupled with the lack of faults with significant displacements and the apparent absence of pre-11 Ma eruptive loci in the interior of the plateau, all hint at a relative lack of extension on the Owyhee Plateau and suggest that the plateau has more or less behaved as a coherent lithospheric block since at least the middle Miocene.

SAMPLE SUITE Sampling was conducted with the dual purpose of representing, as completely as possible, the eruptive history of basaltic volcanism in the Owyhee Plateau region as well as the spatial distribution of basalt eruptive loci. For the former objective, careful sampling and documentation of flow-on-flow sequences exposed in canyons and fault scarps, combined with K-Ar and 40Ar/39Ar method geochronology, allowed the construction of a composite stratigraphic column representing the history of Owyhee Plateau basaltic volcanism from its inception to the present day. A suite of chronostratigraphically well-constrained basalts was already in hand as the result of previous investigations (Hart and Mertzman, 1982, 1983; Hart et al., 1984; Hart and Carlson, 1983, 1985; Carlson and Hart, 1987). Additional sampling of flow sequences in canyons focused on filling in apparent “gaps” in the record and clarifying ambiguous stratigraphic relationships. For the latter objective, few basalt eruptive centers had been previously sampled, thus representing as many centers as possible was a primary focus of the time spent in the field. These vents are typically small (relief normally <100 meters), eroded shield volcanoes, often with some remnant crater features at the summit. Many exhibit red,

115 oxidized agglutinated material, or other evidence of small-volume pyroclastic activity at their summits. These vents tend to occur in clusters, where all vents present have undergone a similar degree of erosion, leading this author to believe that within these clusters, all vents are of similar age. In some cases flows from individual vents could be correlated with temporally constrained basalt flows exposed in nearby canyons, and thus their ages could be determined. Where vent ages could not be constrained in this fashion, 40Ar/39Ar method geochronology was performed on selected samples to constrain the ages of individual vents or vent groups.

DATA Geochronology and elemental and isotope geochemistry Major and trace element geochemical analyses have been performed on 170 basalt to samples from within or adjacent to the boundaries of the Owyhee Plateau, as defined in Figure 3-2. Additionally, a subset of these lavas were analyzed for Sr, Nd, and Pb isotopes. A detailed discussion of the elemental and isotope geochemistry of these samples is not presented here, but is contained in a separate chapter of this dissertation. Of critical importance to the discussion in this chapter are the eleven new 40Ar/39Ar method ages determined on Owyhee Plateau basalts as part of this study. These data are summarized in Table 3-1; complete analytical data are presented in Appendix 2D. These new age determinations, combined with previously reported K-Ar (Hart and Mertzman, 1982, 1983; Hart and Carlson, 1983, 1985; Hart et al., 1984) and 40Ar/39Ar (Shoemaker and Hart, 2003) age determinations (see Appendix 2E), allows a reconstruction of the eruptive history of the Owyhee Plateau. Middle Miocene to Recent basaltic volcanism in the Owyhee Plateau region has produced a variety of distinct geochemical types (Figures 3-3 and 3-4), including strongly differentiated basalts to basaltic andesites erupted between 17-11 Ma (Steens Basalt and Oregon-Idaho Graben basalts), and more primitive tholeiitic basalts erupted after 11 Ma. Based on criteria established by Hart et al. (1984), the post-11 Ma tholeiites can be distinguished into low-K, low-Ti, high alumina olivine tholeiites (HAOT), high-K, high- Ti Snake River Plain-type olivine tholeiites (SROT), and compositions transitional between the two (transitional basalts, TB). Also present are minor volumes of mildly

116

Table 3-1. New 40Ar/39Ar method geochronology.

117 39 39 σ sample # age steps ArK Ar K/Ca Age ± 1 analysis (x 10-15 mol) (%) (Ma) (Ma) KS-96-1 plateau 4 13.3 47.4 0.068 10.44 0.17 KS-96-6C plateau 4 9.6 44.6 0.043 8.34 0.20 KS-96-7 plateau 6 12.2 64.0 0.025 8.36 0.41 KS-96-8D plateau 9 11.83 100.0 0.028 0.65 0.19 CM-97-7 plateau 7 9.99 82.4 0.021 6.54 0.26 CM-97-22 plateau 5 21.6 74.4 0.076 8.15 0.10 118 KS98-4 plateau 9 15.49 100.0 0.033 7.61 0.41 KS98-19 plateau 7 31.3 87.0 0.097 9.71 0.24 KS98-23 plateau 6 22.4 77.5 0.081 10.39 0.27 KS98-28 plateau 7 17.0 79.3 0.049 8.90 0.29 KS98-30 plateau 9 14.33 100.0 0.070 7.56 0.47

Correction factors: 39 37 ( Ar/ Ar)Ca = 0.00089 ± 0.00003 36 37 ( Ar/ Ar)Ca = 0.00028 ± 0.000011 38 39 ( Ar/ Ar)K = 0.01077 40 39 ( Ar/ Ar)K = 0.0002 ± 0.0003

Figure 3-3. MgO-TiO2-K2O ternary diagram depicting Owyhee Plateau region basalt chemical types. SB & OIG, Steens Basalt and Oregon-Idaho Graben basalts (17-11 Ma); HAOT, high-alumina olivine tholeiite (post-11 Ma); TB, transitional basalt (post-11 Ma); SROT, Snake River Plain-type olivine tholeiite (post-11 Ma); AB, alkaline basalt (post-250 ka).

119 MgO x 0.5

HAOT

TB AB SROT

SB & OIG

TiO2 x 1.5 KO2 x 5

120

Figure 3-4. Variations in TiO2 content versus age of eruption. Only samples with K-Ar or Ar/Ar ages, or where ages can be tightly constrained based on stratigraphic relationships, are depicted. Open triangles, SB & OIG; solid squares, 11-5 Ma tholeiites; open squares, 5-0 Ma tholeiites; solid circles, AB. (a.) All Owyhee Plateau basalts. (b.) Detail of post-11 Ma tholeiites. HAOT, TB, and SROT boundaries are from Hart et al. (1984). Note the wide range of compositions that occur on the Owyhee Plateau in close temporal association.

121 a.

3 2

O 2 i T

1 all basalts in this study 0 0 3 6 9 12 15 Age (Ma)

b. T O R S

2 B 2 T O i T

1 T O A H post-11 Ma tholeiites 0 0 3 6 9 Age (Ma)

122 alkaline basalts (AB), erupted after ca. 0.25 Ma from a small group of low shields and tephra cones near the northernmost edge of the Owyhee Plateau. However, because these AB lavas are such a minor component of the total post-17 Ma eruptive package in the Owyhee Plateau region, their treatment is limited in the following discussion. Figure 3-5 depicts the combined incompatible trace element characteristics of representative Steens Basalt, HAOT, and SROT samples on an NMORB-normalized spider diagram. All three samples were collected near the northern margin of the Owyhee Plateau, emphasizing the diversity of chemical types which may occur in close spatial association in this region. However, despite the differences in major element chemistry, the three samples have broadly similar trace element patterns featuring distinct strong enrichments in the LILE, especially in Ba, and negative Nb anomalies. The HAOT, which is most similar to NMORB in Nb though Y, still has LILE abundances in excess of NMORB. Enrichments in the fluid-mobile LILE without corresponding enrichments in fluid-immobile elements such as Nb have been suggested to indicate a back-arc basin tectonomagmatic setting (Hart et al., 1984; Hart, 1985). The general similarity in the patterns observed suggests that similar heterogeneous magma sources and differentiation processes were involved in the generation of these different basalt chemical types, but that the degree of interaction of source materials (or primary magmas) has varied to produce the observed compositions. An important feature of basaltic volcanism in the Owyhee Plateau region is that elemental and isotopic characteristics appear to vary as a function of eruptive age, but these variations are often decoupled (Shoemaker and Hart, 2003). Figure 3-6 illustrates variations in 87Sr/86Sr and K/P of Owyhee Plateau basalts through time. Overall, Owyhee Plateau basalts exhibit a wide range in initial 87Sr/86Sr compositions. Regardless of major element composition (Figure 3-4), the highest Sr isotope ratios occur in those basalts erupted in the 11-5 Ma interval, and while there is considerable variation in K/P values at any given time, the oldest and youngest basalts typically display wider ranges and higher values than the 11-5 Ma tholeiite group.

123

Figure 3-5. NMORB-normalized spider diagram showing the combined incompatible trace element characteristics of representative SB, HAOT, and SROT samples from near the northern margin of the Owyhee Plateau.

124 Rock / NMORB 1 0 1 0 . 0 0 1 1 S r K R b 125 B a N b L a C e P Z r S H S T B R A i O O T T Y

Figure 3-6. Variations in 87Sr/86Sr and K/P versus age of eruption. Symbols are as in Figure 3-4. Note that, regardless of basalt chemical type, the maximum in 87Sr/86Sr, and generally lower K/P, occur in basalts erupted between 11-5 Ma.

126 0.707 r S

6 0.706 8

/

r

S 0.705 7 8

0.704

6 P

/

4 K

2

0 3 6 9 12 15 Age (Ma)

127 Spatial and temporal patterns of volcanism On the basis of eruptive age, geochemical characteristics and eruptive style, basaltic volcanism in the Owyhee Plateau region can be described as occurring in several stages (Figures 3-4, 3-6, and 3-7). Between ca. 17-13 Ma, Oregon Plateau-wide flood basalt volcanism occurred with the eruption of Steens Basalt from large fissure systems as well as isolated fissures. Eruptive loci for Steens Basalt are observed around the margins of the Owyhee Plateau (Carlson and Hart, 1987; Hart and Carlson, 1985; Shoemaker and Hart, 2003), but are not recognized within its boundaries. These magmas are strongly fractionated basalts to basaltic andesites of tholeiitic to calc-alkaline affinity (Carlson and Hart, 1987, 1988; Hart et al., 1989). Between 13-11 Ma, the Steens flood basalt episode was in its waning stages. Steens Basalt lavas produced at this time were geochemically similar to those erupted earlier, but in significantly smaller volumes (Hart and Carlson, 1985; Carlson and Hart, 1987). At the same time, calc-alkaline basalts related to the formation of the Oregon-Idaho Graben (Cummings et al., 2000) were also erupted from fissures and shields near the northern margin of the Owyhee Plateau. A regional change, ca. 11 Ma, from the strongly fractionated Steens Basalt and Oregon-Idaho Graben basalts to relatively little fractionated olivine tholeiite lavas was accompanied by a change in eruptive style, from dominantly fissure-fed eruptions to eruptions from small shield cones, and perhaps small fissures associated with local minor extensional features, located throughout the Owyhee Plateau proper. Basalt compositions produced in this interval represent a complete spectrum from HAOT through TB compositions to SROT, all found in close spatial and temporal association (Hart et al., 1984; Coats, 1985; Draper, 1991). Given the plateau-wide distribution of eruptive loci and the presence of thick flow units exposed in the canyons of the Owyhee River and its tributaries, the interval between 11-5 Ma is inferred to be the period of greatest magmatic output on the Owyhee Plateau. Between approximately 5-2 Ma, the Owyhee Plateau appears to have been magmatically quiescent, although HAOT to TB eruptions occurred marginal to the plateau. For example, Hart and Mertzman (1983) previously reported 4.0-4.5 Ma HAOT and TB from a segment of the Owyhee River Canyon known as The Hole in the Ground, and a 3.8 Ma TB from the wall of the valley now occupied by the Jordan Craters lava

128

Figure 3-7. Temporal summary of eruptive style, eruptive loci, and eruptive products found in the Owyhee Plateau region, 17-0 Ma.

129 0 Ma t d s e t o n i B o i m m i g l n A

r e s r e

n

h t P o d r i t e o W t p n a

u O r c o t o E l

2 Ma s e r P s u n s W o s i i t O f

p f P d u o

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O e l f B i

f r o o

o s a T i

e

d s c l d s n n n a i P n n u & e n g a o o

i i r

W d t , i b

4 Ma g a

r s p v O T

n a e u m e i

r

o n t h m O e o t E o i h c N t w

A r d e n d H u n i o r c

a l l

r a o

m f s o

n

6 Ma , o s s i e d g i l r e e r a i

d h P s n

l u W l o a O b

m e e s r i

h t t e

n t n e e i

r t h c t u i 8 Ma s i o w d h

g T m u o o r r O f

h t s

R n s o n i S t o i p t u p r u r E 10 Ma E

e t e r c P s

12 Ma i W d

s O

d t f n l o

a

P s a s s n n i W s o m i g t O r e a

t p f a s u o b y

r m

s s e

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s s u a I e d n s n 14 Ma d c n s e o n i i O n f u e t u

t e o p o e r S d u i b

g a r

f & r

v

e n o d a e i

l

t

e l h e t o t B i a s m a N s a w o c S a r h o f l b

p

s s d n t n i o n o a i o e t l f M v 16 Ma p u r E

130 field; both locations are north of Antelope Rim, and the lavas may have been erupted from vents off of the Owyhee Plateau. Also, Hart et al. (1984) reported a 4.4 Ma HAOT from the Midas Trough, a feature which defines the southern boundary of the Owyhee Plateau. Following the 5-2 Ma magmatic hiatus, small volume eruptions of HAOT and TB resumed on the Owyhee Plateau, but were limited to the northernmost part of the plateau. Additionally, AB lavas were erupted from low shields and tephra cones in the vicinity of the northern margin of the Owyhee Plateau after ca. 0.25 Ma (Hart and Mertzman, 1983; Hart et al., 1984; Russell et al., 1988; Hart, 1996). Another feature of post-11 Ma volcanism on the Owyhee Plateau is the occurrence of groups of basaltic vents of apparently similar age, which have alignments consistent with regional strain regimes; it is probable that these vents are rooted on deep lithospheric fractures (Korme et al., 1997), likely resulting from localized dike injection (Kuntz, 1992). Three such basaltic vent alignments are illustrated in Figure 3-8, and select characteristics of each sampled vent are provided in Table 3-2. Two of these alignments, the Grassy Mountain (GM) and Northern Nevada (NN) alignments, contain pairs of vents with essentially indistinguishable eruption ages (i.e., within analytical error), but from which basalts of distinct chemical and isotopic composition were erupted. The Grassy Mountain alignment, on the northern Owyhee Plateau, consists of two small shields (Mousetrap Butte and Mustang Butte) flanking a larger shield, Grassy Mountain. The three shields have an apparent alignment trending N 35° W. This trend, consistent with the trend of the Northern Nevada Rift (Zoback et al., 1994), is also consistent with a series of en echelon normal faults mapped by Walker and Repenning (1966), north and east of the alignment; it may also be intersected by a series of northeast-striking normal faults. Both Grassy Mountain and Mousetrap Butte erupted TB lavas; however, Grassy Mountain is an SROT-like TB eruptive center, and Mousetrap Butte is an HAOT-like TB eruptive center (Mustang Butte was not sampled).

Furthermore, the similar K2O concentrations and distinct Sr isotopic compositions (Grassy Mountain = 0.70713; Mousetrap Butte = 0.70642) suggest that these basalts are not related simply by differences in melting and crystallization histories, even though

131

Figure 3-8. Owyhee Plateau basaltic vent alignments. Unsampled but documented vents are shown as open triangles on the insets, but are not depicted on the main map. JC, Jordan Craters alignment; GM, Grassy Mountain alignment; NN, Northern Nevada alignment. Other abbreviations are as in Figure 3-2.

132 Mustang Butte Jordan Craters Grassy Mountain OIG Clarks Mousetrap Butte Butte Rocky Butte JC WSRP Three Mile Hill

OM GM SM

OR ID NV NV SR NN NNR “Pipeline Butte” 50 km “Ant Hill” Bartome BR-T Corral Lake Knoll NNR Butte MT Monument “Guzzler Butte” Hills Wolf Butte

133

Table 3-2. Summary of isotopic and average geochemical characteristics of aligned basaltic vents.

134 Age (Ma) 87 86 TiO2 K2O Mg # Sr/ Sr ± 2 σ Jordan Craters alignment: Jordan Craters (a) 2.17 0.71 69.5 0.70384 0.15 (max) † Jordan Craters (b) 1.77 0.99 71.1 0.70402 0.15 (max) † Clarks Butte 1.94 1.79 64.1 0.70390 0.25 ± 0.05 † Rocky Butte 2.11 1.23 62.8 0.70486 0.03 (max) † Three Mile Hill 1.92 0.72 63.9 0.70549 1.86 ± 0.19 †

Grassy Mountain alignment: Mustang Butte not sampled Grassy Mountain 2.15 0.38 63.7 0.70713 7.20 ± 0.76 † Mousetrap Butte 1.55 0.31 67.1 0.70642 7.56 ± 0.94

Northern Nevada alignment: Pipeline Butte * 1.62 0.40 66.5 0.70569 Ant Hill * 1.39 0.43 68.1 0.70733 10.39 ± 0.54 Corral Lake Butte 2.09 0.35 63.0 0.70719 Guzzler Butte * 2.06 0.37 63.5 0.70663 Wolf Butte 1.27 0.19 68.3 0.70614 Bartome Knoll 2.17 0.48 61.4 0.70786 9.71 ± 0.48 Monument Hills not sampled

* Informal names; all other names appear on USGS maps † K-Ar method ages (reported in Hart and Mertzman, 1983)

135 Grassy Mountain and Mousetrap Butte yield analytically indistinguishable ages of 7.20 ± 0.76 Ma and 7.56 ± 0.94 Ma, respectively. The Northern Nevada alignments, on the southern Owyhee Plateau, consist of at least seven discrete vents on two parallel and adjacent alignments trending N 30° W, again consistent with the trend of the Northern Nevada Rift. The six sampled vents of the Northern Nevada alignments erupted magma compositions ranging from true HAOT (Wolf Butte) through SROT-like TB (Bartome Knoll). As in the Grassy Mountain alignment, the Northern Nevada alignments contain analytically indistinguishable aged eruptive centers producing different basalt compositions: Bartome Knoll is dated at 9.71 ± 0.48 Ma, while Ant Hill (an HAOT-like TB center) is dated at 10.39 ± 0.54 Ma. Although Bartome Knoll and Ant Hill lie on different, though adjacent alignments, their proximity in both space and time is noteworthy. The third set of aligned vents, the Jordan Craters (JC) alignment, consists of three major <250,000 year old AB vents (Jordan Craters, Rocky Butte, and Clarks Butte) and an older ca. 2 Ma TB vent (Three Mile Hill) trending N 15° W. Isotopic variability between these vents, and even within the Jordan Craters vent itself, again suggest that processes beyond crystallization and melting must be considered. However, the location of these vents near the indistinct boundary between the Owyhee Plateau and the Oregon- Idaho Graben, together with the anomalous bulk chemistry of the lavas produced by three of the vents as compared to the other post-11 Ma Owyhee Plateau basalts, suggests that this alignment may not entirely overlie the Owyhee Plateau lithosphere.

DISCUSSION Basaltic volcanism in the Owyhee Plateau region initiated ca. 17 Ma with the voluminous eruption of the Steens Basalt from vents throughout the Oregon Plateau, contemporaneous with the eruption of the Columbia River Basalt Group to the north (Carlson and Hart, 1987; Brueseke and Hart, 2000). At approximately 11 Ma, a significant change in basalt chemical characteristics occurred, as eruptions of strongly fractionated basalt to basaltic andesite were replaced by small volume eruptions of less fractionated olivine tholeiites (Hart and Carlson, 1987, 1992; Carlson and Hart, 1987, 1988; Draper, 1991). The change from strongly fractionated to less fractionated eruptive

136 products coincides with the regional change from large volume, fissure-fed eruptions to small volume eruptions from discrete eruptive centers, as well as the onset of diffuse lithospheric extension across the entire Oregon Plateau and northern Basin and Range (Hart and Carlson, 1987; Hooper, 1990; Draper, 1991; Parsons, 1995). Throughout the Oregon Plateau and High Lava Plains region, these less fractionated tholeiite compositions are dominated by HAOT (Hart et al, 1984), while on the Snake River Plain, they are dominated by SROT (Leeman, 1982b). On the Owyhee Plateau, HAOT is found in close spatial and temporal association with SROT, and a full spectrum of compositions intermediate between the HAOT and SROT endmembers (transitional basalts, TB) is also observed. One potential explanation is that the two endmember magma types may be generated from a similar, relatively homogeneous mantle source, but have experienced different melting or crystallization histories. However, such an interpretation is inconsistent with the observed bulk chemical and the isotopic characteristics (e.g., see Figure 3-8 and Table 3-2) of the erupted basalts. Prior investigations of HAOT to SROT magmatism have demonstrated that these compositions cannot be related by simple magmatic differentiation, but instead require distinct mantle source materials: the bulk chemical characteristics of HAOT are suggestive of an incompatible element depleted mantle source, while SROT bulk chemical characteristics point to an enriched mantle source (Hart, 1985). In the Owyhee Plateau region, the association of these two magma types in close temporal and spatial proximity, and the frequent occurrence of compositions intermediate between the two (TB), suggests a complex juxtaposition of diverse mantle materials. Indeed, on the basis of isotopic mapping, Leeman et al. (1992) demonstrated that, while the cratonic boundary is steep, it is also complex, and likely involves a mantle-level decollement thrust over a horizontal distance of ca. 150 km as a consequence of the accretion of Phanerozoic oceanic terranes onto the edge of the Precambrian Wyoming Craton. Subsequent studies of the HAOT and SROT endmembers have shown that the endmembers themselves are complex. The composition of HAOT is consistent with the contamination of MORB-like melts with small volume melts of mafic material in the lithosphere resulting from prior episodes of melt metasomatism (Hart et al., 1997).

137 SROT compositions indicate derivation from an enriched SCLM source which has also experienced melt metasomatism (Reid, 1995, 1997). As previously indicated, Owyhee Plateau basalts display a diversity of Sr isotope compositions regardless of major element compositions, with the highest 87Sr/86Sr being associated with those basalts erupted following the regional change from strongly fractionated basalt and basaltic andesite eruptions to small volume, less fractionated olivine tholeiite eruptions. One possible interpretation of these isotope variations is that those basalts erupted in the 11-5 Ma interval have experienced some degree of upper crustal contamination, thus modifying their isotope signatures to the observed values. However, trace element ratios such as K/P, which should be elevated in basalts contaminated with evolved crustal materials, actually display their lowest values in lavas erupted during this interval, while the highest values for this ratio occur in early Steens Basalt and young AB lavas. This decoupling may be better explained by variations in the relative contributions from various source reservoirs. Specifically, the high 87Sr/86Sr values in the 11-5 Ma basalts may be the consequence of melting of source materials in the subcontinental lithospheric mantle (SCLM), and little subsequent interaction of these melts with upper crustal materials. If this is the case, then the oldest lavas (Steens Basalt) and youngest lavas (AB) would likely reflect a larger contribution from a sublithospheric mantle reservoir (asthenosphere?), plus relatively greater, although still minimal, amounts of local lithospheric contamination. Any model which is to adequately explain temporal and spatial patterns of pre- and post-11 Ma basaltic volcanism in the Owyhee Plateau region must account for a number of observations, which are detailed below. The presence of large silicic volcanic systems, and Steens-age basaltic eruptive loci, around the margins of the Owyhee Plateau, but the apparent lack of such centers within the boundaries of the plateau. A feature common to most models of Snake River Plain and High Lava Plains magmatism (e.g., Pierce and Morgan, 1992; Humphreys et al., 2000; Christiansen et al., 2002) is that upwelling mantle became focused beneath the origin of both the Snake River Plain and the High Lava Plains; that is, beneath the Owyhee Plateau. If the Owyhee Plateau was behaving coherently as a lithospheric “block” during this time, the large volumes of basaltic magma being produced beneath

138 the Owyhee Plateau would have ascended through the nearest weak zones in the lithosphere: the already-extended margins of the plateau. Alternatively, for it must be emphasized that there is no conclusive evidence that these older silicic and basaltic eruptive centers actually are absent from the interior of the plateau, magmas may have also risen through the Owyhee Plateau lithosphere, along fractures developed in response to regional extension. This raises an important question: has the Owyhee Plateau lithosphere been significantly extended, and if so, to what degree, and when? The location of post-11 Ma basaltic vents along alignments consistent with the strike of the Northern Nevada Rift suggests that the Owyhee Plateau experienced some amount of brittle deformation during (or immediately preceding) the flood basalt episode, but given the current database with regard to geophysical data it is essentially impossible to quantify the amount of extension. On one hand, the Owyhee Plateau block may have behaved rigidly, while the rest of the northern Basin and Range collapsed around it; on the other hand, the Owyhee Plateau may be as extended as the adjacent northern Basin and Range, with the extension fully accommodated through dike injection. In this latter case, the apparent lack of older eruptive loci within the boundaries of the plateau may simply be an artifact of the limited exposures of pre-11 Ma materials beneath the younger basalt flows. Regardless of the degree of extension of the plateau before, during, and even after the flood basalt episode, the buoyant residuum body resulting from the extraction of the flood basalt magmas would have added a relative degree of isostatic stability for the Owyhee Plateau during the ensuing 11 Ma. The decoupling of Owyhee Plateau basalt isotopic and trace element characteristics from bulk geochemical characteristics through time. This observation is linked to variations in the relative contributions of different mantle source materials to the production of basaltic magmas through the history of Owyhee Plateau basaltic volcanism. The initial flood basalts, erupted between ca. 17 Ma and 13 Ma, have isotopic and trace element signatures indicative of derivation from a sublithospheric source. Because of the large volumes of magma involved during the early stage of Steens volcanism, interaction with melts derived from the SCLM would exert little control on the observed isotope and trace element chemistry, but the smaller volumes of basalt erupted during the waning stages of the Steens flood basalt episode (ca. 13-11 Ma) reflect

139 a greater influence from the SCLM. The onset of the second phase of northern Basin and Range extension ca. 11 Ma would allow melting of SCLM materials, thus the higher 87Sr/86Sr observed in basalts erupted during the change from evolved to more primitive chemistries. The expenditure of the easily melted portion of the SCLM combined with heat flow beneath the residuum and continued regional extension would allow the remaining fusible components of the residuum to exert a greater influence on the isotopic and trace element characteristics of the younger basalts. The regional change in basalt characteristics which occurred ca. 11 Ma, from strongly fractionated basalt to basaltic andesites to relatively undifferentiated olivine tholeiites. A massive upwelling of hot sublithospheric mantle ponding and melting at the base of the lithosphere in the Owyhee Plateau region triggered the Steens Mountain flood basalt episode ca. 17 Ma; during their long traverse through the lithosphere to the surface, Steens primary magmas would have had ample time to fractionate, thus the more evolved compositions of lavas erupted prior to the regional change ca. 11 Ma. Subsequently, the cessation of flood basalt volcanism would have left a body of largely, but not completely, melt-depleted residuum attached to the SCLM beneath the Owyhee Plateau, the consequence of the extraction of such large volumes of magma from the mantle. However, the Owyhee Plateau SCLM (and, for that matter, the residuum itself) could be reasonably expected to contain trapped partial melts of Steens primary magmas, as well as trapped partial melts from prior melting events. Regional lithospheric thinning related to the initiation of the second phase of extension in the northern Basin and Range ca. 11 Ma (Parsons, 1995) would have allowed adiabatic melting to occur in the complex and modified Owyhee Plateau SCLM (and in the residuum body, if it was not depleted of fusible components), generating the characteristic olivine tholeiite magmas. The injection of dikes of these magmas into the Owyhee Plateau lithosphere would occur, consistent with regional strain directions, and leakage of these magmas at the surface would produce the observed vent alignments. The rapid ascent of these magmas through the crust would have allowed little time for fractionation. The temporally indistinguishable eruption of different basalt chemical types (HAOT-SROT), which cannot be related by simple fractionation, from vents in close proximity to one another after 11 Ma. As a consequence of the accretion of Phanerozoic

140 oceanic terranes onto the edge of the Precambrian Wyoming Craton, the mantle beneath the Owyhee Plateau consists of interthrust “slices” of ancient cratonic SCLM and younger accreted SCLM (Leeman et al., 1992). Superimposed on this primary complexity are several additional considerations: (1) some 200 million years of subduction, much of it with a low-angle slab, allowing fluid metasomatism and hydration of the Owyhee Plateau SCLM; (2) hot asthenospheric upwelling related to slab roll-back (Humphreys, 1995), and the opening of a slab window (Bohannon and Parsons, 1995); and (3) melt metasomatism of the SCLM related to the flood basalt episode, and potentially also to slab melting, which would have been possible since the slab was young and hot (Peacock et al., 1994; Bourgois et al., 1996; Lagabrielle et al., 2000) because the Farallon Ridge was being consumed (Engebretson et al., 1985; Bohannon and Parsons, 1995). Thus in this environment, discrete mantle domains (e.g., depleted mantle and enriched mantle) with a complex history of modification exist in close proximity. Melting related to or enhanced by heat flux from below, regional extension, the presence of fluids, or some combination of these factors tapping into these different mantle source materials at any particular time would lead to the concurrent eruption of contrasting basalt types from adjacent vents. In such a scenario the mixing of primary HAOT and SROT magmas to produce the observed transitional basalt chemistries is reasonable (e.g., Hart, 1985). The apparent cessation of eruptive activity across the entire Owyhee Plateau by ca. 5 Ma, and the resumption of eruptive activity on only the northernmost Owyhee Plateau from ca. 2 Ma to the present. The cessation of Owyhee Plateau-wide basaltic volcanism ca. 5 Ma may be related to a magma-supply issue, with the exhaustion of readily fusible mantle materials beneath the central and southern Owyhee Plateau occurring more quickly than beneath the northern Owyhee Plateau. Alternatively or additionally, this pattern may be related to the motion of the North American Plate and the subduction geometry of the Farallon Plate. The southwesterly motion of North America and the widening of the slab window created by the roll-back and breakup of the Farallon Plate would cause the focus of mantle upwelling beneath the Owyhee Plateau to shift northward relative to the plateau. The eruption of HAOT to TB magmas marginal to the plateau between 5-2 Ma may indicate that these areas experienced a renewed phase of

141 extension, however minor, allowing rapid ascent of magmas around the edges of the more rigid Owyhee Plateau.

CONCLUSIONS The Owyhee Plateau is a discrete tectonomagmatic entity within the North American Cordillera, but also is intimately related to other late Cenozoic volcanic provinces of the northwestern United States. Understanding the origin and history of volcanism on the Owyhee Plateau is crucial to understanding the tectonomagmatic evolution of the northwestern United States as a whole. Given the observed temporal and spatial patterns of Owyhee Plateau basaltic volcanism, as well as the geochemical and isotopic variations through time, the history of Owyhee Plateau can be interpreted. The accretion of oceanic terranes onto cratonic North America during the Mesozoic created a complex interthrusting of old and young lithospheric mantles, while subhorizontal subduction led to the fluid- and melt- metasomatism of this complex mantle. The return to a steep subduction angle and the collision of North America with the Farallon Ridge induced asthenospheric upwelling as a consequence of slab roll-back and the opening of a slab window. The geometry of this subduction scenario coupled with the motion of the plates would have focused this upwelling beneath the Owyhee Plateau. With the initiation of flood basalt volcanism in the northwestern United States ca. 17 Ma, basaltic magmas were extracted from this sublithospheric mantle beneath the Owyhee Plateau, and rose through the extended crust surrounding the plateau. The resulting buoyant residuum body aided in isostatically stabilizing the plateau. With the onset of diffuse extension throughout the northern Basin and Range and Oregon Plateau ca. 11 Ma, small amounts of melting in the complex and modified SCLM and residuum body were induced by heat transfer from upwelling mantle, the presence of fluids, and decompression, while minor extension of the Owyhee Plateau itself, either leading to or as a consequence of dike injection into the lithosphere, led to the eruption of HAOT-SROT magmas from aligned vents. The cessation of volcanism on the Owyhee Plateau is related to continued slab roll-back, the southwestward motion of North America, the exhaustion of easily fusible materials in the

142 mantle beneath the Owyhee Plateau, and the insulation of the lower Owyhee Plateau lithosphere with melt-depleted residuum. This model of Owyhee Plateau evolution is compatible with and provides additional support for the models of Humphreys et al. (2000) and Christiansen et al. (2002) for the evolution of the Snake River Plain and the Oregon High Lava Plains. Furthermore, it establishes the Owyhee Plateau as an important tectonic and magmatic link between these provinces. An important question remains to be answered: given the seemingly coherent behavior of the Owyhee Plateau since the middle Miocene, does the Owyhee Plateau represent a true allochthonous terrane, similar to the Blue Mountains, or is it simply a lithospheric block which was better able to withstand to the deformation of the northern Basin and Range than the lithosphere surrounding it? The eruptions of large volumes of silicic magmas and flood basalts from its extended margins hint that the plateau acted as a unique block during the early phase of Basin and Range extension, and the lack of large-displacement faults suggest that the lithosphere of the Owyhee Plateau is in some way different from the lithosphere adjacent to it. However, given the lack of geophysical data at this time regarding the structure and materials of the Owyhee Plateau’s mid- to lower crust and lithospheric mantle, it is at best premature and at worst flawed to propose that this block represents a distinct terrane. More likely, the stability of the Owyhee Plateau is a consequence of the plateau being underlain by a keel of thicker Precambrian lithospheric mantle and a buoyant body of melt depleted residuum resulting from the extraction of large volumes of basaltic magma.

143 CHAPTER 4:

THE TECTONOMAGMATIC EVOLUTION OF THE LATE CENOZOIC OWYHEE PLATEAU, NORTHWESTERN UNITED STATES: SUMMARY AND SUGGESTIONS FOR FUTURE RESEARCH

SUMMARY Late Cenozoic basaltic volcanism in the Owyhee Plateau region records evidence for a complexly structured lithospheric mantle and a complex history of fluid- and melt- metasomatism of the lithospheric and sublithospheric mantle. Sevier thrusting emplaced young, accreted oceanic lithospheric mantle over a shelf of thick Precambrian lithospheric mantle protruding from the western edge of the Wyoming Craton. This lithospheric block would become the Owyhee Plateau. Subhorizontal subduction of the Farallon Plate during the Laramide Orogeny trapped a layer of asthenosphere between the descending slab and the base of the lithosphere which evolved isotopically while fluids and possibly melts metasomatized this trapped layer and the subcontinental lithospheric mantle. Removal and foundering of the slab during post-Laramide time induced upwelling of hot, fertile asthenosphere which interacted with the formerly trapped, fluid-rich layer, triggering a massive melting event. The focus of upwelling was likely the western edge of the Owyhee Plateau, where the thicker, sub-Owyhee Plateau lithosphere was adjacent to thinner lithosphere. Thus, the largest volumes of the Steens flood basalts were erupted to the west of the Owyhee Plateau. As melt volumes waned the subcontinental lithospheric mantle began to exert a greater control on the isotopic compositions of the basalts erupted. The onset of diffuse regional extension ca. 11 Ma brought about the eruption of isotopically evolved, little fractionated basalts from vents in the interior of the Owyhee Plateau. These basalts represent both depleted mantle (HAOT) and enriched mantle (SROT) melts, with a full spectrum of basalt compositions transitional between the two (TB). Despite the differences in mantle source materials, some of these basalts were erupted contemporaneously from overlapping shields rooted on deep lithospheric

144 fractures consistent with regional stress fields. While these endmember compositions require different bulk source materials, they may share a common lithospheric mantle contaminant which is responsible for their similarly evolved isotopic characteristics. By 5 Ma, basaltic volcanism in the interior regions of the Owyhee Plateau had ceased, but small volumes of HAOT to TB lavas were erupted at the plateau’s margins. The absence of SROT compositions as well as the less isotopically evolved character of these lavas compared to those erupted between 11-5 Ma points to a significantly decreased role for the Precambrian cratonic lithospheric mantle beneath the Owyhee Plateau. A brief magmatic hiatus was followed by the resumption of basaltic eruptive activity at the Owyhee Plateau’s northern margin ca. 2 Ma, which produced not only more HAOT-TB but also mildly alkaline basalts which are the only basalts of their type observed across the entire Oregon Plateau region. These alkaline lavas have isotopic compositions that are more suggestive of a MORB-like source than any other lavas observed throughout the entire history of basaltic volcanism in the Owyhee Plateau region. To summarize, the primary contributions of this work are: 1. Geochemical, isotopic, and geochronologic characterization of late Cenozoic basaltic volcanism in the Owyhee Plateau region. 2. The definition of the Owyhee Plateau as a discrete tectonomagmatic entity in the northwestern United States, which links the Snake River Plain– Yellowstone and High Lava Plains–Newberry volcanic provinces. 3. The identification of temporally indistinguishable, overlapping basaltic vents on the Owyhee Plateau, aligned with regional extensional structures, which erupted HAOT through SROT compositions that cannot be related by simple differentiation processes but which must require the interplay of heterogeneous source materials. 4. The recognition that basalt eruptions on the Owyhee Plateau proper occurred between 11-5 Ma and produced a full spectrum of HAOT through SROT, and the recognition that eruptions became marginal to the plateau after 5 Ma and only produced HAOT though TB.

145 5. Support for the contention that the sources and processes responsible for late Cenozoic northwestern United States basaltic volcanism reside in the upper mantle and that, in the Owyhee Plateau region at least, a deep mantle plume is unnecessary. 6. Substantiation of the importance of time as a variable in the evolution of continental intraplate basalt provinces.

SUGGESTIONS FOR FUTURE RESEARCH The Owyhee Plateau exists today as a discrete tectonomagmatic entity in the northwestern United States as a consequence of the interthrusting of old and young lithospheric mantles during continental accretion and the basaltic magmatism that subsequently tapped into a modified sublithospheric mantle and this complex lithospheric mantle. The growth of continents by terrane accretion and the addition of crust by continental intraplate volcanism are important but poorly understood problems in geology. The Owyhee Plateau provides a focus area in which these processes have occurred so recently that their causes and effects may be readily identified and interpreted. Among the outstanding questions specific to the Owyhee Plateau region are the following. What is the nature of the boundary between cratonic North America and accreted lithosphere in the Owyhee Plateau region? This boundary has long been considered to be subvertical (e.g., Armstrong et al., 1977), but isotopic mapping indicates that the cratonic and accreted mantle lithospheres in the Owyhee Plateau region are interthrust over a distance of perhaps 150 km (Leeman et al., 1992). Such thrusting would presumably result in the displacement of the cratonic mantle keel westward of the crustal locus of the suture, which would act to stabilize this region (the Owyhee Plateau) against subsequent rifting. However, this model creates a problem. The Owyhee Range, immediately east of the Owyhee Plateau, which should overlie thick, relatively stable cratonic lithosphere, has experienced late Cenozoic uplift; evidence for this is in the form of middle Miocene volcanics unconformably overlying exhumed Mesozoic granitoid basement. One potential explanation is that the overthrusting/interthrusting of accreted terranes onto the edge of the craton acted as a “wedge” to lift the Owyhees

146 upward, but this begs several more questions: (1) What is the nature of the thrusting? Is it a simple overthrust or a complex “interfingering” of cratonic and accreted lithospheres at the mantle level? How did the crust respond to thrusting? (2) If the westward jutting “shelf” of cratonic mantle is responsible for stabilizing the Owyhee Plateau, how could basaltic magmas have ascended and been erupted in the Owyhee Range concurrently with the initial Steens flood basalt episode? Could they have traversed the lithosphere along the decollement, or is there a weak zone in the lithospheric mantle below the crustal locus of the Idaho Suture Zone, and how would such a weak zone affect the subsequent stability of the Owyhee Plateau? (3) What implications does this have for the evolution of large silicic volcanic systems (e.g. the Juniper Mountain volcanic field; Manley and McIntosh, 2003), also located at the eastern margin of the Owyhee Plateau? The acquisition of detailed geophysical data for the Owyhee Plateau region, which at the present time is almost entirely nonexistent, will contribute greatly to the resolution of these issues. To what degree has the Owyhee Plateau been extended, and what is the nature of the extension? The orientation of basaltic vent alignments consistent with regional extensional features alludes to some amount of extension of the Owyhee Plateau, but the near-absence of large displacement normal faults within the plateau’s boundaries suggests that the degree of extension is probably limited. However, this is speculative; fractures and post-11 Ma vent alignments which parallel ca. 17-13 Ma extensional features (e.g. Northern Nevada Rift and Oregon-Idaho Graben; Zoback et al., 1994 and Cummings et al., 2000), indicate that the Owyhee Plateau experienced brittle deformation contemporaneous with the rifting and flood basalt episodes, so it is possible that some simultaneous extension may have been accommodated through dike injection. Regardless, it seems that the Owyhee Plateau was able to resist diffuse lithospheric thinning associated with the phase of northern Basin and Range extension which began ca. 11 Ma; magmas generated at this time simply ascended along the previously established weak zones, producing the observed vent alignments. To fully address this question, however, detailed geophysical studies of the Owyhee Plateau lithosphere are required.

147 What influence, if any, has the presence of melt-depleted residuum at the base of the lithosphere contributed to the stabilization of the Owyhee Plateau? The extraction of massive volumes of flood basalt from the mantle ca. 17 Ma would predictably leave a body of buoyant, melt-depleted residuum at the base of the lithosphere, which may have acted to isostatically stabilize the Owyhee Plateau; however, the thickness and lateral extent of such a body is unknown. If the inferred lithospheric geometry is correct, with thicker lithosphere beneath the Owyhee Plateau than is present to the west, then upwelling and melting were likely focused at the western edge of the Owyhee Plateau, in which case the bulk of the residuum would most likely reside beneath the more obviously thinned lithosphere of the Steens-Sheepsheads-Pueblo Mountains areas. Regardless of its influence on Owyhee Plateau stability, this residuum would influence upper mantle flow and has important implications for Snake River Plain–Yellowstone and High Lava Plains–Newberry development (e.g., Humphreys et al., 2000). Again, detailed geophysical data is need to address this question. What is the nature of the HAOT–SROT association? HAOT lavas are found throughout the northwestern United States, from the Cascades to Yellowstone; however, they are dominant on the Oregon Plateau, and represent only a small fraction of the basaltic output on the Snake River Plain. SROT lavas are observed no further west than the Owyhee Plateau. It is only on the Owyhee Plateau that both basalt types are found in significant quantity, and with a full spectrum of transitional compositions. An obvious control on SROT genesis is the presence of the Precambrian enriched lithospheric mantle of the Wyoming Craton. The nature of HAOT has been more difficult to constrain, but requires a depleted mantle source and a lithospheric mantle contaminant less mafic than peridotite (Hart et al., 1997). A role for a lithospheric mantle contaminant has also been indicated for SROT genesis (Reid, 1995). The identification of spatially and temporally overlapping basaltic vents on the Owyhee Plateau, aligned on apparent lithosphere-scale fractures, which erupted these dissimilar compositions, as well as compositions transitional between them, provides an opportunity to examine the HAOT–SROT association in great detail. While simple differences in isotopic and trace element characteristics of lavas of similar age preclude the evolution of these HAOT and SROT lavas from a single parent magma or even their derivation from the same bulk source

148 materials, the similarity in LILE enrichment as well as the nonsystematic variation of isotopic characteristics with bulk composition in lavas of similar age hints that the contaminant in both basalt chemical types may be the same, as suggested in Chapter 2. Detailed study of HAOT–SROT genesis in the context of one (or more) of these vent groups would eliminate the influence of lateral lithospheric variations and minimize the issue of time, as these appear to be monogenetic fields. As such, the issues of depth of melting, source materials and assimilants/contaminants, and subsequent magma evolution may be addressed more directly than has previously been possible. What is the origin of the <250 ka mildly alkaline basaltic magmatism at the northern margin of the Owyhee Plateau? While these lavas are compositionally similar to those that dominate the central and southern (Leeman and Rogers, 1970), they are anomalous not only on the Owyhee Plateau but across the entire Oregon Plateau. What similarities exist between the lithosphere and sublithospheric mantle in this transition region between the apparently little extended Owyhee Plateau and the Oregon-Idaho Graben, and the Basin and Range far to the south? And how has Owyhee Plateau–Oregon-Idaho Graben transition region evolved so that it has only been possible to generate such alkaline magmas here during the last 0.25 m.y.? Detailed petrologic studies of this young alkaline magmatism are required to address source- and process-related issues, while geophysical studies of this transitional region are necessary to understand the nature of the Owyhee Plateau’s northern boundary and the transition into the Oregon-Idaho Graben.

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161

APPENDIX 1:

ANALYTICAL METHODS

162 APPENDIX 1: ANALYTICAL METHODS

SAMPLE PREPARATION Basalt samples were prepared for analysis as follows. Large hand samples were divided using either a hydraulic rock splitter or a water cooled diamond saw; a representative sample of each was archived for reference and possible future use. Slabs were cut for thin sections and standard petrographic thin sections were obtained commercially. The remaining sample was trimmed of weathering rinds and if possible vesicles bearing secondary mineralization were removed, using either the splitter or the saw, as appropriate, and the sample was reduced to fragments that could be passed through a jaw crusher. A lap wheel or a silicon carbide belt grinder was used to remove any metal shavings from the fragments prior to crushing. The samples were washed in deionized water and allowed to dry at room temperature for a minimum of 24 hours. The fragments were then passed through a Braun Chipmunk brand jaw crusher with common steel crushing plates. After a primary coarse crush, the sample was handpicked, if necessary, to remove any secondary vesicle mineralization, and the sample was passed again through the jaw crusher to further reduce it. The entire volume of sample from the jaw crusher was then passed through a Braun brand pulverizer (“disk mill”) with alumina ceramic crushing plates. This volume was then divided using a “cone and quartering” method to obtain a 30 ml aliquot. This aliquot was then placed in a Spex brand alumina ceramic shatterbox for 15 minutes to ensure a sufficiently fine grained powder for flux fusions. Some samples used in this study had been previously reduced to powders; these samples were divided, as described above, and final crushing was performed using either the shatterbox or an alumina ceramic ball mill. Final powders were placed in borosilicate glass vials, dried at 110°C for a minimum of 12 hours, capped and stored in a desiccator.

163 LOSS ON IGNITION (LOI) LOIs were determined for each sample by first weighing approximately 1 gram of sample powder into a ceramic crucible and recording the weight. The powder in the crucible was then heated to 950°C for one hour, allowed to cool in a desiccator, and weighed again. The LOI was calculated by the following equation:

LOI = ((weightinitial - weightfinal)/weightinitial)*100 to yield the total loss on ignition, expressed as a weight percent.

MAJOR AND TRACE ELEMENT ANALYSIS Whole-rock major and trace element analyses were performed using both direct current argon plasma spectrometry (DCP) and X-ray fluorescence (XRF). A subset of the samples in this study had been previously analyzed by XRF; the major element analyses of these samples were retained (for a discussion of these methods, see Hart, 1982), but the samples were re-analyzed for trace elements. New XRF trace element analyses (Rb, Sr, Y, Zr, Nb, Ni, Ga, Cu, Zn, U, Th, Pb, Ba, Ce, Co, and La; ± Cr, V, and Sc) were performed on all samples in this study by Dr. Stan Mertzman at Franklin and Marshall College, following the methods described in Boyd and Mertzman (1987). For all other samples, major elements and select trace elements (Cr, V, and Sc, if not determined by XRF) were analyzed by DCP at Miami University using a Beckman SpectraScan V direct current argon plasma spectrophotometer. Samples were prepared for DCP analysis by manually mixing 200 mg of sample powder with 600 mg of purified

LiBO2 flux (Spectroflux 100A, manufactured by Johnson Mathey Materials Technology), and fusing this mixture in a graphite crucible in a 950ºC furnace for 15 minutes. The resulting molten bead was dropped into 50 ml of a 6% HNO3 solution spiked with 3000 ppm Li, 10 ppm Ge, and 20 ppm Cd; shaken vigorously until dissolved; and allowed to stand for at least 12 hours. This was the trace element solution for analysis. For major element analysis, 1 ml of the trace element solution was added to 25 ml of a 6% HNO3 solution spiked with 3000 ppm Li and 30 ppm Ge. The Li spike in the stock solution reagents acts as a plasma enhancer and a matrix suppressant. The Cd (trace elements) and Ge (major elements) spikes are used as internal references to which background

164 corrected element intensities are normalized, minimizing the effects of machine/plasma

drift during the analysis run. One procedural blank (LiBO2 only) and 7-8 international rock standards were prepared along with the samples for each set of analyses. Precise masses of all solids and solutions were recorded throughout so that accurate dilution factors and final concentrations could be determined. Each sample solution was analyzed three times and each standard solution and blank was analyzed four times. Two multi- element cassettes were used, allowing simultaneous determination of major elements (cassette 1) and trace elements (cassette 2). Data collection and reduction were performed by an on-line computer with software written specifically for the Miami University DCP. Both major and trace element concentrations were determined by comparison of unknown intensity ratios to calibration curves constructed from rock standard concentrations determined with each set of samples.

ISOTOPE ANALYSIS Selected samples were analyzed for Sr, Nd, and Pb isotopes. With the exception of those samples previously analyzed (Hart, 1985; Carlson and Hart,1987), Sr isotopes were determined at either Miami University or at the Department of Terrestrial Magnetism, Carnegie Institution of Washington (DTM), and all Nd and Pb isotopes were determined at DTM. Sr isotopes All Sr column chemistry was performed at Miami University. Approximately 50 mg of sample was dissolved in 1 ml purified concentrated HF and 0.5 ml purified concentrated HNO3 by placing sample and acids in a capped 15 ml Teflon beaker on a hot plate on low with a heat lamp for 36-48 hours. Once sample was in solution, the beakers

were uncapped and evaporated to dryness; 0.5-1 ml HNO3 was added and the sample was re-dried; and this final step was repeated. 1 ml of concentrated (12N) HCl was added, the beakers capped and allowed to sit for 1 hour; 2 ml of quartz distilled water was then added, the beakers capped and placed on a hot plate for at least 6 hours. The beakers were then uncapped and evaporated to dryness again. In preparation for loading on the Sr columns, the samples were taken back into solution with 2 ml 1.25N HCl, capped and left

165 to sit overnight, and centrifuged just prior to introduction on the columns. The Sr columns were filled with AG50W-X8 resin, which was backwashed in 2.5N HCl and allowed to settle. 50 ml of 2.5N HCL was added to each column to charge the resin. 2 ml of sample solution was pipetted onto the resin, followed by 5 ml of 2.5N HCl in 1 ml increments. 65 ml of 2.5N HCl was then added and allowed to pass through the resin; this and all previous effluents from the columns were discarded. A clean, labeled collection beaker was then placed below the column and 30 ml of 2.5N HCL was added to flush the Sr from the column. This effluent was then evaporated to dryness using a hot plate and a heat lamp in order to recover the Sr cut from the sample. For Sr isotope analysis by thermal ionization mass spectrometry, Ta filaments

were prepared by drying a small amount of Ta2O5 on the filament. The (dry) sample was

then taken back into solution with HNO3, and 2 µl of this liquid was then loaded, one droplet at a time, on the filament and dried. Sr isotopes were analyzed on a Nuclide single collector mass spectrometer (Miami University) and a VG-354 mass spectrometer using a four collector dynamic measurement routine (DTM). Details of data treatment are described in Appendix 2B. Pb and Nd isotopes All Nd and Pb column chemistry was performed at DTM. Samples were spiked with DTM Sm-Nd spike 97-2-138. Approximately 300 µl of spike and 100 mg of sample were placed in a Teflon beaker. 1 ml purified concentrated HF and 0.5 ml purified

concentrated HNO3 were added to dissolve the samples. The beakers were left uncapped on a hot plate on low for 1 hour, then capped and left on the hot plate with a heat lamp for 36-48 hours. Once sample was in solution, the beakers were uncapped and evaporated to

dryness; 0.5-1 ml HNO3 was added and the sample was re-dried; and this final step was repeated. 0.5 ml of concentrated (12N) HCl was added, the beakers capped and allowed to sit for 1 hour; quartz distilled water was then added to about half the beaker volume, the beakers capped and placed on a hot plate for at least 6 hours to ensure that the entire sample went into solution. The beakers were then uncapped and evaporated to dryness again. In preparation for loading on the Pb columns, 1 ml 0.5N HBr was added to the

166 samples, swirled, and the samples were dried down slowly; this step was repeated. 3 ml HBr was then added and the beakers were capped and left to sit overnight. Samples were centrifuged prior to loading on Pb columns. The Pb columns were made from 5 ml syringe bodies fitted with 5 cm long Teflon tubes with pieces of frit jammed in them. They were filled with approximately 0.1 ml of cleaned AG1-X8 100-200 mesh anion

exchange resin. 12 ml of 8N HNO3 was added in 6 ml increments to clean the column

and strip any environmental Pb from the columns. 6 ml of 0.5N HNO3 was added to - columns and allowed to drain, followed by 0.5 ml of H2O to remove NO3 . The columns were charged with 0.5 ml of 0.5N HBr. The samples were added in 3.0 mL of 0.5N HBr. The columns were then eluted with 0.3 ml of 0.5N HBr; 0.5 ml of 0.5N HBr; and 0.5 ml of 0.5N HBr. These aliquots, which contain the Nd cut, were collected in the original sample beakers and dried on a hot plate. The columns were then eluted with 1.0 ml of 0.5N HBr to release the Pb; this was collected in a 5 ml snap cap Teflon beaker. This beaker containing the Pb cut was then placed on a hot plate to dry. The columns were

recleaned with two 6 ml elutions of 0.5N HBr followed by 0.5 ml of H2O. The resin was again charged with 0.5 ml of 0.5N HBr. 1 ml of 0.5N HBr was added to the Pb sample and this was added to the column. The columns were eluted with 0.5 ml of 0.5N HBr twice, and the 1.0 ml of 0.5N HBr was added to release the Pb. The Pb cut was collected in the snap cap beakers, dried, and capped. Nd was separated from the samples through a two step process using primary columns to first separate the rare earth elements (REE), and secondary columns to separate Nd and Sm from the REE. In preparation for loading on the primary columns, the sample was dissolved in HNO3, redried, and dissolved in 2 ml of 1.25N HCl. The Teflon primary columns were filled with AG50W-X8 cation exchange resin, which was backwashed in 2.5N HCl and allowed to settle. 20 ml of 2.5N HCL was added to each column to charge the resin. 0.5 ml of quartz distilled H2O was pipetted onto the resin and allowed to drain. The sample was pipetted onto the resin in 0.5 ml increments. 2 ml of 2.5N HCl was added in 1 ml increments, then 44 ml of 2.5N HCl was added, followed by 12 ml of 4.0N HCl; all effluents to this point were discarded. 16 ml of 4.0N HCl was added and this was collected in a 30 ml Teflon beaker and placed on a hot plate with a

167 heat lamp to dry; this cut contained the REE. The primary columns were cleaned with 6N HCl. In preparation for loading on the secondary columns, 1 drop of 0.1N HCl was placed on the dry REE cut. The columns consisted of glass tubes with Teflon tubes jammed with frits at the tips; the reservoir was a syringe body. The REE were

selectively eluted with methyllactic acid (MLA) under N2 gas using AG50-X4 200-400 mesh cation exchange resin. The syringe body was attached to the glass tube and filled with water. A stopper attached to the N2 gas was inserted and the gas was turned on to remove air bubbles. The stopper is removed and 1 ml of resin was added as a slurry. The resin was allowed to settle and a Pasteur pipette was used to remove excess water and adjust the height of the resin. The columns were charged with 7 ml of MLA and after making sure the columns were flowing the gas stopper was reinserted and the gas was turned on. When only 1 ml of MLA remained the stopper was removed and a Pasteur pipette was used to remove excess resin to the calibrated level. The syringe body was removed, rinsed with quartz distilled water and set aside. 1 drop of H2O was added to each column and a small pipette was used to remove the air bubble so that the column flowed freely. The samples were loaded, allowed to drain, and then 1 drop of H2O was added. MLA was added to the top of the column and the syringe body was replaced. The syringe was filled to exactly 10 ml with MLA. The stopper was placed back in the syringe and the gas was turned on. The elution from 10.0 to 8.4 ml was discarded; the elution from 8.4 to 6.6 ml contained the Sm and was collected in a small snap-cap Teflon beaker; the elution from 6.6-3.0 ml was discarded; and the elution from 3.0 to 0.0 ml contained the Nd and was collected in a small snap-cap Teflon beaker. The Sm and Nd cuts were dried on a hot plate with the assistance of a heat lamp and stainless stell sleeves surrounding the beakers. When dry the Sm and Nd cuts were refluxed with 1-2 drops of concentrated HNO3. Pb and Nd isotopes were analyzed on a VG-P54 multi-collector inductively coupled plasma mass spectrometer with nine Faraday cups in a three-step dynamic

collection routine. Dry Nd samples were returned to solution using 5% HNO3 and quartz

distilled H2O to yield an optimal concentration of 150 ng/mL. Dry Pb samples were

168 returned to solution using 5% HNO3 to yield an optimal concentration of 100 ng/mL, and thallium was added as an internal isotopic standard to correct for instrumental isotopic mass fractionation. Details of Nd and Pb data treatment are described in Appendix 2B.

40Ar/39Ar GEOCHRONOLOGY 40Ar/39Ar method geochronology was obtained through the New Mexico Geochronological Research Laboratory at New Mexico Tech through an arrangement with Dr. Matthew Heizler. Groundmass concentrates of whole-rocks were obtained by coarsely crushing samples and sieving out the 20 to 48 mesh fraction (finer if necessary, coarser if allowable). This fraction was washed twice in distilled deionized water, then placed in an ultrasonic bath of 10% HCl for 5 minutes to dissolve secondary carbonate. The sample was then again washed twice in distilled deionized water, placed in an ultrasonic water bath for 3 minutes, and washed 3 more times in distilled deionized water to ensure that all traces of HCl were removed. Samples were dried, then a minimum of 100 milligrams of clean, unaltered, phenocryst-free grains were handpicked under a binocular microscope. The groundmass concentrates were then placed in machined Al discs along with interlaboratory standard FC-1 (Fish Canyon Tuff, assigned age 27.84 Ma (Deino and Potts, 1990) relative to Mmhb-1 at 520.4 Ma (Samson and Alexander, 1987)) as the neutron flux monitor. After sealing stacked discs in an evacuated Pyrex tube, the samples and monitors were irradiated for 7 hours in the D-3 position of the reactor at the Nuclear Science Center, Texas A&M University, College Station, Texas. Following irradiation, samples and monitors were placed within an automated ultra-high vacuum extraction system where they were heated using resistance-furnace or laser methods to extract argon. Individual crystals of monitor sanidine were placed in a

copper planchet and fused with a 10W Synrad CO2 continuous laser. Evolved gases were purified of reactive species for two minutes using a SAES GP-50 getter operated at ~450°C. Whole-rock samples were step-heated in a double-vacuum Mo resistance furnace. Evolved gases were purified during heating with a SAES GP-50 getter for eight minutes, followed by six minutes of cleanup using a separate GP-50. Argon isotopic

169 compositions were analyzed by a Mass Analyzer Products 215-50 mass spectrometer operated in static mode. Argon isotopes were detected by an electron multiplier with an overall sensitivity of about 1 x 10-16 moles/pA. Extraction system and mass spectrometer blanks and backgrounds were measured numerous times throughout the course of the analyses. Typical laser blanks (including mass spectrometer backgrounds) were 470, 3.0, 0.6, 3.0, 3.0 x 10-18 moles at masses 40, 39, 38, 37, 36 respectively. Typical furnace blanks were 500, 9.0, 2.7, 6.0, 1.8 x 10-18 moles at masses 40, 39, 38, 37, 36 respectively. J-factors were determined to a precision of ±0.1% (1σ) by analyzing four single crystal aliquots from each of 6 radial positions around the irradiation vessel. Correction for interfering nuclear reactions were determined using K- and Ca-rich glasses and salts. Correction factors are given in Appendix 2D. The reported ages and errors are calculated by using the inverse variance as the weighing scheme (Samson and Alexander, 1987). The decay constant and isotopic abundances are those suggested by Steiger and Jaeger (1977).

170

APPENDIX 2:

DATA

171 APPENDIX 2A: SAMPLE LOCATIONS AND DESCRIPTIONS

Sample locations, descriptions of outcrop/field occurrence and handsamples, and petrographic descriptions of the 170 samples in this study are provided in the following section. Representative samples were point counted using a Swift Automatic Point Counter with a target of 1000 counts and a step interval of 2. Samples are divided into the geochemical types and age groups described in Chapter 2, and ranked within those groups according to increasing analytical TiO2. This is the same order used in Appendix 2B and Appendix 2C.

172

Sample ID: JV96-7 Chemical type: Group 1 (SBOIG) Lat.: 43.12 Long.: 117.03 7.5’ topo quad: Hooker Creek, OR-ID Locality: Sheaville quarry area Description: Exposure in recent quarry cut. Wall exposed ~75-100 m with dominant horizontal/horizontal-U-shaped columnar joint pattern. At southern edge of exposure this pattern merges with vertical jointed, more flow like feature. This area is probably a core/conduit of a vent of fissure system with one exposed flow associated with the vent/fissure. Also exposed in the quarry area is finely laminated tuffaceous sediment with porcelainized areas but relationship to basalt is not obvious. Sample from core of a columnar joint from conduit region. Very fine grained, somewhat finely vesicular, black, appears very fresh. Holocrystalline groundmass of 0.25-0.5 mm plag, cpx and abundant oxides with tiny intergranular olivine. Pervasive clay alteration and secondary chalcedony. Sparse 0.5 mm zoned plag phenos and sparse 0.5 mm cpx (augite).

Sample ID: JV96-6 Chemical type: Group 1 (SBOIG) Lat.: 43.16 Long.: 117.04 7.5’ topo quad: Sheaville, OR-ID Locality: US 95, 5 miles south of Succor Creek Road, near Sheaville Description: Basalt in contact with white, laminated, tuffaceous, lacustrine sediment. Appears that basalt underlies sediment, but lower portion of sedimentary unit in contact with basalt is oxidized and turned from indurated material to massive porcelain-like material – it looks baked. Possible that this basalt exposure, only ~5-8 ft thick in exposure, is an intrusive sill/dike into sedimentary unit; unfortunately lower contact of basalt not exposed. Dark gray, somewhat open textured, abundant pheno- and microphenocrysts of plag. Holocrystalline groundmass of 0.25-0.75 mm plag and cpx with intergranular oxide and olivine. Pervasive clay alteration. Minor secondary chalcedony. Plag phenocrysts up to 2 mm.

Sample ID: JV96-3 Chemical type: Group 1 (SBOIG) Lat.: 43.18 Long.: 117.18 7.5’ topo quad: Mahogany Gap, OR Locality: Spring Mountain Description: Eroded rubble-crop of basalt on south flank of Spring Mountain. Dark gray, massive, generally fine grained with pheno- and microphenocrysts of plag and microphenocrysts of iddingsitized olivine. Some material in rubble crop is more vesicular with significant secondary vesicle filling. Holocrystalline, fine grained groundmass of cpx and 0.25 mm plag with abundant oxide and olivine (<0.1 mm). Sieved plag laths 0.5-1.0 mm and subhedral 0.25-0.5 mm olivine strongly altered to clay.

Sample ID: JV96-4 Chemical type: Group 1 (SBOIG) Lat.: 43.21 Long.: 117.14 7.5’ topo quad: Mahogany Gap, OR Locality: Spring Mountain Description: Top flow forming mesa-like top of Spring Mountain. Massive ~20 ft thick flow (in exposure) with crude columnar joints developed locally. Similar to JV96-3 with pheno- and microphenocrysts and laths of plag and microphenocrysts of iddingsitized olivine. Holocrystalline groundmass of plag laths (<0.25 mm) with much less cpx, abundant intergranular oxide and tiny (<0.1 mm) olivine. Euhedral to subhedral olivine microphenos and phenos 0.25-1.0 mm with oxide inclusions; larger olivines have strongly altered rims, smaller olivines are altered pervasively. Plag 0.5-1.0 mm with minor sieving. Some plag in glomeroporphyritic clumps with olivine.

173

Sample ID: JV96-5 Chemical type: Group 1 (SBOIG) Lat.: 43.21 Long.: 117.14 7.5’ topo quad: Mahogany Gap, OR Locality: Spring Mountain Description: Base of jointed wall exposure overlying mafic tuffaceous breccia unit on northwest flank of Spring Mountain. Sample is massive, appears glassy with abundant feldspar phenocrysts (some up to 5 mm and generally fresh), some feldspar microphenocrysts (slightly yellowish), also microphenocrysts of iddingsitized olivine. Holocrystalline, fine groundmass of cpx, plag (<0.25 mm), abundant oxides and small iddingsitized olivine. Phenocryst assemblage is plag laths 0.5- 1.0 mm; blocky plag zoned with strongly sieved cores; and subhedral to rounded olivine with iddingsitized rims.

Sample ID: H85-6 Chemical type: Group 1 (SBOIG) Lat.: 42.84 Long.: 117.22 7.5’ topo quad: Juniper Ridge, OR Locality: south of Antelope Reservoir Description: Along road heading north out of Dewey along base of Glass Butte. Sample of “Lower Basalt” underlying latite along small stream cut. Dark gray-black, massive, aphanitic groundmass with scattered plag (weathered/altered) phenos >2 cm. Some visible cpx. Also quartz fillings in vesicles. Holocrystalline, subophitic groundmass of cpx (Ti-augite?) and 0.25 mm plag with intergranular oxide and olivine. Abundant clay alteration and secondary chalcedony in vesicles. Blocky plag phenocrysts 1.0-2.5 mm, some appear resorbed.

Sample ID: H85-10A Chemical type: Group 1 (SBOIG) Lat.: 42.73 Long.: 117.05 7.5’ topo quad: Juniper Point, OR-ID Locality: ~17 miles south of Jordan Valley, near Oregon-Idaho border Description: Top flow of at least 4 flows exposed in ~N-S trending fault scarp defining upthrown west side of valley. All flows dip ~35-40º to west. Medium gray, fine-medium grained, crudely platy and jointed along dip. Sugary texture with obvious plag laths (up to 0.5 cm) and dark green/black phenos (cpx?). Holocrystalline, slightly subophitic, very fine grained groundmass of cpx and plag (<0.25 mm) with abundant intergranular oxide and some olivine. Euhedral 1 mm olivine (strongly iddingsitized), blocky 1-2 mm resorbed plag phenocrysts, and 0.5 mm cpx (resorbed/altered?).

Sample ID: H85-10B Chemical type: Group 1 (SBOIG) Lat.: 42.73 Long.: 117.05 7.5’ topo quad: Juniper Point, OR-ID Locality: ~17 miles south of Jordan Valley, near Oregon-Idaho border Description: Lowermost exposed flow of at least 4 flows in ~N-S trending fault scarp defining upthrown west side of valley. Pervasively altered. Outcrop is somewhat columnar jointed and highly fractured. Not as platy as upper flow. Dark/medium gray, massive, fine grained, equigranular patches of oxidized mafics? Holocrystalline subophitic very fine grained groundmass of cpx, plag (.01 mm), oxides and olivine with abundant coarse (0.25 mm) oxides and completely altered olivine. Isolated 0.5 mm plag laths.

Sample ID: CH82-32 Chemical type: Group 1 (SBOIG) Lat.: 43.23 Long.: 116.88 7.5’ topo quad: Captain Butte, ID Locality: south of Squaw Butte, Owyhee Mountains Description: see Hart and Carlson, 1985

174

Sample ID: H9-47 Chemical type: Group 1 (SBOIG) Lat.: 42.92 Long.: 116.98 7.5’ topo quad: Stonehouse Creek, ID Locality: ~5 miles southeast of Jordan Valley, on road to Triangle Description: see Hart, 1982

Sample ID: H85-26A Chemical type: Group 1 (SBOIG) Lat.: 42.93 Long.: 116.76 7.5’ topo quad: Flint, ID Locality: abandoned mine site, head of Flint Creek, Owyhee Mountains Description: Sample from ~50 ft thick exposure of nearly vertically columnar jointed massive basalt overlying apparent basaltic volcanic breccia. Sample has very fine grained black matrix with some visible plag and possible olivine grains. A few vesicles filled with secondary material. Some indication that unit may have been silicified, but sample looks fresh. Holocrystalline, subophitic, very fine grained groundmass of cpx and plag (<0.25 mm) with intergranular oxides and olivine. Some clay alteration and secondary carbonate present. Sparse 0.25 mm olivine microphenocrysts.

Sample ID: JV96-1 Chemical type: Group 1 (SBOIG) Lat.: 42.99 Long.: 117.07 7.5’ topo quad: Jordan Valley, OR-ID Locality: Jordan Valley quarry area Description: New opening in quarry north of Jordan Valley. Contorted columnar jointed basalt and small dikelets of basalt. Entire mass appears to be some shallow intrusive – host is highly brecciated tuffaceous material. Appears to be a vent area. Sample is glassy, plag-phyric basalt from large column. Hypocrystalline, very fine grained groundmass of glass, tiny plag laths, cpx (?) and opaques. Pervasive clay alteration. Phenocryts are plag (up to 2.5 mm, some in glomeroporphyritic clumps), a mafic phase (cpx?) completely altered to clays and oxides (up to 1 mm), and oxides. Mode: groundmass 73.5%; plag phenos 18.5%; altered mafic phenos 6.7%; oxide phenos 1.2%

Sample ID: JV96-2 Chemical type: Group 1 (SBOIG) Lat.: 42.99 Long.: 117.07 7.5’ topo quad: Jordan Valley, OR-ID Locality: Jordan Valley quarry area Description: Old excavation in quarry north of Jordan Valley. Possible outflow from vent area of JV96-1. Some remnant columnar jointing. Inside flow of basalt, similar but less glassy than JV96-1. Holocrystalline, subophitic, very fine grained groundmass of cpx (Ti-augite) and plag (<0.25 mm) with abundant intergranular oxide and sparse tiny olivine. Pervasive clay alteration. Glomeroporphyritic clumps of plag (up to 2 mm; some zoned and with sieved centers) and rounded olivine (0.5 mm).

Sample ID: H85-12B Chemical type: Group 1 (SBOIG) Lat.: 42.84 Long.: 117.15 7.5’ topo quad: Juniper Ridge, OR Locality: Juniper Ridge, east of Antelope Reservoir Description: At small stream gully where it crosses road cutting up southern fault rim by Antelope Reservoir. Basalt flow underlying silicic material. Medium-dark gray with scattered veins of clay alteration. Large (>2 cm) plag phenocrysts. Iddingsitized olivine and some possible cpx microphenocrysts. Holocrystalline, subophitic very fine grained groundmass of cps and plag (<0.25 mm) with abundant intergranular oxides and small red olivines. Zoned, resorbed 1 cm plag phenocrysts and o.25 mm oxidized and iddingsitized olivines.

175

Sample ID: H8-74 Chemical type: Group 1 (SBOIG) Lat.: 42.92 Long.: 118.00 7.5’ topo quad: Johnny Creek Spring, OR Locality: Oregon Highway 78, ~13.5 miles northwest of Burns Junction Description: see Hart, 1982

Sample ID: H9-32 Chemical type: Group 1 (SBOIG) Lat.: 42.92 Long.: 118.04 7.5’ topo quad: Johnny Creek Spring, OR Locality: Oregon Highway 78, ~14 miles northwest of Burns Junction Description: see Hart, 1982

Sample ID: CM97-13 Chemical type: Group 2 (HAOT) Lat.: 42.07 Long.: 117.30 7.5’ topo quad: Oregon Butte, OR Locality: Twin Buttes Description: Downslope from southwest summit of Twin Buttes into saddle between summits. Exposure of in-place flow. Fine grained, medium gray with purplish tint, open textured, phenocrysts of olivine and plag, large glomeroporphyritic clumps of olivine and plag. Holocrystalline, ophitic- subophitic, diktytaxitic groundmass of cpx and plag (0.25 mm) with very abundant intergranular oxides and tiny (<0.1 mm) strongly oxidized olivine. Anhedral <0.5 mm olivine phenocrysts with strongly oxidized rims and glomeroporphyritic clumps of plag laths <0.5 mm.

Sample ID: CM97-14 Chemical type: Group 2 (HAOT) Lat.: 42.07 Long.: 117.30 7.5’ topo quad: Oregon Butte, OR Locality: Twin Buttes Description: Northeast summit of Twin Buttes. Rubbly outcrop on summit. Fine grained, medium gray, open textured, pockets of vesicles with secondary mineralization, small phenos of plag and olivine and small glomeroporphyritic clusters. Holocrystalline, ophitic-subophitic, diktytaxitic groundmass of cpx and plag (<0.25 mm)with abundant intergranular olivine (fresh to slightly oxidized) and minor oxides. Glomeroporphyritic clumps of plag (0.5-1.0 mm) with rounded olivine (0.25 mm).

Sample ID: CM97-15 Chemical type: Group 2 (HAOT) Lat.: 42.07 Long.: 117.30 7.5’ topo quad: Oregon Butte, OR Locality: Twin Buttes Description: High point on northeast summit of Twin Buttes. Fine grained, medium to dark gray, slightly open textured, weathered olivine phenocrysts and small plag laths; small glomeroporphyritic clusters. Holocrystalline, ophitic-subophitic, diktytaxitic groundmass of cpx and plag (<0.25 mm) with abundant intergranular olivine (0.1 mm, fresh to slightly oxidized) and minor oxides. Glomeroporphyritic clumps of plag (0.5-1.0 mm) with rounded olivine (0.25 mm) with oxidized rims. Minor secondary carbonate in vesicles.

Sample ID: H9-27 Chemical type: Group 2 (HAOT) Lat.: 42.14 Long.: 117.31 7.5’ topo quad: Guadalupe Meadows, OR Locality: West Little Owyhee River at Louse (LaRosa) Canyon Description: see Hart, 1982

176

Sample ID: H9-30 Chemical type: Group 2 (HAOT) Lat.: 42.66 Long.: 117.80 7.5’ topo quad: Anderson Reservoir, OR Locality: Rattlesnake Creek Road at north end of Bowden Hills, ~3.5 miles from U.S. 95 Description: Road cut in flow in valley. Medium gray, very open textured, abundant iddingsitized olivine with plag and cpx. Holocrystalline, ophitic-subophitic, diktytaxitic groundmass of cpx and 2 mm plag laths with abundant 0.1 mm iddingsitized olivine and intergranular oxide. Minor clay alteration. Euhedral to subhedral 0.5-1.0 mm olivine phenocrysts with iddingsitized rims. Mode: plag 34.4%; olv 21.2%; cpx 35.5%; oxides 8.7%

Sample ID: CM97-12 Chemical type: Group 2 (HAOT) Lat.: 42.06 Long.: 117.31 7.5’ topo quad: Oregon Butte, OR Locality: Twin Buttes Description: Southwest summit of Twin Buttes. Rubbly, oxidized, vesicular outcrop. Fine grained, medium to dark gray, very open textured, olivine phenocrysts, small glomeroporphyritic clumps of olivine and plag. Some vesicles with secondary mineralization. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and 0.25 mm plag with abundant intergranular 0.1 mm olivine (fresh to slightly oxidized) and oxides. Subhedral fractured olivines (<0.75 mm) with slightly altered rims and plag laths up to 1 mm; some olivine and plag in glomeroporphyritic clumps.

Sample ID: H9-28A Chemical type: Group 2 (HAOT) Lat.: 42.68 Long.: 117.87 7.5’ topo quad: Anderson Reservoir, OR Locality: Crooked Creek at U.S. 95, south of Burns Junction Description: Basalt flow overlying platy, highly weathered basaltic andesite. Sample is finely vesicular, open textured, with olivine and plagioclase in glomeroporphyritic clumps; olivine appears fairly fresh. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and 0.5 mm plag with abundant 0.2 mm rounded olivine (iddingsitized) and minor intergranular oxide. Some secondary mineralization in vesicles and minor clay alteration. Euhedral to subhedral 1-3 mm fractured olivine phenocrysts with iddingsitized rims.

Sample ID: KS96-8B Chemical type: Group 2 (HAOT) Lat.: 42.91 Long.: 117.74 7.5’ topo quad: Owyhee Butte, OR Locality: small side canyon on west side of Owyhee Canyon, ~8 miles northwest of Rome Description: Flow exposed approximately 50 ft above KS96-8A. Flow is ~20 ft thick, blocky to spheroidally weathered. Sample is light gray, medium grained with abundant feldspar. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (augite) and plag (<0.25 mm) with small intergranular olivine and oxides. Pervasive clay alteration and some secondary chalcedony in vesicles. Plag phenocrysts up to 1.5 mm.

Sample ID: KS96-8D Chemical type: Group 2 (HAOT) Lat.: 42.91 Long.: 117.74 7.5’ topo quad: Owyhee Butte, OR Locality: small side canyon on west side of Owyhee Canyon, ~8 miles northwest of Rome Description: Flow capping rim of canyon, approximately 100 ft above KS96-8C. Entire flow exposed, approximately 12 ft thick, rubbly bottom grading up into massive with a vesicular top. Sample is medium gray, medium grained and slightly vesicular, some olivine present. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (augite) and plag (<0.25 mm)with intergranular oxides and olivine. Subhedral to euhedral olivine phenocrysts up to 3 mm, fractured with oxide inclusions. Mode: plag 40.6%; olv 22.1%; cpx 32.1%; oxides 5.0%

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Sample ID: H8-73 Chemical type: Group 2 (HAOT) Lat.: 42.91 Long.: 117.99 7.5’ topo quad: Palomino Lake, OR Locality: Oregon Highway 78, ~13.5 miles northwest of Burns Junction Description: see Hart, 1982

Sample ID: KS96-8C Chemical type: Group 2 (HAOT) Lat.: 42.91 Long.: 117.74 7.5’ topo quad: Owyhee Butte, OR Locality: small side canyon on west side of Owyhee Canyon, ~8 miles northwest of Rome Description: Flow directly overlying KS98-8B. Flow is ~10 ft thick, blocky to spheroidally weathered. Sample is dark blue-gray with abundant plagioclase. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and plag (<0.5 mm). Pervasive clay alteration and secondary chalcedony in vesicles. Plag phenocrysts up to 2.5 mm and sparse olivine up to 0.5 mm.

Sample ID: H9-36A Chemical type: Group 2 (HAOT) Lat.: 42.83 Long.: 117.74 7.5’ topo quad: Rome, OR Locality: Crooked Creek at U.S. 95, northeast of Burns Junction Description: see Hart, 1982

Sample ID: H9-34 Chemical type: Group 2 (HAOT) Lat.: 42.92 Long.: 118.03 7.5’ topo quad: Johnny Creek Spring, OR Locality: Oregon Highway 78, ~14 miles northwest of Burns Junction Description: see Hart, 1982

Sample ID: CM97-4 Chemical type: Group 2 (HAOT) Lat.: 42.15 Long.: 117.13 7.5’ topo quad: Stoney Corral, OR Locality: ridge between Hoodoo Butte and Lookout Butte Description: Rubbly flow top exposure. Medium-dark gray, fine grained, slightly open textured, phenocrysts of weathered olivine (orange), no obvious plag, some secondary mineralization in vesicles. Hypocrystalline (?) subophitic, slightly diktytaxitic zones of very fine grained glass (?), cpx, 0.1 mm plag, tiny olivine and oxides interspersed with coarser zones of cpx and 0.25 mm plag with intergranular oxide and olivine. Appears to be more plag in the coarser zones. Olivine is strongly oxidized throughout. Anhedral 0.5 mm olivine phenocrysts with strongly oxidized rims.

Sample ID: KS96-8A Chemical type: Group 2 (HAOT) Lat.: 42.91 Long.: 117.74 7.5’ topo quad: Owyhee Butte, OR Locality: small side canyon on west side of Owyhee Canyon, ~8 miles northwest of Rome Description: Lowermost flow sampled in small canyon. Exposure is ~40 ft thick, somewhat spheroidally weathered. Sample is medium to dark gray, medium grained, weathered and altered, feldspars have a greenish cast, olivine is altered to clay. Nearly holocrystalline, subophitic, groundmass of cpx and plag (<0.5 mm) with small intergranular olivine and oxides. Pervasive clay alteration. Euhedral olivine phenocrysts up to 1.5 mm, highly fractured and strongly altered. PLag laths 2 mm, fractured. Mode: plag 40.3%; olv 17.6%; cpx 39.4%; oxides 2.5%

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Sample ID: KS96-5 Chemical type: Group 2 (HAOT) Lat.: 43.14 Long.: 117.64 7.5’ topo quad: The Hole In The Ground, OR Locality: along road between Riley Horn Reservoir and Mud Creek Description: Flow unit capping plain. Exposure is ~15 ft thick, massive with well jointed columns. Sample is medium gray, open textured, vesicular on one surface. Large fresh olivine phenocrysts are present. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (augite) and plag (<0.5 mm) with minor intergranular oxides and olivine. Very fresh, fractured euhedral olivine phenocrysts up to 4 mm with oxide inclusions. Mode: plag 43.6%; olv 17.2%; cpx 37.1%; oxides 1.9%

Sample ID: KS98-4 Chemical type: Group 2 (HAOT) Lat.: 41.80 Long.: 117.16 7.5’ topo quad: Maiden Butte, NV Locality: Maiden Butte Description: Summit of Maiden Butte. Exposure is very rubbly, with reddish oxidized material suggestive of fire fountaining. Sample is massive (possibly welded spatter?), medium-dark gray with a little reddish claylike(?) material. Glomeroporphyritic clumps of plag and olivine. Holocrystalline, subophitic groundmass of cpx and plag (<0.5 mm)with abundant intergranular oxides and strongly iddingsitized olivine. Glomeroporphyritic clumps of plag (2 mm) and olivine (subhedral, 0.5 mm). Olivine is fractured with red alteration products around rims and cracks. Mode: plag 35.9%; olv 27.2%; cpx 26.8%; oxides 9.9%

Sample ID: H8-98 Chemical type: Group 2 (HAOT) Lat.: 42.77 Long.: 117.87 7.5’ topo quad: Burns Junction, OR Locality: U.S. 95, 1 mile south of Burns Junction Description: see Hart, 1982

Sample ID: KS98-5 Chemical type: Group 2 (HAOT) Lat.: 41.79 Long.: 117.17 7.5’ topo quad: Maiden Butte, NV Locality: Maiden Butte Description: Southwestern flank of Maiden Butte. Rubbly flow-top exposure. Sample is medium-dark gray, slightly open textured with abundant olivine including some large glomeroporphyritic clumps. A few vesicles, some containing secondary carbonate. Holocrystalline, subophitic, slightly diktytaxitic groundmass of cpx and plag (<0.5 mm) with intergranular oxides and iddingsitized olivine. Euhedral to subhedral fractured and iddingsitized olivine phenocrysts.

Sample ID: CM97-8 Chemical type: Group 2 (HAOT) Lat.: 42.03 Long.: 117.14 7.5’ topo quad: Lookout Lake, OR Locality: Willow Creek Butte Description: Rubbly outcrop at summit of Willow Creek Butte. Fine grained, medium gray, massive with pockets of vesicles containing some secondary mineralization. Phenocrysts of plag and sparse phenocrysts of olivine. Sparse glomeroporphyritic clusters. Holocrystalline, ophitic-subophitic, diktytaxitic groundmass of cpx and plag (<0.25 mm) with abundant intergranular oxide and small (0.1 mm) strongly oxidized olivine. Lathlike and acicular plag phenocrysts up to 1.5 mm and sparse rounded strongly oxidized olivines less than 0.5 mm.

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Sample ID: CM97-2A Chemical type: Group 2 (HAOT) Lat.: 42.13 Long.: 117.18 7.5’ topo quad: Stoney Corral, OR Locality: Barking Butte Description: Exposure to west of summit of apparent vent (abundant oxidized scoriaceous material present). Medium gray, fine grained, open textured with vesicle pockets up to 3 cm diameter, weathered olivine phenocrysts (orange). Holocrystalline (?), very fine grained groundmass of cpx and plag (<0.1 mm) with abundant intergranular oxide and tiny strongly oxidized olivine. Subhedral to anhedral olivine phenocrysts 0.25-1.0 mm (fractured, strongly oxidized) with oxide inclsuions.

Sample ID: H85-11C Chemical type: Group 2 (HAOT) Lat.: 42.90 Long.: 117.25 7.5’ topo quad: Antelope Reservoir, OR Locality: Antelope Rim Description: Lowermost of three basalt flows exposed along rim bounding north side of western thumb of Antelope Reservoir. Contacts between flows show only brecciated zones, no soil horizons. Flow is ~10-15 ft thick. Sample from massive portion of flow. Light-medium gray, open textured, medium grained with abundant large glomeroporphyritic clumps of iddingsitized olivine and plag up to 2 x 2 cm. Some plag phenocrysts up to 0.5 cm and abundant smaller laths and scattered intergranular iddingsitized olivine. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (augite) and plag (<0.5 mm) with abundant intergranular olivine (0.1 mm, iddingsitized) and oxides. Plag laths 2.5 to 4 mm and subhedral to anhedral 1.5-2.5 mm olivine phenocrysts, highly fractured and with iddingsite along rims and fractures.

Sample ID: KS98-6A Chemical type: Group 2 (HAOT) Lat.: 41.75 Long.: 117.13 7.5’ topo quad: Maiden Butte, NV Locality: Little Owyhee River at Twin Valley Spring Description: Lowermost of three very similar-looking “flows” separated by platy, slightly vesicular zones (may actually be three parts of a compound flow unit). Each “flow” is ~10-15 ft thick. Sample is medium gray, open textured with an almost “sugary” appearance. Lots of olivine in glomeroporphyritic clumps. Some vesicles with secondary minerals. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (augite) and plag (<0.5 mm) with intergranular oxide and oxidized olivine. Subhedral olivine phenocrysts, some highly fractured, up to 1.5 mm with red alteration around rims.

Sample ID: KS98-2 Chemical type: Group 2 (HAOT) Lat.: 41.81 Long.: 117.18 7.5’ topo quad: Maiden Butte, NV Locality: Maiden Butte Description: Near top of high point just southeast of Maiden Butte. Outcrop is very rubbly, appears to be an eroded, deflated flow lobe. Sample is medium gray, slightly open textured with a few small vesicles containing secondary carbonate. Olivine is reddish but not completely altered to clays. Holocrystalline, ophitic/subophitic, diktytaxitic groundmass of cpx and plag (<0.5 mm) with abundant small intergranular oxides and iddingsitized olivine. Olivine phenocrysts up to 2 mm, highly fractured and strongly iddingsitized.

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Sample ID: KS98-6C Chemical type: Group 2 (HAOT) Lat.: 41.75 Long.: 117.13 7.5’ topo quad: Maiden Butte, NV Locality: Little Owyhee River at Twin Valley Spring Description: “Flow” above KS98-6B. Sample is medium gray, open textured with abundant olivine, including some slightly larger individual crystals. Holocrystalline, ophitic/subophitic, diktytaxitic groundmass of cpx and plag (<0.5 mm) with abundant small intergranular olivine and minor oxides. Euhedral to subhedral olivine up to 5 mm, some highly fractured, slightly oxidized around rims.

Sample ID: H85-11A Chemical type: Group 2 (HAOT) Lat.: 42.90 Long.: 117.25 7.5’ topo quad: Antelope Reservoir, OR Locality: Antelope Rim Description: Uppermost of three basalt flows exposed along rim bounding north side of western thumb of Antelope Reservoir. Contacts between flows show only brecciated zones, no soil horizons. Flow is ~10-15 ft thick. Sample from massive portion of flow. Medium gray, open textured with abundant discrete grains of iddingsitized olivine. Scattered small glomeroporphyritic clumps of olivine+plag. Holocrystalline, subophitic, slightly diktytaxitic groundmass of cpx (augite) and 0.5 mm plag with abundant intergranular oxide and small oxidized olivine. Subhedral 1 mm slightly oxidized olivine phenocrysts and sparse 1 mm plag laths.

Sample ID: H8-43 Chemical type: Group 2 (HAOT) Lat.: 43.21 Long.: 117.60 7.5’ topo quad: The Hole In The Ground, OR Locality: Hole in the Ground Description: Flow above lowermost flow cut by road in the Hole in the Ground canyon. Dark, finely vesicular lots of iddingsitized olivine and plag in glomeroporphyritic clumps. Holocrystalline ophitic-subophitic, diktytaxitic groundmass of cpx and plag (<0.5 mm) with abundant 0.1 mm rounded olivine (iddingsitized) and minor intergranular oxide. Subhedral 1-2 mm fractured olivine phenocrysts with iddingsitized rims and sparse 0.75 mm plag laths. Minor secondary carbonate in vesicles. Mode: plag 38.2%; olv 21.6%; cpx 34.2%; oxides 5.8%

Sample ID: H8-45 Chemical type: Group 2 (HAOT) Lat.: 43.21 Long.: 117.60 7.5’ topo quad: The Hole In The Ground, OR Locality: Hole in the Ground Description: see Hart, 1982

Sample ID: H85-11B Chemical type: Group 2 (HAOT) Lat.: 42.90 Long.: 117.25 7.5’ topo quad: Antelope Reservoir, OR Locality: Antelope Rim Description: Middle flow of three basalt flows exposed along rim bounding north side of western thumb of Antelope Reservoir. Contacts between flows show only brecciated zones, no soil horizons. Flow is ~15-20 ft thick. Sample from massive portion of flow. Medium gray, open textured with some discrete grains of slightly iddingsitized olivine. Minor glomeroporphyritic clumps of olivine+plag. Holocrystalline , subophitic, diktytaxitic groundmass of cpx and plag (0.25 mm) with abundant intergranular olivine (0.1 mm) and oxides. Subhedral 1 mm olvine phenocrysts with oxidized rims and 0.75 mm plag laths. Mode: plag 37.1%; olv 23.3%; cpx 34.3%; oxides 5.1%

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Sample ID: KS98-6B Chemical type: Group 2 (HAOT) Lat.: 41.75 Long.: 117.13 7.5’ topo quad: Maiden Butte, NV Locality: Little Owyhee River at Twin Valley Spring Description: “Flow” above KS98-6A. Sample is medium gray, open textured with abundant olivine. Some olivine completely altered to clays. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (augite) and plag (<1 mm) with abundant intergranular oxide and iddingsitized olivine. Secondary carbonate in some vesicles. Euhedral to subhedral olivine phenocrysts, fractured and strongly iddingsitized. Mode: plag 41.6%; olv 17.8%; cpx 34.0%; oxides 6.6%

Sample ID: H9-36B Chemical type: Group 2 (HAOT) Lat.: 42.83 Long.: 117.74 7.5’ topo quad: Rome, OR Locality: Crooked Creek at U.S. 95, northeast of Burns Junction Description: Section of basalt flows and basaltic talus overlying tuffaceous sediment. Sample is from ~10 ft thick flow overlying the lowermost flow. Dark gray, massive, with iddingsitized olivine and plagioclase in glomeroporphyritic clumps and some larger plag phenocrysts. Holocrystalline, subophitic groundmass of cpx and 0.25 mm plag with abundant intergranular oxide and small olivine. Pervasive clay alteration. Subhedral 1 mm olivine phenocrysts with iddingsitized rims and 0.75-1.25 mm tabular plag isolated and in glomeroporphyritic clumps.

Sample ID: KS98-12 Chemical type: Group 2 (HAOT) Lat.: 41.64 Long.: 116.76 7.5’ topo quad: Corral Lake, NV Locality: Wolf Butte Description: Flank of Wolf Butte, southwest of summit. Sample from rubbly outcrop. Sample is medium-dark gray, matrix appears to have pervasive clay alteration. Small vesicle pockets containing secondary minerals. Not particularly open textured. No real outstanding phenocrysts. Hypocrystalline (?) very fine grained pervasively altered subophitic, diktytaxitic groundmass containing cpx, plag and oxides with abundant olivine. Sparse 1 mm plag laths and 1 mm subhedral olivine with oxidized rims.

Sample ID: SM75-12A Chemical type: Group 2 (HAOT) Lat.: 42.92 Long.: 117.38 7.5’ topo quad: Threemile Hill, OR Locality: Jordan Creek, south of Threemile Hill Description: see Hart, 1982

Sample ID: H9-36C Chemical type: Group 2 (HAOT) Lat.: 42.83 Long.: 117.74 7.5’ topo quad: Rome, OR Locality: Crooked Creek at U.S. 95, northeast of Burns Junction Description: see Hart, 1982

Sample ID: KS98-3 Chemical type: Group 2 (HAOT) Lat.: 41.82 Long.: 117.18 7.5’ topo quad: Maiden Butte, NV Locality: Maiden Butte Description: Small gully north of KS98-2 location. Sample is medium-dark gray, very even-grained matrix, open textured with abundant brownish olivine dispersed throughout. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and plag (<1.0 mm) with small intergranular oxides and iddingsitized olivine. Euhedral olivine phenocrysts up to 0.75 mm are strongly fractured with oxide inclusions and iddingsite around rims and fractures.

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Sample ID: CM97-7 Chemical type: Group 2 (HAOT) Lat.: 42.05 Long.: 117.05 7.5’ topo quad: Star Valley Knoll, OR-ID Locality: Star Valley Knoll Description: Rubble crop on north flank of Star Valley Knoll. Medium-dark gray, fine grained, open textured, abundant phenocrysts of relatively fresh looking olivine and plagioclase, glomeroporphyritic clusters of olivine and plag. Holocrystalline, subophitic, slightly diktytaxitic, groundmass of plag (0.5 mm), cpx, abundant 0.1 mm olivine and intergranular oxide. 0.5-1.0 mm fresh olivine phenocrysts with oxide inclusions and 0.5-1.0 mm tabular to acicular plag. Minor secondary carbonate in some vesicles.

Sample ID: CM97-2B Chemical type: Group 2 (HAOT) Lat.: 42.13 Long.: 117.18 7.5’ topo quad: Stoney Corral, OR Locality: Barking Butte Description: Exposure at summit of apparent vent (abundant oxidized scoriaceous material present). Medium gray, fine grained, slightly open textured/almost massive; large fresh olivine phenocrysts, no visible plag. Hypocrystalline (?) extremely fine grained zones of glass (?), cpx, plag (0.05 mm) oxide and olivine (?), alternating with coarser zones of cpx, plag (0.1 mm), oxide and olivine. Subhedral 1 mm olivine phenocrysts with weakly oxidized rims.

Sample ID: KS98-1 Chemical type: Group 2 (HAOT) Lat.: 42.82 Long.: 117.82 7.5’ topo quad: Burns Junction, OR Locality: Scott Butte Description: Eastern crater wall. Appears to be in-place flow. Sample is medium gray, open textured, with very small glomeroporphyritic clumps of very fresh-looking olivine. Also some glomeroporphyritic plagioclase. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (augite) and plag (<0. 5 mm) with abundant intergranular oxides and tiny olivine. Plag phenocrysts up to 2 mm and fresh euhedral to subhedral olivine up to 0.5 mm. Mode: plag 38.5%; olv 23.6%; cpx 27.5%; oxides 10.2%

Sample ID: KS98-11 Chemical type: Group 2 (HAOT) Lat.: 41.65 Long.: 116.75 7.5’ topo quad: Corral Lake, NV Locality: Wolf Butte Description: North summit of Wolf Butte. Sample is medium-dark gray with vesicles occurring in small pockets. Matrix rather fresh despite the presence of a few scattered blebs of secondary clay (?). Fresh green olivine, also a few larger pyroxene crystals. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and plag (<0.25 mm)with abundant small fresh olivine and intergranular oxide. Euhedral 1 mm cpx (augite) partially enclosing plag laths. Very fresh 1 mm subhedral olivine phenocrysts. Mode: plag 34.9%; olv 17.3%; cpx 38.6%; oxides 8.9%

Sample ID: KS96-7 Chemical type: Group 2 (HAOT) Lat.: 42.48 Long.: 117.15 7.5’ topo quad: Drummond Basin, OR Locality: rim of Middle Fork Owyhee River canyon, ~5 miles upstream from Three Forks Description: Flow capping rim of Middle Fork Canyon. Sample is medium gray, fine grained with sparse vesicles. Some olivine phenocrysts altering to clays. Holocrystalline, subophitic, diktytaxitic, groundmass of cpx (augite) and plag (<0.5 mm) with intergranular oxides and strongly iddingsitized olivine. Subhedral olivine phenocrysts up to 1.5 mm with iddingsitized rims; some larger grains completely altered to clays. Mode; plag 39.3%; olv 16.9%; cpx 37.2%; oxides 6.5%

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Sample ID: KS96-10 Chemical type: Group 2 (HAOT) Lat.: 42.60 Long.: 116.98 7.5’ topo quad: Fairylawn, ID Locality: North Fork Owyhee River crossing, near Oregon-Idaho border Description: Rubbly exposure of basalt overlying silicic materials; rocks are vesicular to massive. Sample is medium-dark gray, slightly vesicular, open textured with abundant olivine and feldspar. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (augite) and plag (<0.25 mm) with intergranular oxide and iddingsitized olvine. Some minor clay alteration. Plag phenocrysts up to 3 mm. Euhedral olivine phenocrysts up to 1 mm, iddingsitized along fractures; some with narrow cpx rims. Mode: plag 40.1%; olv 19.9%; cpx 31.9%, oxides 7.9%

Sample ID: CM97-9 Chemical type: Group 2 (HAOT) Lat.: 42.03 Long.: 117.15 7.5’ topo quad: Lookout Lake, OR Locality: Willow Creek Butte Description: Summit of Willow Creek Butte. Fine grained, medium to dark gray, slightly open textured, phenocrysts of olivine and plag in glomeroporphyritic clusters. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and plag (<0.5 mm) with abundant intergranular oxides and small slightly oxidized olivne. Rounded olivine phenocrysts up to 1 mm with slightly oxidized rims and plag laths up to 2.5 mm.

Sample ID: H8-95 Chemical type: Group 2 (HAOT) Lat.: 41.12 Long.: 117.10 7.5’ topo quad: Elevenmile Well, NV Locality: Midas Trough Description: see Hart, 1982

Sample ID: H9-42 Chemical type: Group 2 (HAOT) Lat.: 43.08 Long.: 117.29 7.5’ topo quad: Cow Lakes, OR Locality: Jordan Craters Road, 0.25 mile southeast of Upper Cow Lake Description: see Hart, 1982

Sample ID: H8-47 Chemical type: Group 2 (HAOT) Lat.: 43.21 Long.: 117.60 7.5’ topo quad: The Hole In The Ground, OR Locality: Hole in the Ground Description: see Hart, 1982

Sample ID: CM97-1 Chemical type: Group 2 (HAOT) Lat.: 42.21 Long.: 117.17 7.5’ topo quad: Stoney Corral, OR Locality: Toppin Creek Canyon Description: One basalt flow exposed on both sides of canyon. Flow ~10-15 ft thick, overlying vitrophyric rhyolite. Medium gray, fine to medium grained, open textured, small plag laths and phenocrysts of olivine (0.5 mm). Holocrystalline, subophitic, diktytaxitic groundmass of cpx and 0.25 mm plag laths with intergranular oxide and small olivine. Anhedral olivine phenocrysts up to 2.5 mm (highly fractured, oxide inclusions) and plag laths up to 1.5 mm. Minor secondary carbonate in some vesicles.

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Sample ID: CM97-11 Chemical type: Group 2 (HAOT) Lat.: 42.04 Long.: 117.15 7.5’ topo quad: Lookout Lake, OR Locality: Willow Creek Butte Description: Rubbly flow top exposure on flank of Willow Creek Butte, further downslope from CM97- 10. Fine grained, medium to dark gray, slightly open textured, phenocrysts of olivine and plag in large glomeroporphyritic clusters. Scattered vesicles with some secondary mineralization. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and plag (<0.5 mm) with abundant intergranular oxides and tiny olivine (<0.1 mm). Euhedral fractured 1.5 mm olivine phenocrysts with slightly altered rims, small 0.25 mm rounded olivine with altered rims, and plag laths up to 1.5 mm. Minor secondary carbonate in somevesicles.

Sample ID: KS98-26 Chemical type: Group 2 (HAOT) Lat.: 41.81 Long.: 116.87 7.5’ topo quad: Star Valley Ridge SE, NV Locality: Ant Hill Description: Sample from remnant of rim on southeast side of crater. Sample is medium gray, slightly open textured with some vesicles. Plagioclase in small glomeroporphyritic clumps with fresh olivine. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (augite) and plag (<0.5 mm) with intergranular oxides and small, slightly oxidized olivine. 2mm fresh subhedral olivine phenocrysts. 2 mm plag laths, some isolated and some in glomeroporphyritic clumps with 1 mm olivine.

Sample ID: CM97-10 Chemical type: Group 2 (HAOT) Lat.: 42.04 Long.: 117.15 7.5’ topo quad: Lookout Lake, OR Locality: Willow Creek Butte Description: Flank of Willow Creek Butte, in situ flow unit. Fine grained, medium to dark gray, massive to slightly open textured. Phenocrysts of plag and fresh olivine in large glomeroporphyritic clusters. Scattered vesicles with some secondary mineralization. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and plag (<0.25 mm) with abundant intergranular oxide and small (0.1 mm) slightly oxidized olivine. Rounded, fractured olivine phenocrysts (up to 1.5 mm) with slightly oxidized rims and plag laths up to 2.5 mm. Minor secondary carbonate in somevesicles.

Sample ID: H8-69E Chemical type: Group 2 (TB) Lat.: 42.68 Long.: 117.36 7.5’ topo quad: Skull Creek, OR Locality: Soldier Creek at the Owyhee Canyon Description: Sequence of four sampled basalt flows with interlayered tuffaceous/sedimentary material overlying older silicic material. This sample is from the third basalt flow sampled from the top of the section. Sample is gray, finely vesicular, with visible plagioclase and iddingsitized olivine. Lots of secondary mineralization completely filling some vesicles. Holcrystalline, subophitic, diktytaxitic groundmass of cpx and plag (<0.5 mm) with intergranular oxides and small iddingsitized olivine. Some clay alteration. Vesicles rimmed with chalcedony and filled with carbonate. Completely iddingsitized olivine phenocrysts up to 2 mm. 1-3 mm plag, larger ones are zoned.

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Sample ID: KS98-25 Chemical type: Group 2 (TB) Lat.: 41.82 Long.: 116.86 7.5’ topo quad: Star Valley Ridge SE, NV Locality: Ant Hill Description: Sample from remnant of rim on northeast side of crater. Sample is medium gray, not particularly open textured but with a “sugary” matrix. Abundant plagioclase some 2-3 mm laths. Olivine is sparse but is very fresh and bright green. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and plag (0.25-0.5 mm)with intergranular oxides and abundant small oxidized olivine. Plag phenocrysts are 2-3 mm, some sieved, some in glomeroporphyritic clumps with subhedral 0.75 mm olivine.

Sample ID: KS98-22 Chemical type: Group 2 (TB) Lat.: 41.81 Long.: 116.89 7.5’ topo quad: Star Valley Ridge SW, NV Locality: Ant Hill Description: Summit of Ant Hill is a multiply-breached crater. Sample from remnant of rim on southwest side. Sample is medium gray and very vesicular with large glomeroporphyritic clumps of reddish olivine and some 2-3 mm plag laths. Some secondary mineralization in vesicles. Holocrystalline, subophitic/diktytaxitic groundmass of cpx (augite) and plag (<1 mm) with abundant intergranular oxides and small oxidized olivine. Euhedral, fractured olivine phenocrysts up to 2 mm with strongly oxidized rims. Sparse 3 mm plag laths.

Sample ID: KS98-24 Chemical type: Group 2 (TB) Lat.: 41.82 Long.: 116.87 7.5’ topo quad: Star Valley Ridge SE, NV Locality: Ant Hill Description: Sample from remnant of rim on north side of crater. Sample is medium gray, slightly vesicular and slightly open textured with large distinct plagioclase laths and glomeroporphyritic clumps of fresh green olivine. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and plag (<0.5 mm) with intergranular oxides and abundant small oxidized olivine. Plag phenocrysts up to 2 mm and sparse subhedral 0.5-1.0 mm olivine. Mode: plag 37.1%; olv 22.3%; cpx 36.2%; oxides 4.2%

Sample ID: KS96-3 Chemical type: Group 2 (TB) Lat.: 43.19 Long.: 117.48 7.5’ topo quad: Jordan Craters North, OR Locality: Birch Creek Canyon Road Description: ~10 ft thick flow exposed approximately 20 ft above KS 96-2 along Birch Creek road. May be underlain by a thin tuffaceous/sedimentary unit. Flow unit appears oxidized. Sample is vesicular, medium gray with a reddish tint, medium gray with sparse olivine phenocrysts. Very fine grained, nearly (?) holocrystalline (some unidentifiable material in groundmass), groundmass of cpx and plag <0.25 mm with abundant intergranular oxide and tiny olivine. Slightly iddingstitized, fractured, subhedral olivine phenocrysts up to 0.5 mm. Mode: plag 36.1%; olv 22.7%; cpx 30.8%; oxides 10.2%

Sample ID: H8-56A Chemical type: Group 2 (TB) Lat.: 42.84 Long.: 117.62 7.5’ topo quad: Scott Reservoir, OR Locality: Owyhee River at U.S. 95, Rome Description: see Hart, 1982

186

Sample ID: H8-56B Chemical type: Group 2 (TB) Lat.: 42.84 Long.: 117.62 7.5’ topo quad: Scott Reservoir, OR Locality: Owyhee River at U.S. 95, Rome Description: Sequence of four thin basalt flows overlying white sediments where US 95 climbs out of Rome Basin toward Jordan Valley. This sample is third from the top. Flow is mildly columnar jointed with huge grains of weather olivine and plagioclase. Holocrystalline, subophitic, diktytaxitic groundmass of cps (Ti-augite) and plag (<0.5 mm) with intergranular oxides and small fresh olivines. Subhedral to anhedral fractured and weakly iddingsitized olivine phenocrysts up to 1 mm. Plag up to 1 mm, some with zoning; in glomeroporphyritic clumps with and without olivine.

Sample ID: H8-28A Chemical type: Group 2 (TB) Lat.: 42.88 Long.: 117.46 7.5’ topo quad: Threemile Hill, OR Locality: Jordan Creek, southwest of Threemile Hill Description: Section of five flows exposed in canyon of Jordan Creek; all five flows appear similar , varying from massive to vesicular depending on location within the flow and are separated by thin rubble zones with no soil horizons. This sample is from the second flow from the top. Flow is ~25-30 ft thick. Sample is open textured with olivine and plagioclase in glomeroporphyritic clumps. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and 0.75 mm plag with abundant 0.2-0.3 mm strongly iddingsitized olivine and intergranular oxide. Glomeroporphyritic clumps of subhedral 1.5 mm fractured olivine phenocrysts with iddingsitized rims and 4 mm plag.

Sample ID: KS96-6B Chemical type: Group 2 (TB) Lat.: 42.44 Long.: 117.07 7.5’ topo quad: Deacon Crossing, OR-ID Locality: Middle Fork Owyhee River at BLM crossing, ~10 miles upstream from Three Forks Description: Basalt flow directly overlying silicic ash unit. Sample is medium-dark gray, medium grained, slightly vesicular with abundant wads of yellowish clay. Holocrystalline, ophitic- subophitic, diktytaxitic groundmass of cpx and plag (<0.5 mm) with intergranular oxides and olivine. Euhedral to subhedral olivine phenocrysts with oxide inclusions, up to 3 mm, fractured and iddingsitized around rims and along fractures. Sparse plag laths up to 1 mm. Mode: plag 40.8%; olv 21.9%; cpx 30.9%; oxides 6.2%

Sample ID: KS98-23 Chemical type: Group 2 (TB) Lat.: 41.82 Long.: 116.88 7.5’ topo quad: Star Valley Ridge SW, NV Locality: Ant Hill Description: Sample from remnant of rim on northwest side of crater. Sample is medium gray, but less vesicular and with a finer-grained matrix than KS98-22. Small plagioclase laths and large glomeroporphyritic clumps of mostly fresh green olivine; a few reddish olivines are also present. Holocrystalline, subophitic, strongly diktytaxitic groundmass of cpx (augite) and plag (0.75 mm) with intergranular oxides and fresh olivine. Anhedral to subhedral, relatively fresh to weakly oxidized, fractured 2-4 mm olivine phenocrysts and plag phenocrysts up to 2 mm.

Sample ID: CM97-5 Chemical type: Group 2 (TB) Lat.: 42.15 Long.: 117.16 7.5’ topo quad: Stoney Corral, OR Locality: Lookout Butte Description: Rubbly outcrop at summit of Lookout Butte. Medium-dark gray, fine grained, open textured, Phenocrysts of fresh green olivine, plag laths, glomeroporphyritic clusters of olivine and plag. Some secondary mineralization in vesicles. Holocrystalline, subophitic, diktytaxitic groundmass of cpx, 0.25-0.5 mm plag laths with intergranular oxides and olivine. Subhedral to anhedral 0.5- 1.0 mm fractured olivine phenocrysts with very weakly oxidized rims. Tabular to lathlike, 1-2 mm plag in glomeroporphyritic clumps.

187

Sample ID: H8-42 Chemical type: Group 2 (TB) Lat.: 43.21 Long.: 117.60 7.5’ topo quad: The Hole In The Ground, OR Locality: Hole in the Ground Description: see Hart, 1982

Sample ID: KS98-9 Chemical type: Group 2 (TB) Lat.: 41.96 Long.: 116.72 7.5’ topo quad: Rubber Hill, NV Locality: Rubber Hill Description: Summit of Rubber Hill. Outcrop is limited and somewhat rubbly. All material exposed is oxidized and vesicular. Sample is somewhat banded, with vesicular zones and more massive zones – most likely welded agglutinate material. Medium-dark gray, fine grained matrix with lots of unaltered but reddish oxidized olivine. Vesicles filled with secondary carbonate (?) but matrix is fresh. Zones of very fine grained glass-cpx-plag matrix alternate with zones of coarser, more abundant plag in hypocrystalline, ophitic glass-cpx-plag matrix; abundant intergranular oxide and small red oxidized olivine present throughout. Subhedral, 1 mm olivine phenocrysts with oxidized rims.

Sample ID: CM97-3 Chemical type: Group 2 (TB) Lat.: 42.14 Long.: 117.11 7.5’ topo quad: Defeat Butte, OR-ID Locality: Hoodoo Butte Description: Rubble crop on summit of Hoodoo Butte. Medium to dark gray, fine grained, open textured, phenocrysts of weathered olivine (orange), phenocrysts of plagioclase in glomeroporphyritic clusters, some alteration but generally pretty fresh. Holocrystalline, subophitic, diktytaxitic groundmass of fine grained cpx with plag laths up to 0.25 mm with abundant intergranular oxide and olivine. Anhedral 0.25-0.5 mm olivine phenocrysts and 1.5 mm plag laths isolated and in glomeroporphyritic clumps.

Sample ID: KS98-29 Chemical type: Group 2 (TB) Lat.: 42.58 Long.: 117.27 7.5’ topo quad: Squaw Flat, OR Locality: Mousetrap Butte Description: Summit of Mousetrap Butte. Abundant outcrop, but very weathered and altered. Sample is least altered available. Medium gray, appears to have some vesicular/nonvesicular banding. Matrix is clayey, and clay also appears in discrete blebs. Zones of holocrystalline, subophitic, diktytaxitic cpx-plag (0.25 mm) groundmass with intergranular oxides and small olivine alternate with zones of hypocrystalline (?) very fine grained glass(?)-olivine-oxide-plag-cpx(?). Some clay alteration present and some carbonate in vesicles. Subhedral to anhedral 2 mm fractured, iddingsitized olivine phenocrysts and plag laths up to 1 mm.

Sample ID: H8-56D Chemical type: Group 2 (TB) Lat.: 42.84 Long.: 117.62 7.5’ topo quad: Scott Reservoir, OR Locality: Owyhee River at U.S. 95, Rome Description: Sequence of four thin basalt flows overlying white sediments where US 95 climbs out of Rome Basin toward Jordan Valley. This sample is uppermost flow. Flow is extremely weathered. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (Ti-augite) and 0.5 mm plag with intergranular oxide and small olivine. Pervasive clay alteration and lots of secondary carbonate in vesicles. Glomeroporphyritic clumps of 1 mm plag and 1.0-1.5 mm strongly altered subhedral olivine.

188

Sample ID: KS98-32B Chemical type: Group 2 (TB) Lat.: 42.78 Long.: 117.53 7.5’ topo quad: Scott Reservoir, OR Locality: Sand Hollow Description: Flow below KS98-32A. More plagioclase and less olivine than KS98-32A, not as glomeroporphyritic. Lots of clay present in matrix. Holocrystalline, subophitic, diktytaxitic matrix of cpx and plag (<0.5 mm) with intergranular oxide and small olivine. 1.5 mm plag and 0.5-1.0 mm olivine in glomeroporphyritic clumps.

Sample ID: H8-69D Chemical type: Group 2 (TB) Lat.: 42.68 Long.: 117.36 7.5’ topo quad: Skull Creek, OR Locality: Soldier Creek at the Owyhee Canyon Description: see Hart, 1982

Sample ID: H9-21E Chemical type: Group 2 (TB) Lat.: 41.93 Long.: 116.68 7.5’ topo quad: Rubber Hill, NV Locality: South Fork Owyhee River at Devil's Corral, southeast of Rubber Hill Description: see Hart, 1982

Sample ID: H9-21B Chemical type: Group 2 (TB) Lat.: 41.93 Long.: 116.68 7.5’ topo quad: Rubber Hill, NV Locality: South Fork Owyhee River at Devil's Corral, southeast of Rubber Hill Description: see Hart, 1982

Sample ID: CM97-6 Chemical type: Group 2 (TB) Lat.: 42.03 Long.: 117.04 7.5’ topo quad: Star Valley Knoll, OR-ID Locality: Star Valley Knoll Description: Rubble crop at summit of Star Valley Knoll. Scoriaceous oxidized (red/black banded) material suggests vent area. Medium-dark gray, fine grained, pockets of vesicles, matrix ranges from open textured to massive. Phenocrysts of olivine and plag. Some secondary mineralization in vesicles. Hypocrystalline, ophitic/subophitic, diktytaxitic; large distinct 1 mm cpx crystals enclosing plag (<0.25 mm) in matrix of plag (0.25 mm), cpx, abundant oxides and oxidized olivine. Also zones of tiny <0.1 mm plag, cpx, olivine, oxides and altered glass. Sparse 0.25 mm strongly oxidized olivine phenocrysts.

Sample ID: H8-35 Chemical type: Group 2 (TB) Lat.: 42.55 Long.: 117.18 7.5’ topo quad: Three Forks, OR Locality: Three Forks Description: Sample from flow underlying rim-forming flow along road leading into Three Forks Canyon. Sample is medium gray with abundant fresh olivine and olivine+plagioclase in glomeroporphyritic clumps. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and 0.5 mm plag with minor intergranular oxide and tiny olivine. Large (up to 4 mm) rounded vesicles. Subhedral 0.25-0.75 mm weakly iddingsitized olivine phenocrysts. Sparse 1.5 mm plag laths. Mode: plag 36.1%; olv 17.7%; cpx 34.1%; oxides 11.9%

Sample ID: H8-29 Chemical type: Group 2 (TB) Lat.: 42.85 Long.: 117.32 7.5’ topo quad: Little Grassy Mountain, OR Locality: Three Forks Road, ~2.5 miles north of Little Grassy Reservoir Description: see Hart, 1982

189

Sample ID: H9-44 Chemical type: Group 2 (TB) Lat.: 43.14 Long.: 117.39 7.5’ topo quad: Jordan Craters North, OR Locality: older flow exposed in wall of canyon now occupied by Jordan Craters lava field, east of Coffeepot Crater Description: see Hart, 1982

Sample ID: KS98-30 Chemical type: Group 2 (TB) Lat.: 42.58 Long.: 117.27 7.5’ topo quad: Squaw Flat, OR Locality: Mousetrap Butte Description: North side of Mousetrap Butte, just below summit. Small eroded bench of basalt. Sample is medium gray, open textured and vesicular, much fresher than material collected at summit. Holocrystalline, subophitic, slightly diktytaxitic groundmass of cpx and plag (0.25 mm) with intergranular oxides and small olivine. Subhedral, fractured, oxidized 0.75 mm olivine phenocrysts and 5 mm plag laths. Mode: plag 37.8%; olv 11.5%; cpx 42.8%; oxides 7.6%

Sample ID: H8-28E Chemical type: Group 2 (TB) Lat.: 42.88 Long.: 117.46 7.5’ topo quad: Threemile Hill, OR Locality: Jordan Creek, southwest of Threemile Hill Description: see Hart, 1982

Sample ID: H8-50E Chemical type: Group 2 (TB) Lat.: 42.89 Long.: 117.60 7.5’ topo quad: Arock, OR Locality: Jordan Creek, southwest of Arock Description: see Hart, 1982

Sample ID: H8-50C Chemical type: Group 2 (TB) Lat.: 42.89 Long.: 117.60 7.5’ topo quad: Arock, OR Locality: Jordan Creek, southwest of Arock Description: see Hart, 1982

Sample ID: H8-50D Chemical type: Group 2 (TB) Lat.: 42.89 Long.: 117.60 7.5’ topo quad: Arock, OR Locality: Jordan Creek, southwest of Arock Description: Sequence of basalt flows underlain by ~100 ft of sediment and pillow lavas. This sample is from flow underlying rim-forming flow. Open textured, olivine and plagioclase isolated and in glomeroporphyritic clumps. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (Ti- augite) and plag (<0.5 mm) with abundant intergranular oxide and small fresh olivine. Anhedral fractured olivine phenocrysts up to 0.75 mm with weakly altered rims and 0.75 mm plag with sieved centers.

190

Sample ID: KS98-10 Chemical type: Group 2 (TB) Lat.: 41.84 Long.: 116.90 7.5’ topo quad: Star Valley Ridge SW, NV Locality: Pipeline Butte Description: Sample from flow lobe extending across road from Pipeline Butte. Sample is very fresh, medium-dark gray, open textured, somewhat “sugary” appearance. Holocrystalline, diktytaxitic, subophitic groundmass of plag laths up to 1.5 mm with less cpx and olivine (slightly oxidized) up to 0.25 mm with intergranular oxides. Subhedral, slightly oxidized olivine phenocrysts up to 0.75 mm). Mode: plag 41.0%; olv 19.0%; cpx 35.2%; oxides 4.8%

Sample ID: KS96-4 Chemical type: Group 2 (TB) Lat.: 43.19 Long.: 117.48 7.5’ topo quad: Jordan Craters North, OR Locality: Birch Creek Canyon Road Description: Uppermost flow along Birch Creek road, caps plain in this area. Sample is medium-light gray, open textured, small plag and olivine present. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and plag (<0.5 mm) with some intergranular oxide and olivine. Subhedral olivine phenocrysts up to 0.5 mm. Mode: plag 42.6%; olv 16.0%; cpx 38.2%; oxides 3.1%

Sample ID: KS96-6C Chemical type: Group 2 (TB) Lat.: 42.44 Long.: 117.07 7.5’ topo quad: Deacon Crossing, OR-ID Locality: Middle Fork Owyhee River at BLM crossing, ~10 miles upstream from Three Forks Description: Flow above KS96-6B (no clear contact between flows). Sample is medium-dark gray, slightly open textured and slightly vesicular. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and plag (<0.75 mm) with minor intergranular oxides and olivine (completely iddingsitized). Clusters of mostly anhedral olivine phenocrysts up to 0.75 mm, strongly iddingsitized around rims. Mode: plag 38.1%; olv 19.2%; cpx 36.3%; oxides 6.2%

Sample ID: H8-56C Chemical type: Group 2 (TB) Lat.: 42.84 Long.: 117.62 7.5’ topo quad: Scott Reservoir, OR Locality: Owyhee River at U.S. 95, Rome Description: Sequence of four thin basalt flows overlying white sediments where US 95 climbs out of Rome Basin toward Jordan Valley. This sample is second from the top. Sample is dark gray, finely vesicular, with iddingsitized olivine and plagioclase. Thin section not available

Sample ID: KS98-7 Chemical type: Group 2 (TB) Lat.: 41.85 Long.: 116.92 7.5’ topo quad: Star Valley Ridge SW, NV Locality: Pipeline Butte Description: Summit of Pipeline Butte. Outcrop is very limited. Sample is medium-dark gray, open textured with abundant feldspars. Olivine is somewhat brownish. A few clots of secondary vesicular material (carbonate). Holocrystalline, subophitic, diktytaxitic groundmass of plag laths up to 0.75 mm with less cpx, with intergranular oxide and small red olivine. Plag phenocrysts up to 3 mm and fractured, iddingsitized olivine up to 1 mm.

191

Sample ID: H8-50B Chemical type: Group 2 (TB) Lat.: 42.89 Long.: 117.60 7.5’ topo quad: Arock, OR Locality: Jordan Creek, southwest of Arock Description: Sequence of basalt flows underlain by ~100 ft of sediment and pillow lavas. This sample is from pillow basalt from base of formation. Massive, dark gray, highly altered, with visible olivine and plagioclase.

Sample ID: H8-28C Chemical type: Group 2 (TB) Lat.: 42.88 Long.: 117.46 7.5’ topo quad: Threemile Hill, OR Locality: Jordan Creek, southwest of Threemile Hill Description: see Hart, 1982

Sample ID: KS98-32A Chemical type: Group 2 (TB) Lat.: 42.78 Long.: 117.53 7.5’ topo quad: Scott Reservoir, OR Locality: Sand Hollow Description: Uppermost flow in a sequence in small canyon on southwest side of Owyhee River, upstream of Rome. Sample is medium gray, very open textured with very abundant plagioclase and fresh green olivine. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (augite) and plag (<0.5 mm) with intergranular oxides and small olivine. 2 mm plag (some with sieved centers) and highly fractured but very fresh olivine up to 4 mm occur isolated and in glomeroporphyritic clumps. Mode: plag 44.7%; olv 13.7%; cpx 36.3%; oxides 5.1%

Sample ID: H9-21C Chemical type: Group 2 (TB) Lat.: 41.93 Long.: 116.68 7.5’ topo quad: Rubber Hill, NV Locality: South Fork Owyhee River at Devil's Corral, southeast of Rubber Hill Description: see Hart, 1982

Sample ID: H8-50A Chemical type: Group 2 (TB) Lat.: 42.89 Long.: 117.60 7.5’ topo quad: Arock, OR Locality: Jordan Creek, southwest of Arock Description: Sequence of basalt flows underlain by ~100 ft of sediment and pillow lavas. This sample is from talus at creek bed – may actually be a flow underlying the sediment/pillow unit. Hypocrystalline groundmass of glass, oxide, 0.25 mm plag, cpx, and olivine. Glomeroporphyritic clumps of 1 mm euhedral olivine and 1 mm plag laths.

Sample ID: H8-39 Chemical type: Group 2 (TB) Lat.: 42.73 Long.: 117.05 7.5’ topo quad: Juniper Point, OR-ID Locality: Soldier Creek near Oregon-Idaho border, ~17 miles south of Jordan Valley Description: see Hart, 1982

192

Sample ID: H9-46 Chemical type: Group 2 (TB) Lat.: 42.95 Long.: 117.00 7.5’ topo quad: Jordan Valley, OR-ID Locality: ~3.5 miles east-southeast of Jordan Valley, on road to Triangle Description: Rim flow along Jordan Creek near Idaho border. Sample from east side of road, appears to be on top of silicic material. Medium gray, finely vesicular, open textured, iddingsitized olivine and plag isolated and in glomeroporphyritic clumps. Holocrystalline, ophitic-subophitic, diktytaxitic groundmass of cpx and 0.25 mm plag with abundant intergranular oxide and small (0.1 mm) olivine. Euhedral to subhedral olivine phenocrysts (with oxide inclusions) up to 4 mm, fractured and with iddingsitized rims. Sparse 0.5 mm plag laths with sieved centers in glomeroporphyritic clumps.

Sample ID: H8-36 Chemical type: Group 2 (TB) Lat.: 42.55 Long.: 117.18 7.5’ topo quad: Three Forks, OR Locality: Three Forks Description: see Hart, 1982

Sample ID: H8-28D Chemical type: Group 2 (TB) Lat.: 42.88 Long.: 117.46 7.5’ topo quad: Threemile Hill, OR Locality: Jordan Creek, southwest of Threemile Hill Description: see Hart, 1982

Sample ID: H8-30B Chemical type: Group 2 (TB) Lat.: 42.76 Long.: 117.29 7.5’ topo quad: Little Grassy Mountain, OR Locality: Soldier Creek, ~0.8 miles east of Owyhee Canyon Description: Section of four flows exposed in valley of Soldier Creek; creek cuts down through lower flows, upper flows exposed on surrounding hills. This sample is from the second flow from the bottom. Flow is ~20 ft thick. Sample is dark gray, relatively massive with lots of olivine and olivine and plagioclase in glomeroporphyritic clumps. Holocrystalline, ophitic/subophitic, diktytaxitic groundmass of cpx and 0.25 mm plag laths with abundant small intergranular olivine and oxide. Euhedral to subhedral 0.5-1.0 mm olivine phenocrysts with strongly iddingsitized rims. Acicular 0.75 mm plag with more tabular, sieved 1 mm laths in small glomeroporphyritic clumps.

Sample ID: KS98-13 Chemical type: Group 2 (TB) Lat.: 41.70 Long.: 116.76 7.5’ topo quad: Corral Lake, NV Locality: Guzzler Butte Description: Summit of Guzzler Butte. Sample is medium-dark gray, very open textured with big fresh olivine phenocrysts and plag laths throughout. Small amount of white secondary mineralization in vesicles. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and plag up to 1 mm with intergranular oxide and small oxidized olivine. 4 mm plag laths and 2 mm fractured subhedral olivine phenocrysts with slightly oxidized rims.

Sample ID: H9-29 Chemical type: Group 2 (TB) Lat.: 42.63 Long.: 117.73 7.5’ topo quad: The Basin, OR Locality: Rattlesnake Creek Road at north end of Bowden Hills, ~7.5 miles from U.S. 95 Description: see Hart, 1982

193

Sample ID: H8-28B Chemical type: Group 2 (TB) Lat.: 42.88 Long.: 117.46 7.5’ topo quad: Threemile Hill, OR Locality: Jordan Creek, southwest of Threemile Hill Description: Section of five flows exposed in canyon of Jordan Creek; all five flows appear similar , varying from massive to vesicular depending on location within the flow and are separated by thin rubble zones with no soil horizons. This sample is from the third flow from the top. Flow is ~25- 30 ft thick. Sample is open textured with olivine and plagioclase in glomeroporphyritic clumps. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and 0.5 mm plag with abundant intergranular oxide and iddingsitized olivine. Euhedral to subhedral 1.0-1.5 mm highly fractured olivine phenocrysts with strongly iddingsitized rims and 2-4 mm plag (larger ones are zoned); some olivine and plag in glomeroporphyritic clumps.

Sample ID: H8-34 Chemical type: Group 2 (TB) Lat.: 42.55 Long.: 117.18 7.5’ topo quad: Three Forks, OR Locality: Three Forks Description: see Hart, 1982

Sample ID: H8-30A Chemical type: Group 2 (TB) Lat.: 42.76 Long.: 117.29 7.5’ topo quad: Little Grassy Mountain, OR Locality: Soldier Creek, ~0.8 miles east of Owyhee Canyon Description: Section of four flows exposed in valley of Soldier Creek; creek cuts down through lower flows, upper flows exposed on surrounding hills. This sample is from the lowermost flow. Flow is ~15 ft thick. Sample is dark gray, relatively massive with lots of olivine and olivine and plagioclase in glomeroporphyritic clumps. Holocrystalline, subophitic-ophitic, slighty diktytaxitic groundmass of cpx and plag (<0.25 mm) with abundant intergranular oxide and tiny iddingsitized olivine. Subhedral to anhedral, strongly iddingsitized olivine phenocrysts up to 2.5 mm and plag up to 0.75 mm.

Sample ID: H9-48 Chemical type: Group 2 (TB) Lat.: 42.89 Long.: 116.82 7.5’ topo quad: Flint, ID Locality: ~12 miles southeast of Jordan Valley, on road to Triangle Description: Road cut through basalt flow. Dark black, aphanitic, no visible phenocrysts. Holocrystalline, ophitic groundmass of cpx and 0.25 mm palg with 0.25 mm oxides and small intergranular olivine. Pervasive clay alteration. Glomeroporphyritic clumps of 0.5 mm olivine and plag.

Sample ID: H9-37D Chemical type: Group 2 (TB) Lat.: 42.79 Long.: 117.60 7.5’ topo quad: Scott Reservoir, OR Locality: Owyhee Canyon, 7 miles upstream from Rome Description: see Hart, 1982

Sample ID: H9-49 Chemical type: Group 2 (TB) Lat.: 42.96 Long.: 117.41 7.5’ topo quad: Threemile Hill, OR Locality: Threemile Hill Description: see Hart, 1982

194

Sample ID: H8-69F Chemical type: Group 2 (TB) Lat.: 42.68 Long.: 117.36 7.5’ topo quad: Skull Creek, OR Locality: Soldier Creek at the Owyhee Canyon Description: Sequence of four sampled basalt flows with interlayered tuffaceous/sedimentary material overlying older silicic material. This sample is from the second basalt flow sampled from the top of the section. Sample is gray, open textured, with visible plagioclase laths and abundant olivine; some olivine and plagioclase in glomeroporphyritic clumps. Holocrystalline, ophitic-subophitic, slightly diktytaxitic groundmass of cpx and 0.25 mm plag with abundant intergranular oxide and small olivine. Some 1 mm rounded vesicles. Subhedral highly fractured 1-2 mm olivine with iddingsitized rims and 1 mm plag laths with some apparent sieving near rims.

Sample ID: H8-69G Chemical type: Group 2 (TB) Lat.: 42.68 Long.: 117.36 7.5’ topo quad: Skull Creek, OR Locality: Soldier Creek at the Owyhee Canyon Description: see Hart, 1982

Sample ID: CM97-25 Chemical type: Group 2 (TB) Lat.: 42.16 Long.: 117.24 7.5’ topo quad: Stoney Corral, OR Locality: Black Butte Description: Summit of butte. Fine grained, medium to dark gray, open textured, olivine and plag in glomeroporphyritic clumps. Hypocrystalline(?), very fine grained groundmass of cpx, plag (<0.05 mm) oxide, olivine, and glass(?). Rounded fractured olivine phenocrysts (0.25-0.75 mm) with oxidized rims and 0.25-1.0 mm plag laths.

Sample ID: CM97-17 Chemical type: Group 2 (TB) Lat.: 42.01 Long.: 117.35 7.5’ topo quad: Oregon Butte, OR Locality: Oregon Butte Description: Sample taken from southern remnant of crater rim. Fine grained, medium to dark gray, open textured, with phenocrysts of plag and small red olivine. Holocrystalline, subophitic groundmass of cpx and 0.25-0.5 mm plag with abundant intergranular oxides and 0.1 mm red olivine. Subhedral to anhedral 0.25-1.0 mm highly fractured and altered olivine phenocrysts and 1.0-1.5 mm plag laths.

Sample ID: H9-19A Chemical type: Group 2 (SROT) Lat.: 41.62 Long.: 116.35 7.5’ topo quad: Wilson Reservoir, NV Locality: Deep Creek, near South Fork Owyhee River Description: Outcrop of four basalt flows with talus beneath on north side of creek. Thin oxidized zone between flows, no soil horizons or significant accumulations of rubble. Sample from uppermost flow. Flow is ~10-15 ft thick. Sample is gray to reddish gray, open textured, finely vesicular with visible plag, cpx, and iddingsitized olivine; some olivine and plag in glomeroporphyritic clumps. Holocrystalline, slightly subophitic, diktytaxitic groundmass of plag (0.75-1.0 mm) and cpx (Ti- augite) with iddingsitized olivine (0.25 mm) and intergranular oxides. Plag microphenocrysts up to 1.5 mm.

195

Sample ID: H8-30D Chemical type: Group 2 (TB) Lat.: 42.76 Long.: 117.29 7.5’ topo quad: Little Grassy Mountain, OR Locality: Soldier Creek, ~0.8 miles east of Owyhee Canyon Description: Section of four flows exposed in valley of Soldier Creek; creek cuts down through lower flows, upper flows exposed on surrounding hills. This sample is from the uppermost flow. Sample is vesicular with secondary mineralization filling in larger vesicles. Less olivine and generally finer grained than H8-30 A&B. Hypocrystalline, subophitic, diktytaxitic groundmass contains glass, cpx (Ti-augite) plag (<0.25 mm), intergranular oxide and small red olivine. Sparse 2 mm plag laths. Abundant subrounded 3 mm vesicles, some containing secondary carbonate.

Sample ID: KS98-28 Chemical type: Group 2 (TB) Lat.: 41.74 Long.: 117.10 7.5’ topo quad: Maiden Butte SE, NV Locality: Hill 5751 Description: Flow exposed along road to north of Hill 5751, which is likely source. Unlike KS98-27, sample is very open textured but does contain plag laths up to 5 mm long. Plagioclase and olivine occur in glomeroporphyritic clumps. Olivine is brownish but does not appear to be iddingsitized. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (Ti-augite) and plag (<0.5 mm) with intergranular oxides and strongly oxidized small olivine. Subhedral to anhedral 1-3 mm highly fractured and strongly oxidized olivine phenocrysts and plag laths up to 5 mm long.

Sample ID: CM97-24 Chemical type: Group 2 (TB) Lat.: 42.02 Long.: 117.25 7.5’ topo quad: Lookout Lake, OR Locality: Mahogany Butte Description: Northwest flank of butte ~200 ft below summit. Fine grained, medium to dark gray, open textured, abundant relatively fresh olivine and plag, glomeroporphyritic clumps. Holocrystalline, ophitic-subophitic, diktytaxitic groundmass of cpx and 0.1 mm plag with intergranular oxides and olivine. Euhedral to subhedral 0.5-1.0 mm highly fractured olivine phenocrysts with slight alteration around rims and 0.5-2.0 mm plag laths; some olivine and plag in glomeroporphyritic clumps.

Sample ID: H9-19B Chemical type: Group 2 (TB) Lat.: 41.62 Long.: 116.35 7.5’ topo quad: Wilson Reservoir, NV Locality: Deep Creek, near South Fork Owyhee River Description: Outcrop of four basalt flows with talus beneath on north side of creek. Thin oxidized zone between flows, no soil horizons or significant accumulations of rubble. Sample from second flow from top. Flow is ~15 ft thick. Sample is gray to reddish gray, open textured, finely vesicular with visible plag, cpx, and iddingsitized olivine; some olivine and plag in glomeroporphyritic clumps. Holocrystalline, subophitic, diktytaxitic groundmass of plag (1 mm) and cpx (Ti-augite) with iddingsitized olivine (0.25 mm) and intergranular oxides. Anhedral 1 mm fractured olivine microphenocrysts with iddingsite around rims and along fractures. 2 mm plag in glomeroporphyritic clumps.

Sample ID: H9-21A Chemical type: Group 2 (TB) Lat.: 41.93 Long.: 116.68 7.5’ topo quad: Rubber Hill, NV Locality: South Fork Owyhee River at Devil's Corral, southeast of Rubber Hill Description: Basalt flow exposed in middle of section in lower “bench” of canyon cut in two levels. Flow is ~10 ft thick. Holocrystalline, subophitic, diktytaxitic groundmass of plag (0.5 mm) and cpx with coarse intergranular oxides (0.25 mm) and red olivine. Sparse 1.5 mm plag laths and sparse subhedral 0.75 mm olivine with oxidized rims.

196

Sample ID: H9-38 Chemical type: Group 2 (TB) Lat.: 42.62 Long.: 117.33 7.5’ topo quad: Squaw Flat, OR Locality: Grassy Mountain Description: see Hart, 1982

Sample ID: KS98-21 Chemical type: Group 2 (TB) Lat.: 41.81 Long.: 116.80 7.5’ topo quad: Star Valley Ridge SE, NV Locality: Bartome Knoll Description: Rubbly outcrop of flow lobe at road, north of Bartome Knoll. Sample is medium-dark gray, very open textured with abundant plagioclase and sparse olivine. PLag appears somewhat tabular and has a bluish look. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and plag (<0.5 mm) with abundant intergranular oxides and small oxidized olivine. Very strongly oxidized 1 mm olivine phenocrysts. Plag up to 4 mm, some sieved, some in glomeroporphyritic clumps.

Sample ID: KS98-15 Chemical type: Group 2 (TB) Lat.: 41.74 Long.: 116.81 7.5’ topo quad: Corral Lake, NV Locality: Corral Lake Butte Description: Summit of Corral Lake Butte. Abundant exposures of vesicular oxidized basalt in a topographic depression suggestive of a breached crater. Sample is dark gray and vesicular with a “banded” appearance. Somewhat open textured with small reddish olivine and disseminated wads of clay alteration. Holocrystalline (?) very fine grained cpx-plag matrix (possibly with some glass), appears to have some clay alteration. 1 mm plag laths and 0.5 mm oxidized/iddingsitized olivine phenocrysts occurring alone and in glomeroporphyritic clumps.

Sample ID: H9-19C Chemical type: Group 2 (SROT) Lat.: 41.62 Long.: 116.35 7.5’ topo quad: Wilson Reservoir, NV Locality: Deep Creek, near South Fork Owyhee River Description: Outcrop of four basalt flows with talus beneath on north side of creek. Thin oxidized zone between flows, no soil horizons or significant accumulations of rubble. Sample from third flow from top. Flow is ~15 ft thick. Sample is gray to reddish gray, open textured, finely vesicular with visible plag, cpx, and iddingsitized olivine; some olivine and plag in glomeroporphyritic clumps. Holocrystalline, ophitic-subophitic groundmass of cpx (Ti-augite) and plag (0.5-1.0 mm) wit abundant iddingsitized olivine (0.25 mm) and intergranular oxide. 2.5 mm plag phenocrysts.

Sample ID: H9-20E Chemical type: Group 2 (TB) Lat.: 41.70 Long.: 116.42 7.5’ topo quad: Wilson Reservoir, NV Locality: Owyhee River at Poole Valley Description: see Hart, 1982

Sample ID: KS98-16 Chemical type: Group 2 (TB) Lat.: 41.77 Long.: 116.75 7.5’ topo quad: Star Valley Ridge SE, NV Locality: Bartome Knoll Description: Summit of Bartome Knoll is topped by a horseshoe-shaped rim, obviously a breached crater. Sample is from north end of this rim. Sample is pervasively oxidized to a deep maroon color. Vesicular and slightly open textured. Abundant small plag laths. Some secondary carbonate in vesicles. Holocrystalline, very fine grained cpx-plag-olivine-oxide matrix. Two populations of plag phenocrysts: laths up to 0.5 mm in glomeroporphyritic clumps and sparse 1.5 mm laths. Subhedral olivine microphenocrysts 0.25 mm. Groundmass and phenocryst olivine is all very strongly oxidized.

197

Sample ID: CM97-18 Chemical type: Group 2 (TB) Lat.: 42.02 Long.: 117.35 7.5’ topo quad: Oregon Butte, OR Locality: Oregon Butte Description: Sample taken from flow descending from northern part of crater rim. Fine grained, medium gray, open textured with abundant plag and less weathered olivine, glomeroporphyritic clusters. Sample appears weathered. Holocrystalline(?) very fine grained groundmass of cpx, oxides, olivine and plag (0.05-0.25 mm). Euhedral completely oxidized olivine phenocrysts (0.5-1.0 mm) and plag laths (0.5-1.5 mm).

Sample ID: H8-30C Chemical type: Group 2 (TB) Lat.: 42.76 Long.: 117.29 7.5’ topo quad: Little Grassy Mountain, OR Locality: Soldier Creek, ~0.8 miles east of Owyhee Canyon Description: Section of four flows exposed in valley of Soldier Creek; creek cuts down through lower flows, upper flows exposed on surrounding hills. This sample is from the third flow from the bottom. Sample is vesicular with zeolite (?) filling in larger vesicles. Less olivine and generally finer grained than H8-30 A&B. Holocrystalline, ophitic, very fine grained groundmass of cpx and plag (<0.25 mm) with abundant intergranular oxide and tiny olivine. Euhedral 1-2 mm highly fractured and oxidized olivine phenocrysts. 1.5 mm plag laths in glomeroporphyritic clumps. Minor secondary carbonate in vesicles.

Sample ID: H9-20C Chemical type: Group 2 (TB) Lat.: 41.70 Long.: 116.42 7.5’ topo quad: Wilson Reservoir, NV Locality: Owyhee River at Poole Valley Description: see Hart, 1982

Sample ID: KS98-17 Chemical type: Group 2 (TB) Lat.: 41.77 Long.: 116.75 7.5’ topo quad: Star Valley Ridge SE, NV Locality: Bartome Knoll Description: South end of Bartome Knoll summit rim, next to survey marker. Sample is dark gray, not open textured but vesicular on one surface. Small, very red oxidized olivine throughout. Some apparent clay alteration of the matrix. Holocrystalline, slightly diktytaxitic fine grained groundmass of cpx and 0.25 mm plag with abundant oxides and small olivine. Subhedral fractured and oxidized olivine phenocrysts up to 1 mm.

Sample ID: H9-37B Chemical type: Group 2 (TB) Lat.: 42.79 Long.: 117.60 7.5’ topo quad: Scott Reservoir, OR Locality: Owyhee Canyon, 7 miles upstream from Rome Description: ~20 ft thick basalt flow directly overlying lowest basalt flow exposed in Owyhee Canyon. Medium gray, vesicular, abundant iddingsitized olivine, cpx, and plag. Lots of plag in larger phenos and clumps±olivine. Thin section not available.

Sample ID: CM97-23 Chemical type: Group 2 (SROT) Lat.: 42.01 Long.: 117.25 7.5’ topo quad: Lookout Lake, OR Locality: Mahogany Butte Description: Rubbly outcrop at summit of butte. Fine grained, medium to dark gray, very open textured, a few vesicles with secondary mineralization, abundant plag and less olivine, glomeroporphyritic clumps. Holocrystalline, subophitic groundmass of cpx (Ti-augite) and plag (<0.5 mm) with intergranular oxide and 0.1-0.2 mm olivine. Subhedral to anhedral 1.0-2.0 mm highly fractured olivine phenocrysts with oxidized rims and 1.0-2.5 mm plag laths; some olivine and plag in glomeroporphyritic clumps.

198

Sample ID: KS98-18 Chemical type: Group 2 (TB) Lat.: 41.77 Long.: 116.75 7.5’ topo quad: Star Valley Ridge SE, NV Locality: Bartome Knoll Description: North flank of Bartome Knoll, just below summit. Sample is medium-dark gray, very open textured with abundant plagioclase and sparse olivine. Holocrystalline, slightly subophitic, very diktytaxitic groundmass of cpx (Ti-augite), 0. 5 mm plag and intergranular oxides and small olivine. Plag phenocrysts 2-3 mm and strongly fractured, weakly oxidized olivine 0.25-0.5 mm. Mode: plag 47.9%; olv 13.5%; cpx 34.4%; oxides 4.0%

Sample ID: CM97-22 Chemical type: Group 2 (TB) Lat.: 42.02 Long.: 117.25 7.5’ topo quad: Lookout Lake, OR Locality: Mahogany Butte Description: Flow exposed in stream cut on flank of butte. Fine grained, medium to dark gray, open textured, vesicular, abundant fresh olivine and plag, very small glomeroporphyritic clumps. Holocrystalline very fine grained groundmass of cpx, oxide, olivine and plag (<0.05 mm). Euhedral to subhedral 0.5-1.0 mm olivine phenocrysts with slightly oxidized rims and 0.5-2.5 mm plag laths (some in glomeroporphyritic clumps).

Sample ID: H9-39 Chemical type: Group 2 (TB) Lat.: 42.62 Long.: 117.33 7.5’ topo quad: Squaw Flat, OR Locality: Grassy Mountain Description: see Hart, 1982

Sample ID: CM97-19 Chemical type: Group 2 (TB) Lat.: 42.02 Long.: 117.35 7.5’ topo quad: Oregon Butte, OR Locality: Oregon Butte Description: Sample taken from flow on northern flank of butte, further downslope than CM97-18. Fine grained, medium gray, open textured, with abundant weathered olivine and plag laths, large glomeroporphyritic clusters. Some apparent alteration. Holocrystalline, subophitic groundmass of cpx and plag (0.25 mm)with abundant tiny olivine and intergranular oxides. Subhedral to anhedral highly fractured and weakly oxidized 0.5 mm olivine phenocrysts and 0.5-1.5 mm plag laths; some olivine and plag in glomeroporphyritic clumps.

Sample ID: KS98-20 Chemical type: Group 2 (TB) Lat.: 41.79 Long.: 116.75 7.5’ topo quad: Star Valley Ridge SE, NV Locality: Bartome Knoll Description: North flank of Bartome Knoll, approximately 300 ft elevation below summit. Sample is dark gray but with a reddish cast, vesicular and slightly open textured, with very small olivine and plagioclase. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and plag (<0.25 mm) with intergranular oxides and small olivine. Isoplated subhedral 0.5 mm olivine phenocrysts and glomeroporphyritic clumps of o.25 mm olivine and 1 mm sieved plag.

Sample ID: CM97-16 Chemical type: Group 2 (TB) Lat.: 42.01 Long.: 117.35 7.5’ topo quad: Oregon Butte, OR Locality: Oregon Butte Description: Sample taken just downslope from the southern remnant of crater rim at summit of Oregon Butte. Fine grained, medium gray, open textured with phenocrysts of fresh olivine and plag in glomeroporphyritic clusters. Overall sample appears relatively fresh. Holocrystalline(?), very fine grained groundmass of cpx, oxides, olivine and plag (0.05-0.25 mm). Euhedral to subhedral 1-2 mm very fresh olivine phenocrysts and 1-3 mm plag laths.

199

Sample ID: H9-20D Chemical type: Group 2 (TB) Lat.: 41.70 Long.: 116.42 7.5’ topo quad: Wilson Reservoir, NV Locality: Owyhee River at Poole Valley Description: Sequence of basalt flows overlying silicic material. Sample is from ~10 ft thick flow, second flow below rim-forming flow. Holocrystalline, subophitic, slightly diktytaxitic groundmass of plag (0.5-1.0 mm) and cpx (Ti-augite) with intergranular oxide and iddingsitized olivine. Anhedral highly fractured 0.5 mm strongly iddingsitized olivine phenocrysts and sparse 2 mm plag with sieved centers.

Sample ID: CM97-20 Chemical type: Group 2 (TB) Lat.: 42.02 Long.: 117.35 7.5’ topo quad: Oregon Butte, OR Locality: Oregon Butte Description: Sample taken from flow on northern flank of butte, further downslope than CM97-19. Fine grained, medium gray, open textured with small pockets of vesicles, relatively fresh olivine and plag, small glomeroporphyritic clumps. Relatively fresh sample. Holocrystalline, subophitic groundmass of cpx and plag (0.25 mm)with abundant tiny olivine and intergranular oxides. Subhedral to anhedral, abundant, highly fractured, fresh 0.5 mm olivine phenocrysts and 0.5-1.5 mm plag laths; some olivine and plag in glomeroporphyritic clumps.

Sample ID: KS98-31 Chemical type: Group 2 (TB) Lat.: 42.72 Long.: 117.38 7.5’ topo quad: Indian Fort, OR Locality: small valley on west side Deadhorse Butte Description: Sample taken from outcrop in small valley to the west of Deadhorse Butte. Source is uncertain, but may be Grassy Mountain. Sample is medium gray, vesicular and slightly open textured. Lots of secondary material. A few small weathered plagioclase and olivine grains. Hypocrystalline zones with glassy matrix containing plag (<0.1 mm), cpx, oxides and olivine alternating with holocrystalline zones with subophitic, diktytaxitic groundmass of cpx and plag (<0.5 mm) with oxides and oxidized olivine. Subhedral to euhedral, highly fractured 1.5 mm olivine phenocrysts with oxidized rims throughout. Plag laths up to 1 mm, some in glomeroporphyritic clumps.

Sample ID: KS98-19 Chemical type: Group 2 (SROT) Lat.: 41.79 Long.: 116.75 7.5’ topo quad: Star Valley Ridge SE, NV Locality: Bartome Knoll Description: North flank of Bartome Knoll, approximately 250 ft elevation below summit. Sample is medium-dark gray, very open textured with abundant plagioclase and abundant olivine. Holocrystalline, subophitic, diktytaxitic groundmass of plag laths (0.5 mm), cpx (Ti-augite), oxide and small olivine. 2-4 mm plag phenocrysts, some exhibiting sieve textures. Highly fractured, slightly iddingsitized 0.5-1.0 mm olivine phenocrysts.

Sample ID: KS98-27 Chemical type: Group 2 (TB) Lat.: 41.76 Long.: 117.06 7.5’ topo quad: Maiden Butte SE, NV Locality: Hill 5751 Description: Flow lobe crossing road to the north of Hill 5751, which appears to be the source. Unlike anything else observed on the southern part of the Owyhee Plateau. Sample is dark gray with a very dense matrix and abundant acicular plag 2-5 mm long. Lots of glomeroporphyritic olivine, reddish-brown in color. Holocrystalline, subophitic, very fine grained matrix of cpx and plag (<0.25 mm) with oxides and tiny red olivine. Euhedral to anhedral, 1-3 mm highly fractured and strongly oxidized olivine phenocrysts and plag laths 2-5 mm long.

200

Sample ID: KS98-14 Chemical type: Group 2 (TB) Lat.: 41.69 Long.: 116.77 7.5’ topo quad: Corral Lake, NV Locality: Guzzler Butte Description: South of summit of Guzzler Butte, next to weather station. Sample is medium-dark gray, open textured with more phenocrystic plag and less olivine than KS98-13. Holocrystalline, subophitic, diktytaxitic groundmass of cpx and plag (1 mm) with abundant intergranular oxides and small intergranular olivine. Some olivine appears to have cpx rims. Plag laths up to 3 mm. Mode: plag 43.7%; olv 16.4 %; cpx 30.8%; oxides 8.9%

Sample ID: CM97-21 Chemical type: Group 2 (TB) Lat.: 42.02 Long.: 117.35 7.5’ topo quad: Oregon Butte, OR Locality: Oregon Butte Description: Sample taken from flow lobe extending downslope from summit on northeast side of butte. Fine grained, medium grained, open textured, abundant fresh olivine and plag, small glomeroporphyritic clumps. Appears unaltered. Holocrystalline, subophitic, slightly diktytaxitic groundmass of cpx and plag (<0.25 mm)with abundant intergranular oxide and minor tiny intergranular olivine. Subhedral to anhedral 0.5 mm olivine phenocrysts with slightly oxidized rims and 1 mm plag laths (some in glomeroporphyritic clumps).

Sample ID: H9-37A Chemical type: Group 2 (TB) Lat.: 42.79 Long.: 117.60 7.5’ topo quad: Scott Reservoir, OR Locality: Owyhee Canyon, 7 miles upstream from Rome Description: see Hart, 1982

Sample ID: KS98-32D Chemical type: Group 2 (TB) Lat.: 42.78 Long.: 117.53 7.5’ topo quad: Scott Reservoir, OR Locality: Sand Hollow Description: Flow below KS98-32C. Sample is medium gray, not open textured. Abundant plagioclase as laths and some acicular crystals 2-3 mm long. Some small reddish olivine. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (Ti-augite) and plag (<0.25 mm) with abundant intergranular oxide and small red olivine. Some clay alteration. 0.5 mm highly fractured and iddingsitized olivine and 2-3 mm plag occur isolated and in glomeroporphyritic clumps.

Sample ID: H9-19D Chemical type: Group 2 (SROT) Lat.: 41.62 Long.: 116.35 7.5’ topo quad: Wilson Reservoir, NV Locality: Deep Creek, near South Fork Owyhee River Description: Outcrop of four basalt flows with talus beneath on north side of creek. Thin oxidized zone between flows, no soil horizons or significant accumulations of rubble. Sample from lowermost flow. Flow is ~5 ft thick. Sample is gray to reddish gray, open textured, finely vesicular with visible plag, cpx, and iddingsitized olivine; some olivine and plag in glomeroporphyritic clumps. Hypocrystalline (?), subophitic, slightly diktytaxitic groundmass of glass(?), cpx (Ti-augite), plag (0.5 mm), intergranular oxides and iddingsitized olivine. Anhedral completely iddingsitized 1 mm olivine and 0.75-1.0 mm plag in glomeroporphyritic clumps.

201

Sample ID: KS98-32C Chemical type: Group 2 (TB) Lat.: 42.78 Long.: 117.53 7.5’ topo quad: Scott Reservoir, OR Locality: Sand Hollow Description: Flow below KS98-32B. Sample is medium gray, open textured, with abundant small feldspars and olivines. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (Ti-augite) and plag (<0.25 mm) with abundant intergranular oxide and small red olivine. Some clay alteration and minor secondary chalcedony in some vesicles. 0.5 mm highly fractured and iddingsitized olivine and 1.0-2.5 mm plag laths occur isolated and in glomeroporphyritic clumps. Mode: plag 38.2%; olv 30.2%; cpx 21.4%; oxide 10.0%

Sample ID: H85-5A Chemical type: Group 2 (SROT) Lat.: 42.84 Long.: 117.22 7.5’ topo quad: Juniper Ridge, OR Locality: south of Antelope Reservoir Description: Upper of two flows exposed in scarp defining eastern edge of stream valley. Open textured, light-medium gray, generally equigranular. Obvious abundant iddingsitized olivine. Holocrystalline, subophitic/ophitic, slightly diktytaxitic groundmass of cpx (Ti-augite) and 0.5 mm plag with coarse intergranular oxide (0.25-0.5 mm) and small iddingsitized olivine. Anhedral 2 mm iddingsitized olivine phenocrysts and sparse 1-3 mm plag with sieved centers.

Sample ID: KS96-1 Chemical type: Group 2 (SROT) Lat.: 43.19 Long.: 117.48 7.5’ topo quad: Jordan Craters North, OR Locality: Birch Creek Canyon Road Description: Lowermost in sequence of flows exposed along road leading to Birch Creek campground. Exposure is 15-20 ft thick and grades from massive at base of flow to more vesicular near top. Sample is medium gray, medium grained with a few vesicles. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (Ti-augite) and plag mostly <0.25 mm with intergranular oxides and slightly iddingsitized olivine. Microphenocryst plag laths (slightly resorbed) up to 0.5 mm and subhedral olivine (iddingsitized) up to 0.5 mm. Mode: plag 48.0%; olv 15.7%; cpx 30.0%; oxides 6.0%

Sample ID: KS98-32E Chemical type: Group 2 (TB) Lat.: 42.78 Long.: 117.53 7.5’ topo quad: Scott Reservoir, OR Locality: Sand Hollow Description: Flow below KS98-32D. Very rubbly outcrop. Similar to KS98-32D, but with less plagioclase and more olivine. Matrix is shot with bluish clay. Holocrystalline, subophitic, diktytaxitic groundmass of cpx (Ti-augite) and plag (<0.25 mm) with abundant intergranular oxide and small red olivine. Pervasive clay alteration. 0.5 mm highly fractured and iddingsitized olivine and 1.0-2.5 mm plag laths occur isolated and in glomeroporphyritic clumps.

Sample ID: KS96-2 Chemical type: Group 2 (SROT) Lat.: 43.19 Long.: 117.48 7.5’ topo quad: Jordan Craters North, OR Locality: Birch Creek Canyon Road Description: Flow exposed approximately 200 ft above KS96-1 along Birch Creek road. Exposure is approximately 25 ft thick, massive at base and vesicular at top. Sample is medium grained, medium gray with a few vesicles. Holocrystalline, ophitic-subophitic, diktytaxitic groundmass of cpx and plag mostly <0.25 mm with intergranular oxide and strongly iddingsitized olivine. Phenocrysts of plag up to 1.5 mm and subhedral to anhedral olivine up to 0.75 mm. Mode: plag 37.6%; olv 25.7%; cpx 29.0%; oxides 7.5%

202

Sample ID: H85-5B Chemical type: Group 2 (SROT) Lat.: 42.84 Long.: 117.22 7.5’ topo quad: Juniper Ridge, OR Locality: south of Antelope Reservoir Description: Lower of two flows exposed in scarp defining eastern edge of stream valley. May unconformably overlie silicic material. Slightly coarser grained than H85-5A, open textured, medium gray. Obvious abundant microphenocrysts of olivine (iddingsitized) and glomeroporphyritic clumps of olivine and olivine+plag. Holocrystalline, subophitic/ophitic, diktytaxitic groundmass of cpx (Ti-augite), and plag laths (<0.5 mm) with abundant coarse intergranular oxide (0.25 mm) and iddingsitized olivine (0.2 mm). Blocky to tabular 1.5 mm plag phenocrysts (some sieved centers) and anhedral 0.75 mm olivine altered to iddingsite.

Sample ID: H9-37C Chemical type: Group 2 (SROT) Lat.: 42.79 Long.: 117.60 7.5’ topo quad: Scott Reservoir, OR Locality: Owyhee Canyon, 7 miles upstream from Rome Description: see Hart, 1982

Sample ID: 85-1 Chemical type: Group 2 (SROT) Lat.: 43.10 Long.: 117.21 7.5’ topo quad: Downey Canyon, OR Locality: Table Mountain Description: Thin section not available.

Sample ID: JC-5 Chemical type: Group 3 (AB) Lat.: 43.15 Long.: 117.46 7.5’ topo quad: Jordan Craters North, OR Locality: Jordan Craters Description: Sample from outflow unit of main crater. Thin section not available.

Sample ID: H8-70 Chemical type: Group 3 (AB) Lat.: 43.03 Long.: 117.43 7.5’ topo quad: Jordan Craters South, OR Locality: Clarks Butte Description: see Hart, 1982

Sample ID: H8-60A Chemical type: Group 3 (AB) Lat.: 42.98 Long.: 117.27 7.5’ topo quad: Danner, OR Locality: Skinner Hill Description: see Hart, 1982

Sample ID: H8-63 Chemical type: Group 3 (AB) Lat.: 43.15 Long.: 117.46 7.5’ topo quad: Jordan Craters North, OR Locality: Jordan Craters Description: Second-furthest spatter cone in alignment of small spatter cones to the southwest of Coffeepot Crater. Spatter cone is ~10 ft high. Sample from material on floor area of pit – agglutinated spatter. Thin section not available.

203

Sample ID: H8-57 Chemical type: Group 3 (AB) Lat.: 43.00 Long.: 117.42 7.5’ topo quad: Threemile Hill, OR Locality: Rocky Butte Description: see Hart, 1982

Sample ID: JC-4 Chemical type: Group 3 (AB) Lat.: 43.15 Long.: 117.46 7.5’ topo quad: Jordan Craters North, OR Locality: Jordan Craters Description: see Hart, 1982

Sample ID: H8-18 Chemical type: Group 3 (AB) Lat.: 42.93 Long.: 117.40 7.5’ topo quad: Threemile Hill, OR Locality: Jordan Creek, south of Threemile Hill Description: see Hart, 1982

204 APPENDIX 2B: SUMMARIZED GEOCHEMICAL, ISOTOPIC, AND GEOCHRONOLOGIC DATA

One hundred seventy (170) samples were analyzed in this study; major and trace element, Sr, Nd, and Pb isotope, and geochronologic data are presented here. Samples are divided into the geochemical types and age groups described in Chapter 2, and ranked

within those groups according to increasing analytical TiO2. This is the same order used in Appendix 2A and Appendix 2C. Major elements were analyzed by XRF and DCP. All trace elements with the exception of Sc, V, and Cr were analyzed by XRF; these three elements were analyzed by either XRF or DCP. All Sr isotope ratios are age-corrected initial ratios corrected for mass fractionation to 86Sr/88Sr = 0.1194. Samples analyzed at Miami University are reported 87 86 relative to Sr/ Sr = 0.708000 for the E&A SrCO3 standard; 2σ uncertainties based on the long-term reproducibility of this standard are ±0.00003. Samples analyzed at the Department of Terrestrial Magnetism, Carnegie Institution of Washington are reported relative to 87Sr/86Sr = 0.710250 for the NBS 987 standard; 2σ uncertainties based on the long-term reproducibility of this standard are ±0.00002. Nd isotope ratios measured at the Department of Terrestrial Magnetism are corrected for mass fractionation to 146Nd/144Nd = 0.72190 and are reported relative to 143Nd/144Nd = 0.511860 for the La Jolla Nd standard; 2σ uncertainties based on the reproducibility of this standard during the time period during which samples for this study were analyzed are ±0.00002. 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios measured at the Department of Terrestrial Magnetism are corrected for mass fractionation by 0.11%, 0.10%, and 0.30%, respectively, and are reported relative to 206Pb/204Pb = 16.9356, 207Pb/204Pb = 15.4891, and 208Pb/204Pb = 36.7006 for the NBS 981 standard as reported by Todt et al. (1996); 2σ uncertainties based on the reproducibility of this standard during the time period during which samples for this study were analyzed are ±0.02 for 206Pb/204Pb, ±0.02 for 207Pb/204Pb, and ±0.08 for 208Pb/204Pb.

205

Notes: a. Major elements previously determined by XRF, with FeO determined by titration (Hart, 1982; and unpublished data) b. Sc, V, and Cr determined by DCP c. New 40Ar/39Ar method age determination (see Appendix 2 D) d. Previously reported K-Ar or 40Ar/39Ar method age determination (see Appendix 2E) e. Estimated age based on stratigraphic context (see Chronostratigraphy section in Chapter 2)

206 GROUP 1: Steens Basalts and Oregon-Idaho Graben basalts (17-11 Ma)

JV96-7 JV96-6 JV96-3 JV96-4 JV96-5 H85-6 H85-10A SiO2 55.38 50.74 52.11 51.89 51.12 49.95 53.82 TiO2 1.04 1.18 1.21 1.21 1.22 1.97 2.00 Al2O3 16.19 16.39 16.55 16.71 16.70 14.73 14.72 Fe2O3 8.00 9.67 9.51 9.58 9.62 2.93 3.45 FeO ------10.41 7.09 MnO 0.15 0.20 0.17 0.17 0.17 0.25 0.15 MgO 4.36 5.52 5.49 4.88 5.80 2.78 5.01 CaO 7.09 8.54 8.14 8.23 8.89 7.28 7.08 Na2O 3.36 3.22 3.48 3.57 3.27 3.15 3.52 K2O 1.86 0.96 1.34 1.36 1.42 1.60 1.41 P2O5 0.62 0.68 0.58 0.67 0.62 0.43 0.47 L.O.I. 1.37 1.38 0.56 0.80 0.65 4.75 1.33 TOTAL 99.42 98.46 99.13 99.06 99.48 100.23 100.05

JV96-7 JV96-6 JV96-3 JV96-4 JV96-5 H85-6 H85-10A Rb 40 24 16 15 17 44 28 Sr 595 637 666 678 627 482 455 Y 28303231303436 Zr 213 193 207 206 193 206 263 Nb 14.5 14.1 14.9 14.7 14.1 13.2 11.9 Ni 47 63 59 58 76 63 94 Ga 18.8 18.9 19.4 19.4 18.9 22.0 21.6 Cu 45 56 62 55 63 55 76 Zn 103 109 109 111 113 119 113 U 1.4 0.8 0.2 0.6 1.2 0.3 0.8 Th 5.4 2.6 1.1 1.8 3.9 5.3 2.6 Pb------Ba 776 651 747 782 613 662 791 Ce 73 67 77 74 71 53 45 Co 26 29 29 30 32 35 31 La 37 33 41 34 34 25 23 Cr 80 115 113 114 129 - - - - V 158 205 206 204 216 - - - - Sc 19.2 24.8 24.4 24.5 25.7 - - - -

JV96-7 JV96-6 JV96-3 JV96-4 JV96-5 H85-6 H85-10A 87Sr / 86Sr 0.70460 - - - - 0.70442 - - 0.70418 0.70432 143Nd / 144Nd 0.512632 - - - - 0.512687 ------206Pb / 204Pb 18.921 - - - - 18.858 - - - - 18.984 207Pb / 204Pb 15.616 - - - - 15.604 - - - - 15.609 208Pb / 204Pb 38.656 - - - - 38.556 - - - - 38.629

Age (Ma) 13.87 14 15 14.61 15 16 16.27

NOTES: b, c b, e b, e b, c b, e a, b, e a, b, c

207 GROUP 1 (continued)

H85-10B CH82-32 H9-47 H85-26A JV96-1 JV96-2 H85-12B SiO2 48.72 48.46 49.41 46.13 49.05 48.00 49.26 TiO2 2.07 2.20 2.32 2.41 2.49 2.53 2.54 Al2O3 15.76 15.83 15.18 15.12 14.29 14.20 16.45 Fe2O3 7.61 12.19 4.85 3.99 13.56 14.05 9.83 FeO 5.33 - - 7.56 9.28 - - - - 3.02 MnO 0.18 0.15 0.19 0.20 0.19 0.20 0.14 MgO 5.67 5.72 4.69 8.23 6.25 6.62 2.77 CaO 9.98 8.17 9.05 10.35 10.15 10.24 8.66 Na2O 3.09 3.00 3.19 2.18 2.57 2.51 3.69 K2O 0.48 0.93 0.92 0.38 0.52 0.48 1.06 P2O5 0.26 0.41 0.43 0.38 0.34 0.44 0.42 L.O.I. 1.49 3.16 2.37 1.17 0.23 0.11 2.61 TOTAL 100.64 100.22 100.16 99.82 99.64 99.37 100.45

H85-10B CH82-32 H9-47 H85-26A JV96-1 JV96-2 H85-12B Rb 6.9 20 15 6.3 9.7 6.2 17 Sr 459 440 466 232 305 307 561 Y 29 32 36 37 40 40 38 Zr 146 189 218 145 154 153 216 Nb 9.5 13.8 15.0 13.3 11.3 10.8 14.9 Ni 93 95 49 161 88 86 62 Ga 22.4 22.5 23.1 18.9 19.7 20.7 24.3 Cu 59 74 52 60 81 79 72 Zn 105 124 132 115 133 133 116 U 0.3 1.4 1.7 0.7

H85-10B CH82-32 H9-47 H85-26A JV96-1 JV96-2 H85-12B 87Sr / 86Sr 0.70397 0.70409 0.70426 - - - - 0.70638 0.70436 143Nd / 144Nd - - 0.512810 ------0.512369 - - 206Pb / 204Pb 18.831 18.990 19.017 - - - - 18.336 - - 207Pb / 204Pb 15.565 15.590 15.612 - - - - 15.605 - - 208Pb / 204Pb 38.440 38.650 38.710 - - - - 39.064 - -

Age (Ma) 16 16.23 15.01 - - 12 12.37 16

NOTES: a, b, e b, d a, b, d a, b b, e b, c a, e

208 GROUP 1 (continued)

H8-74 H9-32 SiO2 46.13 49.80 TiO2 2.86 3.00 Al2O3 14.80 15.40 Fe2O3 15.47 7.59 FeO - - 6.40 MnO 0.20 0.23 MgO 5.01 3.51 CaO 8.75 7.24 Na2O 3.25 3.50 K2O 1.02 1.60 P2O5 0.98 1.37 L.O.I. 0.08 0.73 TOTAL 98.55 100.37

H8-74 H9-32 Rb 16 23 Sr 424 468 Y 48 64 Zr 270 350 Nb 23.2 30.5 Ni 59 19 Ga 22.2 22.8 Cu 46 31 Zn 138 167 U 0.6 1.4 Th 2.4 1.2 Pb - - - - Ba 842 1337 Ce 65 90 Co 41 30 La 38 61 Cr 70 - - V 393 - - Sc 29.9 - -

H8-74 H9-32 87Sr / 86Sr 0.70521 0.70557 143Nd / 144Nd 0.512479 0.512526 206Pb / 204Pb - - 18.722 207Pb / 204Pb - - 15.593 208Pb / 204Pb - - 38.780

Age (Ma) 11.70 11.20

NOTES: b, d a, d

209 GROUP 2: HAOT-TB-SROT (<11 Ma)

CM97-13 CM97-14 CM97-15 H9-27 H9-30 CM97-12 H9-28A SiO2 48.54 48.23 48.62 47.71 48.22 48.65 48.50 TiO2 0.73 0.76 0.79 0.81 0.84 0.86 0.89 Al2O3 17.43 17.20 17.03 16.54 16.24 16.39 16.40 Fe2O3 10.65 10.74 10.92 1.63 10.87 11.54 11.13 FeO ------8.72 ------MnO 0.18 0.18 0.18 0.17 0.20 0.20 0.18 MgO 8.44 9.16 8.93 9.22 9.45 8.88 9.33 CaO 12.04 12.18 12.16 12.19 11.53 12.00 11.43 Na2O 2.24 2.28 2.29 2.29 2.35 2.32 2.44 K2O 0.05 0.09 0.04 0.11 0.20 0.09 0.21 P2O5 0.24 0.19 0.10 0.11 0.08 0.16 0.26 L.O.I. 0.24 -0.07 -0.14 0.89 0.03 -0.25 -0.14 TOTAL 100.79 100.93 100.90 100.39 100.02 100.84 100.62

CM97-13 CM97-14 CM97-15 H9-27 H9-30 CM97-12 H9-28A Rb 2.6 2.3 2.0 2.1 4.8 2.1 3.7 Sr 172 164 152 159 181 153 180 Y 20 20 20 22 19 22 20 Zr 36 37 37 45 50 41 50 Nb 4.1 3.9 3.7 1.8 2.8 4.1 2.7 Ni 142 146 133 152 179 128 168 Ga 16.5 16.4 16.5 16.3 16.6 16.9 17.7 Cu 91 71 113 79 98 76 97 Zn 72 77 68 62 68 82 67 U 0.5 0.8 0.4

CM97-13 CM97-14 CM97-15 H9-27 H9-30 CM97-12 H9-28A 87Sr / 86Sr ------0.70591 ------143Nd / 144Nd ------0.512622 ------206Pb / 204Pb ------18.715 ------207Pb / 204Pb ------15.631 ------208Pb / 204Pb ------38.838 ------

Age (Ma) 5 5 5 5.04 - - 5 - -

NOTES: e e e a, b, d b e b

210 GROUP 2 (continued)

KS96-8B KS96-8D H8-73 KS96-8C H9-36A H9-34 CM97-4 SiO2 47.52 46.92 48.17 46.83 47.42 47.37 46.98 TiO2 0.91 0.92 0.92 0.94 0.94 0.96 1.00 Al2O3 15.99 15.79 16.12 15.84 16.52 15.73 15.60 Fe2O3 11.11 10.92 1.23 10.83 4.44 1.49 11.44 FeO - - - - 8.80 - - 6.24 8.48 - - MnO 0.18 0.17 0.18 0.17 0.17 0.15 0.19 MgO 8.37 9.62 9.28 8.32 8.81 9.52 9.80 CaO 11.15 11.71 11.77 10.67 11.59 11.67 11.11 Na2O 2.50 2.40 2.47 2.32 2.48 2.47 2.13 K2O 0.30 0.16 0.28 0.24 0.17 0.24 0.17 P2O5 0.20 0.20 0.08 0.19 0.10 0.15 0.23 L.O.I. 1.09 0.39 0.58 2.16 0.65 1.17 0.15 TOTAL 99.32 99.19 99.88 98.50 99.53 99.40 98.78

KS96-8B KS96-8D H8-73 KS96-8C H9-36A H9-34 CM97-4 Rb 2.9 2.9 4.4 3.2 2.7 2.0 3.3 Sr 205 185 237 192 191 243 206 Y 19 19 19 20 20 16 23 Zr 53 53 67 57 53 58 65 Nb 5.9 3.4 6.0 6.4 3.3 6.6 6.3 Ni 134 190 112 128 147 172 125 Ga 17.7 17.0 18.0 17.2 18.4 18.5 16.7 Cu 92 106 79 97 84 86 75 Zn 72 74 67 73 71 70 78 U

KS96-8B KS96-8D H8-73 KS96-8C H9-36A H9-34 CM97-4 87Sr / 86Sr - - 0.70534 0.70498 - - 0.70523 - - - - 143Nd / 144Nd ------206Pb / 204Pb ------18.884 - - - - 207Pb / 204Pb ------15.618 - - - - 208Pb / 204Pb ------38.758 - - - -

Age (Ma) - - 0.65 0.43 - - 0.91 - - - -

NOTES: b b, c a, b, d b a, b, d a

211 GROUP 2 (continued)

KS96-8A KS96-5 KS98-4 H8-98 KS98-5 CM97-8 CM97-2A SiO2 46.39 47.49 48.55 46.94 48.55 47.87 47.70 TiO2 1.01 1.02 1.06 1.08 1.12 1.14 1.16 Al2O3 15.66 16.46 16.26 15.95 16.19 16.41 14.81 Fe2O3 11.00 10.62 11.14 2.00 11.19 11.64 11.68 FeO ------8.52 ------MnO 0.17 0.18 0.18 0.19 0.17 0.19 0.19 MgO 10.02 9.46 8.39 8.95 9.40 8.63 10.67 CaO 10.12 11.23 11.69 11.32 11.13 11.73 10.81 Na2O 2.20 2.44 2.45 2.27 2.35 2.31 2.08 K2O 0.32 0.26 0.16 0.16 0.23 0.19 0.28 P2O5 0.19 0.16 0.14 0.08 0.24 0.14 0.28 L.O.I. 1.98 -0.19 0.21 1.51 -0.02 0.04 0.13 TOTAL 99.06 99.11 100.23 98.97 100.56 100.27 99.79

KS96-8A KS96-5 KS98-4 H8-98 KS98-5 CM97-8 CM97-2A Rb 4.5 3.2 1.9 0.2 4.5 3.9 5.1 Sr 241 215 225 187 197 164 162 Y 19 22 21 18 21 23 25 Zr 66 76 63 51 74 65 88 Nb 9.4 7.3 5.6 5.6 7.1 6.6 8.9 Ni 201 153 130 177 172 145 188 Ga 15.8 16.1 19.4 19.9 19.0 17.7 16.2 Cu 93 86 109 277 80 88 94 Zn 78 66 85 66 86 88 93 U

KS96-8A KS96-5 KS98-4 H8-98 KS98-5 CM97-8 CM97-2A 87Sr / 86Sr - - - - 0.70669 ------143Nd / 144Nd ------206Pb / 204Pb ------207Pb / 204Pb ------208Pb / 204Pb ------

Age (Ma) - - - - 7.61 - - 8 - - - -

NOTES: bbcae

212 GROUP 2 (continued)

H85-11C KS98-6A KS98-2 KS98-6C H85-11A H8-43 H8-45 SiO2 47.90 48.37 48.06 48.46 47.79 47.29 47.48 TiO2 1.17 1.18 1.18 1.19 1.19 1.20 1.20 Al2O3 16.66 15.99 16.08 15.64 16.59 16.45 16.82 Fe2O3 3.68 11.18 11.15 11.16 2.67 11.31 8.34 FeO 7.16 ------8.16 - - 2.86 MnO 0.18 0.18 0.17 0.18 0.19 0.19 0.17 MgO 8.03 9.68 9.24 9.62 8.08 8.36 8.90 CaO 11.49 10.93 11.04 10.97 11.49 11.73 11.14 Na2O 2.83 2.29 2.37 2.27 2.63 2.47 2.65 K2O 0.27 0.27 0.24 0.26 0.26 0.29 0.33 P2O5 0.14 0.27 0.15 0.24 0.13 0.17 0.16 L.O.I. 1.04 -0.08 0.13 -0.09 0.61 0.77 0.60 TOTAL 100.55 100.26 99.81 99.90 99.79 100.23 100.65

H85-11C KS98-6A KS98-2 KS98-6C H85-11A H8-43 H8-45 Rb 4.6 4.6 4.0 4.3 4.6 5.7 5.9 Sr 227 194 202 193 207 230 273 Y 22 22 23 22 23 24 23 Zr 76 81 79 81 77 73 96 Nb 7.7 7.6 7.3 7.5 7.9 6.0 8.4 Ni 131 192 172 188 121 133 149 Ga 16.3 19.4 19.5 18.9 17.0 17.3 16.3 Cu 95 84 93 100 99 52 64 Zn 69 93 88 90 74 75 75 U 0.5 0.4 1.6 0.3

H85-11C KS98-6A KS98-2 KS98-6C H85-11A H8-43 H8-45 87Sr / 86Sr ------0.70457 143Nd / 144Nd ------206Pb / 204Pb ------207Pb / 204Pb ------208Pb / 204Pb ------

Age (Ma) - - - - 8 - - - - 4 4.09

NOTES: a, b e a, b b, e a, d

213 GROUP 2 (continued)

H85-11B KS98-6B H9-36B KS98-12 SM75-12A H9-36C KS98-3 SiO2 47.30 48.39 45.93 48.29 47.65 47.48 48.71 TiO2 1.21 1.22 1.22 1.25 1.25 1.25 1.27 Al2O3 16.26 15.67 15.67 16.17 16.26 16.03 15.86 Fe2O3 11.53 11.23 10.80 11.39 2.74 2.76 11.31 FeO ------7.33 8.00 - - MnO 0.18 0.18 0.20 0.18 0.17 0.18 0.17 MgO 8.06 9.29 9.07 8.76 9.89 8.73 9.04 CaO 11.47 10.91 10.14 11.61 11.49 10.45 10.95 Na2O 2.60 2.29 2.42 2.33 2.28 2.69 2.38 K2O 0.25 0.28 0.34 0.18 0.34 0.38 0.27 P2O5 0.08 0.19 0.36 0.25 0.15 0.19 0.21 L.O.I. 0.28 0.08 3.14 -0.09 0.72 1.46 0.13 TOTAL 99.22 99.71 99.28 100.31 100.27 99.60 100.29

H85-11B KS98-6B H9-36B KS98-12 SM75-12A H9-36C KS98-3 Rb 5.3 5.4 5.4 1.5 6.5 5.7 5.7 Sr 216 199 216 193 201 241 204 Y 23 23 21 25 24 22 24 Zr 74 86 88 85 77 86 88 Nb 7.5 7.9 10.5 7.3 5.6 10.5 8.2 Ni 120 190 129 144 166 127 174 Ga 16.9 19.3 16.8 19.2 16.5 17.5 19.6 Cu 144 85 83 97 89 85 81 Zn 76 94 77 86 77 72 92 U

H85-11B KS98-6B H9-36B KS98-12 SM75-12A H9-36C KS98-3 87Sr / 86Sr ------0.70614 0.70667 0.70476 - - 143Nd / 144Nd ------0.512445 - - - - 206Pb / 204Pb ------18.679 - - - - 207Pb / 204Pb ------15.653 - - - - 208Pb / 204Pb ------39.021 - - - -

Age (Ma) - - - - 1 - - 8.51 1.25 8

NOTES: b b, e a, b, d a, b, d e

214 GROUP 2 (continued)

CM97-7 CM97-2B KS98-1 KS98-11 KS96-7 KS96-10 CM97-9 SiO2 47.40 48.06 47.60 48.19 47.62 47.08 48.39 TiO2 1.27 1.27 1.28 1.29 1.29 1.29 1.30 Al2O3 15.76 14.94 16.67 15.75 15.39 16.17 16.24 Fe2O3 11.92 11.70 11.06 11.70 11.29 12.29 12.25 FeO ------MnO 0.19 0.18 0.17 0.20 0.18 0.19 0.20 MgO 9.52 10.45 8.56 8.71 9.19 8.97 8.69 CaO 11.39 10.93 10.90 11.77 11.17 10.60 11.56 Na2O 2.24 2.18 2.72 2.32 2.16 2.47 2.33 K2O 0.21 0.34 0.39 0.21 0.28 0.21 0.22 P2O5 0.16 0.27 0.19 0.22 0.21 0.41 0.26 L.O.I. -0.25 -0.29 -0.15 -0.26 0.34 0.09 -0.32 TOTAL 99.81 100.05 99.39 100.08 99.10 99.77 101.12

CM97-7 CM97-2B KS98-1 KS98-11 KS96-7 KS96-10 CM97-9 Rb 3.9 6.9 5.7 1.8 5.5 3.5 4.7 Sr 175 160 282 244 193 190 167 Y 25 28 19 27 26 27 25 Zr 74 99 85 86 87 84 76 Nb 7.0 9.6 14.0 7.3 6.6 7.0 7.3 Ni 166 153 148 131 135 175 134 Ga 18.2 17.1 19.7 19.1 17.6 18.5 18.4 Cu 72 83 87 105 78 86 86 Zn 96 96 84 88 81 91 91 U

CM97-7 CM97-2B KS98-1 KS98-11 KS96-7 KS96-10 CM97-9 87Sr / 86Sr 0.70663 ------0.70735 - - - - 143Nd / 144Nd ------206Pb / 204Pb ------207Pb / 204Pb ------208Pb / 204Pb ------

Age (Ma) 6.54 - - 1 - - 8.36 - - - -

NOTES: c e b, c b

215 GROUP 2 (continued)

H8-95 H9-42 H8-47 CM97-1 CM97-11 KS98-26 CM97-10 SiO2 48.08 46.77 47.79 47.87 47.78 49.02 48.01 TiO2 1.30 1.30 1.30 1.31 1.31 1.33 1.35 Al2O3 16.46 16.58 15.75 14.67 16.08 16.17 15.96 Fe2O3 2.27 1.68 2.52 11.65 12.12 11.34 12.32 FeO 8.24 8.80 8.00 ------MnO 0.19 0.17 0.18 0.17 0.19 0.17 0.19 MgO 8.65 8.68 9.52 10.11 8.61 9.04 8.50 CaO 11.60 11.19 11.03 10.94 11.38 10.83 11.36 Na2O 2.14 2.55 2.60 2.11 2.29 2.36 2.31 K2O 0.23 0.26 0.32 0.28 0.20 0.39 0.23 P2O5 0.17 0.17 0.26 0.35 0.15 0.25 0.26 L.O.I. 0.59 0.80 0.59 0.04 0.02 0.04 -0.20 TOTAL 99.92 98.95 99.86 99.50 100.11 100.93 100.30

H8-95 H9-42 H8-47 CM97-1 CM97-11 KS98-26 CM97-10 Rb 2.3 4.4 5.0 6.7 3.6 5.8 4.7 Sr 200 242 229 181 166 223 167 Y 23 23 27 26 24 25 25 Zr 59 88 103 96 74 116 78 Nb 5.9 7.3 10.4 9.6 6.6 9.7 7.6 Ni 137 137 138 129 119 167 122 Ga 18.7 18.7 17.4 17.9 19.0 19.9 19.4 Cu 102 79 61 65 102 59 83 Zn 75 75 76 92 82 95 88 U 0.9 0.5 1.0 0.8

H8-95 H9-42 H8-47 CM97-1 CM97-11 KS98-26 CM97-10 87Sr / 86Sr - - 0.70435 0.70528 ------143Nd / 144Nd ------206Pb / 204Pb ------207Pb / 204Pb ------208Pb / 204Pb ------

Age (Ma) 4.40 0.44 4.06 - - - - 10 - -

NOTES: a, d a, b, d a, b, d e

216 GROUP 2 (continued)

H8-69E KS98-25 KS98-22 KS98-24 KS96-3 H8-56A H8-56B SiO2 45.08 49.18 48.84 48.82 47.28 48.05 47.30 TiO2 1.36 1.38 1.38 1.40 1.43 1.43 1.43 Al2O3 16.57 15.92 15.73 15.70 15.68 15.85 16.16 Fe2O3 12.80 11.46 11.52 11.44 12.27 2.27 11.80 FeO ------8.16 - - MnO 0.18 0.18 0.18 0.18 0.19 0.17 0.18 MgO 7.79 8.33 8.41 8.22 8.02 9.49 9.60 CaO 10.41 10.88 11.06 10.83 10.73 10.70 10.64 Na2O 2.36 2.33 2.29 2.34 2.61 2.87 2.62 K2O 0.29 0.45 0.40 0.46 0.34 0.39 0.36 P2O5 0.42 0.30 0.24 0.31 0.26 0.23 0.20 L.O.I. 2.71 0.04 0.16 -0.06 0.32 0.75 0.33 TOTAL 99.97 100.46 100.20 99.63 99.11 100.36 100.61

H8-69E KS98-25 KS98-22 KS98-24 KS96-3 H8-56A H8-56B Rb 4.4 6.6 5.6 7.2 6.7 6.5 5.5 Sr 210 260 270 244 233 229 228 Y 27 27 27 27 27 26 24 Zr 98 122 121 126 98 113 107 Nb 7.7 10.7 9.9 10.8 8.6 11.8 11.6 Ni 117 144 146 139 170 188 203 Ga 18.3 19.8 19.9 20.1 18.8 17.1 17.9 Cu 49 87 86 91 79 82 80 Zn 95 95 97 96 87 90 89 U

H8-69E KS98-25 KS98-22 KS98-24 KS96-3 H8-56A H8-56B 87Sr / 86Sr 0.70656 ------143Nd / 144Nd ------206Pb / 204Pb ------207Pb / 204Pb ------208Pb / 204Pb ------

Age (Ma) 9101010------

NOTES: b, e e e e b a, b b

217 GROUP 2 (continued)

H8-28A KS96-6B KS98-23 CM97-5 H8-42 KS98-9 CM97-3 SiO2 47.45 47.72 49.34 48.12 47.90 47.55 47.29 TiO2 1.44 1.44 1.46 1.46 1.46 1.48 1.49 Al2O3 16.03 14.76 16.09 15.57 16.81 15.86 15.67 Fe2O3 11.55 12.02 11.62 11.72 2.74 11.70 12.18 FeO ------8.16 - - - - MnO 0.17 0.18 0.18 0.18 0.18 0.18 0.20 MgO 8.16 10.02 8.57 9.19 8.76 9.19 8.12 CaO 11.39 9.93 11.11 11.11 10.95 11.36 11.66 Na2O 2.19 2.25 2.39 2.26 2.44 2.31 2.40 K2O 0.27 0.42 0.45 0.33 0.44 0.23 0.23 P2O5 0.16 0.43 0.28 0.20 0.33 0.30 0.15 L.O.I. 0.89 -0.05 -0.22 -0.23 0.70 -0.15 0.07 TOTAL 99.69 99.12 101.27 99.91 100.87 99.99 99.46

H8-28A KS96-6B KS98-23 CM97-5 H8-42 KS98-9 CM97-3 Rb 4.6 9.8 6.8 6.9 7.8 3.4 3.5 Sr 225 175 210 193 246 198 206 Y 27 29 27 27 28 25 26 Zr 102 123 126 105 110 104 85 Nb 8.8 9.2 10.3 10.1 10.7 8.9 8.7 Ni 142 216 133 117 131 158 99 Ga 17.7 17.7 20.5 17.9 18.3 20.1 18.6 Cu 71 69 83 53 70 84 45 Zn 96 97 96 98 81 99 101 U

H8-28A KS96-6B KS98-23 CM97-5 H8-42 KS98-9 CM97-3 87Sr / 86Sr - - - - 0.70733 - - 0.70522 0.70631 - - 143Nd / 144Nd ------206Pb / 204Pb ------207Pb / 204Pb ------208Pb / 204Pb ------

Age (Ma) 7 9 10.39 - - 4.49 6 - -

NOTES: b, e b, e c a, b, d e

218 GROUP 2 (continued)

KS98-29 H8-56D KS98-32B H8-69D H9-21E H9-21B CM97-6 SiO2 48.60 46.65 47.88 46.35 46.89 46.90 47.51 TiO2 1.49 1.49 1.51 1.52 1.52 1.53 1.54 Al2O3 14.84 16.08 16.83 16.84 16.64 13.96 16.00 Fe2O3 12.00 11.92 11.67 7.46 2.93 2.66 12.61 FeO ------5.20 8.24 9.72 - - MnO 0.18 0.24 0.18 0.18 0.17 0.18 0.20 MgO 8.86 8.15 7.02 7.40 8.11 12.71 8.37 CaO 11.24 10.90 11.51 10.80 11.18 9.26 11.28 Na2O 2.23 2.52 2.61 2.34 2.28 2.03 2.44 K2O 0.27 0.50 0.28 0.25 0.25 0.45 0.21 P2O5 0.21 0.36 0.39 0.30 0.22 0.28 0.25 L.O.I. -0.09 1.68 -0.02 1.13 1.02 1.10 -0.21 TOTAL 99.83 100.50 99.86 99.77 99.45 100.78 100.18

KS98-29 H8-56D KS98-32B H8-69D H9-21E H9-21B CM97-6 Rb 2.7 8.7 2.0 3.4 3.0 6.5 3.1 Sr 196 229 229 241 205 188 201 Y 27 26 26 30 25 27 27 Zr 95 116 115 111 105 130 91 Nb 9.7 12.5 12.9 9.4 8.9 12.6 9.1 Ni 103 155 90 100 146 343 135 Ga 19.4 17.9 21.1 18.9 20.4 18.6 19.3 Cu 66 83 92 58 56 64 90 Zn 99 86 96 92 96 106 109 U 0.9 0.8 0.3 0.0 1.6 1.2 0.6 Th 3.7 0.2 3.8 0.5 5.1 4.1 0.4 Pb 4.2 - - 2.2 - - 2.8 5.2 5.8 Ba 301 287 383 297 261 348 490 Ce 17 33 26 36 24 23 17 Co 49 49 46 48 54 66 44 La 7141215111611 Cr 378 203 210 89 281 820 303 V 300 238 264 226 302 230 291 Sc 35.5 31.4 31.9 31.4 34.7 27.9 34.6

KS98-29 H8-56D KS98-32B H8-69D H9-21E H9-21B CM97-6 87Sr / 86Sr ------143Nd / 144Nd ------206Pb / 204Pb ------207Pb / 204Pb ------208Pb / 204Pb ------

Age (Ma) 8 - - - - 9.94 6.01 7 7

NOTES: e b a, b, d a, d a, e e

219 GROUP 2 (continued)

H8-35 H8-29 H9-44 KS98-30 H8-28E H8-50E H8-50C SiO2 48.26 49.11 46.96 49.19 47.58 47.87 47.53 TiO2 1.56 1.57 1.57 1.58 1.58 1.58 1.59 Al2O3 15.61 15.89 16.26 14.94 15.55 16.13 16.16 Fe2O3 12.12 4.77 3.98 12.14 4.17 1.35 2.05 FeO - - 6.64 7.20 - - 6.96 10.00 9.12 MnO 0.18 0.19 0.17 0.19 0.18 0.19 0.19 MgO 9.04 7.57 8.94 8.39 8.01 8.60 8.82 CaO 10.48 11.67 10.86 11.25 12.15 10.91 11.19 Na2O 2.37 2.38 2.43 2.22 2.48 2.93 2.97 K2O 0.49 0.34 0.39 0.35 0.36 0.42 0.40 P2O5 0.35 0.21 0.31 0.38 0.21 0.22 0.16 L.O.I. 0.13 0.93 0.44 -0.28 1.44 0.49 1.24 TOTAL 100.57 101.27 99.51 100.33 100.67 100.69 101.42

H8-35 H8-29 H9-44 KS98-30 H8-28E H8-50E H8-50C Rb 10 7.4 7.9 5.5 8.2 6.3 7.0 Sr 189 235 275 195 215 261 252 Y 30 32 28 30 31 28 28 Zr 130 115 123 104 114 105 110 Nb 9.8 9.7 11.7 10.0 9.5 9.8 10.2 Ni 151 118 141 93 130 125 157 Ga 17.8 18.5 18.4 19.5 17.8 19.0 17.5 Cu 61 78 80 66 73 73 86 Zn 94 96 83 101 93 89 96 U 0.9 0.7 0.4 0.6 1.7

H8-35 H8-29 H9-44 KS98-30 H8-28E H8-50E H8-50C 87Sr / 86Sr - - 0.70693 0.70502 0.70642 0.70676 0.70533 - - 143Nd / 144Nd - - 0.512387 0.512610 ------206Pb / 204Pb - - 18.702 18.693 - - - - 18.774 - - 207Pb / 204Pb - - 15.660 15.638 - - - - 15.643 - - 208Pb / 204Pb - - 39.242 38.940 - - - - 39.006 - -

Age (Ma) 8 9.87 3.84 7.56 7.05 1.55 2

NOTES: b, e b, d a, b, d c a, b, d a, b, d a, b, e

220 GROUP 2 (continued)

H8-50D KS98-10 KS96-4 KS96-6C H8-56C KS98-7 H8-50B SiO2 47.51 48.36 47.19 48.14 46.87 48.39 46.31 TiO2 1.59 1.60 1.60 1.61 1.61 1.61 1.64 Al2O3 16.17 15.74 15.78 15.66 16.59 15.69 15.97 Fe2O3 12.36 11.86 11.72 11.96 12.56 11.61 12.32 FeO ------MnO 0.19 0.18 0.18 0.18 0.19 0.17 0.19 MgO 8.38 8.33 9.19 8.08 8.35 7.84 8.16 CaO 10.58 11.16 10.45 10.49 10.70 11.26 11.25 Na2O 2.65 2.56 2.50 2.39 2.53 2.51 2.57 K2O 0.42 0.35 0.39 0.45 0.32 0.44 0.28 P2O5 0.28 0.24 0.46 0.28 0.23 0.21 0.33 L.O.I. -0.15 -0.31 -0.14 0.16 0.59 0.09 0.49 TOTAL 99.98 100.07 99.31 99.40 100.53 99.83 99.51

H8-50D KS98-10 KS96-4 KS96-6C H8-56C KS98-7 H8-50B Rb 5.8 7.0 7.3 9.1 6.8 8.4 4.2 Sr 258 230 282 226 246 221 252 Y 28 27 29 31 27 28 29 Zr 105 124 133 134 126 128 110 Nb 9.6 9.0 13.1 9.3 13.6 9.5 9.9 Ni 154 120 159 127 157 104 146 Ga 18.7 20.5 18.4 18.8 17.5 21.2 17.9 Cu 76 86 65 58 73 92 85 Zn 93 89 96 93 98 93 107 U

H8-50D KS98-10 KS96-4 KS96-6C H8-56C KS98-7 H8-50B 87Sr / 86Sr ------0.70654 - - 0.70569 - - 143Nd / 144Nd ------206Pb / 204Pb ------207Pb / 204Pb ------208Pb / 204Pb ------

Age (Ma) 2 - - - - 8.34 - - - - 2

NOTES: b, e b b, c b b, e

221 GROUP 2 (continued)

H8-28C KS98-32A H9-21C H8-50A H8-39 H9-46 H8-36 SiO2 48.27 47.45 48.24 47.67 48.39 47.67 47.55 TiO2 1.64 1.65 1.66 1.66 1.70 1.72 1.75 Al2O3 15.54 15.88 16.27 15.78 15.25 14.55 15.07 Fe2O3 12.26 12.69 4.99 12.53 4.19 12.68 2.25 FeO - - - - 6.68 - - 7.40 - - 8.72 MnO 0.19 0.20 0.18 0.19 0.18 0.18 0.18 MgO 7.96 8.27 6.61 8.15 8.41 10.20 8.53 CaO 11.04 11.09 10.74 10.82 11.46 10.13 11.57 Na2O 2.29 2.60 2.51 2.51 2.65 2.14 2.26 K2O 0.42 0.34 0.54 0.53 0.25 0.44 0.28 P2O5 0.32 0.36 0.43 0.21 0.20 0.36 0.24 L.O.I. 0.26 -0.63 1.36 - - 1.09 -0.19 0.97 TOTAL 100.19 99.89 100.21 100.04 101.17 99.88 99.37

H8-28C KS98-32A H9-21C H8-50A H8-39 H9-46 H8-36 Rb 7.9 2.6 7.6 - - 3.7 6.6 5.6 Sr 232 214 235 - - 374 262 213 Y 31 27 30 - - 24 30 33 Zr 116 124 134 - - 112 113 144 Nb 9.9 14.0 11.8 - - 7.1 8.5 9.6 Ni 128 107 127 - - 216 180 88 Ga 18.8 21.4 20.7 - - 20.0 18.0 18.5 Cu 86 81 90 - - 74 64 54 Zn 104 96 100 - - 95 107 98 U

H8-28C KS98-32A H9-21C H8-50A H8-39 H9-46 H8-36 87Sr / 86Sr 0.70682 ------0.70658 143Nd / 144Nd ------0.512437 206Pb / 204Pb ------18.733 207Pb / 204Pb ------15.650 208Pb / 204Pb ------39.145

Age (Ma) 8.14 - - 8.11 2 - - - - 7.58

NOTES: b, d a, d e a, b b a, b, d

222 GROUP 2 (continued)

H8-28D H8-30B KS98-13 H9-29 H8-28B H8-34 H8-30A SiO2 47.76 47.10 47.67 47.08 49.03 48.42 47.28 TiO2 1.77 1.79 1.80 1.80 1.82 1.82 1.86 Al2O3 15.47 14.76 15.50 16.16 15.56 15.78 15.14 Fe2O3 8.70 13.18 12.46 1.28 12.19 4.09 13.21 FeO 3.48 - - - - 9.36 - - 7.80 - - MnO 0.20 0.20 0.18 0.17 0.19 0.19 0.20 MgO 5.57 9.13 8.11 9.08 7.06 7.35 9.08 CaO 11.94 10.29 11.22 11.00 11.51 11.82 10.20 Na2O 2.47 2.17 2.35 2.54 2.39 2.06 2.20 K2O 0.25 0.34 0.35 0.50 0.51 0.31 0.37 P2O5 0.52 0.37 0.43 0.23 0.32 0.27 0.45 L.O.I. 2.07 0.19 0.16 0.82 0.27 1.05 0.08 TOTAL 100.20 99.52 100.21 100.02 100.85 100.96 100.07

H8-28D H8-30B KS98-13 H9-29 H8-28B H8-34 H8-30A Rb 6.9 4.3 4.5 8.7 9.9 5.8 6.4 Sr 278 230 243 282 223 222 230 Y 36 30 30 22 33 35 32 Zr 140 113 137 94 131 156 116 Nb 10.5 9.5 13.4 13.7 10.6 10.5 9.3 Ni 96 153 92 140 97 98 145 Ga 18.9 17.6 21.3 18.3 19.1 18.0 18.5 Cu 97 66 63 74 72 69 60 Zn 107 112 107 77 100 102 109 U 0.8

H8-28D H8-30B KS98-13 H9-29 H8-28B H8-34 H8-30A 87Sr / 86Sr - - - - 0.70664 0.70479 - - 0.70675 - - 143Nd / 144Nd ------0.512434 - - 206Pb / 204Pb ------18.730 - - 207Pb / 204Pb ------15.664 - - 208Pb / 204Pb ------39.241 - -

Age (Ma) - - 8 - - 0.36 8 8.21 8

NOTES: a, b b, e a, b, d b, e a, b, d b, e

223 GROUP 2 (continued)

H9-48 H9-37D H9-49 H8-69F H8-69G CM97-25 CM97-17 SiO2 47.78 46.07 46.97 47.19 48.34 46.92 46.40 TiO2 1.87 1.88 1.92 1.92 1.94 2.00 2.00 Al2O3 15.72 15.98 15.89 15.11 15.53 14.56 15.40 Fe2O3 12.50 1.91 4.81 13.38 3.36 13.35 13.79 FeO - - 10.08 7.28 - - 9.28 - - - - MnO 0.17 0.18 0.17 0.20 0.20 0.21 0.21 MgO 7.31 8.61 7.88 9.09 8.93 9.99 8.91 CaO 9.52 11.14 10.51 10.18 10.24 10.21 10.01 Na2O 2.85 2.79 2.63 2.25 2.23 2.37 2.63 K2O 0.43 0.43 0.72 0.41 0.42 0.41 0.34 P2O5 0.48 0.41 0.34 0.54 0.28 0.29 0.35 L.O.I. 0.98 0.86 0.85 0.08 1.03 -0.39 -0.47 TOTAL 99.61 100.34 99.97 100.35 101.78 99.91 99.54

H9-48 H9-37D H9-49 H8-69F H8-69G CM97-25 CM97-17 Rb 7.7 8.1 15 8.0 7.5 7.5 5.9 Sr 424 257 305 226 260 211 219 Y 25 33 27 32 35 32 32 Zr 125 156 131 124 123 133 128 Nb 7.8 15.6 16.0 10.1 9.5 14.0 10.8 Ni 130 130 124 142 145 157 177 Ga 21.3 20.2 18.3 18.4 18.3 17.7 20.2 Cu 81 79 60 70 63 52 72 Zn 101 92 95 102 105 119 111 U

H9-48 H9-37D H9-49 H8-69F H8-69G CM97-25 CM97-17 87Sr / 86Sr - - 0.70541 0.70549 - - 0.70668 - - - - 143Nd / 144Nd - - 0.512605 0.512599 - - 0.512543 - - - - 206Pb / 204Pb - - 18.709 18.638 - - 18.732 - - - - 207Pb / 204Pb - - 15.634 15.621 - - 15.662 - - - - 208Pb / 204Pb - - 39.009 38.730 - - 39.289 - - - -

Age (Ma) - - 1.49 1.86 9 8.42 - - - -

NOTES: b a, b, d a, b, d b, e a, b, d

224 GROUP 2 (continued)

H9-19A H8-30D KS98-28 CM97-24 H9-19B H9-21A H9-38 SiO2 47.50 45.74 47.50 47.84 47.53 47.62 45.86 TiO2 2.01 2.02 2.03 2.03 2.04 2.04 2.07 Al2O3 15.08 15.86 15.55 14.79 16.12 14.94 15.40 Fe2O3 14.10 13.31 13.08 13.83 13.56 13.60 4.21 FeO ------8.72 MnO 0.18 0.19 0.19 0.19 0.20 0.19 0.18 MgO 7.63 7.83 8.30 9.75 7.48 8.61 8.45 CaO 10.02 11.26 10.67 10.01 10.59 10.40 10.02 Na2O 2.49 2.38 2.31 2.42 2.50 2.33 2.23 K2O 0.54 0.32 0.34 0.42 0.44 0.46 0.35 P2O5 0.41 0.45 0.44 0.26 0.63 0.43 0.39 L.O.I. -0.05 0.75 -0.16 -0.59 -0.01 0.00 0.99 TOTAL 99.90 100.11 100.23 100.94 101.08 100.63 98.87

H9-19A H8-30D KS98-28 CM97-24 H9-19B H9-21A H9-38 Rb 7.8 4.8 5.1 9.1 5.1 5.5 5.7 Sr 253 236 232 209 256 227 242 Y 30 33 31 31 32 31 40 Zr 135 142 138 126 128 144 146 Nb 12.9 10.9 13.5 11.8 12.2 13.2 14.5 Ni 150 83 142 187 133 151 173 Ga 22.0 18.7 21.2 19.7 22.2 21.5 18.8 Cu 58 47 52 67 58 73 76 Zn 111 106 106 107 115 115 115 U 1.9 1.5 1.2 0.1 1.2 2.0

H9-19A H8-30D KS98-28 CM97-24 H9-19B H9-21A H9-38 87Sr / 86Sr - - - - 0.70699 ------143Nd / 144Nd ------206Pb / 204Pb ------207Pb / 204Pb ------208Pb / 204Pb ------

Age (Ma) - - - - 8.90 8 - - 7 7

NOTES: b c e e a, b, e

225 GROUP 2 (continued)

KS98-21 KS98-15 H9-19C H9-20E KS98-16 CM97-18 H8-30C SiO2 47.44 46.15 47.45 46.51 46.75 47.32 46.24 TiO2 2.07 2.07 2.08 2.08 2.10 2.10 2.10 Al2O3 15.54 15.50 15.52 16.26 15.25 15.28 15.97 Fe2O3 13.72 14.23 13.61 4.11 13.66 14.30 13.74 FeO ------8.48 ------MnO 0.20 0.20 0.19 0.17 0.19 0.20 0.20 MgO 7.34 8.51 7.83 7.15 7.83 8.00 6.98 CaO 9.97 9.97 10.14 10.48 10.10 10.02 10.89 Na2O 2.47 2.59 2.55 2.49 2.47 2.52 2.42 K2O 0.47 0.34 0.52 0.46 0.43 0.30 0.28 P2O5 0.51 0.47 0.48 0.43 0.51 0.35 0.61 L.O.I. 0.03 -0.55 -0.13 1.16 0.27 0.45 0.63 TOTAL 99.77 99.48 100.23 99.78 99.55 100.82 100.06

KS98-21 KS98-15 H9-19C H9-20E KS98-16 CM97-18 H8-30C Rb 5.7 2.5 7.4 5.1 6.3 6.1 3.7 Sr 347 224 263 276 255 224 268 Y 33 34 32 31 32 33 34 Zr 158 148 141 142 156 130 149 Nb 14.4 14.5 13.7 13.8 14.6 10.8 11.7 Ni 136 133 152 143 148 176 86 Ga 22.4 21.7 22.2 21.3 21.6 18.5 19.0 Cu 60 73 54 82 68 67 49 Zn 115 121 113 110 126 114 116 U 1.2 1.7 1.8 0.5 0.9 0.5

KS98-21 KS98-15 H9-19C H9-20E KS98-16 CM97-18 H8-30C 87Sr / 86Sr - - 0.70719 ------143Nd / 144Nd ------206Pb / 204Pb ------207Pb / 204Pb ------208Pb / 204Pb ------

Age (Ma) 10 - - - - 8.93 10 - - - -

NOTES: e a, d e b

226 GROUP 2 (continued)

H9-20C KS98-17 H9-37B CM97-23 KS98-18 CM97-22 H9-39 SiO2 47.15 47.53 46.62 48.33 47.14 47.74 46.34 TiO2 2.10 2.11 2.11 2.12 2.14 2.14 2.15 Al2O3 15.67 15.51 16.03 15.34 15.28 14.92 14.84 Fe2O3 4.32 13.78 13.61 12.90 13.91 13.84 2.81 FeO 8.64 ------10.00 MnO 0.19 0.19 0.20 0.19 0.20 0.19 0.20 MgO 7.62 7.66 7.55 7.04 8.00 8.59 8.87 CaO 10.21 10.07 10.72 10.57 9.95 9.94 10.20 Na2O 2.38 2.48 2.30 2.58 2.46 2.50 2.15 K2O 0.45 0.48 0.47 0.62 0.44 0.36 0.40 P2O5 0.35 0.52 0.51 0.18 0.59 0.29 0.42 L.O.I. 0.92 -0.20 0.25 0.21 -0.28 -0.22 1.53 TOTAL 100.00 100.12 100.36 100.07 99.82 100.28 99.91

H9-20C KS98-17 H9-37B CM97-23 KS98-18 CM97-22 H9-39 Rb 6.9 6.2 9.9 16 6.2 11 6.7 Sr 249 285 237 228 273 212 292 Y 29 33 34 33 31 34 40 Zr 131 159 147 135 157 137 152 Nb 12.6 14.6 12.0 12.7 14.3 12.7 14.9 Ni 118 126 139 97 147 165 165 Ga 22.3 22.2 19.4 20.8 21.3 19.0 18.7 Cu 58 69 62 62 53 68 74 Zn 107 121 106 108 120 113 119 U 1.3

H9-20C KS98-17 H9-37B CM97-23 KS98-18 CM97-22 H9-39 87Sr / 86Sr ------0.70599 0.70713 143Nd / 144Nd ------206Pb / 204Pb ------207Pb / 204Pb ------208Pb / 204Pb ------

Age (Ma) 9.34 10 10 8 10 8.15 7.20

NOTES: a, d e b, e e e c a, b, d

227 GROUP 2 (continued)

CM97-19 KS98-20 CM97-16 H9-20D CM97-20 KS98-31 KS98-19 SiO2 46.99 47.01 46.24 47.63 46.93 47.23 47.12 TiO2 2.17 2.18 2.18 2.22 2.23 2.27 2.28 Al2O3 14.99 15.31 14.84 15.55 15.08 14.88 15.02 Fe2O3 14.38 14.01 14.23 13.96 14.61 14.18 14.26 FeO ------MnO 0.20 0.20 0.21 0.20 0.21 0.20 0.21 MgO 9.21 8.10 8.62 6.90 8.94 8.20 7.38 CaO 10.00 9.91 9.91 10.57 10.07 9.98 9.85 Na2O 2.54 2.47 2.64 2.57 2.58 2.37 2.48 K2O 0.27 0.49 0.36 0.47 0.24 0.42 0.53 P2O5 0.32 0.48 0.24 0.60 0.25 0.56 0.60 L.O.I. -0.40 -0.13 -0.38 -0.01 -0.38 -0.39 -0.28 TOTAL 100.68 100.03 99.10 100.66 100.75 99.89 99.44

CM97-19 KS98-20 CM97-16 H9-20D CM97-20 KS98-31 KS98-19 Rb 5.7 7.3 6.3 5.9 5.7 4.4 6.4 Sr 219 312 216 259 217 242 311 Y 33 34 36 33 32 38 36 Zr 125 169 143 147 124 170 176 Nb 10.8 15.4 12.4 14.1 10.3 18.3 15.8 Ni 156 151 179 132 148 104 127 Ga 20.8 21.8 19.6 22.5 21.2 21.6 22.4 Cu 67 67 131 68 70 53 65 Zn 109 121 113 111 106 131 123 U 1.1 0.5 0.8 1.4 1.0 0.9 1.0 Th

CM97-19 KS98-20 CM97-16 H9-20D CM97-20 KS98-31 KS98-19 87Sr / 86Sr ------0.70786 143Nd / 144Nd ------206Pb / 204Pb ------207Pb / 204Pb ------208Pb / 204Pb ------

Age (Ma) - - 10 - - 9 - - - - 9.71

NOTES: ee c

228 GROUP 2 (continued)

KS98-27 KS98-14 CM97-21 H9-37A KS98-32D H9-19D KS98-32C SiO2 47.28 47.76 47.10 45.77 46.03 47.90 45.97 TiO2 2.29 2.31 2.34 2.40 2.58 2.59 2.62 Al2O3 14.87 14.93 14.92 15.28 15.26 14.19 15.44 Fe2O3 13.67 13.94 14.81 6.11 13.96 14.58 14.18 FeO ------7.44 ------MnO 0.20 0.20 0.20 0.19 0.20 0.21 0.20 MgO 7.67 7.90 8.84 8.36 7.87 5.71 7.43 CaO 10.98 10.89 9.43 10.92 10.35 10.64 10.31 Na2O 2.39 2.40 2.64 2.20 2.36 2.64 2.40 K2O 0.38 0.39 0.34 0.48 0.46 0.83 0.48 P2O5 0.43 0.52 0.40 0.49 0.70 0.65 0.68 L.O.I. 0.22 -0.24 -0.39 1.42 -0.09 0.53 0.02 TOTAL 100.38 100.99 100.63 101.06 99.66 100.47 99.71

KS98-27 KS98-14 CM97-21 H9-37A KS98-32D H9-19D KS98-32C Rb 5.2 5.7 8.8 9.9 5.5 7.9 6.5 Sr 235 258 246 242 258 248 266 Y 34 35 35 36 36 43 36 Zr 155 161 151 167 180 184 180 Nb 15.5 15.4 12.6 14.8 18.7 17.0 18.1 Ni 110 93 146 130 130 77 111 Ga 22.2 22.5 21.2 18.4 21.7 22.2 21.3 Cu 80 81 71 61 62 92 57 Zn 116 116 123 117 126 127 125 U 1.3 0.7 0.6 0.6

KS98-27 KS98-14 CM97-21 H9-37A KS98-32D H9-19D KS98-32C 87Sr / 86Sr ------0.70688 ------143Nd / 144Nd ------206Pb / 204Pb ------207Pb / 204Pb ------208Pb / 204Pb ------

Age (Ma) ------10------

NOTES: a, b, e

229 GROUP 2 (continued)

H85-5A KS96-1 KS98-32E KS96-2 H85-5B H9-37C 85-1 SiO2 47.79 46.49 46.19 46.53 47.23 45.48 46.27 TiO2 2.63 2.66 2.67 2.71 2.75 2.80 2.83 Al2O3 14.72 13.87 15.07 14.11 14.39 16.25 14.25 Fe2O3 1.58 14.69 14.22 14.89 2.24 5.58 2.29 FeO 11.74 ------11.44 8.48 11.57 MnO 0.20 0.22 0.21 0.21 0.21 0.20 0.22 MgO 7.71 8.38 7.88 8.37 7.64 7.50 7.60 CaO 9.53 9.50 10.20 9.71 9.50 9.77 9.74 Na2O 2.53 2.21 2.33 2.29 2.57 2.53 2.56 K2O 0.65 0.54 0.47 0.52 0.64 0.53 0.65 P2O5 0.48 0.50 0.62 0.53 0.48 0.50 0.77 L.O.I. 0.90 0.10 -0.22 0.06 1.13 0.88 0.71 TOTAL 100.46 99.15 99.63 99.92 100.22 100.50 99.46

H85-5A KS96-1 KS98-32E KS96-2 H85-5B H9-37C 85-1 Rb 11 7.5 5.7 7.0 11 8.9 12 Sr 295 274 279 274 307 252 298 Y 39 38 36 39 40 36 49 Zr 184 187 182 184 185 159 265 Nb 15.5 13.3 18.6 12.9 15.1 14.0 24.2 Ni 108 176 114 166 107 94 110 Ga 20.8 20.5 21.7 20.8 20.8 19.6 20.7 Cu 63 58 79 53 68 57 41 Zn 124 128 128 127 127 119 137 U 1.1 0.6 0.3 0.9

H85-5A KS96-1 KS98-32E KS96-2 H85-5B H9-37C 85-1 87Sr / 86Sr ------0.70723 - - 143Nd / 144Nd ------0.512363 - - 206Pb / 204Pb ------18.585 - - 207Pb / 204Pb ------15.654 - - 208Pb / 204Pb ------39.290 - -

Age (Ma) - - 10.44 - - 10 - - 9.57 - -

NOTES: a, b b, c b, e a, b a, b, d a

230 GROUP 3: Alkaline basalts (<0.25 Ma)

JC-5 H8-70 H8-60A H8-63 H8-57 JC-4 H8-18 SiO2 48.34 48.79 48.08 47.66 48.47 47.53 47.22 TiO2 1.78 1.94 2.08 2.11 2.11 2.16 2.37 Al2O3 16.25 16.75 16.20 15.98 16.35 15.98 15.51 Fe2O3 1.75 6.41 4.83 2.92 2.74 2.31 1.93 FeO 7.84 4.61 5.52 7.47 9.04 8.55 10.28 MnO 0.16 0.17 0.16 0.15 0.18 0.17 0.18 MgO 8.62 6.69 7.71 8.53 7.18 9.29 7.40 CaO 9.98 8.82 9.24 9.26 10.09 9.93 9.04 Na2O 2.67 3.08 3.33 2.85 2.93 3.08 3.43 K2O 1.00 1.79 1.14 0.71 1.23 0.69 0.99 P2O5 0.41 0.54 0.37 0.37 0.40 0.29 0.35 L.O.I. 0.80 0.61 0.57 1.19 0.87 0.15 0.36 TOTAL 99.60 100.20 99.23 99.20 101.59 100.13 99.06

JC-5 H8-70 H8-60A H8-63 H8-57 JC-4 H8-18 Rb 17 41 22 12 27 12 19 Sr 486 476 541 586 460 655 466 Y 26 26 22 24 30 23 25 Zr 162 198 144 96 174 117 146 Nb - - 43.6 36.9 - - 32.3 17.9 32.9 Ni 118 86 138 166 85 156 119 Ga - - 17.9 19.4 - - 18.5 18.1 22.0 Cu - -5157- -636558 Zn - -8395- -958497 U - - 2.2 2.1 - - 1.0 0.7 1.7 Th - - 3.3 4.8 - - 2.3 0.4 4.3 Pb - - - - 2.9 ------3.3 Ba 251 653 320 254 499 262 363 Ce 40 51 31 29 43 27 32 Co - -4345- -424950 La 18 29 17 12 22 12 19 Cr 264 129 200 208 132 188 144 V 226 180 215 207 231 208 204 Sc - - 23.0 26.7 - - 28.4 27.9 23.1

JC-5 H8-70 H8-60A H8-63 H8-57 JC-4 H8-18 87Sr / 86Sr 0.70398 0.70390 0.70439 0.70385 0.70486 0.70382 - - 143Nd / 144Nd 0.512804 0.512841 0.512711 0.512836 0.512699 0.512850 - - 206Pb / 204Pb 18.838 18.644 18.829 18.794 18.765 18.957 - - 207Pb / 204Pb 15.567 15.579 15.596 15.600 15.581 15.578 - - 208Pb / 204Pb 38.561 38.640 38.673 38.545 38.710 38.545 - -

Age (Ma) 0 0.25 0 0 <0.03 <0.15 - -

NOTES: a, e a, b, d a, e a, e a, b, d a, b, d a

231 APPENDIX 2C: NORMATIVE MINERALOGIES

CIPW weight percent norms (Cross et al., 1902) were calculated according to the method described by Cox et al. (1979). Prior to calculation, analyses were normalized to 100% anhydrous with total Fe divided according to the method described by LeMaitre (1976). Samples are divided into the geochemical types and age groups described in

Chapter 2, and ranked within those groups according to increasing analytical TiO2. This is the same order used in Appendix 2A and Appendix 2B.

232 GROUP 1: Steens Basalts and Oregon-Idaho Graben basalts (17-11 Ma)

JV96-7 JV96-6 JV96-3 JV96-4 JV96-5 H85-6 H85-10A H85-10B CH82-32 H9-47

SiO2 56.77 52.61 53.19 53.13 52.04 52.19 54.47 49.29 50.35 50.54

TiO2 1.07 1.22 1.23 1.24 1.24 2.06 2.02 2.09 2.29 2.37

Al2O3 16.60 16.99 16.89 17.10 17.01 15.39 14.90 15.95 16.45 15.53

Fe2O3 3.21 3.51 3.59 3.64 3.55 5.45 4.35 4.51 4.26 4.66 FeO 4.49 5.86 5.50 5.55 5.61 8.73 6.40 8.26 7.57 8.00 MnO 0.16 0.20 0.17 0.18 0.18 0.26 0.15 0.18 0.16 0.19 MgO 4.47 5.72 5.60 5.00 5.91 2.90 5.07 5.74 5.94 4.80 CaO 7.27 8.85 8.31 8.43 9.05 7.61 7.17 10.10 8.49 9.26

Na2O 3.45 3.34 3.55 3.65 3.33 3.29 3.56 3.13 3.12 3.26 233 K2O 1.90 0.99 1.37 1.40 1.44 1.67 1.43 0.49 0.97 0.94

P2O5 0.63 0.71 0.59 0.69 0.63 0.45 0.48 0.26 0.43 0.44 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

JV96-7 JV96-6 JV96-3 JV96-4 JV96-5 H85-6 H85-10A H85-10B CH82-32 H9-47 Q 8.79 3.41 2.75 2.91 1.11 6.07 6.98 0.40 1.98 2.80 or 11.25 5.86 8.10 8.25 8.52 9.88 8.43 2.87 5.71 5.56 ab 29.15 28.24 30.07 30.89 28.16 27.85 30.15 26.46 26.38 27.61 an 24.21 28.45 26.10 26.17 27.21 22.28 20.45 28.04 28.03 24.94 ne------di 6.20 8.74 9.06 8.99 10.90 10.42 9.66 16.54 9.13 14.75 hy 12.26 16.24 15.02 13.57 15.12 10.66 13.09 14.57 17.28 12.05 ol------mt 4.66 5.09 5.21 5.28 5.15 7.90 6.31 6.54 6.17 6.76 il 2.02 2.32 2.34 2.36 2.36 3.91 3.84 3.98 4.34 4.51 ap 1.47 1.64 1.37 1.59 1.47 1.04 1.10 0.61 0.99 1.02

%AN 45.36 50.18 46.47 45.86 49.14 44.45 40.41 51.46 51.52 47.46 GROUP 1 (continued)

H85-26A JV96-1 JV96-2 H85-12B H8-74 H9-32

SiO2 46.75 49.81 48.83 50.61 47.34 50.09

TiO2 2.44 2.53 2.58 2.61 2.94 3.02

Al2O3 15.32 14.51 14.44 16.90 15.19 15.49

Fe2O3 4.31 4.41 4.49 4.83 5.30 5.42 FeO 9.16 8.42 8.82 7.84 9.52 8.43 MnO 0.20 0.19 0.20 0.14 0.21 0.23 MgO 8.34 6.34 6.74 2.85 5.14 3.53 CaO 10.49 10.30 10.41 8.90 8.98 7.28

Na2O 2.21 2.61 2.55 3.79 3.34 3.52 234 K2O 0.39 0.52 0.49 1.09 1.05 1.61

P2O5 0.39 0.35 0.45 0.43 1.01 1.38 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00

H85-26A JV96-1 JV96-2 H85-12B H8-74 H9-32 Q - - 3.28 1.92 2.74 - - 4.71 or 2.28 3.09 2.90 6.44 6.19 9.51 ab 18.70 22.11 21.57 32.08 28.23 29.79 an 30.76 26.31 26.52 25.88 23.38 21.71 ne ------di 15.16 18.26 18.00 12.73 11.93 4.43 hy 16.84 14.96 16.66 7.17 12.24 13.07 ol 4.49 ------2.46 - - mt 6.25 6.40 6.51 7.01 7.68 7.86 il 4.64 4.80 4.89 4.96 5.57 5.73 ap 0.89 0.80 1.04 1.00 2.33 3.19

%AN 62.20 54.34 55.15 44.65 45.31 42.16 GROUP 2: HAOT-TB-SROT (<11 Ma)

CM97-13 CM97-14 CM97-15 H9-27 H9-30 CM97-12 H9-28A KS96-8B KS96-8D H8-73

SiO2 48.64 48.11 48.48 47.86 48.60 48.51 48.50 48.76 47.86 48.40

TiO2 0.73 0.76 0.78 0.81 0.85 0.86 0.89 0.94 0.93 0.92

Al2O3 17.47 17.15 16.98 16.59 16.37 16.34 16.40 16.41 16.11 16.20

Fe2O3 3.22 3.23 3.29 3.41 3.35 3.50 3.44 3.51 3.35 3.43 FeO 6.71 6.73 6.83 7.15 6.85 7.20 6.92 7.10 7.00 6.87 MnO 0.18 0.18 0.18 0.17 0.20 0.20 0.18 0.18 0.18 0.18 MgO 8.46 9.13 8.91 9.25 9.53 8.86 9.33 8.59 9.81 9.32 CaO 12.06 12.15 12.12 12.23 11.62 11.97 11.43 11.44 11.95 11.83

Na2O 2.25 2.28 2.28 2.30 2.37 2.32 2.44 2.57 2.45 2.48 235 K2O 0.05 0.09 0.04 0.11 0.20 0.09 0.21 0.30 0.16 0.28

P2O5 0.24 0.19 0.10 0.11 0.08 0.16 0.26 0.21 0.20 0.08 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

CM97-13 CM97-14 CM97-15 H9-27 H9-30 CM97-12 H9-28A KS96-8B KS96-8D H8-73 Q ------or 0.29 0.53 0.23 0.65 1.18 0.53 1.22 1.79 0.97 1.66 ab 19.03 19.25 19.28 19.44 20.02 19.61 20.68 21.72 20.75 21.00 an 37.42 36.33 36.00 34.64 33.45 33.93 33.17 32.36 32.47 32.23 ne ------di 16.80 18.26 18.92 20.38 19.05 19.67 17.54 18.54 20.48 20.82 hy 14.93 9.61 11.68 6.86 10.16 12.26 11.27 10.12 4.80 5.95 ol 4.92 9.46 7.40 11.29 9.51 6.93 8.84 8.13 13.45 11.43 mt 4.67 4.69 4.78 4.95 4.86 5.08 4.98 5.10 4.86 4.97 il 1.39 1.44 1.49 1.54 1.61 1.63 1.68 1.78 1.77 1.76 ap 0.55 0.43 0.23 0.26 0.18 0.37 0.61 0.48 0.46 0.19

%AN 66.29 65.36 65.12 64.05 62.55 63.37 61.60 59.84 61.01 60.54 GROUP 2 (continued)

KS96-8C H9-36A H9-34 CM97-4 KS96-8A KS96-5 KS98-4 H8-98 KS98-5 CM97-8

SiO2 48.99 48.00 48.13 48.03 48.17 48.18 48.92 48.09 48.65 48.15

TiO2 0.98 0.95 0.98 1.02 1.05 1.03 1.07 1.11 1.12 1.14

Al2O3 16.58 16.72 15.98 15.95 16.26 16.70 16.38 16.34 16.23 16.51

Fe2O3 3.40 3.51 3.39 3.45 3.40 3.30 3.46 3.50 3.45 3.55 FeO 7.13 7.20 6.93 7.41 7.22 6.73 6.99 7.42 6.98 7.34 MnO 0.18 0.17 0.15 0.19 0.18 0.18 0.18 0.19 0.17 0.19 MgO 8.70 8.92 9.67 10.01 10.40 9.60 8.46 9.17 9.42 8.68 CaO 11.16 11.73 11.86 11.35 10.51 11.39 11.78 11.60 11.15 11.80

Na2O 2.43 2.51 2.51 2.17 2.28 2.48 2.47 2.33 2.35 2.32 236 K2O 0.25 0.17 0.24 0.17 0.33 0.26 0.16 0.16 0.23 0.19

P2O5 0.20 0.10 0.15 0.23 0.20 0.16 0.14 0.08 0.24 0.14 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

KS96-8C H9-36A H9-34 CM97-4 KS96-8A KS96-5 KS98-4 H8-98 KS98-5 CM97-8 Q ------or 1.47 1.02 1.44 1.00 1.96 1.53 0.93 0.97 1.37 1.10 ab 20.57 21.24 21.24 18.38 19.33 20.95 20.87 19.68 19.93 19.64 an 33.58 33.86 31.63 33.27 33.13 33.70 33.15 33.67 33.02 34.08 ne ------di 16.54 19.08 21.03 17.30 14.21 17.47 19.62 18.79 16.64 18.95 hy 15.14 6.21 4.58 13.56 13.36 7.61 12.59 10.38 14.79 11.08 ol 5.46 11.47 12.96 9.00 10.64 11.64 5.45 9.15 6.57 7.52 mt 4.93 5.09 4.92 5.01 4.93 4.78 5.02 5.08 5.00 5.14 il 1.86 1.81 1.85 1.95 1.99 1.96 2.04 2.10 2.13 2.17 ap 0.45 0.23 0.35 0.54 0.46 0.36 0.33 0.19 0.56 0.33

%AN 62.01 61.44 59.83 64.41 63.15 61.67 61.36 63.11 62.37 63.44 GROUP 2 (continued)

CM97-2A H85-11C KS98-6A KS98-2 KS98-6C H85-11A H8-43 H8-45 H85-11B KS98-6B

SiO2 48.26 48.14 48.58 48.60 48.84 48.14 47.93 47.68 48.20 48.95

TiO2 1.18 1.18 1.18 1.20 1.20 1.20 1.21 1.21 1.23 1.23

Al2O3 14.99 16.74 16.06 16.26 15.77 16.71 16.67 16.89 16.57 15.85

Fe2O3 3.54 3.70 3.44 3.45 3.45 3.68 3.51 3.61 3.62 3.48 FeO 7.45 7.20 7.01 7.04 7.02 7.33 7.16 7.16 7.31 7.08 MnO 0.19 0.18 0.18 0.17 0.18 0.19 0.19 0.17 0.18 0.18 MgO 10.79 8.07 9.72 9.34 9.69 8.14 8.47 8.94 8.21 9.40 CaO 10.94 11.55 10.98 11.16 11.06 11.57 11.89 11.19 11.69 11.04

Na2O 2.10 2.84 2.30 2.40 2.29 2.65 2.50 2.66 2.65 2.32 237 K2O 0.28 0.27 0.27 0.24 0.26 0.26 0.29 0.33 0.25 0.28

P2O5 0.28 0.14 0.27 0.15 0.24 0.13 0.18 0.16 0.08 0.19 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

CM97-2A H85-11C KS98-6A KS98-2 KS98-6C H85-11A H8-43 H8-45 H85-11B KS98-6B Q ------or 1.67 1.60 1.61 1.42 1.55 1.55 1.71 1.96 1.51 1.65 ab 17.77 24.07 19.42 20.29 19.38 22.42 21.14 22.52 22.42 19.60 an 30.63 32.12 32.72 32.90 31.96 32.93 33.43 33.17 32.56 32.02 ne ------di 17.47 19.55 16.01 17.25 17.11 19.06 19.66 17.11 20.06 17.26 hy 16.18 3.12 15.77 13.16 16.62 6.41 6.02 4.00 5.67 16.74 ol 8.26 11.64 6.59 7.36 5.55 9.73 10.24 13.36 10.01 4.91 mt 5.13 5.36 4.99 5.01 5.00 5.34 5.09 5.23 5.25 5.05 il 2.24 2.23 2.25 2.27 2.27 2.28 2.31 2.29 2.34 2.33 ap 0.65 0.33 0.64 0.34 0.56 0.30 0.41 0.37 0.19 0.44

%AN 63.28 57.16 62.75 61.86 62.26 59.50 61.26 59.56 59.23 62.03 GROUP 2 (continued)

H9-36B KS98-12 SM75-12A H9-36C KS98-3 CM97-7 CM97-2B KS98-1 KS98-11 KS96-7

SiO2 48.16 48.48 47.84 48.34 49.02 47.77 48.29 48.19 48.42 48.60

TiO2 1.27 1.25 1.25 1.27 1.27 1.28 1.28 1.30 1.29 1.32

Al2O3 16.43 16.24 16.32 16.32 15.96 15.88 15.01 16.88 15.82 15.71

Fe2O3 3.41 3.49 3.33 3.72 3.52 3.61 3.58 3.53 3.59 3.47 FeO 7.12 7.15 6.84 7.32 7.07 7.56 7.36 6.90 7.35 7.25 MnO 0.21 0.18 0.17 0.18 0.18 0.19 0.18 0.17 0.20 0.18 MgO 9.51 8.80 9.93 8.89 9.09 9.60 10.50 8.66 8.75 9.38 CaO 10.63 11.65 11.54 10.64 11.02 11.48 10.98 11.04 11.83 11.40

Na2O 2.53 2.34 2.29 2.74 2.39 2.26 2.19 2.75 2.33 2.20 238 K2O 0.36 0.18 0.34 0.39 0.27 0.22 0.34 0.40 0.21 0.28

P2O5 0.37 0.25 0.15 0.19 0.21 0.16 0.27 0.19 0.22 0.21 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

H9-36B KS98-12 SM75-12A H9-36C KS98-3 CM97-7 CM97-2B KS98-1 KS98-11 KS96-7 Q ------or 2.13 1.04 2.02 2.29 1.60 1.27 2.02 2.35 1.22 1.66 ab 21.44 19.80 19.37 23.17 20.22 19.11 18.55 23.27 19.73 18.62 an 32.39 33.28 33.26 31.09 32.03 32.56 30.11 32.53 32.10 32.16 ne ------di 14.36 18.42 18.41 16.43 17.05 18.76 18.10 16.84 20.24 18.49 hy 11.50 13.99 7.72 8.22 16.80 10.55 14.52 5.59 12.71 15.81 ol 9.96 5.47 11.67 10.54 4.28 9.71 8.45 11.40 5.84 5.25 mt 4.94 5.06 4.83 5.40 5.11 5.23 5.19 5.12 5.20 5.03 il 2.42 2.37 2.38 2.42 2.42 2.44 2.43 2.46 2.45 2.50 ap 0.87 0.58 0.35 0.45 0.49 0.37 0.63 0.45 0.51 0.49

%AN 60.17 62.71 63.20 57.30 61.29 63.01 61.89 58.30 61.93 63.33 GROUP 2 (continued)

KS96-10 CM97-9 H8-95 H9-42 H8-47 CM97-1 CM97-11 KS98-26 CM97-10 H8-69E

SiO2 47.64 48.11 48.35 47.56 48.09 48.53 48.14 48.96 48.18 46.79

TiO2 1.31 1.29 1.31 1.32 1.31 1.32 1.32 1.33 1.35 1.41

Al2O3 16.36 16.14 16.55 16.86 15.85 14.88 16.20 16.15 16.02 17.19

Fe2O3 3.78 3.73 3.46 3.56 3.58 3.55 3.70 3.55 3.77 3.92 FeO 7.79 7.60 7.22 7.28 7.11 7.43 7.66 7.00 7.74 8.43 MnO 0.19 0.20 0.19 0.17 0.18 0.18 0.19 0.17 0.19 0.19 MgO 9.07 8.64 8.70 8.83 9.58 10.25 8.67 9.03 8.53 8.09 CaO 10.72 11.50 11.66 11.38 11.10 11.09 11.47 10.82 11.40 10.81

Na2O 2.50 2.31 2.15 2.59 2.62 2.14 2.31 2.36 2.32 2.45 239 K2O 0.21 0.22 0.23 0.26 0.32 0.28 0.20 0.39 0.23 0.30

P2O5 0.42 0.26 0.17 0.17 0.26 0.35 0.15 0.25 0.26 0.43 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

KS96-10 CM97-9 H8-95 H9-42 H8-47 CM97-1 CM97-11 KS98-26 CM97-10 H8-69E Q ------or 1.24 1.30 1.37 1.56 1.90 1.66 1.17 2.28 1.36 1.75 ab 21.18 19.58 18.21 21.94 22.14 18.08 19.54 19.94 19.62 20.73 an 32.78 33.01 34.82 33.59 30.55 30.17 33.26 32.36 32.63 35.04 ne ------di 14.25 18.02 17.65 17.49 18.29 18.11 18.26 15.82 17.96 12.74 hy 12.56 13.65 15.63 4.97 7.24 17.89 13.31 17.32 14.58 10.22 ol 9.06 5.99 4.43 12.39 11.60 5.62 6.26 4.04 5.23 10.15 mt 5.48 5.41 5.02 5.16 5.19 5.15 5.36 5.14 5.46 5.69 il 2.49 2.45 2.48 2.51 2.48 2.51 2.50 2.53 2.57 2.68 ap 0.97 0.59 0.40 0.40 0.61 0.81 0.35 0.57 0.59 1.01

%AN 60.74 62.77 65.66 60.48 57.98 62.52 62.99 61.88 62.45 62.82 GROUP 2 (continued)

KS98-25 KS98-22 KS98-24 KS96-3 H8-56A H8-56B H8-28A KS96-6B KS98-23 CM97-5

SiO2 49.36 49.21 49.36 48.27 48.17 47.55 48.42 48.53 49.00 48.45

TiO2 1.38 1.39 1.41 1.46 1.43 1.44 1.47 1.47 1.45 1.47

Al2O3 15.98 15.85 15.87 16.01 15.89 16.24 16.36 15.01 15.98 15.68

Fe2O3 3.62 3.61 3.63 3.89 3.64 3.69 3.54 3.74 3.65 3.61 FeO 7.10 7.19 7.14 7.77 6.95 7.36 7.42 7.63 7.10 7.36 MnO 0.18 0.18 0.18 0.19 0.17 0.18 0.18 0.18 0.18 0.18 MgO 8.36 8.47 8.31 8.19 9.51 9.65 8.33 10.19 8.51 9.25 CaO 10.92 11.14 10.95 10.95 10.73 10.70 11.62 10.10 11.03 11.19

Na2O 2.34 2.31 2.37 2.66 2.88 2.63 2.23 2.29 2.38 2.28 240 K2O 0.45 0.40 0.47 0.35 0.39 0.36 0.27 0.42 0.45 0.33

P2O5 0.30 0.25 0.32 0.26 0.23 0.21 0.16 0.44 0.28 0.20 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

KS98-25 KS98-22 KS98-24 KS96-3 H8-56A H8-56B H8-28A KS96-6B KS98-23 CM97-5 Q ------or 2.65 2.39 2.75 2.05 2.31 2.12 1.62 2.49 2.66 1.96 ab 19.80 19.53 20.05 22.52 24.35 22.26 18.89 19.39 20.12 19.26 an 31.78 31.68 31.29 30.70 29.29 31.45 33.82 29.42 31.59 31.59 ne ------di 16.43 17.70 16.87 17.69 17.96 16.28 18.33 14.28 17.15 18.13 hy 20.12 18.72 19.51 10.62 4.18 5.72 15.26 19.64 16.91 14.87 ol 0.65 1.54 0.85 7.40 13.38 13.62 3.79 5.55 2.89 5.69 mt 5.25 5.24 5.26 5.64 5.28 5.35 5.14 5.43 5.30 5.24 il 2.62 2.64 2.69 2.77 2.72 2.74 2.79 2.79 2.74 2.79 ap 0.70 0.57 0.73 0.61 0.53 0.48 0.38 1.02 0.64 0.47

%AN 61.61 61.86 60.94 57.68 54.61 58.55 64.17 60.28 61.09 62.12 GROUP 2 (continued)

H8-42 KS98-9 CM97-3 KS98-29 H8-56D KS98-32B H8-69D H9-21E H9-21B CM97-6

SiO2 47.77 47.87 47.99 49.05 47.61 48.33 47.16 47.60 46.99 47.74

TiO2 1.46 1.49 1.51 1.51 1.52 1.52 1.55 1.54 1.53 1.55

Al2O3 16.77 15.96 15.90 14.97 16.41 16.99 17.13 16.89 13.99 16.07

Fe2O3 3.67 3.57 3.75 3.71 3.76 3.67 4.03 3.68 4.03 3.86 FeO 7.30 7.39 7.74 7.56 7.56 7.30 8.50 7.73 8.51 7.93 MnO 0.18 0.18 0.20 0.18 0.24 0.18 0.18 0.17 0.18 0.20 MgO 8.74 9.25 8.24 8.94 8.32 7.09 7.53 8.23 12.73 8.41 CaO 10.92 11.44 11.84 11.34 11.12 11.61 10.99 11.35 9.28 11.33

Na2O 2.43 2.33 2.43 2.25 2.57 2.63 2.38 2.31 2.03 2.45 241 K2O 0.44 0.23 0.23 0.27 0.51 0.29 0.25 0.25 0.45 0.21

P2O5 0.33 0.30 0.15 0.21 0.37 0.39 0.31 0.22 0.28 0.25 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

H8-42 KS98-9 CM97-3 KS98-29 H8-56D KS98-32B H8-69D H9-21E H9-21B CM97-6 Q ------or 2.59 1.34 1.37 1.60 3.03 1.69 1.50 1.50 2.66 1.22 ab 20.59 19.68 20.60 19.03 21.76 22.29 20.15 19.59 17.21 20.71 an 33.53 32.45 31.78 29.96 31.71 33.68 35.31 34.96 27.70 32.27 ne ------di 14.88 17.96 20.93 20.05 16.94 17.27 13.96 16.05 13.20 18.03 hy 10.96 12.09 9.46 18.92 7.20 11.90 13.15 12.46 15.38 11.46 ol 8.61 7.80 7.20 1.72 10.15 4.05 6.45 6.67 14.45 7.20 mt 5.32 5.17 5.44 5.38 5.46 5.32 5.84 5.34 5.85 5.60 il 2.77 2.82 2.88 2.86 2.90 2.89 2.94 2.93 2.91 2.95 ap 0.76 0.69 0.36 0.49 0.86 0.91 0.71 0.52 0.65 0.58

%AN 61.95 62.25 60.67 61.16 59.31 60.17 63.67 64.09 61.68 60.91 GROUP 2 (continued)

H8-35 H8-29 H9-44 KS98-30 H8-28E H8-50E H8-50C H8-50D KS98-10 KS96-4

SiO2 48.45 48.99 47.42 49.30 47.97 47.65 47.36 47.86 48.57 47.84

TiO2 1.56 1.57 1.59 1.58 1.59 1.57 1.58 1.60 1.61 1.62

Al2O3 15.67 15.85 16.42 14.97 15.68 16.06 16.10 16.29 15.81 16.00

Fe2O3 3.80 3.79 3.71 3.77 3.71 3.99 3.89 3.91 3.74 3.67 FeO 7.53 7.49 7.55 7.55 7.46 7.58 7.42 7.69 7.35 7.39 MnO 0.18 0.19 0.17 0.19 0.18 0.19 0.19 0.19 0.18 0.18 MgO 9.07 7.55 9.03 8.41 8.08 8.56 8.79 8.44 8.36 9.32 CaO 10.52 11.64 10.97 11.27 12.25 10.86 11.15 10.65 11.21 10.59

Na2O 2.38 2.37 2.45 2.23 2.50 2.92 2.96 2.67 2.57 2.53 242 K2O 0.49 0.34 0.39 0.35 0.36 0.42 0.40 0.42 0.35 0.40

P2O5 0.35 0.21 0.31 0.38 0.21 0.22 0.16 0.28 0.25 0.46 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

H8-35 H8-29 H9-44 KS98-30 H8-28E H8-50E H8-50C H8-50D KS98-10 KS96-4 Q ------0.64 ------or 2.88 2.00 2.33 2.06 2.15 2.47 2.36 2.50 2.09 2.34 ab 20.15 20.09 20.76 18.83 21.16 24.68 23.84 22.57 21.75 21.42 an 30.62 31.59 32.62 29.84 30.49 29.48 29.47 31.23 30.55 31.12 ne ------0.65 ------di 15.48 20.01 15.86 19.02 23.29 18.47 19.92 15.95 18.88 14.78 hy 16.39 16.84 8.91 20.26 5.71 2.24 - - 9.24 12.17 12.04 ol 5.18 0.51 10.42 - - 8.32 13.39 14.75 9.16 5.53 8.84 mt 5.51 5.50 5.37 5.47 5.38 5.78 5.64 5.67 5.42 5.32 il 2.97 2.97 3.01 3.01 3.03 2.99 3.01 3.04 3.05 3.08 ap 0.81 0.49 0.73 0.87 0.49 0.51 0.37 0.65 0.57 1.07

%AN 60.31 61.13 61.11 61.31 59.03 54.43 55.29 58.04 58.41 59.23 GROUP 2 (continued)

KS96-6C H8-56C KS98-7 H8-50B H8-28C KS98-32A H9-21C H8-50A H8-39 H9-46

SiO2 48.91 47.31 48.91 47.18 48.72 47.62 48.85 48.07 48.37 48.06

TiO2 1.63 1.62 1.63 1.67 1.66 1.66 1.68 1.68 1.70 1.74

Al2O3 15.92 16.74 15.86 16.27 15.68 15.94 16.48 15.91 15.24 14.67

Fe2O3 3.79 3.89 3.69 3.82 3.83 3.96 3.98 3.96 3.89 3.89 FeO 7.53 7.91 7.23 7.86 7.69 7.90 7.73 7.80 7.66 8.01 MnO 0.18 0.19 0.17 0.19 0.19 0.20 0.18 0.19 0.18 0.19 MgO 8.21 8.43 7.93 8.31 8.03 8.30 6.69 8.22 8.41 10.28 CaO 10.66 10.80 11.38 11.46 11.14 11.13 10.88 10.91 11.45 10.21

Na2O 2.43 2.56 2.54 2.62 2.31 2.61 2.54 2.53 2.65 2.16 243 K2O 0.46 0.33 0.44 0.29 0.42 0.34 0.55 0.54 0.25 0.44

P2O5 0.29 0.23 0.22 0.34 0.32 0.36 0.44 0.21 0.20 0.37 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

KS96-6C H8-56C KS98-7 H8-50B H8-28C KS98-32A H9-21C H8-50A H8-39 H9-46 Q ------0.20 ------or 2.70 1.92 2.60 1.69 2.51 2.03 3.23 3.18 1.48 2.59 ab 20.56 21.63 21.49 22.16 19.56 22.06 21.51 21.38 22.41 18.28 an 31.18 33.25 30.56 31.80 31.17 30.77 31.93 30.50 28.96 29.03 ne ------di 15.96 15.18 19.71 18.43 17.74 17.85 15.54 17.91 21.37 15.43 hy 18.42 8.67 12.96 5.40 18.44 8.94 17.62 10.26 9.75 19.02 ol 1.94 10.10 3.72 11.04 1.16 8.65 - - 7.36 6.69 5.87 mt 5.49 5.64 5.36 5.54 5.55 5.74 5.77 5.75 5.65 5.64 il 3.10 3.08 3.09 3.17 3.14 3.15 3.19 3.18 3.23 3.30 ap 0.66 0.53 0.50 0.78 0.75 0.83 1.01 0.49 0.46 0.85

%AN 60.26 60.59 58.71 58.94 61.45 58.25 59.75 58.78 56.37 61.36 GROUP 2 (continued)

H8-36 H8-28D H8-30B KS98-13 H9-29 H8-28B H8-34 H8-30A H9-48 H9-37D

SiO2 48.26 48.91 47.87 48.06 47.35 49.16 48.47 47.73 48.87 46.21

TiO2 1.78 1.81 1.82 1.81 1.81 1.82 1.82 1.88 1.92 1.89

Al2O3 15.29 15.84 15.00 15.62 16.25 15.60 15.80 15.28 16.07 16.03

Fe2O3 3.67 3.96 4.02 3.85 3.66 3.87 3.87 4.03 4.08 4.09 FeO 7.60 8.02 8.43 7.84 7.28 7.52 8.01 8.37 7.83 8.15 MnO 0.18 0.20 0.20 0.18 0.17 0.19 0.19 0.20 0.18 0.18 MgO 8.66 5.70 9.28 8.17 9.13 7.08 7.36 9.17 7.47 8.64 CaO 11.74 12.23 10.45 11.31 11.06 11.54 11.83 10.30 9.73 11.17

Na2O 2.29 2.53 2.21 2.37 2.55 2.39 2.06 2.22 2.91 2.80 244 K2O 0.28 0.26 0.34 0.35 0.50 0.51 0.31 0.37 0.44 0.43

P2O5 0.24 0.53 0.38 0.43 0.23 0.32 0.27 0.45 0.49 0.41 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

H8-36 H8-28D H8-30B KS98-13 H9-29 H8-28B H8-34 H8-30A H9-48 H9-37D Q - - 1.62 ------0.63 0.88 ------or 1.68 1.51 2.02 2.06 2.97 3.02 1.83 2.21 2.62 2.55 ab 19.41 21.41 18.70 20.02 21.62 20.26 17.45 18.79 24.65 22.64 an 30.60 31.12 30.01 30.99 31.39 30.31 32.93 30.63 29.47 29.90 ne ------0.56 di 20.96 21.24 15.60 18.00 17.62 20.10 19.42 14.08 12.58 18.40 hy 13.68 12.70 19.35 15.49 4.74 15.89 17.80 19.87 18.10 - - ol 4.43 - - 4.17 3.43 12.39 - - - - 3.96 1.88 15.48 mt 5.32 5.74 5.83 5.58 5.31 5.61 5.61 5.85 5.92 5.94 il 3.37 3.44 3.45 3.44 3.44 3.46 3.46 3.57 3.64 3.58 ap 0.56 1.23 0.88 0.99 0.54 0.73 0.63 1.05 1.14 0.95

%AN 61.19 59.25 61.61 60.76 59.22 59.94 65.36 61.98 54.45 56.91 GROUP 2 (continued)

H9-49 H8-69F H8-69G CM97-25 CM97-17 H9-19A H8-30D KS98-28 CM97-24 H9-19B

SiO2 47.42 47.51 47.94 47.22 46.84 47.99 46.47 47.75 47.56 47.46

TiO2 1.94 1.93 1.92 2.01 2.02 2.03 2.05 2.04 2.02 2.04

Al2O3 16.04 15.21 15.40 14.65 15.54 15.23 16.12 15.63 14.70 16.09

Fe2O3 4.14 4.09 4.18 4.10 4.27 4.46 4.04 4.01 4.27 4.21 FeO 8.00 8.43 8.44 8.40 8.68 8.80 8.53 8.22 8.53 8.39 MnO 0.17 0.20 0.20 0.21 0.21 0.18 0.19 0.19 0.19 0.19 MgO 7.96 9.16 8.86 10.05 8.99 7.71 7.95 8.34 9.70 7.47 CaO 10.61 10.25 10.16 10.28 10.10 10.12 11.44 10.73 9.95 10.57

Na2O 2.66 2.27 2.21 2.38 2.65 2.51 2.42 2.32 2.41 2.50 245 K2O 0.73 0.41 0.42 0.42 0.34 0.55 0.33 0.34 0.42 0.44

P2O5 0.34 0.54 0.28 0.29 0.35 0.41 0.46 0.44 0.25 0.63 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

H9-49 H8-69F H8-69G CM97-25 CM97-17 H9-19A H8-30D KS98-28 CM97-24 H9-19B Q ------or 4.30 2.41 2.46 2.46 2.00 3.22 1.93 2.03 2.47 2.59 ab 22.47 19.20 18.71 20.15 22.44 21.26 20.47 19.61 20.35 21.14 an 29.71 30.12 30.87 28.06 29.51 28.67 32.16 31.22 28.09 31.40 ne ------di 16.68 13.84 14.26 16.89 14.74 15.28 17.46 15.40 15.79 13.71 hy 6.55 18.80 20.54 11.70 9.32 17.71 8.05 18.21 14.91 16.64 ol 9.83 4.78 2.80 10.32 11.14 2.59 9.12 2.83 7.80 3.08 mt 6.00 5.93 6.06 5.95 6.20 6.46 5.86 5.81 6.19 6.11 il 3.68 3.67 3.65 3.81 3.84 3.86 3.90 3.87 3.83 3.87 ap 0.80 1.25 0.64 0.67 0.82 0.95 1.06 1.02 0.59 1.46

%AN 56.94 61.06 62.26 58.21 56.80 57.42 61.10 61.43 57.98 59.77 GROUP 2 (continued)

H9-21A H9-38 KS98-21 KS98-15 H9-19C H9-20E KS98-16 CM97-18 H8-30C H9-20C

SiO2 47.77 46.86 48.02 46.59 47.72 47.16 47.54 47.62 46.96 47.59

TiO2 2.05 2.12 2.10 2.09 2.09 2.11 2.13 2.11 2.13 2.12

Al2O3 14.99 15.73 15.73 15.65 15.61 16.49 15.51 15.38 16.22 15.82

Fe2O3 4.21 4.22 4.32 4.39 4.30 4.22 4.27 4.43 4.20 4.32 FeO 8.49 8.99 8.61 8.98 8.45 8.55 8.66 8.96 8.78 8.76 MnO 0.19 0.18 0.20 0.21 0.19 0.17 0.19 0.20 0.21 0.19 MgO 8.64 8.63 7.43 8.59 7.87 7.25 7.96 8.05 7.09 7.69 CaO 10.43 10.24 10.09 10.07 10.19 10.63 10.27 10.08 11.06 10.31

Na2O 2.33 2.28 2.50 2.61 2.57 2.52 2.51 2.53 2.45 2.40 246 K2O 0.47 0.36 0.48 0.35 0.53 0.47 0.44 0.30 0.28 0.45

P2O5 0.44 0.40 0.51 0.47 0.48 0.44 0.52 0.35 0.62 0.35 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

H9-21A H9-38 KS98-21 KS98-15 H9-19C H9-20E KS98-16 CM97-18 H8-30C H9-20C Q ------or 2.75 2.11 2.82 2.05 3.11 2.76 2.60 1.78 1.67 2.68 ab 19.74 19.28 21.17 22.12 21.73 21.36 21.22 21.41 20.77 20.33 an 29.06 31.65 30.30 29.96 29.52 32.28 29.76 29.71 32.41 31.03 ne ------di 15.99 13.36 13.29 13.67 14.45 14.25 14.37 14.58 14.94 14.39 hy 17.79 16.36 20.40 10.89 15.65 13.01 16.74 17.65 15.26 17.58 ol 3.68 6.19 0.61 9.90 4.23 5.22 3.88 3.64 3.38 2.88 mt 6.10 6.11 6.26 6.36 6.23 6.11 6.19 6.42 6.09 6.26 il 3.89 4.02 3.98 3.98 3.97 4.01 4.05 4.01 4.05 4.03 ap 1.01 0.92 1.18 1.09 1.12 1.01 1.21 0.80 1.43 0.82

%AN 59.55 62.14 58.87 57.52 57.60 60.17 58.37 58.11 60.94 60.42 GROUP 2 (continued)

KS98-17 H9-37B CM97-23 KS98-18 CM97-22 H9-39 CM97-19 KS98-20 CM97-16 H9-20D

SiO2 47.83 47.01 48.83 47.55 47.95 47.04 46.95 47.39 46.94 47.76

TiO2 2.13 2.13 2.14 2.15 2.15 2.18 2.17 2.20 2.22 2.23

Al2O3 15.61 16.16 15.50 15.41 14.99 15.06 14.98 15.43 15.07 15.60

Fe2O3 4.32 4.17 4.17 4.34 4.31 4.21 4.40 4.37 4.44 4.39 FeO 8.59 8.60 7.98 8.73 8.63 8.93 8.97 8.78 9.00 8.65 MnO 0.19 0.20 0.19 0.20 0.19 0.20 0.20 0.20 0.21 0.20 MgO 7.71 7.62 7.11 8.07 8.63 9.00 9.20 8.16 8.75 6.92 CaO 10.13 10.81 10.68 10.03 9.98 10.35 9.99 9.99 10.06 10.60

Na2O 2.49 2.31 2.60 2.48 2.51 2.18 2.54 2.49 2.68 2.58 247 K2O 0.48 0.47 0.62 0.45 0.36 0.41 0.27 0.49 0.37 0.47

P2O5 0.52 0.51 0.18 0.59 0.29 0.43 0.32 0.49 0.24 0.60 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

KS98-17 H9-37B CM97-23 KS98-18 CM97-22 H9-39 CM97-19 KS98-20 CM97-16 H9-20D Q ------or 2.85 2.78 3.67 2.64 2.14 2.40 1.58 2.90 2.18 2.78 ab 21.10 19.58 22.02 21.02 21.21 18.47 21.49 21.09 22.71 21.82 an 29.97 32.33 28.78 29.59 28.58 30.11 28.69 29.48 27.99 29.60 ne ------di 13.67 14.55 18.57 13.18 15.37 14.90 15.15 13.66 16.41 15.44 hy 18.97 15.21 15.52 18.81 17.86 17.60 13.05 16.69 9.15 17.52 ol 1.94 4.29 0.91 3.01 3.84 5.29 8.80 4.55 10.35 0.85 mt 6.26 6.05 6.04 6.29 6.26 6.11 6.38 6.34 6.44 6.37 il 4.04 4.05 4.06 4.09 4.08 4.15 4.12 4.17 4.21 4.24 ap 1.21 1.18 0.43 1.37 0.67 0.99 0.74 1.13 0.56 1.39

%AN 58.68 62.27 56.65 58.47 57.40 61.98 57.17 58.30 55.21 57.56 GROUP 2 (continued)

CM97-20 KS98-31 KS98-19 KS98-27 KS98-14 CM97-21 H9-37A KS98-32D H9-19D KS98-32C

SiO2 46.88 47.56 47.72 47.65 47.63 47.10 46.02 46.60 48.41 46.57

TiO2 2.23 2.29 2.31 2.31 2.30 2.34 2.41 2.61 2.62 2.65

Al2O3 15.06 14.99 15.21 14.99 14.89 14.92 15.36 15.45 14.34 15.64

Fe2O3 4.47 4.38 4.49 4.22 4.29 4.60 4.32 4.27 4.77 4.36 FeO 9.11 8.90 8.95 8.60 8.64 9.19 9.12 8.87 8.96 9.00 MnO 0.21 0.20 0.21 0.20 0.20 0.20 0.19 0.20 0.21 0.20 MgO 8.92 8.26 7.47 7.73 7.88 8.84 8.41 7.97 5.77 7.52 CaO 10.06 10.05 9.98 11.07 10.86 9.43 10.98 10.48 10.75 10.44

Na2O 2.57 2.39 2.51 2.41 2.39 2.64 2.21 2.38 2.67 2.43 248 K2O 0.24 0.42 0.54 0.39 0.39 0.34 0.48 0.46 0.83 0.48

P2O5 0.25 0.56 0.61 0.43 0.52 0.40 0.49 0.71 0.66 0.69 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

CM97-20 KS98-31 KS98-19 KS98-27 KS98-14 CM97-21 H9-37A KS98-32D H9-19D KS98-32C Q ------1.24 - - or 1.42 2.47 3.20 2.29 2.30 2.02 2.85 2.73 4.93 2.86 ab 21.77 20.23 21.25 20.36 20.24 22.35 18.72 20.17 22.61 20.54 an 28.84 28.93 28.64 28.96 28.74 27.85 30.57 30.10 24.68 30.36 ne ------di 15.66 13.92 13.67 18.68 17.57 13.15 16.71 13.92 19.82 13.69 hy 12.18 20.57 20.02 15.89 17.57 15.66 10.70 15.96 13.32 15.80 ol 8.85 1.89 0.91 2.32 1.78 6.94 8.48 4.33 - - 3.80 mt 6.48 6.35 6.51 6.12 6.23 6.66 6.26 6.20 6.92 6.32 il 4.23 4.34 4.39 4.38 4.38 4.44 4.58 4.95 4.97 5.03 ap 0.59 1.30 1.41 1.00 1.20 0.93 1.14 1.64 1.52 1.61

%AN 56.99 58.84 57.40 58.72 58.67 55.48 62.02 59.87 52.19 59.64 GROUP 2 (continued)

H85-5A KS96-1 KS98-32E KS96-2 H85-5B H9-37C 85-1

SiO2 47.85 47.43 46.73 47.09 47.54 45.70 46.74

TiO2 2.63 2.71 2.70 2.74 2.77 2.81 2.86

Al2O3 14.74 14.15 15.24 14.27 14.49 16.33 14.39

Fe2O3 4.65 4.54 4.36 4.59 4.75 4.61 4.77 FeO 9.00 9.40 9.02 9.43 9.27 9.42 9.48 MnO 0.20 0.22 0.21 0.21 0.21 0.20 0.22 MgO 7.72 8.54 7.97 8.47 7.69 7.54 7.68 CaO 9.54 9.69 10.31 9.82 9.56 9.82 9.84

Na2O 2.53 2.26 2.36 2.32 2.59 2.54 2.59 249 K2O 0.65 0.55 0.47 0.53 0.64 0.53 0.66

P2O5 0.48 0.51 0.63 0.54 0.48 0.50 0.78 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00

H85-5A KS96-1 KS98-32E KS96-2 H85-5B H9-37C 85-1 Q ------or 3.85 3.24 2.80 3.10 3.81 3.15 3.88 ab 21.44 19.10 19.95 19.62 21.89 21.51 21.88 an 26.93 26.86 29.62 26.99 26.01 31.57 25.73 ne ------di 13.95 14.41 14.09 14.72 14.77 11.21 14.56 hy 20.46 22.73 17.29 20.12 18.49 11.01 16.07 ol 0.54 0.76 3.38 2.36 1.77 8.37 3.74 mt 6.74 6.58 6.31 6.65 6.89 6.68 6.92 il 5.00 5.15 5.12 5.21 5.26 5.34 5.43 ap 1.11 1.19 1.45 1.24 1.12 1.16 1.80

%AN 55.67 58.45 59.76 57.91 54.30 59.47 54.04 GROUP 3: Alkaline basalts (<0.25 Ma)

JC-5 H8-70 H8-60A H8-63 H8-57 JC-4 H8-18

SiO2 48.84 49.10 48.78 48.59 48.05 47.46 47.71

TiO2 1.80 1.95 2.11 2.15 2.09 2.16 2.39

Al2O3 16.42 16.86 16.44 16.29 16.21 15.96 15.67

Fe2O3 3.49 4.15 3.85 3.72 4.32 3.88 4.61 FeO 6.37 6.71 6.55 6.95 7.52 7.12 8.00 MnO 0.16 0.17 0.16 0.15 0.18 0.17 0.18 MgO 8.71 6.73 7.82 8.70 7.12 9.28 7.48 CaO 10.08 8.88 9.38 9.44 10.00 9.92 9.13

Na2O 2.70 3.10 3.38 2.91 2.90 3.08 3.47 250 K2O 1.01 1.80 1.16 0.72 1.22 0.69 1.00

P2O5 0.41 0.54 0.38 0.38 0.40 0.29 0.35 TOTAL 100.00 100.00 100.00 100.00 100.00 100.00 100.00

JC-5 H8-70 H8-60A H8-63 H8-57 JC-4 H8-18 Q ------or 5.97 10.65 6.84 4.28 7.21 4.07 5.91 ab 22.83 26.23 28.59 24.59 24.58 26.03 29.22 an 29.71 26.76 26.27 29.28 27.59 27.70 24.25 ne ------0.06 di 14.06 11.02 14.21 12.07 15.65 15.66 15.09 hy 10.07 6.39 1.68 12.16 4.84 0.70 - - ol 7.93 7.98 11.95 7.29 8.99 15.46 13.43 mt 5.06 6.02 5.58 5.39 6.27 5.62 6.68 il 3.42 3.71 4.01 4.09 3.97 4.10 4.55 ap 0.96 1.26 0.87 0.87 0.92 0.67 0.82

%AN 56.55 50.50 47.88 54.35 52.88 51.56 45.36 APPENDIX 2D: NEW 40Ar/39Ar GEOCHRONOLOGY

Whole-rock 40Ar/39Ar method geochronology on eleven samples was obtained through the New Mexico Geochronological Research Laboratory at New Mexico Tech through an arrangement with Dr. Matthew Heizler. Isotopic ratios are corrected for blank, radioactive decay, and mass discrimination, and are not corrected for interfering reactions. Ages are calculated relative to FC-1 Fish Canyon Tuff sanidine interlaboratory standard at 27.84 Ma. 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 root MSWD where MSWD>1. Plateau and integrated ages incorporate uncertainties in interfering reaction corrections and J factors. Decay constants and isotopic abundances after Steiger and Jaeger (1977). Number symbol (#) preceding sample ID denotes analyses excluded from plateau age calculations. Discrimination = 1.00468 ± 0.00093 Correction factors: 39 37 ( Ar/ Ar)Ca = 0.00089 ± 0.00003 36 37 ( Ar/ Ar)Ca = 0.00028 ± 0.000011 38 39 ( Ar/ Ar)K = 0.01077 40 39 ( Ar/ Ar)K = 0.0002 ± 0.0003

251 40 39 37 39 36 39 39 40 39 σ ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1 (°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

KS-96-1, C2:89, 51.03mg groundmass concentrate, J=0.0007914, D=1.0017099, NM-89, Lab#=9158-01 # A 625 2457.4 2.142 8322.1 1.553 0.24 -0.1 5.6 -2 13 # B 700 121.2 2.874 399.4 2.87 0.18 2.8 15.8 4.80 0.82 # C 750 56.02 5.164 171.2 2.69 0.099 10.4 25.5 8.35 0.47 D 800 43.55 6.049 124.9 2.96 0.084 16.3 36.1 10.15 0.39 E 875 27.02 7.389 68.95 4.02 0.069 26.7 50.4 10.33 0.26 F 975 23.70 7.989 56.78 4.38 0.064 31.8 66.1 10.78 0.25 G 1075 22.57 10.45 55.54 1.89 0.049 30.8 72.9 9.98 0.46 # H 1250 35.08 24.51 104.9 3.76 0.021 17.0 86.4 8.63 0.39 252 # I 1650 58.05 46.11 185.7 3.81 0.011 11.6 100.0 9.88 0.60 Integrated age ± 1σ n=9 27.9 K2O=0.27 % 8.61 0.90 Plateau ± 1σ steps D-G n=4 MSWD=1.20 13.3 0.068 47.4 10.44 0.17

KS-96-6C, C4:89, 54.66mg groundmass concentrate, J=0.0007897, D=1.0017099, NM-89, Lab#=9160-01 # A 625 1753.1 3.091 5965.5 1.81 0.17 -0.5 8.4 -14 10 # B 700 149.5 2.729 501.6 3.01 0.19 1.0 22.3 2.20 0.92 # C 750 79.92 4.237 259.1 3.42 0.12 4.6 38.2 5.28 0.58 D 800 52.45 8.318 159.8 2.88 0.061 11.2 51.5 8.37 0.46 E 875 26.73 13.07 75.30 2.91 0.039 20.5 65.0 7.87 0.35 F 975 15.00 15.91 34.52 2.90 0.032 40.1 78.4 8.65 0.30 G 1075 12.31 16.53 26.14 0.950 0.031 47.6 82.8 8.42 0.75 # H 1250 16.00 29.52 46.75 1.84 0.017 27.8 91.3 6.46 0.50 # I 1650 206.8 81.74 692.5 1.87 0.006 4.1 100.0 12.7 1.6 Integrated age ± 1σ n=9 21.6 K2O=0.19 % 5.4 1.1 Plateau ± 1σ steps D-G n=4 MSWD=0.95 9.6 0.043 44.6 8.34 0.20 40 39 37 39 36 39 39 40 39 σ ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1 (°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

KS-96-7, C3:89, 51.03mg groundmass concentrate, J=0.000791, D=1.0017099, NM-89, Lab#=9159-01 # A 625 2207.1 4.033 7453.0 1.411 0.13 0.2 7.4 7 12 # B 700 128.9 4.095 426.7 2.73 0.12 2.4 21.8 4.45 0.89 # C 750 69.98 6.641 225.4 2.71 0.077 5.5 36.0 5.55 0.57 D 800 53.04 12.10 165.4 2.63 0.042 9.6 49.8 7.34 0.53 E 875 32.94 18.71 95.61 2.98 0.027 18.6 65.5 8.84 0.36 F 975 31.93 20.77 93.55 2.66 0.025 18.4 79.5 8.50 0.44 G 1075 47.80 22.64 143.4 0.826 0.023 15.0 83.8 10.3 1.2 H 1250 44.76 38.05 146.3 1.223 0.013 10.0 90.3 6.52 0.78 253 I 1650 180.5 98.57 612.8 1.85 0.005 3.9 100.0 10.7 1.6 Integrated age ± 1σ n=9 19.0 K2O=0.18 % 7.5 1.2 Plateau ± 1σ steps D-I n=6 MSWD=3.17 12.2 0.025 64.0 8.36 0.41

KS-96-8D, C1:89, 49.46mg groundmass concentrate, J=0.0007904, D=1.0017099, NM-89, Lab#=9157-01 A 625 1623.6 10.61 5474.7 0.248 0.048 0.4 2.1 9.6 9.7 B 700 26.90 13.38 95.03 0.791 0.038 -0.6 8.8 -0.23 0.97 C 750 10.58 12.81 37.69 1.016 0.040 4.1 17.4 0.62 0.73 D 800 6.303 13.39 23.24 1.95 0.038 7.4 33.8 0.67 0.40 E 875 5.490 13.63 20.76 2.18 0.037 7.4 52.2 0.58 0.35 F 975 6.308 16.52 23.71 1.72 0.031 9.1 66.8 0.82 0.46 G 1075 6.210 24.15 26.76 0.653 0.021 2.5 72.3 0.2 1.1 H 1250 11.63 32.06 47.43 1.186 0.016 0.7 82.4 0.11 0.67 I 1650 17.06 88.89 75.77 2.08 0.006 8.8 100.0 2.28 0.91 Integrated age ± 1σ n=9 11.83 K2O=0.12 % 1.00 0.40 Plateau ± 1σ steps A-I n=9 MSWD=0.73 11.83 0.028 100.0 0.65 0.19 40 39 37 39 36 39 39 40 39 σ ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1 (°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

CM-97-7, C6:89, 53.06mg groundmass concentrate, J=0.0007891, D=1.0017099, NM-89, Lab#=9162-01 # A 625 1738.3 8.496 5826.0 0.633 0.060 1.0 5.2 25 10 # B 700 112.1 8.947 376.6 1.500 0.057 1.3 17.6 2.1 1.0 C 750 62.70 14.52 200.8 1.70 0.035 7.1 31.6 6.43 0.70 D 800 40.85 19.80 128.9 1.74 0.026 10.4 45.9 6.14 0.56 E 875 30.04 23.09 90.86 1.89 0.022 16.5 61.5 7.18 0.50 F 975 30.59 24.84 94.01 1.73 0.021 15.4 75.8 6.83 0.50 G 1075 47.63 24.78 152.4 0.621 0.021 9.5 80.9 6.5 1.4 H 1250 55.93 53.25 191.0 1.287 0.010 6.4 91.5 5.28 0.87 254 I 1650 182.2 155.5 647.7 1.026 0.003 1.5 100.0 4.4 2.1 Integrated age ± 1σ n=9 12.12 K2O=0.11 % 6.70 0.85 Plateau ± 1σ steps C-I n=7 MSWD=0.92 9.99 0.021 82.4 6.54 0.26

CM-97-22, C5:89, 51.93mg groundmass concentrate, J=0.0007888, D=1.0017099, NM-89, Lab#=9161-01 # A 625 2272.9 2.095 7715.7 1.316 0.24 -0.3 4.5 -10 12 # B 700 60.39 3.773 192.1 2.45 0.14 6.5 13.0 5.57 0.65 C 750 36.23 5.125 106.0 2.99 0.100 14.6 23.3 7.53 0.38 D 800 21.46 7.214 54.76 4.64 0.071 27.2 39.2 8.31 0.24 E 875 14.45 7.039 31.43 7.35 0.072 39.5 64.5 8.14 0.15 F 975 13.68 6.570 28.41 5.37 0.078 42.3 83.0 8.26 0.17 G 1075 17.98 8.713 45.00 1.266 0.059 29.8 87.4 7.65 0.60 # H 1250 28.89 30.09 92.69 1.69 0.017 13.2 93.2 5.53 0.55 # I 1650 53.49 48.61 179.0 1.97 0.010 8.1 100.0 6.35 0.71 Integrated age ± 1σ n=9 29.0 K2O=0.27 % 6.80 0.62 Plateau ± 1σ steps C-G n=5 MSWD=1.08 21.6 0.076 74.4 8.15 0.10 40 39 37 39 36 39 39 40 39 σ ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1 (°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

KS98-4, A2:110, 29.57mg groundmass, J=0.0007762, D=1.00754, NM-110, Lab#=50456-01 A 625 679.9 7.563 2225.0 0.411 0.067 3.4 2.1 32.2 8.0 B 700 41.96 8.404 129.2 0.749 0.061 10.7 5.9 6.3 2.5 C 750 24.29 8.358 59.84 0.601 0.061 30.0 9.0 10.3 2.9 D 800 15.79 9.567 37.97 1.147 0.053 33.9 15.1 7.6 1.5 E 875 10.50 12.33 21.23 1.922 0.041 49.9 25.9 7.41 0.93 F 975 9.853 12.42 17.58 2.19 0.041 57.7 39.0 8.03 0.80 G 1075 10.79 14.81 22.98 1.929 0.034 48.4 51.4 7.40 0.90 H 1250 15.31 17.79 38.03 2.19 0.029 36.2 66.4 7.87 0.83 255 I 1700 29.95 64.85 103.1 4.35 0.008 16.2 100.0 7.21 0.62 Integrated age ± 1σ n=9 15.49 K2O=0.26 % 8.24 0.48 Plateau ± 1σ steps A-I n=9 MSWD=1.43 15.49 0.033 100.0 7.61 0.41

KS98-19, A5:110, 28.97 mg groundmass, J=0.0007792, D=1.00468, NM-110, Lab#=50459-01 # A 625 695.5 0.9119 2370.1 1.176 0.56 -0.7 3.3 -6.8 4.9 # B 700 36.26 1.377 110.8 3.50 0.37 10.0 13.0 5.10 0.62 C 750 25.56 1.843 66.48 2.52 0.28 23.7 20.0 8.53 0.85 D 800 20.16 2.504 48.15 3.04 0.20 30.4 28.4 8.63 0.63 E 875 17.53 3.482 35.54 3.44 0.15 41.7 38.0 10.29 0.53 F 975 18.03 5.201 39.60 4.95 0.098 37.5 51.8 9.52 0.38 G 1075 18.54 8.726 40.63 3.10 0.058 39.2 60.3 10.26 0.59 H 1250 26.05 8.862 66.08 7.12 0.058 27.9 80.1 10.26 0.32 I 1700 46.05 28.14 142.1 7.16 0.018 13.9 100.0 9.19 0.40 Integrated age ± 1σ n=9 36.0 K2O=0.61 % 8.63 0.31 Plateau ± 1σ steps C-I n=7 MSWD=1.97 31.3 0.10 87.0 9.71 0.24 40 39 37 39 36 39 39 40 39 σ ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1 (°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

KS98-23, A1:110, 30.63 mg groundmass, J=0.0007767, D=1.00754, NM-110, Lab#=50455-01 # A 625 289.9 1.365 961.8 0.753 0.37 2.0 2.6 8.2 3.7 # B 700 24.23 2.161 65.22 2.68 0.24 21.2 11.9 7.20 0.73 C 750 22.84 2.918 56.96 2.79 0.17 27.3 21.6 8.75 0.70 D 800 19.76 4.628 43.23 3.64 0.11 37.3 34.2 10.33 0.52 E 875 13.70 6.952 22.42 4.48 0.073 55.8 49.7 10.75 0.43 F 975 10.76 8.242 12.90 5.84 0.062 70.9 69.9 10.74 0.32 G 1075 10.50 11.06 13.60 3.10 0.046 70.4 80.6 10.44 0.58 H 1250 12.68 16.12 25.17 2.53 0.032 51.9 89.4 9.32 0.72 256 # I 1700 40.47 66.78 136.9 3.06 0.008 13.7 100.0 8.23 0.82 Integrated age ± 1σ n=9 28.9 K2O=0.47 % 9.68 0.27 Plateau ± 1σ steps C-H n=6 MSWD=1.92 22.4 0.081 77.5 10.39 0.27

KS98-28, A4:110, 29.88 mg groundmass, J=0.0007788, D=1.00468, NM-110, Lab#=50458-01 # A 625 229.0 1.893 759.7 1.302 0.27 2.0 6.1 6.5 2.5 # B 700 13.38 2.806 33.20 3.13 0.18 28.4 20.7 5.35 0.57 C 750 11.92 3.960 24.54 1.613 0.13 41.9 28.3 7.0 1.1 D 800 11.18 7.039 18.04 2.41 0.072 57.5 39.5 9.07 0.71 E 875 9.583 10.74 13.31 2.96 0.047 68.2 53.4 9.25 0.58 F 975 9.051 11.33 11.29 3.68 0.045 73.5 70.5 9.42 0.48 G 1075 10.70 13.51 21.52 2.04 0.038 51.0 80.1 7.75 0.84 H 1250 12.49 23.63 27.54 2.64 0.022 50.5 92.5 9.02 0.69 I 1700 32.31 117.0 124.9 1.615 0.004 15.7 100.0 7.9 1.3 Integrated age ± 1σ n=9 21.4 K2O=0.35 % 8.08 0.30 Plateau ± 1σ steps C-I n=7 MSWD=1.20 17.0 0.049 79.3 8.90 0.29 40 39 37 39 36 39 39 40 39 σ ID Temp Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Ar Age ±1 (°C) (x 10-3) (x 10-15 mol) (%) (%) (Ma) (Ma)

KS98-30, A3:110, 24.12 mg groundmass, J=0.0007773, D=1.00754, NM-110, Lab#=50457-01 A 625 285.7 2.111 958.1 1.078 0.24 1.0 5.9 3.9 3.5

257 B 700 27.28 3.419 80.45 2.28 0.15 13.9 19.1 5.32 0.88 C 750 25.11 5.193 68.25 1.332 0.098 21.4 27.3 7.5 1.4 D 800 20.41 9.381 51.55 1.599 0.054 29.2 37.6 8.4 1.1 E 875 15.03 13.65 35.36 1.442 0.037 38.0 47.5 8.1 1.2 F 975 13.10 15.12 27.39 1.627 0.034 47.8 59.2 8.9 1.1 G 1075 11.74 18.59 23.29 1.198 0.027 54.5 68.4 9.1 1.4 H 1250 15.86 25.19 41.55 1.950 0.020 35.7 84.1 8.11 0.91 I 1700 44.43 100.4 163.7 1.819 0.005 9.8 100.0 6.7 1.3 Integrated age ± 1σ n=9 14.33 K2O=0.29 % 7.32 0.54 Plateau ± 1σ steps A-I n=9 MSWD=1.49 14.33 0.070 100.0 7.56 0.47 APPENDIX 2E: PREVIOUSLY REPORTED K-Ar AND 40Ar/39Ar GEOCHRONOLOGY

Thirty-five (35) previously reported K-Ar method age determinations (Hart and Mertzman, 1982, 1983; Hart and Carlson, 1983, 1985; Hart et al., 1984) and four (4) previously reported 40Ar/39Ar method age determinations (Shoemaker and Hart, 2003; see Appendix 3), combined with the eleven new 40Ar/39Ar method age determinations (Appendix 2D), provide the chronostratigraphic framework for this study. Because of their critical importance to the work at hand, these previously reported ages, with their errors, are provided here. They are ordered from oldest to youngest.

258 sample age (Ma) ±2σ method reference H85-10A 16.27 0.17 40Ar/39Ar Shoemaker and Hart, 2003 CH82-32 16.23 0.80 K-Ar Hart and Carlson, 1985 H9-47 15.01 0.90 K-Ar Hart and Carlson, 1985 JV96-4 14.61 0.35 40Ar/39Ar Shoemaker and Hart, 2003 JV96-7 13.87 0.39 40Ar/39Ar Shoemaker and Hart, 2003 JV96-2 12.37 0.48 40Ar/39Ar Shoemaker and Hart, 2003 H8-74 11.70 0.65 K-Ar Hart and Mertzman, 1982 H9-32 11.20 0.65 K-Ar Hart and Mertzman, 1982 H8-69D 9.94 0.98 K-Ar Hart and Mertzman, 1983 H8-29 9.87 1.10 K-Ar Hart and Mertzman, 1983 H9-37C 9.57 0.60 K-Ar Hart and Mertzman, 1983 H9-20C 9.34 0.68 K-Ar Hart et al., 1984 H9-20E 8.93 0.63 K-Ar Hart et al., 1984 SM75-12A 8.51 1.02 K-Ar Hart and Mertzman, 1983 H8-69G 8.42 0.74 K-Ar Hart and Mertzman, 1983 H8-34 8.21 0.85 K-Ar Hart and Mertzman, 1983 H8-28C 8.14 0.65 K-Ar Hart and Mertzman, 1983 H9-21C 8.11 0.62 K-Ar Hart et al., 1984 H8-36 7.58 0.70 K-Ar Hart and Mertzman, 1983 H9-39 7.20 0.76 K-Ar Hart et al., 1984 H8-28E 7.05 0.61 K-Ar Hart and Mertzman, 1983 H9-21E 6.01 0.80 K-Ar Hart et al., 1984 H9-27 5.04 0.84 K-Ar Hart et al., 1984 H8-42 4.49 0.38 K-Ar Hart and Mertzman, 1983 H8-95 4.40 0.87 K-Ar Hart et al., 1984 H8-45 4.09 0.34 K-Ar Hart and Mertzman, 1983 H8-47 4.06 0.41 K-Ar Hart and Mertzman, 1983 H9-44 3.84 0.57 K-Ar Hart and Mertzman, 1983 H9-49 1.86 0.19 K-Ar Hart and Mertzman, 1983 H8-50E 1.55 0.11 K-Ar Hart et al., 1984 H9-37D 1.49 0.18 K-Ar Hart and Mertzman, 1983 H9-36C 1.25 0.28 K-Ar Hart and Mertzman, 1983 H9-36A 0.91 0.36 K-Ar Hart and Mertzman, 1983 H9-42 0.44 0.16 K-Ar Hart and Mertzman, 1983 H8-73 0.43 0.09 K-Ar Hart and Mertzman, 1982 H9-29 0.36 0.13 K-Ar Hart and Carlson, 1983 H8-70 0.25 0.05 K-Ar Hart and Mertzman, 1983 JC-4 0.15 (max) - - K-Ar Hart and Mertzman, 1983 H8-57 0.03 (max) - - K-Ar Hart and Mertzman, 1983

259

APPENDIX 3:

preprint of: Shoemaker, K.A. and Hart, W.K., 2002, Temporal controls on basalt genesis and evolution on the Owyhee Plateau, Idaho and Oregon, in Bonnichsen, B., White, C.M., and McCurry, M. (eds.), Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, in press.

260 Temporal Controls on Basalt Genesis and Evolution on the Owyhee Plateau, Idaho and Oregon

Kurt A. Shoemaker1 and William K. Hart2

ABSTRACT more primitive high-alumina olivine tholeiites after 11 Ma. A preliminary model is offered that calls for tempo- The development of the Snake River Plain volcanic ral variations in magma production, lithospheric struc- province has been attributed to the passage of the North ture, and the relative contributions from lithospheric and American plate over the Yellowstone mantle plume. As sublithospheric source reservoirs. this province was being formed by an eastward progres- Key words: basalt, Owyhee Plateau, lithosphere, Sr sion of younger silicic volcanic centers, basaltic volcan- isotopes, magmatic evolution, plume track ism has continued through the present day in the western regions of the “plume track.” Of particular interest is the Owyhee Plateau of southeastern Oregon, located at the INTRODUCTION westernmost end of the SRP volcanic province. The Owyhee Plateau preserves the best documented continu- The initiation of flood basalt volcanism in the north- ous record of mid-Miocene to Recent basaltic volcanism western United States at approximately 17.5 Ma marks in the northwestern United States, allowing an investiga- the onset of a time and longitudinally transgressive se- tion of variations in magmatic sources and processes quence of volcanic activity attributed to the passage of through time. In addition, this region is underlain by litho- the North American plate over the hypothesized sphere transitional between that of the Proterozoic- Yellowstone mantle plume (Pierce and Morgan, 1992; Archean Wyoming Craton to the east and accreted ter- Geist and Richards, 1993; Camp, 1995). The earliest pulse ranes younger than 200 Ma to the west. Basaltic prod- of flood basalt eruptions (17.5-14 Ma) are represented ucts that erupted from a variety of vents on the Owyhee by the Clarkston Basalt of the Columbia Plateau (Hooper Plateau illustrate chemical and Sr isotopic diversity that and Hawkesworth, 1993) and Steens Mountain Basalt of cannot be solely attributed to lateral lithospheric hetero- the Oregon Plateau (Carlson and Hart, 1987, 1988; Hart geneities. Rather, this diversity appears to be a function and Carlson, 1987). These basalts were erupted from a of eruptive age. For example, Sr isotope data show a sys- narrow array of vents extending for nearly 1,000 km from tematic increase in 87Sr/86Sr from 0.704 to 0.707 with southeastern Washington to central Nevada. This array is decreasing age from 17.5-11 Ma, but decreasing 87Sr/86Sr believed to represent a rift in the back-arc region of the to 0.704 from 11-0 Ma. The “peak” in Sr values corre- Cascade arc (Christiansen and McKee, 1978; Carlson and sponds to a regional change in the dominant basalt type Hart, 1987; Zoback and others, 1994). After 14 Ma, erup- erupted, from large volume, strongly fractionated basalts tive activity along this rift contracted in a north-south and basaltic andesites before 11 Ma to smaller volume, direction, becoming focused in the Owyhee Plateau of the Oregon-Idaho-Nevada border region (Figure 1). Sub- sequent magmatism to the east formed the Snake River Editors’ note: The manuscript was submitted in June 1998 and has been revised at the authors’ discretion. Plain (SRP). It is important to note that, as the SRP was 1Geology Department, Saint Joseph’s College, Rensselaer, IN 47978 being formed by an eastward progression of silicic erup- 2Geology Department, Miami University, Oxford OH 45056 tive centers, basaltic volcanism has continued essentially

Shoemaker, K.A., and W.K. Hart, 2002, Temporal controls on basalt genesis and evolution on the Owyhee Plateau, Idaho and Oregon, in Bill Bonnichsen, C.M. White, and Michael McCurry, eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, p. 00?-00?. 261 2 Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province

through the present day in the western regions of the “plume track.” Determining the ultimate mantle sources of and the processes responsible for the evolution of these basalts has been hindered by the complication of contamination of the primary magmas by chronologically and chemi- cally heterogeneous lithospheric materials (Leeman and others, 1992). Regional geochemical and isotopic infor- mation, combined with regional patterns of crustal de- formation, has led some researchers to argue that the ini- tial, large volume flood basalt eruptions were related to the arrival of the Yellowstone plume head at the base of the lithosphere (Geist and Richards, 1993; Hooper and Hawkesworth, 1993; Camp, 1995). Other researchers have used the same evidence to argue that these basalts were erupted as a result of rifting behind the Cascade volcanic arc (Christiansen and McKee, 1978; Carlson and Hart, 1987). Obviously, the current data base is ambigu- ous with regard to these issues, and questions concern- ing sources and processes cannot be answered until the influence of the lithosphere on the primary magmas can be adequately determined (Hart, 1997; Hart and others, 1997). The Owyhee Plateau (Figure 1) provides an excel- lent opportunity to explore variations in the geochemical and isotopic characteristics of late Cenozoic northwest- ern United States basalts through time. The basalts in this study constitute a geochemically and isotopically diverse suite erupted over the past 17.5 Ma from vents located within a geographically restricted area between approxi- mately lat 42º00′N. and 43º15′N. and long 116º45′W. and 118º15′W. This area is underlain by lithosphere transi- tional between that of the Proterozoic-Archean Wyoming craton to the east and accreted terranes younger than 200 Ma to the west (Leeman and others, 1992). Here, the ba- saltic magmas are assumed to have passed through, and consequently to have had the opportunity to interact with, the same chronologically similar “package” of litho- spheric materials. Thus, by eliminating the distinct isoto- pic signatures attributed to regional, lateral lithospheric variations, a significant variable to investigate is time.

Figure 1. Location of the study area, in relation to major volcanic and tectonic features of the western United States. (a) Generalized map of the major late Cenozoic volcanic provinces of the northwestern United States. C: Cascade arc; CP: Columbia Plateau; OP: Oregon Plateau; WSRP: Western Snake River Plain; BR: Basin and Range; MTJ: Mendocino Triple Junction. (b) Digital elevation model of the Owyhee Plateau study area, modified from Sterner (1997). SM: Steens Moun- tain; OM: Owyhee Mountains; OIG: Oregon-Idaho Graben; NNR: Northern Nevada Rift; O: Owyhee Plateau and Owyhee River Canyon region. Locations of tectonic and structural features adapted from Bohannon and Parsons (1995), Zoback and others (1994), Carlson and Hart (1987), and Ferns (1997). 262 Shoemaker and Hart—Basalt Genesis and Evolution on the Owyhee Plateau 3

Furthermore, to our knowledge, the Owyhee Plateau con- tains the most complete record of mid-Miocene to Re- cent basaltic volcanism in the northwestern United States. Thus, it is uniquely suited to an exploration of the tem- poral variations in lithospheric and sublithospheric con- tributions to this late Cenozoic basaltic volcanism.

SPATIAL AND TEMPORAL RELATIONSHIPS Previous investigations of post-17.5 Ma Oregon Pla- teau basaltic volcanism by Carlson and Hart (1987, 1988), Hart and Carlson (1987, 1992), and Draper (1991) docu- mented a significant change about 11 Ma in the domi- nant basalt type erupted (Figure 2a). The older eruptions (Steens Mountain Basalt) produced strongly fractionated tholeiitic basalts and basaltic andesites. Post-11 Ma erup- tions were dominated by less fractionated low-K, high alumina olivine tholeiites (HAOT). This change coincides not only with the regional shift from large volume, fis- sure-fed eruptions to small volume eruptions from dis- crete eruptive centers, but also with the onset of regional lithospheric thinning due to diffuse extension through- out the entire Oregon Plateau area (Hart and Carlson, 1987; Hooper, 1990; Draper, 1991). Leeman and others (1992) documented that the maxi- mum diversity in 87Sr/86Sr for late Cenozoic northwest- ern United States basalts occurs in a longitudinal “band” corresponding to approximately 117º to 118º west longi- tude (Figure 2b). This region of maximum diversity co- incides with the transition between the Proterozoic- Archean cratonic lithosphere to the east and accreted ter- ranes younger than 200 Ma to the west (Leeman and oth- ers, 1992; Elison and others, 1990; Wright and Wooden, 1991). Importantly, the Owyhee Plateau overlies this re- Figure 2. Important regional geochemical and isotopic observations. gion of transitional lithosphere (Figure 1). (a) Illustration of the regional change in basalt geochemistry on the Hart (1997) further explored the relationships between Oregon Plateau, from strongly fractionated basalts and basaltic andes- age and location of eruption and Sr isotope compositions ites before approximately 11 Ma to relatively unfractionated basalts of northwestern United States basalts (Figure 3). Figure after approximately 11 Ma, after Hart and Carlson (1987). (b) Longitu- 3 substantiates the observations of Leeman and others dinal variation in Sr isotope compositions of basaltic rocks, after Leeman 87 86 and others (1992). The area of focus for this study coincides with the (1992) that a wide range in basalt Sr/ Sr values occurs cratonic boundary, where the greatest diversity in 87Sr/86Sr is observed. along approximately 117º west longitude, with higher and lower 87Sr/86Sr values occurring east and west of this line, found when the data are plotted versus the age of erup- respectively. Additionally, Figure 3 shows that those tion. Sr-isotope values increase with decreasing age from basalts representing the earliest pulse of volcanism the initiation of volcanism to about 11 Ma, at which point (Steens, Clarkston, and Picture Gorge basalts) have lower the data diverge. Except for the young Yellowstone area 87Sr/86Sr values than many of the samples representing basalts that span nearly the entire observed range in younger eruptions (e.g., SRP and Saddle Mountains 87Sr/86Sr, SRP basalts retain radiogenic signatures while basalts). Furthermore, samples from the Owyhee Plateau Owyhee Plateau basalts trend toward lower 87Sr/86Sr val- define a wide range in 87Sr/86Sr, from less than 0.704 to ues from 11 Ma to 0 Ma. The rest of this paper will focus greater than 0.707. Another interesting relationship is only on the Owyhee Plateau basalts. 263 4 Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province

Figure 3. Compilation of Sr isotope data for northwestern United States basalts, after Hart (1997). Where possible, the longitude of the eruption site is used. Data are from Carlson (1984), Carlson and Hart (1987), Carlson and others (1981), Hart (1985), Hart and others (1989), Hooper and Hawkesworth (1993), Lambert and others (1995), Leeman and others (1992), Leeman (1982), Leeman and Manton (1971), Mark and others (1975), Noble and others (1973), and this study.

DATA On the basis of eruptive age, the sample suite has been divided into five groups as indicated in Figure 4. These groups are described briefly below: CHRONOSTRATIGRAPHY 17.5-14 Ma. This interval represents the initial, large volume eruptive phase of Oregon Plateau basaltic volca- Careful sampling of basalt flow sequences exposed nism, temporally correlative with the eruption of the in fault scarps and in canyons of the Owyhee River and Clarkston Basalt of the Columbia Plateau (Hart and its tributaries (see Figure 1b) combined with new (Table Carlson, 1985; Hart and others, 1989; Hooper and 1) and previously reported (Hart and Mertzman, 1982, Hawkesworth, 1993). These flood basalts typically were 1983; Hart and Carlson, 1983, 1985; and Hart and oth- erupted from isolated fissures and large fissure systems. ers, 1984) chronologic data has allowed us to construct a Magmas erupted at this time were strongly fractionated composite chronostratigraphic section representing the basalts to basaltic andesites with tholeiitic to calc-alka- history of basaltic volcanism on the Owyhee Plateau over line affinities. 40 39 the last 17.5 Ma. Four new Ar/ Ar method age deter- 14-11 Ma. This interval represents the waning stages minations (Table 1) were obtained in order to better con- of Steens Mountain Basalt eruptions. Basalt compositions strain the timing of the older pulse of Owyhee Plateau are similar to those of the older age group, but the vol- basaltic volcanism. Age estimates were interpolated for umes erupted are significantly smaller and individual samples with relatively tight (approximately 2 m.y. or flows are less areally extensive (Hart and Mertzman, less) stratigraphic constraints based on K-Ar or 40Ar/39Ar 1982; Hart and Carlson, 1985). age data. These estimated ages are marked with an aster- 11-6 Ma. This interval marks the regional change in isk (*) in Table 2. dominant basalt type erupted, from the strongly fraction- 264 Shoemaker and Hart—Basalt Genesis and Evolution on the Owyhee Plateau 5

Table 1. New 40Ar/39Ar age data.

Sample Age 39ArK Age ± 2 s.d. number Analysis steps (x 10-15 mol) % 39Ar K/Ca (Ma) (Ma) H85-10A plateau 6 46.6 82.2 0.33 16.27 0.17 JV96-2 plateau 5 33.4 75.2 0.09 12.37 0.48 JV96-4 plateau 7 27.8 73.1 0.20 14.61 0.35 JV96-7 plateau 7 90.1 66.0 0.28 13.87 0.39 Correction factors: (39Ar/37Ar)Ca = 0.00070 ± 0.00005 (36Ar/37Ar)Ca = 0.00026 ± 0.00002 (38Ar/39Ar)K = 0.0119 (40Ar/39Ar)K = 0.0002 ± 0.0003 ated basalts and basaltic andesites of the early phase of volcanism to relatively unfractionated basalts. This is accompanied by a change from primarily fissural flood basalt activity to a combination of eruptions from local fissures associated with extensional features and from isolated low shield cones similar to those found within the SRP proper. Compositions range from low-K, low- Ti, high alumina olivine tholeiites (HAOT) to high-K, high-Ti Snake River Plain-type olivine tholeiites (SROT). A full spectrum of compositions transitional between these end members (transitional basalts, TB) is present (Hart and others, 1984; Hart, 1985). 6-3 Ma. This interval is defined primarily by increased magmatism between 5-4 Ma following a brief period of low magmatic output (Hart and others, 1984; Figure 3). Magmatism during this interval is dominated by small Figure 4. Total alkali-silica diagram of Le Bas and others (1986), show- volume HAOT to SROT eruptions from local fissures ing classification of Owyhee Plateau basalts. Total Fe has been parti- and low shield cones (Hart and others, 1984; Hart, 1985). tioned prior to normalization of analyses to 100 percent anhydrous, 3-0 Ma. This interval again follows a brief period of according to the method of Le Maitre (1976). Age groups are described in the text. low magmatic output and is dominated by small volume HAOT eruptions (Hart and others, 1984). In addition, the only alkaline basalts observed on the Oregon Plateau were produced during this interval (after 1 Ma) and were by titration. Samples for which only Fe2O3 is reported erupted from low shield and tephra cones (Hart and were analyzed for major elements by DCP at Miami Uni- Mertzman, 1983; Russell and others, 1988; Hart, 1996). versity by the method of external standards, using 7-8 international standards to define calibration curves. All trace elements were analyzed by XRF, with the excep- ELEMENTAL AND SR-ISOTOPE tion of Sc, V, and Cr, which were determined by DCP GEOCHEMISTRY using external standards. Sr isotope ratios were deter- mined by thermal ionization mass spectrometry at Mi- Major and trace element geochemical, Sr-isotope, and ami University and at the Department of Terrestrial Mag- chronologic data for the sample suite of this study are netism, Carnegie Institution of Washington. Geochronol- given in Table 2. Major element analyses were performed ogy by the 40Ar/39Ar method was performed at the New by both X-ray fluorescence and direct current argon Mexico Geochronologic Research Laboratory. plasma spectrometry (DCP) methods. Samples for which Most of the samples are basalts, although some pre- both Fe2O3 and FeO are reported were analyzed for ma- 11 Ma samples plot in the basaltic andesite field of Le jor elements by XRF at Franklin and Marshall College Bas and others (1986; Figure 4). The samples are domi- according to the methods described by Boyd and nantly subalkaline, with the exception of some of the Mertzman (1987); FeO for these samples was determined samples representing the most recent pulse of eruptive 265 6 Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province trations are reported as weight t and Mertzman, 1982, 1983; Hart timated ages are marked with an asterisk age-corrected initial ratios reported relative 0.00006 or less. ± standard; 2-sigma uncertainties are 3 2836 2094 32 4476 34 95 6.9 74 29 63 1745 5531 38 9323 15 43 59 42 36 62 15 23 53 72 35 31 49 40 25 40 52 46 28 58 6.2 16 48 55 32 40 47 16 26 48 45 39 48 86 27 23 74 79 30 64 59 7.4 34 73 46 26 32 19 8.9 37 43 31 45 118 36 6.5 18 65 78 41 94 24 7.5 38 90 57 30 166 5.8 35 61 26 89 46 145 12 35 43 53 63 98 24 26 42 69 10 23 52 13 27 48 15 0.82.6 1.4 3.0 0.3 5.3 0.3 0.6 0.9 2.5 1.7 4.9 0.6 1.8 1.4 5.4 0.1 — 0.6 2.4 1.4 1.2 0.7 0.6 — 0.9 — 1.6 — — — 0.1 113 124 119 105 116 132 111 103 133 138 167 96 119 77 105 102 455263 440 189 482 206 459 146791 561 216 466 416177 218 678209 662 206 50 595 247 280 213 49 307 578 238 153 538 424 33 307 270 782 468 — — 350 776 235 273 52 115 548 252 204 114 159 842 201 158 80 77 1337 305 260 221 773 393 123 222 70 574 — 156 — 220 285 275 769 290 53 352 240 360 286 320 291 288 2.003.45 2.207.09 12.190.15 1.975.01 —7.08 0.16 2.93 2.07 5.72 10.41 8.17 7.61 0.25 2.54 2.78 5.33 7.28 9.83 0.18 2.32 5.67 3.02 9.98 4.85 0.14 1.21 2.77 7.56 8.66 0.19 9.58 1.04 4.69 — 9.05 8.00 0.17 2.53 4.88 14.05 8.23 — 0.15 2.86 4.36 15.47 7.09 0.20 — 3.00 6.62 10.24 7.59 0.20 1.57 5.01 — 4.77 8.75 0.23 2.80 3.51 6.40 5.58 7.24 0.19 1.25 7.57 11.67 6.64 2.74 0.20 7.50 1.94 9.77 8.48 3.36 0.17 11.49 9.89 7.33 1.82 4.09 0.20 10.24 8.93 9.28 0.19 11.82 7.35 7.80 0.471.33 0.41 3.16 0.43 4.75 0.26 1.49 0.42 2.61 0.43 2.37 0.67 0.80 0.62 1.37 0.44 0.11 0.98 0.08 1.37 0.73 0.21 0.93 0.50 0.88 0.15 0.72 0.28 1.03 0.27 1.05 3.52 3.00 3.15 3.09 3.69 3.19 3.57 3.36 2.51 3.25 3.50 2.38 2.53 2.28 2.23 2.06 1.41 0.93 1.60 0.48 1.06 0.92 1.36 1.86 0.48 1.02 1.60 0.34 0.53 0.34 0.42 0.31 11.9 13.8 13.2 9.5 14.9 15.0 14.7 14.5 10.8 23.2 30.5 9.7 14.0 5.6 9.5 10.5 21.6 22.5 22.0 22.4 24.319.5 23.1 25.2 19.4 21.8 18.8 26.9 20.7 — 22.2 26.0 22.8 24.5 18.5 19.2 19.6 36.6 16.5 29.9 18.3 — 18.0 37.4 32.0 34.7 36.6 37.6 16.2742.73 16.23 43.23 *16 42.84 *16 42.73 42.84 *16 42.92 15.01 43.21 14.61 43.12 13.87 42.99 12.37 42.92 11.70 42.92 11.20 42.85 9.87 42.79 9.57 42.92 8.51 42.68 8.42 42.55 8.21 Trace elements in ppm Trace 53.82 48.4614.72 15.88 49.95 48.72 14.73 49.26 15.76 49.41 16.45 15.18 51.89 55.38 16.71 48.00 16.19 14.20 46.13 49.80 14.80 49.11 15.40 15.89 45.48 47.65 16.25 16.26 48.34 15.53 48.42 15.78 117.05 116.88 117.22 117.05 117.15 116.98 117.14 117.03 117.07 118.00 118.04 117.32 117.60 117.38 117.36 117.18 Major oxides in wt. % 0.70432 0.70409 0.70418 0.70397 0.70436 0.70426 0.70442 — 0.70638 0.70521 0.70557 0.70693 0.70723 0.70667 0.70668 0.70675 100.05 100.28 100.23 100.64 100.45 100.16 99.06 99.42 99.37 98.55 100.37 101.27 100.50 100.27 101.78 100.96 H85-10A CH82-32 H85-6 H85-10B H85-12B H9-47 JV96-4 JV96-7 JV96-2 H8-74 H9-32 H8-29 H9-37C SM75-12A H8-69G H8-34 Samples: Sr = 0.70800 for the E&A SrCO 86 Sr 3 3 86 2 5 2 O Sr/ O O 2 O 2 2 O 2 87 2 Sr/ SiO Al FeO MnO MgO CaO Na TiO Fe Sr Y Zr Nb Ni Ga Cu Zn U Th Ba Ce Co La Cr V Sc Rb L.O.I. TOTAL K P 87 Longitude Age (Ma) Latitude percent oxides; trace elements are reported in ppm. Samples arranged according to eruptive age, from oldest youngest. Es Table 2. Major and trace element geochemical, Sr-isotope, and chronologic data for Owyhee Plateau basalts. Major element concen 2. Major and trace element geochemical, Sr-isotope, Table (see text). With the exception of estimated ages and ages reported in Table 1, all ages are previously published K-Ar (Har Table the exception of estimated ages and reported in With (see text). All Sr isotope ratios are Carlson, 1983, 1985). Sr isotope data are from Hart (1985), Carlson and (1987), this study. to 266 Shoemaker and Hart—Basalt Genesis and Evolution on the Owyhee Plateau 7 26111111149 8 132922 33 408854 22 165981196275768395899272717675778395 74 23 15223 149 79 274312166101217131015124 138 31 64 28 52 141 17 61 27 49 124 25 80 28 50 125 30 60 33 130 47 37 73 22 127 45 34 79 147 20 51 27 183 85 21 48 137 34 84 23 47 140 23 117 22 45 86 79 13 26 43 85 74 15 50 30 21 51 50 28 63 50 51 43 43 42 .—13.04——.—.—2.21.0 5.6 6.79.6 2.1 14.9 5.9 1.80.4——1.31.00.4————0.7—0.5— 5.02.0 8.4 7.9 — 10.4 15 11.7 1.3 6.3 16.0 0.1 8.1 9.8 1.3 15.6 5.7 0.1 10.5 2.7 1.5 3.3 2.3 0.4 2.6 4.4 0.9 7.3 8.7 — 13.7 0.2 41 43.6 — 27 32.3 — 1.4 3.3 2.3 213144 292 152 159 273 45 229 96321 275 103 2559 305 123306 454275 261 131 320 141 277 257 105 278 259 243 241 156 210 — — 191 404 86 313 249 196 236 301 53 254 242 249 253 69 268 282 130 219 88 265 476 113 232 254 94 192 270 460 247 198 116 318 253 174 207 278 229 240 653 254 235 499 248 129 180 132 231 1.75 2.15 0.81 1.20 1.30 1.57 1.92 1.58 1.88 1.25 0.94 0.88 1.30 1.80 1.94 2.11 2.258.720.18 2.81 10.008.53 0.20 1.632.26 8.87 8.72 8.34 0.17 2.86 2.15 9.22 0.17 2.52 8.00 8.90 2.29 3.98 0.18 7.20 2.65 9.52 0.17 4.81 8.94 7.28 2.60 0.17 1.35 10.00 2.43 7.88 1.91 10.08 0.19 2.63 8.60 0.18 2.76 8.00 8.61 2.93 0.18 4.44 6.24 2.79 8.73 1.95 8.32 0.17 2.69 8.81 0.18 1.68 8.80 9.25 2.48 0.17 1.28 9.36 2.50 8.68 0.17 6.41 4.61 2.55 9.08 0.17 2.74 9.04 2.54 6.69 0.18 3.08 7.18 2.93 0.240.97 0.42 1.53 0.11 0.89 0.16 0.60 0.26 0.59 0.31 0.44 0.34 0.85 0.22 0.49 0.41 0.86 0.19 1.46 0.10 0.65 0.13 0.93 0.17 0.80 0.23 0.82 0.54 0.61 0.40 0.87 0.28 0.40 0.11 0.33 0.32 0.39 0.72 0.42 0.43 0.38 0.17 0.28 0.26 0.50 1.79 1.23 18.5 18.7 16.3 16.3 17.4 18.438.1 18.3 34.1 19.0 37.7 20.2 — 17.5 36.3 18.4 37.0 16.3 31.5 18.7 34.1 18.3 34.0 17.9 32.1 18.5 35.9 34.6 35.6 31.9 23.0 28.4 7.58 7.20 5.04 4.09 4.06 3.84 1.86 1.55 1.49 1.25 0.91 0.44 0.44 0.36 0.25 *0 42.55 42.62 42.14 43.21 43.21 43.14 42.96 42.89 42.79 42.83 42.83 43.22 43.08 42.63 43.03 43.00 11.57 10.20 12.19 11.14 11.03 10.86 10.51 10.91 11.14 10.45 11.59 11.51 11.19 11.00 8.82 10.09 H8-36 H9-3947.55 H9-27 46.3415.07 H8-45 14.84 47.71 H8-47 H9-44 47.48 16.54 H9-49 47.79 16.82 H8-50E 46.96 15.75 H9-37D H9-36C 16.26 46.97 H9-36A H8-75A 47.87 15.89 H9-42 46.07 16.13 H9-29 15.98 47.48 H8-70 47.42 16.03 H8-57 48.68 16.52 16.43 46.77 47.08 16.58 16.16 48.79 48.47 16.75 16.35 99.37 99.91 100.39 100.65 99.86 99.51 99.97 100.69 100.34 99.60 99.53 101.04 98.95 100.02 100.20 101.59 117.18 117.33 117.31 117.60 117.60 117.39 117.41 117.60 117.60 117.74 117.74 118.34 117.29 117.73 117.43 117.42 Major oxides in wt. % Trace elements in ppm Trace 0.70658 0.70713 0.70591 0.70457 0.70528 0.70502 0.70549 0.70533 0.70541 0.70476 0.70523 0.70462 0.70435 0.70479 0.70390 0.70486 Samples: Sr 3 3 86 2 5 2 O O O 2 O 2 2 O 2 2 Sr/ SiO Sr Y Zr Nb Ni Ga Cu Zn U Th Ba Ce Co La Cr V Sc Rb TiO Fe Al FeO MnO MgO CaO K Na L.O.I. TOTAL P 87 Latitude Longitude Age (Ma) Table 2. Continued. Table 267 8 Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province ————————— ————————— ————————— 1223 20 28 2665————————————— 26 1284————————————— 25 12 18111218—12171612121211 272339272740—29343527292726 26 1749————————————— 129 26 17 25 12 24 19 25 16 25 15 35 13 30 15 25 20 34 *0 *0 *0 *0 *0 *0 *0 *0 *0 *0 *0 *0 *0 *0 0.7————————————— 0.4————————————— 117 131 180 121 128 162 157 96 126 136 126 127 91 134 156 159 118 180 189 118 120 166 155 120 178 158 153 187 655 670 484 668 653262 486 215 498188 230208 586 202 240 209 528 251 212 239 502 187 251 207 675 204 240 223 671 264 254 226 585 245 238 227 655 208 244 207 289 259 191 261 252 207 194 277 209 178 225 210 187 196 200 226 2.16 2.21 1.86 2.11 2.16 1.78 1.79 2.11 1.72 1.69 2.11 2.21 2.17 2.17 2.318.550.17 1.819.29 8.979.93 0.16 1.52 9.00 8.44 9.94 2.41 0.17 8.34 8.24 10.45 0.17 2.47 8.95 8.29 9.40 1.75 0.17 7.84 9.11 9.94 0.16 2.21 8.62 7.60 9.98 0.16 2.92 8.53 7.47 10.26 2.04 0.15 7.46 8.53 9.26 0.15 2.25 9.13 7.22 9.85 0.16 4.03 9.00 10.23 6.88 4.16 0.16 6.81 8.74 9.27 0.17 3.85 8.51 9.60 6.62 0.15 6.05 8.83 9.68 5.30 0.17 9.46 9.84 0.290.15 0.32 0.40 0.39 0.57 0.31 0.51 0.30 0.69 0.41 0.80 0.41 0.58 0.37 1.19 0.43 0.31 0.37 0.58 0.29 1.10 0.32 0.51 0.36 0.63 0.30 0.34 3.08 2.93 3.03 3.21 3.04 2.67 3.01 2.85 3.07 2.80 3.10 3.39 3.26 2.86 0.69 0.72 1.02 0.68 0.69 1.00 1.00 0.71 0.96 0.94 0.71 0.74 0.70 0.69 17.918.1 — — — — — — — — 27.9 — — — — JC-4 H86-12 H86-10 H8-62 H86-1 JC-5 JC-3 H8-63 JC-30B JC-33 H8-64 85-28B H8-66 H86-2 43.15 43.15 43.15 43.15 43.15 43.15 43.15 43.15 43.16 43.15 43.15 43.15 43.15 43.15 47.53 47.7315.98 16.31 48.20 47.36 16.63 46.99 16.01 48.34 15.97 16.25 48.25 47.66 15.97 48.43 15.98 15.97 48.11 47.18 16.20 47.24 16.02 48.08 15.96 47.35 16.12 15.90 117.46 117.50 117.50 117.50 117.50 117.50 117.50 117.50 117.40 117.50 117.50 117.50 117.50 117.50 Major oxides in wt. % Trace elements in ppm Trace 0.70382 0.70385 0.70405 0.70384 0.70383 0.70398 0.70398 0.70385 0.70406 0.70401 0.70385 0.70382 0.70384 0.70383 Samples: 100.13 100.50 100.32 99.46 99.82 99.60 99.77 99.20 99.52 99.55 99.59 99.62 100.45 100.43 Sr 3 3 86 2 5 2 O O O 2 O 2 2 O 2 2 Sr/ SiO Al TiO Fe FeO MnO MgO CaO Na L.O.I. Total K P Rb Ni Ga Cu Sr Y Zr Nb Zn U Th Ba Ce Co La Cr V Sc 87 Longitude Age (Ma) Latitude Table 2. Continued. Table

268 Shoemaker and Hart—Basalt Genesis and Evolution on the Owyhee Plateau 9 activity, and some more evolved members representing indicative of differentiation from a common parental the earlier pulse of activity. magma. However, major element variations within each

Figures 5 and 6 illustrate the relationships between age group (Figure 5), such as decreasing SiO2 and CaO selected major and trace elements and MgO. MgO is used and increasing TiO2, K2O, and P2O5 with decreasing MgO, here as an indicator of overall degree of basalt differen- do suggest that fractional crystallization has played a sig- tiation. Given the wide range in ages, vent locations, and nificant role in Owyhee Plateau basalt evolution, particu- 87Sr/86Sr ratios of these basalts (Figures 3 and 4), there is larly in the two older (Steens Basalt) age groups. In addi- no reason to consider trends between age groups to be tion, the wide range of CaO, Al2O3, TiO2, K2O, and P2O5

Figure 5. Major element variations with MgO content. Symbols are explained in Figure 4. All oxides reported in weight percent. 269 10 Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province contents at the high MgO end of the spectrum suggests clinopyroxene removal was important during the pro- that variable depths and degrees of partial melting or the tracted crystallization experienced by pre-11 Ma basalts. melting of heterogeneous source lithologies, or both, have The large ion lithophile (Ba), light rare-earth (La), and exerted a primary control on basalt chemical signatures. high field strength (Zr) element variations also are con- The trace element characteristics illustrated in Figure sistent with variable degrees of crystallization, but the 6 support the above suggestions. For example, the corre- scatter in Ba and Zr above approximately 8 weight per- lated decreases in Ni and MgO within most age groups cent MgO suggests that multiple geochemical reservoirs and decreases in Sc and MgO within the 14-17.5 Ma group probably contributed to the overall geochemical patterns. suggest olivine fractionation was ubiquitous and hint that The most intriguing relationships are displayed in the Sr

Figure 6. Trace element variations with MgO content. Symbols are explained in Figure 4. All trace elements reported in ppm. 270 Shoemaker and Hart—Basalt Genesis and Evolution on the Owyhee Plateau 11 versus MgO plot. The basalt suite is divided into high Sr restricted to the area outlined in Figure 2b reveals an in- (greater than 400 ppm) and low Sr (less than 300 ppm) triguing relationship between the initial 87Sr/86Sr compo- groups. The high Sr samples span the entire observed sition and the age of eruption (Figure 8). The most radio- range of differentiation and include all of the younger genic Sr is associated with the basalts erupted between than 1 Ma mildly alkaline basalts and most of the older 11 and 6 Ma, coincident with the regional change in domi- than 11 Ma lavas. These relationships, particularly the nant basalt type erupted (Figure 2a). The systematic de- occurrence of high (approximately 650 ppm) and low crease in 87Sr/86Sr from this maximum to values less than (approximately 200 ppm) Sr content samples at identical or equal to 0.704 in both the oldest and youngest basalts high (approximately 9 weight percent) MgO contents, implies a decoupling between isotope and bulk chemical again suggest an important role for partial melting or characteristics. source composition complexities. The combined incompatible element characteristics of the least fractionated members of each eruptive age group (i.e., samples with high Mg2+/(Mg2++Fe2+) (Mg number), high Ni, and low SiO2) are illustrated in the NMORB normalized trace element diagram of Figure 7. These samples have similar overall patterns character- ized by strong relative enrichments in Ba and lesser rela- tive enrichments in Nb. The sample with the most MORB- like affinities from La through Y (less than 3 Ma HAOT) has the lowest concentrations of all elements shown, yet still has Sr, K, Rb, and particularly Ba concentrations in excess of NMORB. These characteristics have previously been suggested to indicate a back-arc basin tectonomagmatic setting (Hart and others, 1984; Hart, 1985). Although similar patterns are observed through Figure 8. Variations in Sr isotope composition of Owyhee Plateau basalts time, the absolute abundances appear to change as a func- with age of eruption. Symbols are explained in Figure 4. Samples plot- tion of time, as do many incompatible element ratios. ted in this figure include only those thought to be erupted from within These features suggest that similar heterogeneous magma the area outlined in Figure 2b, thus not all Oregon Plateau samples 87 86 sources and differentiation processes have been involved, plotted in Figure 3 are included. Note that the peak in Sr/ Sr around 11 Ma coincides with both the change in dominant basalt type erupted but to varying degrees as a function of time, in the evolu- and the onset of Oregon Plateau-wide lithospheric extension. tion of Owyhee Plateau basaltic magmatism. As shown in Figure 3, basalts from the Owyhee Pla- teau have a wide range in Sr-isotope values. Close ex- amination of these Sr-isotope characteristics for basalts DISCUSSION The data so far presented clearly illustrate that Owyhee Plateau basalt bulk chemical, trace element, and isotopic parameters vary as a function of age. These variations, while systematic, often are decoupled. For example, basalts with Mg number values around 60 span nearly the entire Sr isotopic range. The discussion below focuses on these issues in an attempt to understand the processes leading to the observed features. Figure 9 illustrates the relationships between the Sr isotopic composition and the Sr concentration and Rb/Sr ratios of the Owyhee Plateau basalts. Samples with Sr contents less than 300 ppm span a range of 87Sr/86Sr from 0.7043 to the highest value measured at 0.7072 (Figure 9a). These samples have Rb/Sr ratios between approxi- Figure 7. NMORB normalized spider diagram of trace element charac- mately 0.01 and 0.05 (Figure 9b). In contrast, samples teristics of samples representing the least fractionated members of each with Sr contents in excess of 400 ppm have Rb/Sr ratios age group. Symbols are explained in Figure 4. that range from 0.01 to 0.09 and a more limited range in 271 12 Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province

87Sr/86Sr from 0.7038 to 0.7055. In particular, post-1 Ma The above relationships do not rule out a role for felsic and pre-14 Ma high Sr concentration samples (with one upper crustal contamination that in the Owyhee Plateau exception) all have 87Sr/86Sr values less than 0.7045 and area could involve heterogeneous lithologies with 87Sr/ define within age group trends of nearly constant 87Sr/ 86Sr of 0.705 to greater than 0.710 (Leeman and others, 86Sr with increasing Rb/Sr. Similar trends with flat or 1992 and references therein). For example, contamina- slightly positive slopes in 87Sr/86Sr versus Rb/Sr are ob- tion of a low Sr concentration basaltic magma by felsic served for all other age groups with the exception of the material at the radiogenic end of this range certainly could 14-11 Ma Steens Basalts. Although defined by only three contribute to the high 87Sr/86Sr of the 11-6 Ma group. Such samples in Figure 9, the 14-11 Ma group occupies an a contaminant would likely have an elevated Rb/Sr ratio; intriguing “intermediate” position in terms of combined thus the low Rb/Sr ratios of the 11-6 Ma basalts are at Sr concentration, 87Sr/86Sr, and Rb/Sr characteristics. odds with significant felsic crustal addition. This inter- These characteristics may in part reflect mixing between pretation is extended to the remainder of the Owyhee an end member with high 87Sr/86Sr and low Sr and Rb/Sr Plateau basalt suite considering the observed within-group (11-6 Ma basalts) and an end member with low 87Sr/86Sr 87Sr/86Sr, Rb/Sr, and Sr concentration relationships. There- and high Sr and Rb/Sr (high Sr sample suite). fore, we interpret the relationships displayed in Figure 9 to indicate that felsic crustal contamination was not a dominant contributor to the overall geochemical charac- teristics observed, that distinct mantle source reservoirs were involved in Owyhee Plateau magma generation, and that magmas erupted just before the regional change in basalt geochemistry at approximately 11 Ma may pre- serve evidence for a combination of pre- and post-11 Ma source and process inputs. Additional combined chemical and Sr isotope char- acteristics of basalts from the Owyhee Plateau are illus- trated in Figure 10 in the context of the eruptive age groups. The decoupling of isotope and chemical char- acteristics is further highlighted by examining variations in differentiation indices such as Mg number (Figure 10a) and trace element ratios such as K/P and Zr/Nb (Figure 10b, 10c) relative to the age of eruption and Sr isotope compositions. The plot of Mg number versus age (Figure 10a) again illustrates the regional change from strongly fractionated basalts and basaltic andesites before about 11 Ma to rela- tively unfractionated basalts after about 11 Ma. The same Sr isotope composition, however, can occur over a wide range of Mg number values. This implies that upper crustal assimilation accompanying crystal fractionation exerted little influence on the observed Sr isotope com- positions. Thus, the first-order variations in isotopic com- position are likely due to variations in relative contribu- tions from different mantle sources through the course of Owyhee Plateau development. The K/P ratio (Figure 10b) has been used to monitor contamination of low K/P mafic magmas by high K/P upper crustal materials (Carlson and Hart, 1987). In the Owyhee Plateau basalt suite K/P shows the least crustlike signatures at the most radiogenic Sr isotope values, and Figure 9. Variations in Sr isotope composition with Sr concentration and Rb/Sr ratio in the context of the eruptive age groups. (a) Relation- vice versa. Thus, if crustal contamination is occurring, ship between 87Sr/86Sr and Sr concentration. (b) Relationship between either it is not the dominant factor controlling the chemi- 87Sr/86Sr and Rb/Sr ratio. Symbols are explained in Figure 4. cal characteristics of these basalts, or it is taking place at 272 Shoemaker and Hart—Basalt Genesis and Evolution on the Owyhee Plateau 13

Figure 10. Variations in Mg number, K/P, and Zr/Nb in Owyhee Plateau basalts with age and 87Sr/86Sr. Symbols are explained in Figure 4. depth where the crust is likely to be more “basaltic” in (SCLM) that subsequently interacted little with upper composition. The relationships may be best explained by crustal rocks or melts. The older and younger basalts may changes in the relative contributions of various source have a more complex history involving a larger contri- reservoirs. For example, the high 87Sr/86Sr values seen in bution from less radiogenic sublithospheric mantle the basalts erupted during the change in basalt geochem- sources plus relatively greater, but still minimal amounts istry (approximately 11 Ma) may represent melts derived of local crustal contamination. If this is the case, it is dominantly from the subcontinental lithospheric mantle significant that the peak in 87Sr/86Sr values and the change 273 14 Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province in basalt chemistry approximately coincide with the on- derived melt is now much less, melts derived from the set of Oregon Plateau-wide lithospheric extension (Hart SCLM exert a stronger control on the Sr isotopic compo- and Carlson, 1987; Hooper, 1990; Draper, 1991). Such sition and trace element characteristics of the resulting extension would significantly thin the lithosphere, allow- magmas (Figures 9, 10a, and 10c) ing decompression melting of the SCLM (McKenzie and Stage 3. The onset of Oregon Plateau-wide litho- Bickle, 1988; Gallagher and Hawkesworth, 1992). It spheric extension at about 11 Ma allows decompression would also significantly reduce the thickness of crust that melting of the SCLM, particularly easily melted mafic the magmas would have to traverse before eruption, re- constituents (Harry and Leeman, 1995). Basalts erupted sulting in less differentiated basalt chemistries. during the 11-6 Ma interval reflect the higher 87Sr/86Sr In contrast to K/P, Zr/Nb (Figure 10c) appears only values of the SCLM materials as melts derived therefrom to decrease slightly with decreasing age throughout the dominated over sublithospheric melts. Crustal residence full age spectrum. Zr and Nb should not be fractionated time and input are less significant during this interval due from each other during typical melting or crystallization to the active extension of already thinned crust. While processes in relatively anhydrous basaltic systems. While some degree of crustal contamination cannot be ruled out, this trend is also apparently decoupled from the trend in its influence on basalt chemistry is more cryptic. This 87Sr/86Sr values, the implications of this relationship are may reflect the involvement of more mafic crustal litholo- not as clear. Only a few outliers in the data set, however, gies present as a result of the previous pulses of flood approach normal MORB values (Zr/Nb = 30); most basalt activity. samples fall in between values for continental crust and Stage 4. With much of the easily melted portion of ocean island basalts (Weaver, 1991). These characteris- the SCLM removed, small volume magmas are produced tics may lend support to the presence of subduction-modi- that represent varying degrees of mixing between fied sublithospheric upper mantle beneath the Owyhee sublithospheric and SCLM melts, with the contribution Plateau (Carlson and Hart, 1987; Hart and others, 1997). from the SCLM lowest in the young, mildly alkaline The aforementioned relationships suggest that the basalts. Thus, basalts erupted during short intervals be- temporal development of rifting and extension in this tween 6 Ma and the present have more widely varying narrowly defined region of transitional lithosphere is ex- geochemical signatures. erting a strong control on (1) basalt source parameters, in Further testing and refinement of this model await particular the reservoirs involved in the genesis of these further detailed modeling of possible basalt source melt- magmas, and their relative contributions at various stages ing scenarios and melt regime physical parameters as well during the past 17.5 Ma; and (2) the subsequent mag- as Nd, Pb, and Os isotopic work on samples spanning the matic differentiation histories of these basalts. ranges shown in Figures 9-11. In addition, further field In light of these observations, the following prelimi- and analytical work is in progress south of the area shown nary model is offered and is keyed to the Sr isotope ver- in Figure 1b in a region where numerous basalt vent lo- sus age plot of Figure 11. calities have been identified. Stage 1. The initial, 17.5-14 Ma, large volume flood basalts are generated from hot, upwelling sublithospheric mantle. Although these magmas may be contaminated by small amounts of melt from the SCLM and by crustal lithologies, they retain their less radiogenic Sr isotopic signature because the volume of sublithospheric mantle- derived melt is so great. In addition, these basalts have Sr concentrations in excess of 400 ppm, thus further mini- mizing the effects of crustal contamination accompany- ing upper level differentiation on the Sr isotopic compo- sition. Significant residence time in crustal magma cham- bers is suggested by the evolved bulk compositions char- acteristic of this time interval and is consistent with the presence of thick crust at the onset of rifting. Stage 2. The waning stages of the Steens Mountain Basalt eruptions (14-11 Ma) reflect smaller volumes of melt being generated from the sublithospheric mantle. Figure 11. Preliminary model for the development of observed Sr iso- These melts interact with melts of the SCLM as before. tope and geochemical characteristics of Owyhee Plateau basalts. Sym- However, because the volume of sublithospheric mantle- bols are explained in Figure 4. 274 Shoemaker and Hart—Basalt Genesis and Evolution on the Owyhee Plateau 15

CONCLUSIONS et Cosmochimica Acta, v. 48, p. 2357-2372. Carlson, R.W., and W.K. Hart, 1987, Crustal genesis on the Oregon Pla- The observed geochemical and isotopic diversity in teau: Journal of Geophysical Research, v. 92, p. 6191-6206. Owyhee Plateau basalts apparently cannot be attributed ———, 1988, Flood basalt volcanism in the northwestern United States, in J.D. Macdougall, ed., Continental Flood Basalts: Kluwer Aca- solely to lateral lithospheric heterogeneities or to crustal demic Publishers, p. 35-61. contamination accompanying upper-level differentiation. Carlson, R.W., G.W. Lugmair, and J.D. Macdougall, 1981, Columbia Rather, this diversity appears to be a function of the age River volcanism: The question of mantle heterogeneity or crustal of eruption. Furthermore, the first-order processes respon- contamination: Geochimica et Cosmochimica Acta, v. 45, p. 2483- sible for the generation and evolution of these magmas 2499. Christiansen, R.L., and E.H. McKee, 1978, Late Cenozoic volcanic and are apparently controlled by temporal variations not only tectonic evolution of the Great Basin and Columbia intermontane in magmatic volume and lithospheric structure, but also regions, in R.B. Smith and G.P. Eaton, eds., Cenozoic Tectonics and in relative contributions from lithospheric and Regional Geophysics of the Western Cordillera: Geological Society sublithospheric source reservoirs. The results of this study of America Memoir 152, p. 283-311. emphasize the importance of time as a variable in evalu- Draper, D.S., 1991, Late Cenozoic bimodal magmatism in the northern Basin and Range Province of southeastern Oregon: Journal of Vol- ating the evolution of magmas, magmatic processes, and canology and Geothermal Research, v. 47, p. 299-328. magma source regions in complex continental igneous Elison, M.W., R.C. Speed, and R.W. Kistler, 1990, Geologic and isotopic provinces. constraints on the crustal structure of the northern Great Basin: Geo- logical Society of America Bulletin, v. 102, p. 1077-1092. Ferns, M.L., 1997, Field trip guide to the eastern margin of the Oregon- Idaho graben and the middle Miocene calderas of the Lake Owyhee ACKNOWLEDGMENTS volcanic field: Oregon Geology, v. 59, p. 9-20. Gallagher, Kerry, and Chris Hawkesworth, 1992, Dehydration melting The authors gratefully acknowledge financial support and the generation of continental flood basalts: Nature, v. 358, p. 57-59. from the Geological Society of America (Lipman Re- Geist, Dennis, and Mark Richards, 1993, Origin of the Columbia Plateau search Award, 5929-96; Shoemaker), the National Sci- and Snake River Plain: Deflection of the Yellowstone plume: Geol- ence Foundation (EAR-9204780; Hart), the Miami Uni- ogy, v. 21, p. 789-792. Harry, D.L., and W.P. Leeman, 1995, Partial melting of melt versity Graduate School Dissertation Research Support metasomatized subcontinental mantle and the magma source po- Fund (Shoemaker), the Miami University Committee on tential of the lower lithosphere: Journal of Geophysical Research, Faculty Research (Hart), and the Miami University Ge- v. 100, p. 10,255-10,269. ology Department. We would like to thank Stan Mertzman Hart, W.K., 1985, Chemical and isotopic evidence for mixing between depleted and enriched mantle, northwestern U.S.A.: Geochimica et of Franklin and Marshall College for making his XRF Cosmochimica Acta, v. 49, p. 131-144. facility available, and John Morton of Miami University ———, 1996, Petrogenesis of Quaternary Oregon Plateau alkaline basalts: for his assistance with the Sr isotope and DCP major and Geological Society of America Abstracts with Programs, v. 28, p. 73. trace element analyses. Our gratitude is also extended to ———, 1997, Plume-asthenosphere-lithosphere interactions through Matt Heizler of the New Mexico Geochronological Re- time: A comparison of Ethiopian and northwestern USA basalts: 40 39 Geological Society of America Abstracts with Programs, v. 29, search Laboratory for performing the Ar/ Ar analyses. p. 298. Reviews by Roger Stewart, Craig White, and Scott Hart, W.K., J.L. Aronson, and S.A. Mertzman, 1984, Areal distribution Hughes helped to clarify the content and improve the read- and age of low-K, high-alumina olivine tholeiite magmatism in the ability of this contribution. northwestern Great Basin: Geological Society of America Bulletin, v. 95, p. 186-195. Hart, W.K., and R.W. Carlson, 1983, K-Ar ages of late Cenozoic basalts from southeastern Oregon, southwestern Idaho, and northern Ne- vada: Isochron/West, v. 38, p. 23-26. 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