MIAMI UNIVERSITY The Graduate School Certificate for Approving the Dissertation We hereby approve the Dissertation of Matthew E. Brueseke Candidate for the Degree: Doctor of Philosophy _______________________________________ William K. Hart, Director ________________________________________ Alan R. Wallace, Reader ________________________________________ Elisabeth Widom, Reader ______________________________________ Brian S. Currie ______________________________________ Michael R. Brudzinski ______________________________________ Michael J. Pechan, Graduate School Representative ABSTRACT MID-MIOCENE MAGMATIC SYSTEM DEVELOPMENT IN THE NORTHWESTERN UNITED STATES By Matthew E. Brueseke This dissertation investigates the spatial, temporal, geochemical, and petrologic development and evolution of mid-Miocene volcanic systems in the southeastern Oregon Plateau region of Oregon and Nevada. This integrated field and laboratory investigation conclusively demonstrates that flood basalt volcanism occurred on the Oregon Plateau over at least a 2 m.y. duration, and provides the first comprehensive view into the development of a mid-Miocene Oregon Plateau volcanic field and its relationship with regional flood basalt volcanism. The first portion of this study focuses on the geochemical and chronostratigraphic characteristics of flood basalt lava flows in the vicinity of Steens Mountain, Oregon. New 40Ar/39Ar ages and recalculated literature ages from the Steens Basalt type section illustrate that multiple magmatic centers were present locally, and that Oregon Plateau flood basalt activity was coeval with the main phase of Columbia River Basalt Group volcanism. The remainder of this study focuses on the Santa Rosa-Calico volcanic field (SC) of northern Nevada in order to better define and understand the link between mid-Miocene Oregon Plateau mafic and silicic volcanism. In the SC, mafic through silicic eruptive loci and shallow intrusive bodies are exposed along broadly north-south trending alignments, coincident with regional lithospheric structures. At least sixteen physically and compositionally distinct units are exposed in the SC representing approximately 2 m.y. of magma production. Local mafic volcanism was dominated by the eruption of Steens Basalt magmas. SC silicic magmas were produced by basaltic magma induced crustal melting of granitoid upper crust and erupted from diverse vent types and locations. At least four distinct intermediate (andesite-dacite) magmatic systems also are documented. Physical, chemical, and isotopic data indicate that open-system petrogenetic processes played a substantial role in the generation of these magmas and also influenced the chemical characteristics of SC mafic and silicic units. The complex array of physical and chemical characteristics and processes displayed and documented by SC units provide an exceptional example of how compositionally diverse volcanic fields develop. Moreover, these complexities establish an important link between regional mid-Miocene mafic magma production and the generation of silicic-dominated volcanic fields on the Oregon Plateau. MID-MIOCENE MAGMATIC SYSTEM DEVELOPMENT IN THE NORTHWESTERN UNITED STATES A DISSERTATION Submitted to the Faculty of Miami University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Geology by Matthew E. Brueseke Miami University Oxford, Ohio 2006 Dissertation Director: William K. Hart, Ph.D. TABLE OF CONTENTS Chapter 1: Introduction 1 References 4 Chapter 2: Distribution and Geochronology of 6 Oregon Plateau (U.S.A.) Flood Basalt Volcanism: The Steens Basalt Revisited Abstract 7 Body Text 8 References 56 Appendix A: Appendix A: Argon Data Repository 63 Appendix B: Whole-Rock Major and Trace Element Data 71 Appendix C: Sample Locations and Descriptions 85 Chapter 3: Diverse mid-Miocene Silicic Volcanism Associated 100 with the Yellowstone-Newberry Thermal Anomaly Abstract 101 Body Text 102 References 124 Appendix 1: 40Ar/39Ar Analyses of Single Crystal 157 Sanidines From Volcanic Rocks From the Santa Rosa-Calico Volcanic Field Chapter 4: Geology and Petrology of the Mid-Miocene 172 Santa Rosa-Calico Volcanic Field, Northern Nevada Body Text 172 References 321 Appendix 1: Analytical Methods 335 Appendix 2: Sample Locations and Descriptions 344 Appendix 3: Major, Trace, and Isotope Data 379 Appendix 4: 40Ar/39Ar Geochronology 412 Chapter 5: Summary and Suggestions for Future Research 425 ii LIST OF TABLES Chapter 2: Distribution and Geochronology of 6 Oregon Plateau (U.S.A.) Flood Basalt Volcanism: The Steens Basalt Revisited 1 - Summary of 40Ar/39Ar Methods 29 2 - Summary of 40Ar/39Ar Age Results 30 3 - Summary of New and Published Oregon Plateau 31 Flood Basalt Radiometric Ages 4 - Major and Trace Element Geochemical Data 32 for Dated Oregon Plateau Lava Flows Chapter 3: Diverse mid-Miocene Silicic Volcanism Associated 100 with the Yellowstone-Newberry Thermal Anomaly 1 - Summary of Sanidine 40Ar/39Ar Results for 155 Santa Rosa-Calico Volcanic Field Silicic Units 2 - Representative Major and Trace Element Geochemical 156 Data for Santa Rosa-Calico Volcanic Field Silicic Units Chapter 4: Geology and Petrology of the Mid-Miocene 172 Santa Rosa-Calico Volcanic Field, Northern Nevada 1 - Summary of Assigned 40Ar/39Ar Ages 312 2 - Summary of SC Eruptive Units 313 3 - Representative Major and Trace Element Geochemical 314 Data for Santa Rosa-Calico Units Flood Basalt Radiometric Ages 4 - Results of Two Component Mixing Calculations for 320 Two SC Samples iii LIST OF FIGURES Chapter 2: Distribution and Geochronology of 6 Oregon Plateau (U.S.A.) Flood Basalt Volcanism: The Steens Basalt Revisited 1 - Cenozoic Volcanic and Tectonic Features of the Northwest USA 33 2 - 40Ar/39Ar ages and K/Ca Spectra for Newly Dated Lava Flows 35 From Stratigraphic Sections Peripheral to Steens Mountain 3 - 40Ar/39Ar age and K/Ca Spectra and Isotope Correlation Diagrams 38 For Dated Lava Flows From Steens Basalt at Steens Mountain 4 - 40Ar/39Ar age and K/Ca Spectra Diagrams for Low Resolution 40 Laser Heating Analyses for Steens Basalt at Steens Mountain Sample CH82-22G 5 - 40Ar/39Ar age and K/Ca Spectra Diagrams for Low Resolution 42 Laser Heating Analyses for Steens Basalt at Steens Mountain Sample CH82-22B 6 - Petrographic and Stratigraphic Relationships of Sampled Sections 44 7 - Total Alkali vs. Silica Diagram and Plot of Wt. % TiO2 vs. P2O5 46 8 - Chemostratigraphic Variation Diagrams From Sampled 48 Stratigraphic Sections, Discussed in this Study 9 - Chemostratigraphic Variation of the Steens Basalt at 50 Steens Mountain 10 - Select Major and Trace Element Variations of Oregon Plateau 52 Flood Basalt Lava Flows 11 - Stratigraphic Variation of Oregon and Columbia Plateau 54 Flood Basalt Units Chapter 3: Diverse mid-Miocene Silicic Volcanism Associated 100 with the Yellowstone-Newberry Thermal Anomaly 1 - Cenozoic Volcanic and Tectonic Features of the Northwest USA 131 2 - Satellite Imagery and Generalized Geologic Map of the 133 Santa Rosa-Calico Volcanic Field iv 3 - Generalized Stratigraphy of the SC 135 4 - Photographs of SC Silicic Units and Features 137 5 - Mineralogical Characteristics of SC Silicic Units 139 6 - Textural Characteristics of Select SC Silicic Units 141 7 - Coyote Mountain Ash-Flow Tuff Stratigraphy 143 8 - Cold Springs Tuff Exposures From the Central SC 145 9 - Total Alkali vs. Silica Diagram and Plot of A/NK vs. A/CNK 147 10 - Major and Trace Element Variations for Western SC Silicic Units 149 11 - Major and Trace Element Variations for Eastern SC Silicic Units 151 12 - Steens Basalt Normalized Multi-Element Diagrams of 153 SC Silicic Units Chapter 4: Geology and Petrology of the Mid-Miocene 172 Santa Rosa-Calico Volcanic Field, Northern Nevada 1 - Cenozoic Volcanic and Tectonic Features of the Northwest USA 233 2 - Satellite Image and Map of the Santa Rosa-Calico Volcanic Field 235 3 - Generalized Stratigraphy of the SC 237 4 - Photographs of SC Units and Geologic Features 239 5 - Total Alkali vs. Silica Diagram and Plot of Wt. % K2O vs. SiO2 243 6 - Satellite Imagery and Generalized Geologic Map of the 245 Santa Rosa-Calico Volcanic Field 7 - Photomicrographs of Common SC Disequilibrium Textures 248 8 - Photomicrographs of Metamorphic Xenolith in a Tad1 Lava Flow 250 9 - Idealized Ash Flow (Ignimbrite) Flow Unit 252 10 - Coyote Mountain Ash-Flow Tuff Stratigraphy 254 v 11 - Cold Springs Tuff Composite Stratigraphic Section 256 12 - Photographs of SC Tuffaceous Units 258 13 - Plots of ppm Sr vs. Wt.% Silica, Tholeiitic vs. Calc-Alkaline 260 Discrimination Diagram, and A/NK vs. A/CNK 14 - Harker Diagrams Illustrating Major and Trace Element 262 Characteristics of SC Mafic to Intermediate Units 15 - MORB Normalized Trace Element Variation Diagram 265 for SC Mafic and Intermediate Units 16 - Chondrite Normalized REE Diagrams for All SC Units 267 17 - MORB Normalized Comparison of Regionally and 269 Locally Erupted Steens Basalt 18 - Harker Diagrams Illustrating Major and Trace Element 271 Characteristics of SC Mafic to Intermediate Units 19 - Upper Continental Crust Normalized Trace Element 274 Variation Diagram for SC Silicic Units 20 - Initial Sr and Nd Isotopic Characteristics of SC Units 276 21 - Initial Sr and Nd Isotopic Characteristics of SC Units vs. 278 Wt. % Silica 22 - Variations in Pb Isotopic Compositions of SC Units 280 23 - Combined Sr, Nd, and Pb Isotope Characteristics of SC Units 282 24 - Sr and Nd Isotopic Characteristics of Locally Exposed 284 Mesozoic and Younger Units