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The Tectonomagmatic Evolution of the Late Cenozoic Owyhee Plateau, Northwestern United States

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The Tectonomagmatic Evolution of the Late Cenozoic Owyhee Plateau, Northwestern United States 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 UNITED STATES by Kurt A. Shoemaker The Owyhee Plateau (OWP) is an intermontane basalt plateau located at the nexus of the Snake River Plain (SRP) and Oregon 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-Idaho 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 basalts 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
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