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Regional Tectonic Evolution of the Pioneer Core Complex, South-Central Idaho

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Regional Tectonic Evolution of the Pioneer Core Complex, South-Central Idaho REGIONAL TECTONIC EVOLUTION OF THE PIONEER CORE COMPLEX, SOUTH-CENTRAL IDAHO A Thesis submitted to the faculty of San Francisco State University In partial fulfillment of the requirements for the Degree Master of Science In Geoscience by Rui Liu San Francisco, California January 2017 CERTIFICATION OF APPROVAL I certify that I have read Regional Tectonic Evolution of the Pioneer Core Complex, South-Central Idaho by Rui Liu, and that in my opinion this work meets the criteria for approving a thesis submitted in partial fulfillment of the requirement for the degree Master of Science in Geoscience at San Francisco State University. REGIONAL TECTONIC EVOLUTION OF THE PIONEER CORE COMPLEX, SOUTH-CENTRAL IDAHO Rui Liu San Francisco, California 2017 The Pioneer core complex (PCC) is one of lesser-studied Cordilleran metamorphic core complexes (MCCs) located north of the Snake River Plain in the Rocky Mountain region of the western U.S.. Based on analyses of zircon geochronology, Ti thermometry and thermodynamic modeling, the tectonic evolution of the PCC followed a clockwise P-T-t- d path with prograde to peak conditions of 4 kbar and 860°C from 52 to 48 Ma, followed by retrograde metamorphism to 2 kbar and 590°C at 43 Ma. The prograde metamorphism (corresponding to D l) of the PCC began near the start of the Sevier orogeny (before 109 Ma) in the Late Cretaceous as the Farallon plate began to subduct eastward beneath North America. The PCC underwent compressional deformation during the Sevier orogeny at 86-84 Ma (D2) when the subducting Farallon plate flattened and deformation verged eastward. The Eocene Challis magmatism and extension in the PCC (52-43 Ma, corresponding to D3) likely resulted from slab rollback of the Farallon plate and asthenospheric upwelling, under a stalled, accreted microplate (Siletzia). I certify that the Abstract is a correct representation of the content of this thesis. ACKNOWLEDGEMENTS This research was supported by the Pestrong Fund in the Earth & Climate Sciences Department at SF State, and a National Science Foundation CAREER grant (EAR 0847721 to Mary Leech) jointly supported by the Tectonics, Petrology & Geochemistry, and Education & Human Resources programs, for field and lab work. Russell McArthur, Joseph Verdian, Claire Zurkowski, Brandon Swanson, Beth Holmes, Jesse Waco, Nolen Brown, Salvatore Romano provided great assistance in field work. Andrew Nieblas and Jackson Reeder offered valuable help in lab work. TABLE OF CONTENTS List of Tables........................................................................................................................... vii List of Figures.........................................................................................................................viii List of Appendices....................................................................................................................ix Introduction................................................................................................................................. 1 The Pioneer Mountains metamorphic core complex..................................................1 Previous results.......................................................................................................................... 2 Methods.......................................................................................................................................4 Perple_X thermodynamic modeling............................................................................5 Zircon U-Pb dating....................................................................................................... 6 Ti-in-zircon and Ti-in-biotite thermometry................................................................ 9 Sample descriptions................................................................................................................. 11 Lower plate.................................................................................................................. 11 Middle plate.................................................................................................................13 Analytical results..................................................................................................................... 15 Zircon U-Pb ages.........................................................................................................15 Ti-in- zircon and Ti-in-biotite thermometry............................................................. 16 Discussion.................................................................................................................................17 Zircon U-Pb ages.........................................................................................................17 Ti-in-zircon and Ti-in-biotite thermometry.............................................................. 19 v Thermodynamic modeling......................................................................................... 21 Regional tectonic events during the evolution of the Pioneer core complex 23 Cordilleran metamorphic core complexes in proximity to the PCC.......................25 Conclusion................................................................................................................................28 References.................................................................................................................................29 Tables and figures....................................................................................................................37 Appendices................................................................................................................................57 LIST OF TABLES Table Page 1. PCC sample locations....................................................................................... 37 2. Whole rock chemistry data for PCC samples...................................................38 3. U-Th-Pb zircon data for PCC samples............................................................ 39 LIST OF FIGURES Figures Page 1. Location map of the Pioneer core complex....................................................... 41 2. Simplified geological map of the Pioneer core complex.................................. 42 3. Field photos of lithology and structures in the Lower and Middle plates in Pioneer core complex.......................................................................................................... 43 4. Petrographic photomicrographs of Lower plate gneiss and phyllite and quartzite from the Middle plate.............................................................................................46 5. U-Pb concordia diagrams from zircons from selected samples....................... 49 6. Plots of Ti-in-zircon and Ti-in-biotite thermometries........................................52 7. Relationship between mineral crystallization, metamorphic phases and deformation events....................................................................................................................... 53 8. Pseudosection of sample PC-10 andalusite phyllite...........................................54 9. Maps and cross sections of northwestern U.S at 60, 50,40, and 0 Ma............56 LIST OF APPENDICES Appendix Page 1. Summary of ages in the Pioneer core complex from literature...........................57 2. Ti-in-zircon thermometry results for selected samples.........................................60 3. Ti-in-biotite thermometry results for selected samples.........................................61 4. Isopleths of X(Mg) on pseudosections for selected samples................................73 ix 1 INTRODUCTION The Cordilleran metamorphic core complexes (MCCs) are exposures of high- grade, deep crustal rocks exhumed from beneath low-angle detachment faults and a brittle upper plate, that formed during crustal rifting in the Basin and Range province (Armstrong, 1982; Whitney et al., 2013). MCCs are composed of greenschist- to upper amphibolite-facies metamorphic rocks, and they are frequently associated with granitoid intrusions (Haney, 2008). The MCCs of the Basin and Range comprise a belt extending from southern British Columbia, Canada, southward through the western U.S. to Mexico, in the western hinterland of the Cretaceous-Paleogene Sevier-Laramide fold-and-thrust belt (e.g., Armstrong, 1982; Silverberg, 1990; Vogl et al., 2012). The MCCs underwent Mesozoic compressional deformation during the Cretaceous Sevier-Laramide orogeny when the subducting Farallon plate flattened beneath North America; this was followed by Cenozoic extension related to roll-back of the Farallon plate. The extension taking place in the thinned Basin-and-Range crust also triggered volcanism that is associated with the emplacement of plutonic rocks in the Cordilleran core complexes (e.g., Armstrong, 1982; Janecke, 1992, 1994; Lee Armstrong and Ward, 1991; O’neill and Pavlis, 1988; Silverberg, 1990; Vogl et al., 2012; Wust, 1986). The Pioneer Mountains metamorphic core complex The Pioneer core complex (PCC) is one of the lesser-studied Cordilleran core complexes, that is located north of the Snake River Plain in the Rocky Mountains. The 2 complex resides in a transitional zone separating Eocene MCCs to the north and Oligocene-Miocene MCCs to the south (Fig. 1; Armstrong, 1982; Silverberg, 1990; Vogl et al., 2012). The PCC comprises a fan-shaped NW-SE-trending dome that is bounded by the Wildhorse Detachment fault and the high-angle White Mountains
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