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A Mid-Miocene Volcanotectonic Depression, North-Central Mccullough Mountains, Southern Nevada

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UNLV Theses, Dissertations, Professional Papers, and Capstones 12-1991 The Sloan Sag: A Mid-Miocene Volcanotectonic Depression, North-Central McCullough Mountains, Southern Nevada Hayden L. Bridwell University of Nevada, Las Vegas Follow this and additional works at: https://digitalscholarship.unlv.edu/thesesdissertations Part of the Geochemistry Commons, Geology Commons, and the Volcanology Commons Repository Citation Bridwell, Hayden L., "The Sloan Sag: A Mid-Miocene Volcanotectonic Depression, North-Central McCullough Mountains, Southern Nevada" (1991). UNLV Theses, Dissertations, Professional Papers, and Capstones. 1111. http://dx.doi.org/10.34917/2493548 This Thesis is protected by copyright and/or related rights. It has been brought to you by Digital Scholarship@UNLV with permission from the rights-holder(s). You are free to use this Thesis in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s) directly, unless additional rights are indicated by a Creative Commons license in the record and/ or on the work itself. This Thesis has been accepted for inclusion in UNLV Theses, Dissertations, Professional Papers, and Capstones by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. THE SLOAN SAG: A MID-MIOCENE VOLCANOTECfONIC DEPRESSION, NORTH-CENTRAL McCULLOUGH MOUNTAINS, SOUTHERN NEVADA by Hayden L. Bridwell A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology Department of Geoscience University of Nevada, Las Vegas December, 1991 • I ACKNOWLEDGEMENTS I would like to thank Dr. Gene Smith for keeping me enthused about this project. I feel as though he has taught me more about geology in the past two years than I have learned in all of my previous geologic studies combined. Tracey Cascadden, Dan Feurebach and Terry Naumann also contributed substantially to this work. The countless converstions we shared added greatly to my understanding of volcanology and the dynamic geology of the Basin-and-Range province. With great emotion, I thank my son Ian for helping me keep my perspective over the past two years. 11 ABSTRACT In the Hidden Valley area of the north-central McCullough Mountains, southern Nevada, mid-Miocene andesite and dacite domes, flows and pyroclastic units (the Sloan volcanics) partially fill a sag in the underlying Hidden Valley volcanics. The 13.5 km diameter sag formed during and/or after the eruption of the Sloan volcanics. Sagging was accommodated by a combination of movement on the McCullough Wash fault system, and subsidence into evacuated chambers. Major, trace and rare-earth element geochemistry suggests that the rocks of the Sloan volcanics belong to four groups, each of which were produced by partial melting of chemically distinct sources. With the exception of the Center Mountain dome complex, magmas rose rapidly without significant crystal fractionation or crustal contamination. The Mount Hanna andesite member of the Sloan volcanics erupted as a hot, dry aphyric lava by a mechanism of fire-fountaining from a depth of up to 25 km, precluding a Plinian-style ash-flow event. Eruptions of felsic-to-intermediate lavas by a lava-fountaining event have been described in other areas, but the Mount Hanna andesite represents the first documentation of such an eruption in the southern Basin-and-Range. iii CONTENTS INTRODUCITON . 1 Location and Geography . 1 Purpose of Srudy . 3 Geologic Setting . 4 Structural Geology . 4 Intermediate Volcanism . 8 CALDERAS: A GENERAL DISCUSSION . 10 ANALYTICAL METHODS . 14 GEOCHRONOLOGY . 15 THE SLOAN VOLCANICS . 20 Introduction . 20 Pre-Sloan Volcanic Stratigraphy . 20 Stratigraphy of the Sloan Volcanics . 23 Characteristics of the Sloan Volcanic Centers . 26 Petrography . 33 GEOCHEMISTRY . 38 Metasomatism . 38 Major Elements . 38 Trace Elements . 44 Temporal and Spatial Variations in Chemical Trends . 53 MAGMA PETROGENESIS . 64 Variations Between Groups of the Sloan Volcanics . 64 Introduction . 64 Magma Commingling . 64 Fractionation and Partial Melting . 66 Variations Within Groups of the Sloan Volcanics . 69 Introduction . 69 Andesites . 69 Dacites . 70 The Tuff of the Sloan Volcanics . 71 Sources for Partial Melting . 71 Introduction . 71 Partial Melt Models . 74 Discussion . 76 Crustal Contamination . 81 . IY THE SLOAN SAG . 82 Evidence for a Sag in Hidden Valley . 82 Introduction . 82 The Hidden Valley Sag . 82 Conclusion . 83 Classic Caldera, or Volcanotectonic Depression? . 84 Introduction . 84 Discussion . 84 Significance of the Mount Hanna Andesite . 85 Introduction . 85 Origin of the Mount Hanna Andesite . 86 Depth of Magma Generation Versus Depth of Eruption . 91 Formation of the Sloan Sag . 91 Introduction . 91 The McCullough Wash Fault System . 92 Sagging and Tectonic Control . 93 Summary of Events . 94 CONCLUSIONS . 97 FUTURE WORK . 98 REFERENCES CITED . 100 APPENDIX A: SAMPLE LOCATION AND PETROGRAPHY . 107 APPENDIX B: CHEMISTRY TABLES . 127 APPENDIX C: CIPW NORMS................................. 138 APPENDIX D: ROCK MODES . 142 FIGURES 1: Location Map . 2 2: Location of Major Faults in the Lake Mead-Las Vegas Area . 6 3: Collapse Caldera Sequence . 12 4: Ar-Ar Date Data . 18 5: Stratigraphic Column of the North-Central McCullough Mountains . 21 6: Stratigraphic Column of the Sloan Volcanics . 24 7: Photo of the Mount Hanna Andesite Vent Center . 27 8: Sample Location Map . 29 9: Photo of Hydroclastic Tuff at Station 73 . 30 10: Photo of Mount Ian Andesite Hydroclastic Breccia at Station 120 . 31 11: Photo of Platy, Foliated Mount Ian Andesite . 32 12: Photo of Hypabyssal Stock in the Northern Mount Sutor Dacite Field . 34 v 13: Harker Variation Diagrams . 39 14: AFM Diagram . 43 15: Harker Variation Diagrams Showing the Four Chemically Distinct Groups of the Sloan Volcanics . 45 16: Trace Element Diagrams . 46 17: Chondrite-Normalized REE Diagrams . 54 18: Chondrite and MORB-Normalized Spider Diagrams . 56 19: Chondrite and MORB-Normalized Spider Diagrams of the Sloan Andesites and the Hidden Valley Volcanics . 60 20: Chondrite and MORB-Normalized Spider Diagrams of Tertiary Volcanic Rocks in the Lake Mead-Las Vegas Area . 62 21: Cbondrite-Normalized REE Diagram for Melt Model #1: Sloan Andesite From Gabbro . 75 22: Cbondrite-Normalized REE Diagram for Melt Model #2: Sloan Dacite From Gabbro . 78 23: Chondrite-Normalized REE Diagram for Melt Model #3: Sloan Andesite and Dacite From Peridotite (2X Chondrite) . 79 24: Chondrite-Normalized REE Diagram for Melt Model #4: Sloan Andesite and Dacite From Peridotite (Metasomatized Mantle Chondrite Values) . 80 25: Microphoto of the Mount Hanna Andesite . 87 26: Photo of the Streaky, Discontinuous Texture of the Mount Hanna Andesite . 89 27: Formation Sequence of the Sloan.
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