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The Geology of the Tuff of Bridge Spring: Southern Nevada and Northwestern Arizona

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The Geology of the Tuff of Bridge Spring: Southern Nevada and Northwestern Arizona UNLV Theses, Dissertations, Professional Papers, and Capstones 12-1993 The geology of the Tuff of Bridge Spring: Southern Nevada and northwestern Arizona Shirley Ann Morikawa University of Nevada, Las Vegas Follow this and additional works at: https://digitalscholarship.unlv.edu/thesesdissertations Part of the Geochemistry Commons, Geology Commons, Stratigraphy Commons, and the Volcanology Commons Repository Citation Morikawa, Shirley Ann, "The geology of the Tuff of Bridge Spring: Southern Nevada and northwestern Arizona" (1993). UNLV Theses, Dissertations, Professional Papers, and Capstones. 1453. The Geology of the TufT of Bridge Spring: Southern Nevada and Northwestern Arizona by Shirley Ann Morikawa A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology Geoscience Department University of Nevada, Las Vegas December, 1993 © 1994 Shirley Ann Morikawa All Rights Reserved The thesis of Shirley Ann Morikawa for the degree of Master of Science in Geology is approve . r, Ernest M. Duebendorfer, Ph.D. Graduate Faculty Representaf , Donna E.Weistrop, Ph.D. Dean of the Graduate College, Ronald W. Smith, Ph.D. University of Nevada, Las Vegas December, 1993 ii Abstract The Tuff of Bridge Spring (TBS) is a regionally-widespread, andesite to rhyolite (59 .50 to 74.91 wt. %) ash-flow tuff of mid-Miocene age (ca. 15.2 Ma) that is exposed in the northern Colorado River extensional corridor of southern Nevada and northwestern Arizona. Determination of the areal distribution, geochronology, lithology, geochemistry, and internal stratigraphy of the TBS is important for its establishment as a reliable stratigraphic reference horizon for tectonic reconstructions of the extensional corridor during the middle Miocene. Based on reoccurring patterns of major and trace element variation, the TBS is divided into constant Cr/Variable Si02 and variable Cr/variable Si~ chemical members. Reconciliation of chemical member assignments and regional stratigraphic relationships allows the division of the TBS into three stratigraphic members. The regionally-extensive nature of a Zr/Ti vs. Ba chemical horizon in the TBS suggests that its chemical signature is magmatic in origin. The presence of linear isotopic arrays of Nd/Rb/Pb plots, regionally-consistent geochemical trends, and disequilibrium textures in feldspars in the TBS suggests it was formed by magma mixing processes which involved the injection of mafic magma into a normally-zoned felsic magma chamber. The Nd/Rb/Pb isotopic signature of the Tuff of Bridge Spring suggests that the TBS may be cogenetic with either the Aztec Wash pluton, Nevada or the Mt. Perkins pluton, Arizona. Comparison of the isotopic signatures of the TBS with tuffs collected from Salt Spring Wash, Arizona, and the Lucy Gray Range, Nevada suggests that these tuffs are not co genetic with the TBS. Incremental release 40Ar;39 Ar analysis of the tuff of Dolan Springs (16.09 ±0.15 Ma) suggests it was derived from a different source than the TBS. ill Contents Abstract ..................................................................................................................... iii . F•gures ....................................................................................................................... 'VI Tables ......................................................................................................................... viii Acknowledgments ...................................................................................................... ix Introduction ............................................................................................................... 1 The Tuff of Bridge Spring: A Brief Description ................................................ 3 Regional Setting: The Northern Colorado River Extensional Corridor .............. 5 History of Extension ............................................................................. 5 Regional Volcanic/Plutonic Stratigraphy ............................................... 6 Instrumental Techniques .......................................................................................... 12 Analytical Techniques ...................................................................................... 12 Whole Rock Vs. Pumice Sampling ................................................................... 14 Analytical Data .......................................................................................................... 20 Major and Trace Element Geochemistry ........................................................... 20 Tuff of Bridge Spring Geochemistry ..................................................... 21 Other Ash-flow Tuffs ............................................................................ 24 Petrologic Descriptions .................................................................................... 25 Regional Correlation ..................................................................................... :........... 61 Isotopic Correlation of the Tuff of Bridge Spring ............................................. 61 Interstate-15 ......................................................................................... 63 Other Ash-flow Tuffs ....................................................................................... 64 Salt Spring Wash and Lucy Gray Range ................................................ 64 Dolan Springs ....................................................................................... 64 Origin of the Tuff of Bridge Spring Data Array ................................................ 65 Lithic Contamination ........................................................................................ 66 Patsy Mine Volcanics ........................................................................... 66 Alkali Olivine Basalts ............................................................................ 68 Precambrian Basement .......................................................................... 69 Mix.ing Trend ................................................................................................... 70 The Role of Contaminants in the Genesis of the Salt Spring Wash and Lucy Gray Range Tuffs .................................................................................... 70 iv Internal Stratigraphy ................................................................................................ 80 Introduction ..................................................................................................... 80 Eruptive Units .................................................................................................. 82 Chemical Criteria .................................................................................. 82 Field Evidence ...................................................................................... 83 Chemical Members of the Tuff of Bridge Spring ............................................... 86 Regional Members of the Tuff of Bridge Spring ............................................... 88 Fine Scale Chemical Variation .......................................................................... 89 Magma Chamber Processes .............................................................................. 91 Location of Source ..................................................................................................... 109 Introduction ..................................................................................................... I09 Radiogenic Isotopes ......................................................................................... 111 Geochronology ................................................................................................ 112 Discrepancies in Tuff of Bridge Spring Geochronology ......................... 113 Comparative Geochronology ................................................................ 114 Geochemistry ................................................................................................... 114 Chemical path correlation ..................................................................... 115 Field and Petrologic Studies ............................................................................. 117 Field Indicators ..................................................................................... 118 Location of Pluton ................................................................................ 119 Magmatic Mixing Textures ................................................................... 119 The Dolan Springs Volcanic Section ................................................................ 120 Summary and Conclusion ................................................................................. 121 Summary and Conclusion ......................................................................................... 127 Future Work .................................................................................................... 131 References Cited ........................................................................................................ 132 Appendix A: Major and Trace Element Analyses ........................................................ 139 Appendix B: Thin Section Point Counts ..................................................................... 145 Part I: Whole Rock Modal Analyses ................................................................. 145 Part II: Phenocryst Modal Analyses .................................................................
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