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Insights Into Rhyolite Magma Dome Systems Based on Mineral and Whole Rock Compositions at the Mono Craters, Eastern California

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ABSTRACT INSIGHTS INTO RHYOLITE MAGMA DOME SYSTEMS BASED ON MINERAL AND WHOLE ROCK COMPOSITIONS AT THE MONO CRATERS, EASTERN CALIFORNIA The Mono Craters magmatic system, found in a transtensional tectonic setting, consists of small magmatic bodies, dikes, and sills. New sampling of the Mono Craters reveals a wider range of magmatic compositions and a more complex storage and delivery system than heretofore recognized. Space compositional patterns, as well as crystallization temperatures and pressures taken from olivine-, feldspar-, orthopyroxene-, and clinopyroxene-liquid equilibria, are used to create a new model for the Mono Craters magmatic system. Felsic magmas erupted throughout the entire Mono Craters chain, whereas intermediate batches only erupted at Domes 10-12 and 14. Mafic magmas are spatially restricted, having erupted only at Domes 10, 12 and 14. Data from the new whole rock analyses illustrates a linear trend. Fractional crystallization does not replicate this trend but rather the linear trend indicates magma mixing. This study also analyzes samples from the Mono Lake Islands and the June Lake Basalts and compares them to the Mono Craters. Although the Mono Lake Islands fall into the intermediate to felsic group, they contain distinctly higher Al2O3 and Na2O at a given SiO2. Therefore, this study concludes that the Mono Craters represent a distinct magmatic system not directly related to the magmatic activity that created the Mono Lake Islands. Michelle Ranee Johnson May 2017 INSIGHTS INTO RHYOLITE MAGMA DOME SYSTEMS BASED ON MINERAL AND WHOLE ROCK COMPOSITIONS AT THE MONO CRATERS, EASTERN CALIFORNIA by Michelle Ranee Johnson A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology in the College of Science and Mathematics California State University, Fresno May 2017 APPROVED For the Department of Earth and Environmental Sciences: We, the undersigned, certify that the thesis of the following student meets the required standards of scholarship, format, and style of the university and the student's graduate degree program for the awarding of the master's degree. Michelle Ranee Johnson Thesis Author Keith Putirka (Chair) Earth & Environmental Sciences John Wakabayashi Earth & Environmental Sciences Christopher Pluhar Earth & Environmental Sciences For the University Graduate Committee: Dean, Division of Graduate Studies AUTHORIZATION FOR REPRODUCTION OF MASTER’S THESIS X I grant permission for the reproduction of this thesis in part or in its entirety without further authorization from me, on the condition that the person or agency requesting reproduction absorbs the cost and provides proper acknowledgment of authorship. Permission to reproduce this thesis in part or in its entirety must be obtained from me. Signature of thesis author: ACKNOWLEDGMENTS I would like to express gratitude to Keith Putirka for his knowledge, expertise, patience, and ability to push me past my self-imposed limits. I will always be thankful to John Wakabayashi for his enthusiasm helping me with my thesis and my AGU posters, even at the last minute. Many thanks go to Chris Pluhar for his aid in helping me with my grammar and his knowledge of the Mono Basin area. I appreciate Bernard Evans for his help and expertise with helping me understand my thin sections. I am immensely grateful for Margaret Mangan's interest in this project, her knowledgeable answers to my many questions, and allowing me to use the USGS electron microprobe. I am also thankful for her invitation and subsequent opportunity to speak with her and her associates at the USGS concerning this project. Thank you to the USGS scientists Dave Ponce, Darcy McPhee, Jared Peacock, and Amanda Pera McDonnell for sharing their expertise and their geophysical research on the Mono Craters and the surrounding area. Mae Marcaida, USGS, kindly shared several of her samples from the Mono Lake Islands for microprobe research. Tom Sisson, USGS, encouraged and helped me gain perspective. Mike Clynne, USGS, shared his knowledge in volcanology and willingly offered supportive words. It was an honor to meet Wes Hildreth, USGS, who has a remarkable ability to explain and clarify concepts. I am much obliged to Sarah Roeske at UC Davis and Leslie Hayden at the USGS for helping me with the electron microprobes at their locations. I also want to thank them for their knowledge and skill in the use of the machines and for their patience in teaching me how to use their equipment. v v Both Pete van der Water and Bob Dundas kindly helped me navigate graduation requirements. Kerry Workman-Ford is appreciated for her talent in making me smile when I needed a boost. Belinda Rossette, Sue Delcroix, and Dawn Moate thoughtfully assisted me when I needed to complete paperwork and meet deadlines. I also want to thank Sue Bratcher and Kellie Townsend for taking their time to aid me in printing my AGU posters. Many thanks to Sue Bratcher, Kellie Townsend, and Douglas Kliewer who helped me whenever there was a problem with the lab equipment and/or when supplies were needed in the lab. I am grateful for exceptional friends and colleagues whom I have made over the years at CSU Fresno who have influenced and counseled me throughout this journey. I am indebted to my personal friends who helped me collect samples out in the field over the course of 1.5 weeks of the 9 weeks in the field: namely Stuart Wilkinson (who helped me collect samples from Domes 17 & 18), Doug Nidever (Domes 6-8;13-14), Alicia Castro (Dome 19), Kyle Davis and Lindsay Mate (Domes 11&12), Nelly Sangrujiveth (Domes 25;27), and Keith Putirka, Gerardo Torrez, Andrew Wonderly (Dome 12). This paper would not have been possible without the unwavering support of my parents, Joseph and Ranee Johnson. I am obliged to Bouakham Sriri-Perez, who consistently provided sound advice and cheerfully encouraged me throughout the process. TABLE OF CONTENTS Page LIST OF TABLES ................................................................................................. vii LIST OF FIGURES ............................................................................................... viii INTRODUCTION .................................................................................................... 1 Geologic Background ........................................................................................ 6 METHODS ............................................................................................................. 15 Field Observation and Sample Collection ...................................................... 15 Whole Rock Geochemistry ............................................................................. 17 Electron Microprobe Analyses (EPMA) ......................................................... 18 Thermobarometry Analyses and Calculations ................................................ 19 RESULTS ............................................................................................................... 20 Whole Rock Geochemistry Analysis .............................................................. 20 Petrology ......................................................................................................... 27 Thermobarometry Analysis ............................................................................. 28 DISCUSSION ......................................................................................................... 37 CONCLUSION ...................................................................................................... 44 REFERENCES CITED .......................................................................................... 45 APPENDIX: SUPPLEMENTAL TABLES ........................................................... 53 LIST OF TABLES Page Table 1: The number of samples collected per each dome. ................................... 16 Table 2: Number of samples collected at the Mono Craters from Kelleher and Cameron (1990) study versus this newer study at the various textural and mineralogical groupings that Kelleher and Cameron (1990) and Wood (1983) named. ................................................................................ 38 Table A1: Whole rock geochemistry of the Mono Lake Islands (MLI), Mono Craters (MC), and June Lake Basalts (JLB). ........................................... 54 Table A2: Comparison of minerals and their major elements analyzed at UC Davis. ....................................................................................................... 59 Table A3: Comparison of minerals and their major elements analyzed at the USGS, Menlo Park, CA. .......................................................................... 60 Table A4: Clinopyroxene compositions (wt %) ..................................................... 61 Table A5: Orthopyroxene compositions (wt %) .................................................... 66 Table A6: Plagioclase compositions ...................................................................... 67 Table A7: Sanidine compositions ........................................................................... 74 Table A8: Olivine crystal compositions ................................................................. 78 Table A9: Petrology analysis ................................................................................. 80 LIST OF FIGURES Page Figure 1: Generalized map from Mono Lake in the North to Mammoth Mt and Long Valley Caldera in the South ............................................................
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