Source and Magma Evolution of the tuff of Elevenmile Canyon, Stillwater Range, Clan Alpine and northern Desatoya Mountains, western Nevada. Daniel Stepner A thesis submitted to the Faculty of Graduate and Postdoctoral Studies in partial fulfillment of the requirements for the degree of: Master of Science In Earth Sciences University of Ottawa Daniel Stepner, Ottawa, Canada, 2017 Abstract The tuff of Elevenmile Canyon (TEC) is a 25.1 Ma trachydacite to rhyolite intracaldera tuff produced by the largest of 6 Oligocene overlapping calderas that, along with related plutons, constitute the Stillwater Caldera Complex, one of the largest eruptions of the Western Nevada Volcanic Field during the mid-Tertiary ignimbrite flare-up. Typically crystal-rich with a mineral assemblage of plagioclase > quartz sanidine > biotite ± hornblende and clinopyroxene, there are two discernable pumice types throughout the tuff: a lighter crystal-rich pumice and a darker, commonly aphyric pumice type. Rb-Sr and Sm-Nd isotopic compositions of pumice fragments and whole rock samples indicate an 87 86 enriched mantle component ( Sr/ Srin = 0.70495 – 0.70535, Nd[t=25.1Ma] = -1.13 to - 206 204 0.39) similar to that of coeval Cenozoic mafic lavas. Pb isotopes ( Pb/ Pbin = 19.042 – 207 204 19.168, Pb/ Pbin = 15.557 – 15.664) fall along a tight trend between the Northern Hemisphere Reference Line (Hart 1984) and an endmember similar to local granitic units. Major and trace element modelling support a source for the TEC derived from the mixing of anatectic melts of crustal rocks with intruded mantle-derived magmas similar to a local basaltic-andesite. ii Acknowledgements First and foremost, I`d like to thank both of my supervisors for teaching me that there is always room for improvement. Thank you Brian Cousens for accepting me as a relatively unknown undergraduate with an interest in volcanoes and opening my eyes towards a shared academic interest in geochemistry. Without opportunities provided by Brian, both prior to this project and throughout, I would not be here today. Thank you to Jon O`Neil for all of your help, nearly infinite patience and careful attention to detail, without any of which this thesis would never have been completed. Field work would not have been possible without the help of Dr. David John, Dr. Joe Colgan and Dr. Kathryn Watts with the USGS and Dr. Chris Henry with the Nevada Bureau of Mines. Each are resident experts of this area of Nevada and have all provided a wealth of knowledge, additional samples and guidance throughout. A generous thank you is also deserved of Ayesha Landon- Browne for her work as a field assistant and the additional work she performed on our shared samples. To everyone at Carleton University and the University of Ottawa, staff, faculty members and fellow students I`d like to extend my thanks as well. Lastly, thank you to my friends and family. Thank you to my parents, Nate and Karen Stepner, you both have gone above and beyond encouraging my interest in science and satisfying my endless curiosity. And most importantly, thank you to Shelbi Dippold; your unwavering support (and unending patience) in me served as a driving force behind my work. iii Table of Contents Abstract Acknowledgements ................................................................................................................. iii List of Figures ........................................................................................................................... vi List of Tables ............................................................................................................................ ix List of Abbreviations ................................................................................................................. x 1.0 Introduction ...................................................................................................................... 1 2.0 Tectonothermal history of the Southwestern United States ........................................... 3 2.1 Subduction of Farallon plate ....................................................................................................... 3 2.2 Crustal Thickening and Formation of the Nevadaplano ............................................................. 7 2.3 Mid-Cenozoic Ignimbrite Flare-up .............................................................................................. 9 2.4 Basin and Range Extension ....................................................................................................... 14 2.5 Extension-related magmatism .................................................................................................. 17 3.0 Applied Concepts and Theory .......................................................................................... 19 3.1 Models for Generating Felsic Magmas ..................................................................................... 19 3.2 MASH/Deep Crustal Hot zone ................................................................................................... 22 3.3 Introduction to Isotopes ........................................................................................................... 25 Rb-Sr ............................................................................................................................................ 25 Sm-Nd .......................................................................................................................................... 29 (Pb)-(Pb), Common Lead ............................................................................................................. 33 4. Regional Geology ............................................................................................................... 35 4.1 Geological Framework .............................................................................................................. 35 4.2 Stratigraphic Relations .............................................................................................................. 37 4.3 Field observations ..................................................................................................................... 44 4.4 Extension in the Stillwater Caldera Complex ............................................................................ 43 5. Methods ............................................................................................................................. 50 5.1 Samples ..................................................................................................................................... 50 5.2 Major and Trace element analyses ........................................................................................... 51 5.3 Radiogenic Isotope Methods .................................................................................................... 52 Pb Isotope Analysis ..................................................................................................................... 53 iv Sr Isotope Analysis ...................................................................................................................... 54 Nd Isotope Analysis ..................................................................................................................... 55 5.4 Electron Microprobe Methods .................................................................................................. 57 6. Results ................................................................................................................................ 58 6.1 Petrography ............................................................................................................................... 58 6.2 Microprobe Results ................................................................................................................... 61 6.3 Major Element Geochemistry ................................................................................................... 65 6.4 Trace Element Geochemistry .................................................................................................... 71 6.5 Isotope Geochemistry ............................................................................................................... 78 6.6 Chemostratigraphic variation in the tuff of Elevenmile Canyon ............................................... 87 7. Discussion .......................................................................................................................... 90 7.1 Assessment of Element Mobility and Establishing Primary Characteristics ............................. 90 7.2 Isotopic Modelling the Petrogenesis of the tuff of Elevenmile Canyon ................................... 92 7.3 Feldspar Isotopic Characteristics and Constraints on Assimilation .......................................... 99 7.4 Trace Element Modelling ........................................................................................................ 102 7.3 Supporting Clues from Trends in Major Element Chemistry .................................................. 106 8 Conclusions ....................................................................................................................... 111 8.1 The tuff of Elevenmile Canyon ................................................................................................ 111 References ........................................................................................................................... 113 Appendix 1 Supporting