AN ABSTRACT OF THE DISSERTATION OF Jason Frederick Kaiser for the degree of Doctor of Philosophy in Geology presented on June 4, 2014. Title: Understanding Large Resurgent Calderas and Associated Magma Systems: The Pastos Grandes Caldera Complex, Southwest Bolivia. Abstract approved: ______________________________________________________ Shanaka L. de Silva The Pastos Grandes Caldera Complex (PGCC) in southwest Bolivia has produced two large-volume (≥800 km3 DRE) dacite ignimbrites from a nested caldera source over a period of 5.5 Myr. In addition to the large-volume ignimbrites, a small-volume ignimbrite shield and post-climactic lavas define this composite system. Based on detailed field work and analysis of satellite imagery plus biotite 40Ar/39Ar dating, we summarize a revised stratigraphy, areal distribution and volume calculations for ignimbrites. From interpretations of stratigraphy and structures along the caldera margins, we propose an asymmetric collapse hinged along the north and western boundaries. Both the early Chuhuilla (5.45 ± 0.02 Ma), and the younger Pastos Grandes (2.89 ± 0.01 Ma) calderas share the hinge and much of the eastern collapse scarp, however the Chuhuilla caldera defines a much larger area (>1700 km2) compared to the Pastos Grandes (870 km2). It is proposed here that pre-existing regional tectonic weaknesses combined with influences of the magma body caused roof failure and caldera collapse. The vast majority of the ignimbrite volume lies within the Chuhuilla and Pastos Grandes calderas (92 and 75% respectively). Considerable intracaldera fill and the lack of preceding plinian deposits suggest that the caldera collapse was early and the eruptive column was not high and was short-lived. The physical properties of the ignimbrites, the limit of their areal distribution to regional topographic lows along with paleomagnetic characteristics support the idea of dense sluggish pyroclastic flows. New volume calculations using multiple methods update previous estimates, with the Chuhuilla now at approximately1300 km3 and the Pastos Grandes at 800 km3. The ignimbrite volumes and spatial pattern of vents suggest that the caldera complex mirrors the construction of a long-lived composite batholith that focused spatially with time. Eruptions from the PGCC have produced compositionally restricted, high-K dacites with volumetrically minor rhyolites. Combined with granodiorite xenoliths, each caldera cycle contains a progression of textural maturity from ignimbrite, through post- climactic lavas, to remnant pluton. The chemical signatures in the PGCC mirror those of the host Altiplano Puna Volcanic Complex (APVC) and resemble typical arc characteristics (i.e. LIL enriched magmas - Ba/Nb of 4.7-4.8 and Ba/La of 1.6-2.0). However, overprinted on the arc signature are elevated radiogenic isotopes (87Sr/86Sr ~0.708 – 0.709), which suggest high degrees of crustal assimilation that is thought to be related to increased mantle input from melted asthenosphere. It is suggested here that a combination of assimilation and fractional crystallization from the regional mid-crustal parental source would create the magmas erupted in the PGCC. Subtle decreases in the 87Sr/86Sr from the Chuhuilla caldera cycle, to the younger Pastos Grandes cycle suggest higher amounts of crustal assimilation related to more heat flow during the peak of the flare up. Zircon chronochemistry reveals that the climactic, caldera-forming eruptions of the PGCC punctuate the protracted magma accumulation and storage periods. The combination of in-situ zircon U-Pb ages with indicators for geochemical evolution (e.g., Zr/Hf, Yb/Gd, Eu/Eu*, Th, U) magmatic temperatures (e.g., Ti) in zircon, reveals protracted magma presence before and after the climactic 2.89 ± 0.01 Ma Pastos Grandes Ignimbrite (PGI) supereruption (~800 km3 of magma) in southwest Bolivia. Zircons from PGI pumice and a lava dome define a pre-climactic magmatic stage of ~0.7 Myr duration prior to the climactic eruption and formation of the eponymous caldera. A further 0.4 Myr of post-climactic zircon crystallization is recorded in lava domes and cogenetic granodiorite clasts. Zircon crystallization is recorded for approximately 1.3 Myr for the Chuhuilla cycle; however the record is entirely pre-climactic. We propose a model for the Pastos Grandes cycle in which the climactic caldera-forming eruption vented the upper portions of the reservoir that were zircon saturated. Subsequently, deeper “remnant” dacite magma previously outside the zone of zircon saturation but crystallizing other major phases, rose to re-establish lithostatic equilibrium, commenced zircon crystallization anew, and drove resurgent volcanism and uplift. This ~1.1 Myr zircon crystallization history records the minimum duration of the lifetime of the PGI supereruption magma from its accumulation to post-climactic solidification. These data support >1 Myr magma lifetimes and a link between volcanic and plutonic realms in large sub-caldera magma reservoirs in the uppermost crust that feed some supereruptions. ©Copyright by Jason Frederick Kaiser June 4, 2014 All Rights Reserved Understanding Large Resurgent Calderas and Associated Magma Systems: The Pastos Grandes Caldera Complex, Southwest Bolivia by Jason Frederick Kaiser A DISSERTATION Submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Presented June 4, 2014 Commencement June 2014 Doctor of Philosophy dissertation of Jason Frederick Kaiser presented on June 4, 2014 APPROVED: Major Professor, representing Geology Dean of the College of Earth, Ocean, and Atmospheric Sciences Dean of the Graduate School I understand that my dissertation will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my dissertation to any reader upon request. Jason Frederick Kaiser, Author ACKNOWLEDGEMENTS If there is one thing I have learned in my academic career, it is that science cannot move forward without a community of inclusive thinkers and teachers. I have been fortunate to have been allowed to work with such bright, motivating, and inspiring people. Only in working alongside such brilliant and passionate people was I able to discover my own passion and abilities in this world. There are many who have shaped my life; as many as humanly possible are mentioned here. First, I have to mention my parents. The most supportive, caring, and helpful people I could ever imagine having in my life. I know they would be proud of me no matter what I did with my life. It is this unfiltered love and support that has let me explore this world and make my own way. Thank you so much for introducing me to as much as possible and teaching me to think for myself. Maybe we didn’t explore much of the world first- hand when I was a kid, but you sure didn’t hide any of it from me. I cannot thank you enough for everything you have taught me. I have so many teachers to thank for inspiring me from an early age. The first that comes to mind is Walter Long. The most passionate and inspiring history teacher a junior high school could ever ask for. The most important lesson I learned from Mr. Long was that there was more to life than the Mississippi River Valley and that I needed to go see and experience as much of it as I could. I am well on my way Mr. Long – Thank you. Another amazing and inspiring teacher – Mark Pfieffer – made learning more fun than I could have ever thought. These teachers gave me a true passion for knowledge, I owe so much to them. My life forever changed the day I met John Hogan. My first geology instructor and constant mentor taught me how to me a scientist and have fun with it. I stayed in college solely because of John, and am at this stage in my life only because of John. Whether I am in the field, in the lab, or in front of the classroom, his words are echoing in my mind. I still look to John for words of wisdom and advice, thank you Captain Contact. My graduate career has been filled with ups and downs as anyone would expect. Sheila Seaman and Mike Williams were my first guides through this treacherous world. I cannot thank them enough for their kindness and generosity. My committee at Oregon State has been truly amazing. Anita Grunder and Andrew Meigs have given me critical instruction and guidance. Thank you for your brutal honesty and generous scientific insight along the way. I owe a special thanks to Axel Schmitt for being a surrogate advisor for a year. I will never be able to match his energy or attention to detail, but I learned more than I thought possible while trying my best to keep up. Thank you Dale and Stephanie for being with me every step of the way. I would not have survived without your help and support. Thanks for letting me crash your offices with ideas, questions, or just to vent frustration. And finally, the biggest thanks to Shan de Silva (aka Papa Bear). I have finally learned that there is always a method to your madness. Every interaction has been a learning opportunity and your incredibly high standards have pushed me far beyond what I thought I could be as a scientist. Thank you for your brutal honesty and unwavering motivation. I would not be the scientist or man I am today without you. TABLE OF CONTENTS Page 1 General Introduction .................................................................................................... 1 Argument ...........................................................................................................