AN ABSTRACT OF THE THESIS OF Casey R. Tierney for the degree of Master of Science in Geology Presented on June 7, 2011. Title: Timescales of Large Silicic Magma Systems: Implications from Accessory Minerals in Pleistocene Lavas of the Altiplano-Puna Volcanic Complex, Central Andes Abstract approved: _______________________________________ Shanaka L. de Silva Constraining the development, evolution, and timescales of large silicic magma systems is important to understanding the development of granite batholiths, the relationships between volcanoes and their plutonic underpinnings, and the development of the continental crust. The ignimbrite flare up that produced the Altiplano-Puna Volcanic Complex of the Central Andes is characterized by episodic volcanism over a ~11 Ma time-span that climaxed about 4 Ma. Since peak activity, the temporal and spatial record of volcanism suggests a waning of the system with only one other supervolcanic eruption at 2.6 Ma. The most recent phase of volcanism from the APVC comprises a series of late Pleistocene domes that share a general petrochemical resemblance to the ignimbrites. New U-Th/U-Pb data on zircons and high precision 40Ar/39Ar age determinations reveal that these effusive eruptions represent a temporally coherent magmatic episode. The five largest domes (Chao, Chillahuita, Chanka, Chascon-Runtu Jarita, and Tocopuri) have a combined volume >40 km3, and are distributed over a roughly elliptical area of almost 2000km2 centered at 22°S 68°W. They are crystal rich (>50%) dacites to rhyolites. 40Ar/39Ar ages from biotite reveals eruption ages from 108±6 to 120±5 ka while more accurate sanidine for some of the domes reveal eruption ages from 87±4 to 97±2 ka. SIMS U-series crystallization ages from the rims of 215 zircon crystals from the domes show a similar age spectra from dome to dome, with common peaks in zircon ages at ~100ka and ~220ka. Furthermore, the ages reveal a fairly continuous spread of ages from near eruption to >300ka indicating that the residence time of this magma body was likely over a similar time interval. Ubiquitous andesitic inclusions evidence a vital role for recharge in sustaining and maybe eventual eruption of these magmas. Lastly, the interiors of crystals with rim U-Th secular equilibrium ages were re-analyzed and have yielded U-Pb ages of up to 3.5 Ma. The presence of these older interiors suggests that the source region of these magmas retained a record of an earlier history dating back to the last supervolcanic eruption in the region from the nearby Pastos Grandes caldera. This suggests that the thermal history of the system precluded complete resorption of antecrysts. The volcanological, petrological, temporal and spatial coherence of this series of eruptions combined with the similar 40Ar/39Ar and zircon age spectra argue for a long- lived and unitary magma chamber revealing perhaps the waning of this major continental magma system. ©Copyright by Casey R. Tierney June 7, 2011 All Rights Reserved Timescales of Large Silicic Magma Systems: Implications from Accessory Minerals in Pleistocene Lavas of the Altiplano-Puna Volcanic Complex, Central Andes by Casey R. Tierney A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Presented June 7, 2011 Commencement June 2011 Master of Science thesis of Casey R. Tierney presented on June 7, 2011 APPROVED: _____________________________________________________________________ Major Professor, representing Geology _____________________________________________________________________ Chair of the Department of Geosciences _____________________________________________________________________ Dean of the Graduate School I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request _____________________________________________________________________ Casey R. Tierney, Author ACKNOWLEDGEMENTS I would like to sincerely thank everyone who has assisted in making this research possible. First and foremost, I would like to express my appreciation to my advisor, Shan de Silva, for the opportunity to come to Oregon State University and participate in this exciting research. Shan’s continued patience, guidance and encouragement were integral in the success of this project. Thanks to Axel Schmitt for his guidance and humor in the field and to his seemingly countless hours of assistance and troubleshooting on the SIMS. Axel’s insightful editing of various proposals, abstracts, posters and this thesis are also greatly appreciated. To Anita Grunder, for agreeing to serve on my committee and for her enthusiastic teaching, valuable perspectives and vigorous editing of this thesis. Completion of this research and the maintenance of my sanity would also not have been possible without my research mates, Dale Burns, Stephanie Grocke, Jason Kaiser, Bob Peckyno, Jamie Kern, Rodrigo Iriarte, Trish Gregg, Kevin Weldon and Toshio Miyazaki, who were always eager to offer help and support when needed. My gratitude also goes to the various other VIPERS, graduate students, and acquaintances who served as great resources and excellent friends during my time in Corvallis. Thanks to my family and friends back home in Iowa and elsewhere, especially my parents Catherine and Robert, and my brother Shane, without whom I would never have made it this far. Finally, a special thanks goes to Erica for her endless emotional support and for forcing me to have a life outside of school during this process. TABLE OF CONTENTS Page 1.0 Introduction .............................................................................................................. 1 1.1 Research Objectives ............................................................................................. 5 1.2 Geologic Background ........................................................................................... 6 1.2.1 Andes Overview ............................................................................................ 6 1.2.2 Evolution of the Central Volcanic Zone ........................................................ 7 1.2.3 Evolution of the Altiplano-Puna Plateau ....................................................... 9 1.2.4 The Altiplano-Puna Volcanic Complex....................................................... 10 1.2.5 Late Pleistocene Domes of the APVC ......................................................... 12 2.0 Purpose and Scope ................................................................................................. 16 2.1 Hypotheses ......................................................................................................... 16 2.2 Methods .............................................................................................................. 18 3.0 Background ............................................................................................................ 21 3.1 Large Silicic Magma Systems ............................................................................ 21 3.2 Longevities of individual systems ...................................................................... 22 3.3 Zircon as a probe for large silicic magma systems ............................................ 24 4.0 Field and Sample Observations .............................................................................. 28 4.1 Summary ............................................................................................................ 33 5.0 Petrology and Geochemistry .................................................................................. 35 5.1 Thin Section Petrography ................................................................................... 35 5.1.1 Previous work .............................................................................................. 35 5.1.2 Results.......................................................................................................... 38 TABLE OF CONTENTS (Continued) Page 5.1.3 Summary ...................................................................................................... 42 5.2 Whole Rock Chemistry ...................................................................................... 42 5.2.1 Previous Work ............................................................................................. 42 5.2.2 This Study .................................................................................................... 46 5.2.3 Summary ...................................................................................................... 55 5.3 Geothermometry and Geobarometry .................................................................. 56 5.3.1 Previous work .............................................................................................. 56 5.3.2 Determining Intensive Parameters ............................................................... 59 5.3.3 Fe-Ti Oxide Geothermometry Results ......................................................... 60 5.3.4 Amphibole Geobarometry Results .............................................................. 67 5.3.5 Summary ...................................................................................................... 70 5.4 Chapter Summary ............................................................................................... 72 6.0 Geochronology ......................................................................................................