GEOCHEMISTRY OF THE NEOGENE LOS FRAILES IGNIMBRITE COMPLEX ON THE CENTRAL ANDEAN ALTIPLANO PLATEAU A Thesis Presented to the Faculty of the Graduate School of Cornell University In Partial Fulfillment of the Requirements for the Degree of Master of Science by Joseph John Kato August 2013 © 2013 Joseph John Kato ABSTRACT The Los Frailes Ignimbrite Complex sits in the backarc of the Andean Central Volcanic Zone (CVZ) and is the most easterly of the large Altiplano volcanic centers. Despite its large size (2000 km3) and substantial mineralization in its satellite units, the majority of the Los Frailes Complex remains poorly described with conflicting age assessments of the main Los Frailes ignimbrite. Processes related to its emplacement include: variable crustal thickening and uplift over a steepening subducted slab, episodes of delamination of the mantle-lithosphere and lower crust and deep crustal flow. Based on 25 new analyses and the works of previous sub-regional studies, a three tier crustal magma evolution is proposed for the Los Frailes Complex, similar to models suggested for Puna ignimbrites. The crust-to-mantle mixing ratio of the 18 erupted mass is put near 50:50 based on new fractionation corrected δ OQuartz analyses (+9.43-10.79‰). AFC models incorporating new 87Sr/86Sr (0.710-0.713) and 143 144 Nd/ Nd (0.5121-0.5123) ƐNd (-9 to -6) ratios and the strongly peraluminous character of the complex support a metapelitic crustal end-member and silicic crustal base. Melting and mixing near the Moho is established based on steep HREE patterns (Sm/Yb=4-12) and very high Sr content (400-650 ppm Sr) while middle crustal plagioclase removal creates negative Eu anomalies (Eu/Eu*=0.6-0.9). High crystal content and reported cordierite are the results of low pressure crystallization in upper crustal magma chambers prior to eruption. Coherent temporal trends of REE and HFSE ratios display the effects of crustal thickening and delamination on Moho-depth AFC processes. Two distinct ratio changes at 10-12 Ma and 2-4 Ma are proposed to correlate with two discreet delamination events below the Los Frailes Complex. The 10-12 Ma event is supported by corresponding shifts in regional brittle deformation and isotopic character as well as proposed rapid uplift at that time. Major eruptions of the Livicucho and Condor Nasa ignimbrites and emplacement of the large main Los Frailes ignimbrite mark the volcanic expression. The age of the main Los Frailes ignimbrite is established at 1.5-3.5 Ma, in agreement with Barke et al. (2007), based on two new 40Ar/39Ar sanidine ages ~1.521 Ma. In addition, seismic tomographic studies show missing lithosphere directly below the complex which corresponds with a 3He/4He emissions study suggesting recent mantle melting. These findings demonstrate that delamination in the southern Altiplano is episodic with the results of the 2-4 Ma event still observable today. ii BIOGRAPHICAL SKETCH Joseph Kato was born March 28, 1977, in Middletown, New York, son of John and Ruth Kato. His early years were spent in Central and Southeast Alaska where his father worked as a geologist for the Federal Government. He graduated from Juneau- Douglas High School in 1995 and spent a year at Northwestern Preparatory School in Malibu, California as well as the University of Alaska Fairbanks before entering the U.S. Naval Academy with the class of 2000. At Annapolis, he earned a Bachelor of Science in Honors Oceanography and was ordered to flight training to become a Naval Flight Officer following graduation. In Pensacola, Florida, he earned his “Wings of Gold” and was assigned to EA-6B Prowlers in Whidbey Island, Washington as an Electronic Countermeasures Officer. His junior officer tours included VAQ-136 in Atsugi, Japan and the Royal Air Force 12 Bomber Squadron in Lossiemouth, Scotland. In 2010, he was assigned to Cornell University Naval Reserve Officers Training Unit (NROTC) as a Naval Science Instructor and Visiting Lecturer. It was then that, under the guidance of Professor Suzanne Kay, this project and thesis took shape. iii Dedicated to Snes, whose time and support made this possible. iv ACKNOWLEDGMENTS I acknowledge, first and foremost, my advisor, Professor Suzanne M. Kay for her time and patience while teaching me most of what I know about geochemistry and South American geology, and William M. White and Robert W. Kay for the remainder. I would also like to acknowledge the Cornell Earth and Atmospheric Department staff for their time and help, especially Savannah Sawyer. This thesis and the work it describes was made possible by the 2009 and 2010 reconnaissance trip and field work done by Beatriz Coira, Suzanne Kay, Nestor Jiménez, Brenhin Keller, Pablo Caffe, Luis Galvan and Chelsea Allison. In the early stage of development of this thesis, extensive input was borrowed from Brenhin Keller’s (now at Princeton) 2010 Senior Thesis. In the later stages of this project, Brian Jicha (University of Wisconsin-Madison), Chris Harris (University of Cape Town, South Africa) and Brenhin Keller contributed valuable Ar/Ar ages, oxygen isotopic data and zircon analyses. I would like to thank Louis A. Derry and George Hade for their assistance with a temperamental TIMS, as well as John Hunt for his instruction on the use of the microprobe. In addition, my fellow graduate students deserve thanks for endless questions I have directed their way, particularly Ashley Tibbetts, Nicolas Consentino and Sander Hunter. Finally, thank you to my father for reading this about a dozen times during the editing process. v TABLE OF CONTENTS Biographical Sketch ……………………………………………………………... iii Dedication………………………………………………………………………… iv Acknowledgments………………………………………………………………… v Table of Contents…………………………………………………………….…… vi List of Figures…………………………………………………………………….. viii List of Tables……………………………………………………………………... xi List of Abbreviations……………………………………………………………… xii List of Symbols…………………………………………………………………… xiv Chapter 1: Introduction and Geologic Setting…………………………………… 1 Introduction to the Los Frailes Ignimbrite Complex……………………... 1 Geological Setting of the Los Frailes Ignimbrite Complex…………….... 7 Volcanic History of the Los Frailes Ignimbrite Complex……………….. 15 Comparison to Cerro Galán, selected APVC and Morococala Ignimbrites …………………………………………………………………………… 18 Chapter 2: Analytical Methods………………………………………………….. 22 Major Elements Analyses……………………………………………….. 22 Trace Elements Analyses ……………………………………………….. 22 Mineral Analyses …………………………………………………...…... 23 Strontium and Neodymium Isotopic Analyses………………………….. 23 Oxygen Isotopic Analyses ……………………………………………… 25 Zircon Isotopic Analyses………………………………………………... 26 40Ar-39Ar Age Analyses…………………………………………………. 27 Geothermometry and Geobarometry Analyses …………………………. 28 Chapter 3: Analytical Results…………………………………………………… 29 vi Major Elements………………………………………………………….. 30 Trace Elements…………………………………………………………… 30 Mineral Compositions…………………………………………….……… 60 Isotopic Results…………………………………………………..………. 65 Zircon Age Analysis………………………………….……….…………. 71 40Ar-39Ar Age Analysis…………………………………………………… 72 Geothermometry and Geobarometry……………………………………… 72 Chapter 4: Discussion of Analytical Results…………………………………….. 75 Major Elements…………………………………………………………… 75 Trace Elements……………………………………………………………. 78 Isotopic Compositions…………………………………………………….. 89 Mineralogy………………………………………………………………… 99 Geochronology…………………………………………………………….. 99 Geothermometry and Geobarometry………………………………………. 102 Chapter 5: Modeling……………………………………………………………….. 104 AFC Models………………………………………………………………... 104 Magma Evolution and Ignimbrite Genesis Model…………………………. 127 Chapter 6: Conclusions……………………………………………………………. 132 Appendix A: Neodymium Separation Procedure…………………………………. 136 Appendix B: Major and Trace Element Data………………………………………139 Appendix C: Strontium and Neodymium Isotopic Data and Error Analysis……... 144 Appendix D: Oxygen Isotopic Data and Error Analysis………………………….. 146 Appendix E: Zircon Isotopic Data and Error Analysis…………………………… 147 Appendix F: Microprobe Data and Error Analysis……………………………….. 149 Appendix G: 40Ar-39Ar Data and Error Analysis…………………………………. 159 References…………………………………………………………………………. 161 vii LIST OF FIGURES Figure 1.1: Map of South America showing major tectonic features………… 2 Figure 1.2: Map of Central Andean Altiplano-Puna ignimbrites………...…… 3 Figure 1.3: Modified composite map of the Los Frailes Complex……………. 5 Figure 1.4: Lithospheric cross-section for evolution of the southern Altiplano. 9 Figure 1.5: Lithospheric scale cross-section near 21°S……………………….. 10 Figure 1.6: Schematic cross-section cartoon of seismic low velocity zones in the mantle……………………………………………………………… 12 Figure 1.7: A compilation of seismic P-wave tomographic images and figures depicting ongoing delamination of the northern Puna lithosphere… 14 Figure 3.1: Plot of SiO2 versus Al/(K+Na+Ca) depicting the highly peraluminous nature of Los Frailes Complex…………………………………….. 31 Figure 3.2: Plot of K2O versus Na2O displaying the aluminousity variability of the ignimbrites reflected in their Na2O and K2O concentrations……… 33 Figure 3.3: Plot of SiO2 versus K2O displaying high K2O content…………….. 34 Figure 3.4: Primitive mantle normalized plots of trace element concentrations. 35 Figure 3.5: Plot of Sm/Yb ppm ratio versus La/Sm ppm ratio which illustrates variable REE depletion……………………………………………. 37 Figure 3.6: Plot of La ppm versus La/Yb ppm ratio shows no discernible effect of La concentration on the La/Yb ppm ratio…………………………. 39 Figure 3.7: Plot of SiO2 versus La/Yb ppm ratio which shows