CHEMICAL SIGNATURES OF MAGMAS AT TIMES OF FRONTAL ARC MIGRATION: EXAMPLES FROM THE CENTRAL ANDES AND SOUTHERN CENTRAL AMERICA A Dissertation Presented to the Faculty of the Graduate School of Cornell University In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy by Adam Robert Goss January 2008 © 2008 Adam Robert Goss CHEMICAL SIGNATURES OF MAGMAS AT TIMES OF FRONTAL ARC MIGRATION: EXAMPLES FROM THE CENTRAL ANDES AND SOUTHERN CENTRAL AMERICA Adam Robert Goss, Ph. D. Cornell University 2008 Within the Central Andes (27º-28.5º S) and in southeastern Central America (7º-11º N), discrete episodes of late Miocene-Pliocene frontal arc migration were accompanied by backarc slab shallowing and increased rates of forearc subduction erosion. Arc lavas erupting during and following these short-lived periods exhibit adakitic geochemical signatures indicative of high-pressure melting of mafic crust (i.e., steep REE patterns and elevated Sr concentrations). Geochemical and petrologic data from these lavas are used to address fundamental questions regarding the genesis of adakitic magmas spatially and temporally associated with arc migration. Along both margins, enrichment in radiogenic isotopic ratios in late Miocene- Pliocene adakitic lavas compared to early-mid Miocene and Quaternary arc lavas cannot be attributed to melting of subducted oceanic crust or pelagic sediments. Instead, these trends are better explained by partial melting of mafic forearc crust transported to mantle depths during times of accelerated forearc subduction erosion. Mass balance calculations and isotopic modeling results agree with compositional estimates of the Central American and Chilean forearc and indicate that mantle contamination by eroded forearc crust is an inevitable and observable process. In arcs with thick crust such as the central Andes (> 60 km), partial melting of garnet-bearing lower crust contributes to the adakitic signature of lavas erupting during frontal arc migration. Strong HFSE-depletion characteristic of the Central Andean adakites is attributed to HFSE-bearing residual phases in equilibrium with the melt. Geochemical trends and near-chondritic Nb/Ta ratios support a change from a rutile-bearing eclogite to a garnet-bearing amphibolite residue consistent with cooler more-hydrous conditions that evolved over the shallowing slab during the late Miocene. As magmatism diminished along the arc, rising adakitic magmas unreplenished by mantle-derived or lower crustal melts stalled within the Andean upper crust. Here, crustal assimilation and fractional crystallization subdued the primary adakitic signature and enriched the isotopic composition of the perched magma. After a 3-1 Ma period of effusive dome eruptions, rapid magma mixing caused the stalled system to explode at ~ 0.51 Ma. The resultant Incapillo Caldera and Ignimbrite mark the youngest volcanic event within the currently amagmatic flatslab segment of the Central Andes. BIOGRAPHICAL SKETCH Adam Robert Goss was born on January 2, 1979 in Milford, Massachusetts, the first son of Robert Myles Goss Jr. and Deborah Joan Goss. Since he was a child, Adam has been in awe of the ageless functions of the Earth, its slow tectonic processes that formed the metamorphic and igneous rocks in his backyard. Weekend trips to the Milford granite quarries and Purgatory Chasm lit a spark in his mind as to the origin of these and other crustal features of Central New England. How did this happen? The first time he tried to answer this question was a third grade elementary science fair. The title of his presentation: Pangaea and Plate Tectonics. After graduating from Hopedale Jr./Sr. High School in Hopedale, Massachusetts, Adam attended Wesleyan University in Middletown, Connecticut graduating in 2001 with University and High Honors and as a member of Phi Beta Kappa with a B.A. in Earth and Environmental Science. A Watson Fellowship to study the socio-cultural relationship between volcanic hazards and society allowed for a one year independent journey to Europe, Asia, Central and South America, and the Caribbean after college. Upon returning home in the summer of 2002, Adam returned to academic life and matriculated at Cornell University to begin graduate school in geochemistry/volcanology under the direction of Dr. Suzanne Kay. During graduate school, Adam experienced the daily toils of academic research and the joys that accompany scientific discovery. The frustration with mass spectrometers and the excitement of examining fresh new data define the simultaneous highs and lows of graduate work in geochemistry. In his second year, Adam was awarded a NASA Earth System Science Graduate Research Fellowship to fund his doctoral research that took him to a remote corner of the Central Andes to study the chemical evolution of the highest caldera on Earth. Another high moment was that earlier that year, Adam met the woman he would eventual marry, Janice Cruz- iii Cardona, a young and enthusiastic veterinary student from the island of Puerto Rico. With the presentation of this thesis, Adam begins a new chapter in his life as he begins post-doctoral work at the University of Florida on the geochemical interactions between hydrothermal fluids and active mid-ocean ridges. On July 21, 2007 Adam and Janice were married amongst the karst in the bride’s hometown of Isabela, Puerto Rico. iv To Mom , Dad, and Janice v ACKNOWLEDGMENTS I wish to acknowledge my entire family for their incredible and unwavering support during graduate school and the writing of this thesis. Specifically, Mom and Dad for always knowing I would make it and for keeping me going. For all those hot August days endlessly moving furniture, I sincerely thank you. To Janice, my wife, for being my strength, my voice of reason, and keeping me sane. You are my rock, the most interesting and special amongst many rocks. I wish to recognize my many friends and colleagues at Cornell, to whom I will never forget. Also due recognition for their open ears and helpful advice are my trusty roommates, Ves and Cubbie. I would like to acknowledge Dr. Suzanne Kay, for her guidance throughout this work as well as other contributing members of my doctoral committee; Dr. William White, Dr Robert Kay, Dr. Jason Phipps-Morgan, and Dr. Rüdiger Dieckmann. I am truly privileged to have studied under all of you. Lastly, I wish to thank NASA, the National Science Foundation, and CONICYT for providing the financial support that made this research possible. vi TABLE OF CONTENTS BIOGRAPHICAL SKETCH iii DEDICATION v ACKNOWLEDGEMENTS vi TABLE OF CONTENTS vii LIST OF FIGURES viiii LIST OF TABLES xii LIST OF ABBREVIATIONS xiv LIST OF SYMBOLS xvi INTRODUCTION 1 CHAPTER 1 18 CHAPTER 2 53 CHAPTER 3 163 CHAPTER 4 193 APPENDICES 266 REFERENCES 310 vii LIST OF FIGURES Figure 0.1 Schematic cartoon of a continental subduction zone………………. ….3 Figure 0.2 Map of the 15 major tectonic plates of the Earths lithosphere showing accretionary and non-accretionary margins………………………........6 Figure 0.3 Schematic representation of subducted volumes of eroded forearc vs. subducted sediment for the non-accretionary North Chile and Costa Rica margins…..……………………………………………………...11 Figure 0.4 Cartoon showing arc geometries before and after frontal arc migration and subduction erosion……………………………………………….13 Figure 1.1 Tectonic map of Central America showing major volcanic features………………………………………………………………..21 Figure 1.2 Tectonic map of southeastern Costa Rica and western Panama...........22 Figure 1.3 Graphs of (A) La/Yb vs. SiO2 (wt %) and (B) La/Sm vs. Sm/Yb for lavas from southern Costa Rica and west-central Panama…………... 25 Figure 1.4 Graphs of 206Pb/204Pb vs. 208Pb/204Pb for (A) Central American volcanic front lavas and (B) Cretaceous/Tertiary forearc complexes.………… 26 Figure 1.5 Graphs of (A) 206Pb/204Pb and (B) 208Pb/204Pb vs. latitude for Central American arc lavas and forearc ophiolites............................................34 Figure 1.6 Graphs of 206Pb/204Pb vs. (A) Th/U and (B) Nd/Pb for Costa Rican and Panamanian forearc ophiolites………………………………………..35 Figure 1.7 Graph of 206Pb/204Pb vs. 208Pb/204Pb showing the results of 3-component isotopic mixing model for Costa Rican arc lavas........... 38 Figure 1.8 Schematic illustrations of the general model of subduction erosion adapted for the southeastern Costa Rican forearc and of a transect across the Central American arc at ~ 4 Ma through the Osa Peninsula and Cordillera de Talamanca………………………………………… 48 Figure 2.1 SRTM digital elevation model of the Central Andes showing main morphotectonic features……………………………………………....56 Figure 2.2 Thematic Mapper imagery of the Andean northern flat slab transition zone south of the Valle Ancho (A) and schematic maps showing the eruptive extent of Pircas Negras adakitic andesites and late Miocene- Pliocene dacitic arc centers (B-D)………………………………...…..60 viii Figure 2.3 Photographs of Pircas Negras andesite outcrops……………………..65 Figure 2.4 Thematic Mapper imagery of the southernmost Andean CVZ north of the Valle Ancho showing major volcanic centers………………….....73 Figure 2.5 Harker diagrams for mafic and intermediate lavas associated with late Miocene arc migration at 28º S………………………………………. 93 Figure 2.6 Chondrite-normalized REE diagram (A) and MORB-normalized incompatible element spider diagram (B) for experimental adakitic melts compared to Pircas Negras, Dos Hermanos, and Valle Ancho lavas………………………………………………………………….. 96 Figure 2.7 Chondrite-normalized REE diagrams (A and C) and MORB- normalized incompatible element spider diagrams (B and D) for representative late Miocene-Pliocene mafic and intermediate lavas from the northernmost flatslab transition zone (27º S-28.5º S)……………. 97 Figure 2.8 Graphs of La/Ta vs. (A) La/Yb and (B) Ba/Ta for Neogene mafic and intermediate lavas from the northernmost flatslab transition zone…...99 Figure 2.9 Graphs of (A) La/Yb vs. SiO2 (wt %) and (B) La/Sm vs. Sm/Yb for Neogene mafic and intermediate lavas from the northern flatslab transition zone……………………………………………………….101 Figure 2.10 Graphs of (A) Sr (ppm) vs.