Table Of ContentCHEMICAL 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-
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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.
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To Mom , Dad, and Janice
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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.
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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
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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. SiO (wt %) and (B) La/Sm vs. Sm/Yb for
2
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
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Description:rare earth elements (REE), are far less soluble in fluid phases and are therefore
.. from the forearc offshore Guatemala/El Salvador (von Huene et al., 1985).
