Date of Award

5-2009

Document Type

Thesis

Degree Name

Master of Science (MS)

College/School

College of Science and Mathematics

Department/Program

Earth and Environmental Studies

Thesis Sponsor/Dissertation Chair/Project Chair

Matthew L. Gorring

Committee Member

Stefanie Brachfeld

Committee Member

Gregory Pope

Abstract

The Zeballos Complex (47.0°S, 71.4°W) lies on the western side of Meseta del Lago Buenos Aires (MLBA) in the southern Andean backarc region, southeast of the Chile Triple Junction and east of ridge collision that occurred at 6 Ma. The Zeballos Complex is a bimodal, alkaline volcanic complex consisting of multiple lava flows, dikes, volcanic necks, and sub-volcanic intrusives that range from basalt to trachybasalt (47-50% SiO2; 5-7% Na2O+K2O; 3-7% MgO) to highly differentiated trachyandesite, trachydacite, and trachyte (58-65% SiO2; 8-12% Na2O+K2O). Intermediate compositions are relatively rare, but a small population of samples is classified as basaltic trachyandesites (51-56% SiO2; 6-7% Na2O+K2O). Four new 40Ar/39Ar ages reported here combined with several published 40Ar/39Ar ages indicate that Zeballos Complex rocks erupted between 4.0 and 1.7 Ma and, are thus, contemporaneous with post-plateau basalt volcanism of the MLB A documented further east. The 40Ar/39Ar age data are consistent with a tectonic model that ties Zeballos Complex volcanism with ridge collision and the development of an asthenospheric slab window between the Nazca and Antarctic plates (Ramos and Kay, 1992; Gorring et al, 1997). The Zeballos Complex mafic suite (basalts and trachysbasalts) has strong OIB-like signatures (La/Yb = 6-12; Ce/Pb= 9-23; Nb/U = 25- 50; Ba/Nb = 6-17; La/Nb = 0.6-1.5) and are geochemically indistinguishable from MLBA post-plateau lavas. The felsic suite (trachyandesites, trachydacites, and trachytes) rocks have consistently lower Ce/Pb (5-16) and Nb/U (7-28) and higher Ba/Nb (11-27) and La/Nb (1-2.3) indicative of an important subduction and/or continental crustal component in the petrogenesis of these rocks. Geochemical modeling support the hypothesis that most of the Zeballos Complex rocks represent a cogenetic suite that is linked by closed system fractional crystallization processes that occurred in crustal magma chambers. Major (least squares mixing) and trace element (Raleigh fractionation) modeling results indicate that most intermediate and felsic suite trachyandesites can be modeled by 40-60% crystal fractionation of an assemblage dominated by plagioclase (60%), clinopyroxene (20%), olivine (10%), and titanomagnetite (10%) from parent magma similar in composition to the Zeballos Complex mafic suite rocks. However, felsic suite trachydacites and trachytes require additional, open system processes (most likely crustal assimilation) in order to explain their geochemical characteristics.

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