RIDGE SUBDUCTION VOLCANISM IN THE ZEBALLOS COMPLEX; SOUTHERN PATAGONIAN ANDES

The Zeballos Complex (ZC), (47.0^oS, 71.4^oW) 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 6 Ma. The ZC is a bimodal, alkaline volcanic complex consisting of lava flows, dikes, necks, and sub-volcanic intrusives, ranging 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). New ⁴⁰Ar/³⁹Ar ages reported here combined with published ⁴⁰Ar/³⁹Ar ages indicate that ZC rocks erupted between 4.0 and 1.7 Ma and, are thus, contemporaneous with post-plateau basalt volcanism of the MLBA. The ⁴⁰Ar/³⁹Ar ages are consistent with a tectonic model that ties ZC 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 ZC 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 supports the hypothesis that most of the ZC rocks represent a cogenetic suite linked by closed system fractional crystallization. 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 ZC 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.