Lithospheric Origin of Oligocene^Miocene Magmatism in Central Chile: U^Pb Ages and Sr^Pb^Hf Isotope Composition of Minerals

Establishing the petrogenesis of volcanic and plutonic rocks is a key issue in unraveling the evolution of distinct subduction-related tectonic phases occurring along the South American margin.This is particularly true for Cenozoic times when large volumes of magma were produced in the Andean belt. In this study we have focused on Oligo-Miocene magmatism in central Chile at 338S. Our data include field and petrographic observations, whole-rock major and trace element analyses, U^Pb zircon dating, and Pb, Sr, and Hf isotope analyses of plagioclase, clinopyroxene, and zircon mineral separates. Combined with earlier dating results the new zircon ages define a 288^52Ma period of plutonic and volcanic activity that ceased as a consequence of flattening subduction of the Nazca^ Farallon plate. Rare earth elements patterns are variable, with up to 92 times chondrite concentrations for light rare earth elements yielding (La/Yb)N between 36 and 70, and an absence of Eu anomalies. Initial Pb isotope signatures are in the range o