PROTEROZOIC METAVOLCANICS FROM WESTERN SIERRAS-PAMPEANAS TERRANE, ARGENTINA

A series of medium grade metamorphic rocks of the western sector of th e Sierras Pampeanas Terrane in central western Argentina are represent ed by amphibolites, gneisses and schists derived from sedimentary as w ell as from igneous rocks. The metavolcanics consist of amphibolites, quartz-K-feldspar-muscovite schists, and hornblende-biotite and biotit e-epidote-plagioclase schists. Based on petrographic and geochemical d ata they are interpreted as originating as basaltic tholeiites, rhyoli tes and mesosilicic volcanics. The distribution and geochemical behavi or are similar to present day western Pacific lavas, mainly those deve loped on island arcs or heavily attenuated continental crust. Based on these characteristics, an accretionary tectonic model involving a ser ies of island-are collisions is proposed for the Proterozoic. The comp lex Proterozoic tectonic history of the western Sierras Pampeanas has been partially obliterated by the emplacement of the Early Paleozoic m agmatic are rocks.