EARLY EVOLUTION OF THE PROTO-ANDEAN MARGIN OF SOUTH-AMERICA

From a detailed study of a 500 km transect in the Sierras Pampeanas, c entral-west Argentina, two pre-Silurian tectono-magmatic episodes are recognized and defined, each culminating in micro-continental collisio ns against the proto-Andean margin of Gondwana. The Pampean orogeny st arted in Early Cambrian time with short-lived subduction, indicated by ca 535 Ma calc-alkaline granitoids. Following Pampean terrane collisi on, burial to granulite facies conditions (ca, 9 kbar) generated wides pread migmatites and ca 520 Ma highly peraluminous granites in the Eas tern Sierras Pampeanas, After brief quiescence, a second major episode , the Famatinian orogeny, started with subduction ca, 490 Ma, forming a wide continental are and ensialic backarc basin, This heralded the a pproach of Laurentia to Gondwana, during which the Precordillera terra ne separated from the southern Appalachian region, finally colliding w ith Gondwana in Silurian-Devonian time.