JURASSIC AND EARLY CRETACEOUS ISLAND ARC VOLCANISM, EXTENSION, AND SUBSIDENCE IN THE COAST RANGE OF CENTRAL CHILE

More than 2000 km(3) of acid and 9000 km(3) of basic volcanic rocks fo rmed during the Jurassic and Early Cretaceous in the Coast Range of ce ntral Chile, between 32 degrees 30'S and 34 degrees S. These rocks, wh ich constitute the major part of an similar to 15-km-thick pile of alt ernately marine and continental deposits, issued from volcanic arcs si tuated between a land area with Paleozoic basement in the west and a m arginal sea in the east. Asthenospheric upwelling led to extension and bimodal volcanism; the volcanic products were deposited in intra-arc basins subsiding at high rates (100-300 m/m.y.), The source of the mag mas became more depleted with time due to an increase in degree of par tial melting, and their compositions were modified by subduction-relat ed fluids and contamination with a progressively thinner and younger c rust. The basic lavas are of high-Ii calc-alkaline to shoshonitic affi nity, chemically resembling the lavas found in some mature island arcs in the western Pacific. The extension and subsidence resulted in a lo w-relief topography close to sea level, in contrast with the present-d ay convergent type of Andean volcanism at the same latitude where calc -alcanism intermediate lavas erupt from volcanoes at great height abov e a thick crust.