More than 2000 km3 of acid and 9000 km3 of basic volcanic rocks formed during the Jurassic and Early Cretaceous in the Coast Range of central Chile, between 32°30′S and 34°S. These rocks, which constitute the major part of an •15-km-thick pile of alternately marine and continental deposits, issued from volcanic arcs situated between a land area with Paleozoic basement in the west and a marginal 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 magmas became more depleted with time due to an increase in degree of partial melting, and their compositions were modified by subduction-related fluids and contamination with a progressively thinner and younger crust. The basic lavas are of high-K calc-alkaline to shoshonitic affinity, chemically resembling the lavas found in some mature island arcs in the western Pacific. The extension and subsidence resulted in a low-relief topography close to sea level, in contrast with the present-day convergent type of Andean volcanism at the same latitude where calc-alkaline intermediate lavas erupt from volcanoes at great height above a thick crust.

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