Cerro de Vidrio rhyolitic dome:: evidence for Late Pliocene volcanism in the central Andean flat-slab region, Lama-Veladero district, 29°20′S, San Juan Province, Argentina

Magmatic activity in the El Indio-Pascua Au-Ag-Cu belt, situated in the Cordillera Principal at latitudes 29-30degreesS at the center of the southern Central Andean flat subduction regime, did not, as previously assumed, cease at 5-6 Ma but continued locally until the Late Pliocene. New and essentially identical (40)Ar-(39)Ar laser step-heating ages of 2.1 +/- 0.5 Ma (biotite) and 2.0 +/- 0.2 Ma (glass) are recorded for a rhyolitic dome, the Cerro de Vidrio, in the northern Valle del Cura region near the Veladero Au (-Ag) property. The rhyolite is geochemically distinct from local Upper Miocene volcanic rocks; it is slightly but unequivocally peraluminous and does not exhibit significant REE fractionation apart from a pronounced negative Eu anomaly, a feature also shown by the Upper Paleozoic-Lower Mesozoic basement units of the area. This suggests that magma generation occurred in a garnet-free environment, which implies anatexis at shallower levels than for the rhyolites of the Upper Miocene Vallecito Formation. (C) 2002 Elsevier Science Ltd. All rights reserved.