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

Abstract

Magmatic activity in the El Indio-Pascua Au–Ag–Cu belt, situated in the Cordillera Principal at latitudes 29–30°S 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 40Ar– 39Ar 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.