Diversity of volcanism and evidence of discrete eruption centres in the Miocene Andes of Central Chile

Abstract

The geology of the Farellones region in the Andean Cordillera of central Chile, comprise a thick sequence of volcanic and volcaniclastic Miocene rocks. The occurrence of discrete eruption centres within this sequence, whilst suggested, has been difficult to stablish, as in the modern volcanic systems of the SVZ to the southeast of the study area. Here we report, for the first time a previously uncharacterized composite (Cerro Colorado Volcano) associated to a series of intercalated lava flows and pyroclastic deposits of basaltic to rhyolitic composition of Miocene age. Eruptive sequences reach 1.7 km in vertical section, yet the lateral continuity of units is interrupted by a major faults, and gravity collapse deposits. We document a series of large pyroclastic block and ash flows, which are overlain by rhyolitic pyroclastic density currents, ash fall and crystal, lithic and vitric tuffs, all making up more than 300 m of the sequence. A further 300 m thick unit of spherulite-bearing rhyolitic lavas represent the upper most section of the Cerro Colorado volcano. During the early stages of its construction magmas and hydrothermal fluids interacted to form a maar-diatreme system (Quebrada Lunes maar-diatreme). Much of the volcanic units are intruded by E-W and NW-SE striking andesitic and rhyolitic dikes One of the dikes exhibits internal pyroclastic textures evidence of shallow conduit fragmentation. This pyroclastic dike is further evidence of magma-fluid interaction and related explosivity. Similar pyroclastic breccias described elsewhere related to porphyry copper mineralization. Our results indicate that Miocene volcanism in the Andes of Central Chile is linked to discrete eruptive centres which can be identified and characterized through careful field mapping.