Nearly circular plutons emplaced by stoping at shallow crustal levels, Cerro Aspero batholith, Sierras Pampeanas de Córdoba, Argentina

Abstract

The Cerro Aspero batholith (CAB) (440km2) is one of many discordant granites emplaced during the Middle to Late Devonian, marking the closure of igneous activity in the southern part of the Sierras Pampeanas de Córdoba, Argentina. The main feature of the CAB is the conspicuous circular shape of some of its plutons and internal structures, which were developed entirely under pressures lower than 2kb. The aim of this work is to study the processes responsible for the emplacement of these plutons. The CAB is composed of a succession of three major plutons aligned in a NNW–SSE trend that intrude mainly into mylonites corresponding to an Early Paleozoic shear zone. The emplacement of the plutons was controlled by a set of early master fractures, with magma ascent by means of fracture propagation. The space for magma was created by a combination of tectonically created cavities, internal magma-related buoyancy, and heating and thermal fracture of the host rocks. Field and structural evidence suggests that stoping mechanisms were the most effective for the final emplacement of magma. Thermal fracture and stoping were favored by the thermal contrast between granite and host rock and by the magma viscosity as reduced by the presence of fluorite. The apparent circular shape of the plutons is polygonal in detail and was developed from several linear and curviplanar segments as a result of brittle fracturing of the host rocks, as well as by changes in the stress field, from vertical to horizontal, related to magma arrival at shallow crustal levels.