Metamorphic and tectonic evolution at 31°36′S across a deep crustal zone from the Sierra Chica of Córdoba, Sierras Pampeanas, Argentina

Abstract

A geological section at 31°36′S cut the main structures of a representative area of the deep crust in the Sierra Chica (Sierras Pampeanas of Córdoba, Argentina), where associations of high-grade gneisses, migmatites, marbles, amphibolites and ultramafic rocks are recognized. A group of ultramafic bodies composed of serpentinized spinel harzburgites are associated with a basement affected by a M2-D2 regional metamorphism that developed the main S2 metamorphic foliation. Applying different thermobarometers to the varieties of garnet sillimanite gneisses and leucocratic garnet gneisses, a range of P-T conditions of 768-840 °C and 7.3-8.6 kb were obtained for M2. A strong D3 ductile deformation structured the ultramafic bodies, more viscous than the associated metamorphic rocks, generating large west-vergent sheath folds with tubular geometry. This constrictional deformation was developed in a simple shear regime, under high-temperature conditions, with a tectonic transport direction toward the northwest (~N 300°). Later, decompression and cooling down to amphibolite facies conditions occurred in all the rocks of the area during a retrograde M3 event. A range of P-T conditions of 588-710 °C and 5.2-7.3 kb were calculated for M3 in the gneisses of the area. The Sierra Chica ultramafic rocks outcrop as folded bodies, isolated between them and locally parallel to the S2 metamorphic foliation, therefore they cannot be correlated as a regional belt that could be interpreted as an ancient suture. Our hypothesis is that the ultramafic bodies are upper mantle relics of earlier emplacements into an accretionary prism that was entirely reworked during the Pampean orogeny. This same outcrop-pattern can also be recognized in the other ultramafic bodies in the Sierras de Córdoba. Hence, we suggest to avoid the use of the terms Eastern and Western Belts for the treatment of the ultramafic rocks in the Sierras de Córdoba. © 2010 Elsevier Ltd.