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

MARTINO, ROBERTO DONATO; GUERESCHI, ALINA BEATRIZ; ANZIL, PATRICIA ANDREA

Mar del Plata

Congreso; GeoSur 2010: International Geological Congress on the Southern Hemisphere; 2010

Istituto Nazionale di Oceanografia e Geofisica Sperimentale

            A geological section was constructed at latitude 31º 36’ S that cut the main structures of a representative area of the deep crust in the Sierra Chica (Sierras Pampeanas of Córdoba, Argentina) in west-east direction, where associations of high-grade gneisses, migmatites, marbles, amphibolites and ultramafic bodies 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 thermobarometry to the varieties of garnet sillimanite gneisses and leucocratic garnet gneisses, P–T conditions of 768–840 ºC and 7.3–8.6 kb were obtained for M2. A strong D3 postmetamorphic ductile deformation would have structured the ultramafic bodies, more viscous than the associated metamorphic rocks, generating large west-vergent sheath folds with tubular geometry. This constrictional deformation would have been developed in a simple shear regime, under high-temperature conditions, with a tectonic transport direction toward the northwest (~N 300º). Later, decompression and cooling down to amphibolite facies conditions would have occurred in all the rocks of the area during a retrograde M3 event. P–T conditions of 588–710 ºC 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. The ultramafic bodies would have been tectonically emplaced as upper mantle slices probably in an accretion prism. 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. The hangingwall and footwall outcrops, at the present time separated by the Tertiary Sierra Chica fault, have barometric differences that still show the influence of the Cretaceous extensional tectonic in the Sierra Chica.