P-T-t path reconstruction in a syn-deformational migmatization event along the north-central portion of Sierra de Comechingones, Córdoba, Argentina

Abstract

The granulite-complexes of the Sierras de Córdoba, inside context of Sierras Pampeanas, expose the deeply-uplifted internal metamorphic zone of an early Cambrian orogen. One of the largest granulite-complexes in the north-central of Sierra de Comechingones consists mainly of a set of stromatic and diatexitic migmatites all derived from metasedimentary progenitors. Migmatites vary over a spectrum of morphologies ranging from patch, net, and stromatic metatexites to schollen-schlieren and nebulite diatexites with widespread interspersed anatectic granites. The granulite-complex subordinately includes amphibolites, marbles, serpentinized ultramafic rocks, and mafic plutonic rocks. Petrographic analysis, textural and structural interpretation, thermobarometric estimates, and inherited and metamorphic zircon dating reveal the metamorphic history of migmatites in the north-central part of the Sierra de Comechingones. Metastable preservation of kyanite reflects that migmatites were buried (0.69–0.78 GPa) and heated (~650 °C) along a prograde trajectory before the onset of extensive anatexis. Then migmatites were further heated to granulite-facies peak temperatures (≥800 °C) at similar pressures (0.77 ± 0.06 GPa) through the sillimanite stability field. Widespread partial melting of the metasedimentary sequence occurred during peak metamorphism. The entire granulite-complex cooled to temperatures below 550 °C and ascended to middle crustal depths (0.3–0.5 GPa). The prograde heating trajectory and decompression during uplift overcoming cooling and tectonic exhumation are compatible with a predominantly compressive deformation. Coeval widespread partial melting with contractional non-coaxial deformation triggered self-reorganization of anatectic melts giving rise syn-deformational migmatites. In all the analyzed migmatites, U–Pb zircon geochronology yields inherited ages corresponding to those of orogenic systems that characterize the West Gondwana supercontinent. Inherited ages show a polymodal distribution pattern with contributions from Neoproterozoic (551–734 Ma; i.e., Brasiliano - Pan-African) and Mesoproterozoic detrital zircons (850–1182 Ma; i.e., Grenvillian), and minor proportions of Paleoproterozoic zircon grains (1751–1839 Ma). Ages interpreted as metamorphism and anatexis suggest the development of a single metamorphic event and restricted anatexis to a relatively short period of about 15 Ma (ca. 540-525 Ma). Metamorphic ages associated with the metamorphic peak trajectory are related to the Cambrian Pampean Orogeny.