Geothermobarometry of Qtz + Pl + Bt + Grt + Sil paragenesis in high-grade gneisses from central-eastern portion of Sierra de Comechingones

Abstract

The metamorphic basement on the central portion of Sierra de Comechingones (Córdoba), consist mainly of Bt-Grt Bt-Grt (± Sil) gneisses, stromatic migmatites and diatexites. During metamorphic regional peak M2-D2, high-grade conditions have been reached, producing partial melting of metapelitic rocks and developing a pervasive metamorphic foliation S₂. To determine pressure and temperature conditions during M2, Bt-Grt-Sil gneisses samples were selected and main mineral phases were analyzed by microprobe. Grt-Bt thermometry and GASP barometry (Grt-Sil-Qtz-Pl), supplemented by GRAIL barometry (Grt-Rt-Sil-Il-Qtz) were applied to the Qtz + Pl + Bt + Grt + Sil paragenesis. The garnets exhibit varying degrees of resorption with embayed edges and irregular crystal outlines. Partial retrograde resorption is corroborated by garnet profiles, which show strong Fe and minor Mn enrichment at the rims. Grt points inside the modified rim were chosen for thermobarometry to avoid the retrograde effects and are believed to be the closest possible approximation to metamorphic peak conditions. These compositions were paired with matrix biotite and/or plagioclase for the P-T calculations. The thermobarometric results of 776°C and 7,9 kb are consistent with the observed phase equilibria and the migmatized nature of the area. These results for M2-D2 event indicate typically barrovian metamorphic conditions during the climax of collision of Pampia terrane with the western Gondwana margin.