Mineral re-equilibration and P-T path of metagabbros, Sierra de San Luis, Argentina: insights into the exhumation of mafic-ultramafic belt

At La Melada, Sierra de San Luis, Argentina, two lenses of mafic-ultramafic rocks, mainly consisting of metagabbros, are made up of igneous and metamorphic minerals in varying combinations. These rocks are part of a narrow NNE-SSW trending belt along the eastern flank of the Sierra and were affected by Famatinian metamorphism and deformation followed by shear deformation. La Melada metagabbros show a wide variety of microstructures including actinolite + chlorite + quartz pseudomorphs, epidote coronas and epidote + quartz symplectites, actinolite + quartz intergrowths and calcite + chlorite aggregates. These microstructural characteristics witness a complex metamorphic history consisting of a progressive cooling and decompression associated with regional and shear deformation from M0 down to the M1 to M3 greenschist/sub- greenschist stage. Cooling from the M0 stage (T≈780°C and <6.5 < P < 8.0 kbar) led to destabilization of pyroxene with talc formation at the M1 stage (T = 650-670 °C, P < 6 kbar). Further cooling resulted in the formation of actinolite + chlorite + quartz pseudomorphs growing on igneous and metamorphic amphibole (M2 stage <600 < T < 625 °C and P ≈ 3.8 kbar). The third re-equilibration stage led to the formation of epidote coronas and epidote + quartz symplectite around igneous plagioclase (M3 stage< 420 < T < 470 °C and 2 < P < 4 kbar). Calcite + chlorite aggregates partially replacing actinolite are evidence of the last stage of mineral re-equilibration. As compared to P-T path previously reported in the literature, La Melada rocks indicate that mineral re-equilibration reached P-T conditions as low as ∼420-470 °C, with epidote development under (sub-)greenschist-facies P-T conditions. The resulting P-T path is in agreement with the P-T path reconstructed for the country rocks.