Metamorphic evolution of the Río Santa Rosa Granulites, northern Sierra de Comechingones, Argentina

Highly anhydrous granulites from Río Santa Rosa in the eastern Sierras Pampeanas of Argentina occur as a thick lens surrounded by melt-depleted migmatites. Grt-Crd granulite composed of Qtz+Pl+Grt+Crd+Ilm±Spl±Ath±Phl is the dominant rock, whereas Opx-Grt granulite appears as discontinuous lenses in the center of the granulite body. Grt-Crd granulite includes blocks of metabasite that are relics of refractory lithologic beds interlayered in the supracrustal sequence. A distinct assemblage composed of Qtz, Pl, Grt, Crd, Opx, Spl, Crn, Sil, Bt, Phl, Ath, and Fe-Ti oxides in different combinations was generated in a reaction zone between Grt-Crd granulites and metabasites at peak metamorphism (850-900 °C and 7.6±0.5 kbar). The P-T trajectory of Grt-Crd granulites suggests an early prograde garnet-forming stage followed by nearly isothermal decompression that caused garnet breakdown. Melting and melt draining accompanying garnet growth was active during heating (to 900 °C) at intermediate pressures (∼7.6 kbar). Peak P-T estimates for Opx-Grt granulites are similar to those obtained with Grt-Crd granulites, which indicates that both granulites passed through the highest thermal stage. These results constrain the late evolution of Opx-Grt granulite to a garnet-consuming stage. Furthermore, they imply that garnet formation in Opx-Grt granulite happened at an early prograde P-T trajectory. Garnet growth in Opx-Grt granulite cannot result from heating at high pressure, which would lead to an apparent contradiction in the prograde P-T paths of the two granulites. This discrepancy may be solved by demonstrating that Opx-Grt granulite is the product of synmetamorphic mafic magmatism that was contaminated while cooling. The Río Santa Rosa granulites are inferred to have formed in a thickened crust in which mafic magmatic activity providing a local heat input.