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

OTAMENDI, J.E., TIBALDI, A.M., DEMICHELIS, A.H., Y RABBIA, O.M.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

Elsevier Ltd.

Lugar: Amsterdam; Año: 2005 vol. 18 p. 163 - 181

0895-9811

Highly anhydrous granulites from Río Santa Rosa in 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, which 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 presents an early –prograde– garnet-forming stage followed by nearly isothermal decompression causing garnet breakdown. Melting and melt draining accompanying garnet growth was certainly active during heating (up to 900 °C) at intermediate pressure (c. 7.6 kbar). Peak P-T estimates for Opx-Grt granulites are similar to those obtained with Grt-Crd granulites suggesting that both granulites passed through the highest thermal stage. These results allow constrain the late evolution of Opx-Grt granulite as a garnet-consuming stage. Further it implies 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 leads to an apparent contradiction in the prograde P-T paths of the two granulites. This discrepancy could be solved by demonstrating that Opx-Grt granulite is the product of syn-metamorphic mafic magmatism contaminated while cooled. The Río Santa Rosa granulites are shown to have formed in a thickened crust with mafic magmatic activity providing an extra heat input.