High-temperature metamorphic imprint on calc-silicate granulites of Rayagada, Eastern Ghats, India: implication for the isobaric cooling path

Abstract

Calc-silicate granulites from Rayagada, north-central sector of Eastern Ghats granulite belt show a wide range of mineral assemblages and chemical compositions, which can be grouped as Gr. I (grossular- rich garnet-wollastonite-scapolite-calcite-clinopyroxene), Gr. II (andradite-rich garnet-scapolite-calcite-clinopyr- oxene), and Gr. III (scapolite-calcite-clinopyroxene-plagioclase) assemblages. Petrographic features suggest the following several reactions in the CaO–Al2O3–SiO2-vapor system: Mei+4Wo+Cal=3Grs+Qtz +2CO2, Mei+3Wo+2Cal=3Grs+CO2, Mei= 3An+Cal, Wo+CO2=Cal+Qtz, Mei+5Wo =3Grs+2Qtz+CO2, An+Wo=Grs+Qtz, Mei+ 5Cal+3Qtz=3Grs+6CO2, and the following reactions in the CaO–FeO–MgO–Al2O3–SiO2-vapor system: Cpxss+Scp+Wo=Grtss+Qtz+CO2, 4Hd+ 2Cal+O2=2Adr+2Qtz+2CO2, Cpxss+Scp= Grtss+Cal+Qtz. These reactions have been used to estimate peak T-XCO2 condition for these granulites. A maximum temperature of ∼920 °C has been calculated at an estimated pressure of 9 kbar. A T-XCO2 diagram shows an isobaric cooling from ∼920 °C to ∼815 °C. A range of XCO2 (0.50 at 920 °C to 0.25 at 815 °C) has been observed for Gr. I calc-silicate granulites based on the reaction sequences including coronal garnet-forming reactions. This sequence is suggestive of internal fluid buffering rather than external fluid influx and the differences in XCO2 conditions has been thought to be due to local buffering of fluid phases. Group II and Gr. III calc-silicate granulites, on the other hand, exhibit relatively lower temperature conditions.