Aluminous Granulites of the Archean Craton of Kasai (Zaire): Petrology and P-T Conditions

Abstract

Aluminous granulites of the Archean (2·8 Ga) Kasai craton (Zaire) consist of two main mineral assemblages: Grt-Opx and Sil±Grt±Crd rocks. The high-grade metamorphic conditions as deduced from Grt-Opx and Grt-Opx-Pl-Qtz equilibria are 720°C-6·7 kb. Considering the zoning of the same minerals, the slope of the P-T path is estimated at 15 b/°C. Thermobarometry involving Crd is consistent with those P-T conditions. Three cordierite-forming reactions have been observed petrographically: These equilibria are continuous reactions; end-member reactions have slopes less than 15 b/°C; they are decompression reactions occurring after the metamorphic climax. Using available thermodynamic data, (R3) fixes the oxygen fugacity to a value below the QFM buffer (log10fO2 = − 17·6 at 720°C, 6·7 kb and XH2Ofl=0.3 in the graphite stability field. The absence of graphite in the rocks shows that the end of the granulite facies metamorphism did not occur under important CO2 streaming. The polymetamorphic history of this Archean craton is considered.