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:. Grt+Qtz=Crd+Opx(R1)Grt+Sil=Crd+Spl(R2))Grt+Sil=Crd+Mag. (R3)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 (log 10f O2 = - 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 CO 2 streaming.The polymetamorphic history of this Archean craton is considered. © 1988 Oxford University Press.
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