Aluminous Granulites of the Archean Craton of Kasai (Zaire): Petrology and P-T Conditions |
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Authors: | BINGEN BERNARD; DEMAIFFE DANIEL; DELHAL JACQUES |
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Institution: | 1Universit? libre de Bruxelles, laboratoires de g?ologie-p?trologie-g?ochronologie (CP 160) 50 av. F. D. Roosevelt, 1050 Bruxelles, Belgium
2Department of Geology and Mineralogy, Royal Museum of Central Africa 1980 Tervuren, Belgium
3Aspirant au Fonds National de la Recherche Scientifique |
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Abstract: | Aluminous granulites of the Archean (2?8 Ga) Kasai craton (Zaire)consist of two main mineral assemblages: Grt-Opx and Sil?Grt?Crdrocks. The high-grade metamorphic conditions as deduced from Grt-Opxand Grt-Opx-Pl-Qtz equilibria are 720?C-6?7 kb. Consideringthe zoning of the same minerals, the slope of the P-T path isestimated at 15 b/?C. Thermobarometry involving Crd is consistentwith those P-T conditions. Three cordierite-forming reactions have been observed petrographically:
These equilibria are continuous reactions; end-member reactionshave slopes less than 15 b/?C; they are decompression reactionsoccurring after the metamorphic climax. Using available thermodynamic data, (R3) fixes the oxygen fugacityto a value below the QFM buffer (log10fO2 = 17?6 at720?C, 6?7 kb and in the graphite stability field. The absence of graphite in the rocks showsthat the end of the granulite facies metamorphism did not occurunder important CO2 streaming. The polymetamorphic history of this Archean craton is considered. |
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