Theses et Memoires de l'universite d'Antananarivo

Resume

In the south-western part of Madagascar, the Pan-African composite block of Vohibory contains many occurrences of corundum, sometimes of quality gem, scattered in calco-ferro-magnesium rich rocks and in stockworks. The information collected in this study are indispensable concerning the modes of formation of these corundum bearing rocks. The contribution of the structural data puts in evidence the predominance of interferences of folds type I (structures in dome and basin) in the sector of Anavoha. A global shortening oriented NW-SE results in a ductile distortion that would be responsible for the development of the shear zone of Vohitany. This shear zone is contemporary of the major stage of the regional metamorphism of the granulite facies to high amphibolite facies of proterozoic age. The major fractures (mylonitic narrow shear zone) of direction privileged N120, played the role of channels that would have facilitated the migration of the metasomatic fluid responsible for the formation of the hornblende-corundum bearing skarns. The evaluations of the thermodynamic conditions obtained by thermobarometry show predominant values of 730-740°C for the major stage of crystallization of the host rocks. These temperatures are from 730-790°C to 9-10kb whith aH2O = 0.8 - 1 in the deposits of Anavoha and 720-735°C for 8,4kb whith XCO2 £ 0.43 in Vohitany. The CFM rocks contain of the terms of gabbro composition of tholeiitic affinity for the metabasites or dark amphibolites and of the characters typical of the calco-alkali series for corroded amphibolites. These rocks show very near chemical compositions. The composition of the amphibolites partially metasomatised would be imposed by the one of a constant enough fluid in the block of Vohibory. The responsible fluids for the metasomatose are similar to the metamorphic fluids. The crystallization of phlogopite and corundum would be due to reactions of these fluids on the minerals of the amphibolites (hornblende, anorthite, spinel, garnet) and would require the very meaningful presence of water and potassium in the fluids.

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