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METABASITES GRANULITIQUES, ANORTHOSITES ET ROCHES ASSOCIEES DE LA CROUTE INFERIEURE - Exemples pris à Madagascar et dans le Massif Central français - ARGUMENTS EN FAVEUR D'UN METAMORPHISME ASSOCIE A L'EXTENSION LITHOSPHERIQUE

Abstract : In its pre-drift position, before the end of the Palaeozoic, Madagascar was adjacent to Kenya and Tanzania and was located on the eastern front of the Mozambican belt, juxtaposed against the Indian craton. The crust of Archean age was reactivated during the Proterozoic (2.6 Ga, 1.1 Ga ?, 850-550 Ma The metamorphic events were generally high grade and formed extensive granulitic terrains. In the north of the island, granulites associated with belts of basic and ultrabasic magmatic rocks indicate high grade conditions : T=1000°C and P=6kb. Aluminous rocks contained Q, Al rich Opx, Ga, Sill, rut and/or ilm, Sp, Feld and probably osumilite and sapphirine. Primary Opx contain garnet lamellae and are aluminous-rich (7-10 wt % Al2O3). Near isobaric retrogression and hydration produced arthoamphibole-cordierite gneisses. In the south of Madagascar, the metamorphic grade increases from greenschist to granulite grade going from west to east. Supracrustal metabasites (sapphirine-corundum amphibolites, serendibite and clintonite clinopyroxenites, etc.) have undergone a prograde event. The synmetamorphic, anorogenic gabbro-anorthosite intrusive complex is related to an increased geothermal gradient going from W (high-P granulite facies) to E (intermediate-P granulite facies). Isobaric cooling was followed by decompression (P-T path concave towards the T axis). Gneisses with Al rich Opx, Ga, Cord, Sp+Q are common in the SE and suggest high T. A widespread anatectic event produced Q-Kf-Pl-Ga-Cord-Sill-Bio gneisses and biotite rich residues which contain Sapphirine, kornerupine and grandidierite. It is proposed that the magmatic intraplating, the very high T metamorphism, the P-T path concave towards the T axis and the variation in the geothermal gradient are the consequences of continental lithospheric thinning by extension followed by a compressive event.
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https://tel.archives-ouvertes.fr/tel-00787525
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Submitted on : Tuesday, February 12, 2013 - 12:29:44 PM
Last modification on : Tuesday, October 19, 2021 - 6:59:10 PM
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  • HAL Id : tel-00787525, version 1

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Christian Nicollet. METABASITES GRANULITIQUES, ANORTHOSITES ET ROCHES ASSOCIEES DE LA CROUTE INFERIEURE - Exemples pris à Madagascar et dans le Massif Central français - ARGUMENTS EN FAVEUR D'UN METAMORPHISME ASSOCIE A L'EXTENSION LITHOSPHERIQUE. Pétrographie. Université Blaise Pascal - Clermont-Ferrand II, 1988. Français. ⟨tel-00787525⟩

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