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Les métaleucogranites du Haut Vivarais et leur environnement. Évolution et implications géodynamiques

Abstract : The study of the Haut Vivarais crystalline schists, based on a 1: 25,000 scale map of the St Agrève region, confirms the existence of a "clear line" whose elements constitute a "leptyno-gneissic complex": all facies of this complex have identical geochemical characters that allow to relate all to an old leucogranitic pluton witnessing a continental collision. This set is gneissified and then transformed by Vellave anatexis under low pressure conditions (780 ° - 3.8 kbars). The mineralogical and geochemical study of the anatectic facies and some mobilisates shows the primordial role that the fusion plays in the individualization of the "clear line" and makes it possible to apprehend the sequence of disappearance of the minerals as well as the chemical modalities of the fusion . The intercalations of metapelites and metabasites recognized within the leptyno-gneissic complex testify for their part of a different tectono-metamorphic evolution: the paroxysmal conditions of crystallization of these rocks estimated at T = 640 ° C and P = 5 kbars reveal in effect the existence of an early episode. Amphibole zonations and some metamorphic reactions, however, indicate that the rocks in question subsequently underwent the low pressure event. In addition, the geochemistry of amphibolites confirms the emplacement of the protolith in the continental distensive domain. The metamorphic characters and the geochemical typology of the "clear line" and its basic intercalations make it possible to insert the geodynamic evolution of the Vivarais Occidental in that of the Massif Central as it is currently envisaged by many authors: the setting up leucogranites probably seal the Devonian collision, to which it is probably necessary to relate the structuring of pelites and associated basic massifs. Amphibolites testify in West Vivarais of the Ordovician distension. The Vellave phase responsible for the gneissification then anatectomy of leucogranites does not erase the trace of an older episode in all metapelites-metabasites. The polyphase regional evolution ends at about 300 MA by the installation of the Baudinet late-tectonic granite.
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Patrick Lochon. Les métaleucogranites du Haut Vivarais et leur environnement. Évolution et implications géodynamiques. Minéralogie. Université Claude Bernard - Lyon 1, 1985. Français. ⟨tel-02052924⟩

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