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Pétrologie structurale et pétrogenèse des formations plutoniques septentrionales du Massif des Ballons (Vosges, France)

Abstract : The Massif des Ballons (MB), a porphyritic granite pluton, constitutes a major unit of the Southern Vosges Variscan basement. It was emplaced into a thick fossiliferous Viséan series, that recorded a syntectonic volcanic-sedimentary deposition attributed to what Vosges geologists use to call Intra-Viséan Vosges Events (IVVE). Along the northern margin of the MB pluton a narrow zone (NMB: 12 km x 1 km) comprises rock-types ranging from gabbro to quartz monzonite. They exhibit intricate relationships, such as magmatic breccias. Structural mapping of the whole MB helped evidencing the unique, unconformable features of the NMB with respect to the rest of the MB. The NMB might have emplaced earlier, synchronous with the IVVE, as an early Viséan precursor or the putatively Namurian MB. From the petrography, mineralogy and geochemistry of the NMB, a differentiated magmatic suite was evidenced, controlled mostly by crystal fractionation under changing conditions. Anhydrous minerals fractionated first, yielding Fe,Ti-enriched liquids (= Fenner Trend of the dolerite + microdiorite + diorite group). Next, a sharp increase of pH2O, related to the mobilization of the sialic host rocks, impulsed the fractionation of hydrous minerals and generated Al,Si-enriched liquids (monzodiorite + monzonite + quarz-monzonite group). The cumulate group, with gabbro, diorite and vaugnerite types, exhibits successive assemblages that recorded this major evolution of the differentiation controls. Detailed investigations were performed of the magmatic breccias that abound in the NMB plutonic formations. (i) Some breccias (classified as of plastic-dynamic type) denote intimate magma associations, with breccia generated by emission of composite veins. (ii) Located at the roof of the NMB, a breccia (denoted as fragile-static) derives from the dismantling of a consolidated rock by a magmatic venue. (iii) A detailed modeling of flow and differentiation mechanisms in a vein reveals a telescopic intrusion process and suggests the new concept denoted as "vein intra-rims". Magma differentiation and magma emplacement might thus have been controlled by regional tectonic strains. The NMB mafic to intermediate plutonites, genetically related to the other magmatic activities during the Early Viséan times, testify to a mafic magmatism, probably of tholeiite type, that was emitted along weak zones of a mobilized sialic host, here termed "continental tearing".
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Francis André. Pétrologie structurale et pétrogenèse des formations plutoniques septentrionales du Massif des Ballons (Vosges, France). Pétrographie. Université Nancy 1, Faculté des sciences, 1983. Français. ⟨tel-01782377⟩

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