Authors: BERNARD BONIN
Abstract: The Alpine belt comprises several pieces of Pre-Mesozoic basement, which have received various imprints of the successive Paleozoic orogenic episodes. If Pre-Mesozoic palinspatic reconstructions are taken into account, the Alpine basement can be considered as one single area before Mesozoic continental breakup. Contrasting magma associations were emplaced according to space and time. There exists a controversy about the existence of the Caledonian orogenic episode. The Lower Paleozoic orogenesis was advocated through different lines of evidences in the southern Alpine basement, such as emplacement of Ordovician and Silurian protoliths of calc-alkaline and alkaline orthogneisses. The Variscan orogenic episode ended with the Late-Devonian collisional stage. Then, the Alpine basement was subjected to large-scale shearing effects accompanying lithosphere distensional thinning, high geothermal regimes and low-pressure regional metamorphism. The middle crust ultimately underwent "wet" and/or H 2 O-deficient partial melting processes, which may or may not have been induced by the intrusion of mantle-derived melts. Peraluminous crustal melts were associated and/or mixed with high-K basic magmas. Lower to Middle Carboniferous high-K calc alkaline suites characterize the culmination of the post-collisional stage, that was related to uplift and erosion in short-lived transpressional and/or transtensional environments. At the Permian-Carboniferous boundary, near alkaline suites, yielding both alkaline and calc-alkaline features, were emplaced as volcanic-plutonic massifs in a major distensional regime and mark the end of the Variscan orogenesis sensu stricto. Upper Carboniferous to Early Permian calc-alkaline granitoids were emplaced only at the southern flank of the Variscan belt. They yield features characteristic of subduction-related suites and resemble closely the circum-Pacific Cordilleran batholiths. They indicate a renewed ocean-continent plate margin convergence, which could be related to the alleghanian orogenesis. This episode, well developed in the southern Appalachians, can be evidenced in North Africa, the Alpine belt and further east to Asia. Mid-Permian post-orogenic and Late Permian to Triassic early anorogenic alkaline complexes constitute a part of the Western Mediterranean alkaline province. Associated thermal imprints were recorded in the basement mineral isotopic clocks. The anorogenic magmatic activity can be related both to continental consolidation of the European plate and precursory stage in the formation of the Meso-Tethys oceanic basin.
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