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Variscan subduction of an Ordovician granite : the Lévézou massif, French Massif Central

Abstract : The Variscan Belt is usually interpreted as the result of an oceanic subduction followed by continental collision. The subducted ocean is represented by eclogitic lenses enclosed in amphibolite-facies rocks. This rock assemblage, called the Leptyno-Amphibolic Group (LAG), is considered as a suture unit that separates two different continental fragments (Upper Gneiss Unit, UGU and Lower Gneiss Unit, LGU). During the collisional stage, crustal thickening would have led to the emplacement of several synkinematic granites (e.g. Matte, 1986). Such an interpretation has been proposed for the Lévézou area (southern French Massif Central, fig. a) (Nicollet, 1978). The Pinet orthogneiss, initially dated at 360 Ma by lower intercept on zircon fraction (U/Pb, Pin, 1981), is intrusive in both the UGU and the LGU and was considered as a syn-kinematic granite contemporaneous with the Carboniferous thrusting (Pin, 1981; Burg et al. 1984, Burg & Teyssier 1983). Our recent observations show that the deformation pattern of this orthogneiss results from two superimposed deformations (see also Duguet and Faure, 2004) and do not correspond to C-S structures, symptomatic of synkinematic intrusions, as inferred previously (Burg & Teyssier, 1983). Its porphyric character confirms that the Pinet orthogneiss is not a synkinematic intrusion. Geochemical and geochronological analyses of the orthogneisses in both the UGU and the LGU show that they result from the melting of a common source and that they emplaced ca. 470 Ma ago (U/Pb dating on zircon external rims), during the early Ordovician. The geochemical signature of the orthogneisses and the mafic rocks of the LAG show affinities with a magmatic arc and its back arc basin, respectively. In the least deformed orthogneisses, rectangular clusters dominated by phengite, kyanite and garnet suggest a pseudomorphism of former crystals of magmatic cordierite (fig. b). Phase diagrams calculated with THERMOCALC suggest that the pseudomorphs developed in eclogite-facies conditions (ca. 14 - 17 kbar, 650°C, fig. b). These conditions are similar to those obtained from the eclogite lenses of the neighbouring LAG. Both the orthogneisses and the LAG experienced the same high-pressure event. Preliminary Ar-Ar dating on muscovite grains from both orthogneisses show that the exhumation and deformation took place ca 340 Ma ago. This suggests that Variscan tectonics in the Lévézou area corresponds to the subduction and accretion of a magmatic arc and associated narrow basins rather than a true continental collision. Burg, J.P., Delor, C., and Leyreloup, A., (1984), Structures tangentielles en Rouergue Oriental : Arguments tectoniques et métamorphiques, Comptes Rendus de l'Académie des Sciences de Paris, 298, 375-378 Burg, J.P., and Teyssier, C., (1983), Contribution à l'étude tectonique et microtectonique des séries cristallophylliennes du Rouergue oriental : La déformation des laccolites syntectoniques, type Pinet, Géologie de la France, 1,3-30. Duguet, M., and Faure, M., (2004), Granitoid emplacement during a thrusting event: structural analysis, microstructure and quartz c-axis patterns. An example from hercynian plutons in the French Massif Central.. Journal of Structural Geology, 26, 927-945.Matte, P., (1986), Tectonics and plate tectonics model for the Variscan Belt of Europe, Tectonophysics, 126, 347- 374 Nicollet, C., (1978), Pétrologie et tectonique des terrains cristallins anté-permiens du versant sud du dôme du Lévezou (Rouergue, Massif central), Bulletin du B.R.G.M (deuxième série), 1, 225-263. Pin, C., (1981), Old inherited zircons in two synkinematic variscan granitoids : the "granite du Pinet" and the "orthogneiss de Marvejols" (southern french Massif central), N. Jb. Miner. Abh., 142, 27-48
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Submitted on : Tuesday, August 23, 2016 - 10:16:39 AM
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Caroline Lotout, Pavel Pitra, Marc Poujol, Ermete Tondella, Gilles Ruffet, et al.. Variscan subduction of an Ordovician granite : the Lévézou massif, French Massif Central. Variscan 2015, Jun 2015, RENNES, France. ⟨insu-01355370⟩



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