GNGTS 2019 - Atti del 38° Convegno Nazionale

238 GNGTS 2019 S essione 1.4 Differently, data from the French Massif Central with high P/T ratios fit better with P-T predicted in hot subduction, which suggests that either a hotter subduction is necessary to develop P-T conditions compatible with these data or that the amount and accuracy of the available radiometric data are insufficient to propose a comparison between natural geological data and model predictions. The lack of data with high P/T ratios from the FMC in continuous agreement with the slab of the second subduction during phase 3 is in agreement with the geographic distribution of the data, because evidences of HPmetamorphism related to the second subduction should be located further north than the FMC (e.g. in the Armorican Massif; Fig. 2). Although the ocean-continent margins in our model do not include an ocean-continent transition (OCT), data belonging to the upper gneiss unit (UGU) with accurate proposed ages, compatible with the first subduction, show very good fitting with both continental markers eroded from the upper plate - representing here a magmatic arc developed on continental crust of either the southern margin of Armorica or an unknown and lost microcontinent (Lardeaux, 2014) - and oceanic markers of the lower plate, coupled at the trench and successively subducted and exhumed in the mantle wedge. Consequently, our model shows the possibility that rocks from the UGU could have an origin different from an OCT. References Faure M., Lardeaux J.-M. and Ledru P.; 2009: A review of the pre-Permian geology of the Variscan French Massif Central . Comptes Rendus Geoscience, 341 , 202-213. Faure M., Leloix C. and Roig J.-Y.; 1997: L’Evolution polycyclique de la chaine hercynienne . Bulletin de la Société Géologique de France, 168(6) , 695-705. Franke W., Cocks L.R.M. and Torsvik T.H.; 2017: The Palaeozoic Variscan oceans revisited . Gondwana Research, 48 , 257-284. Guillot S., di Paola S., Ménot R.-P., Ledru P., Spalla M.I., Gosso G. and Schwartz S.; 2009: Suture zones and importance of strike-slip faulting for Variscan geodynamic reconstructions of the External Crystalline Massifs of the western Alps . Bulletin de la Societe Geologique de France, 180(6) , 483-500. Fig. 2 - Simplified tectonic sketch of the Variscan belt with the evolution for the FMC and the Alps as suggested by the fitting between natural P-T estimates and P-T conditions predicted by the double subduction model (DS.2.5). Arm: Armorican Massif; FMC: French Massif Central; MT: Maures-Tanneron Massif.