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Âge, durée et enregistrement du métamorphisme de haute pression dans le massif Central

Abstract : Subduction is one of the key stages of the mountain building processes. It leads to the development of high-pressure (HP) metamorphism in the rocks that typically equilibrate in the blueschist or eclogite-facies conditions. Dating the HP metamorphism and estimating its intensity is therefore a major challenge when reconstructing geodynamics through time. Through a petrological and geochronological study, this PhD dissertation aims to better constrain conditions, durations and timings of HP metamorphism in the southern French Massif Central (European Variscan Belt). The massifs of Najac, Montagne Noire and Lévézou were investigated by a multi-method geochronological approach (zircon, rutile and apatite U-Pb dating, garnet Lu-Hf and Sm-Nd dating, biotite and muscovite 40Ar-39Ar dating) associated with a petrological analysis including numerical modelling of phase equilibria (Theriak-Domino and THERMOCALC). The Najac eclogites reached 560-630 °C at 15-20 kbar and the prograde part of the highpressure metamorphic event lasted for ~ 7 Myr starting at ~ 383 and peaking at ~ 376 Ma. Eclogites hosted in sillimanite-bearing migmatites in the Montagne Noire dome (French Massif Central) reached c. 750°C, 21 kbar before significant decompression at high temperatures. However, none of the obtained geochronological dates could be associated with the HP event. The study of the Najac massif and the Montagne Noire Dome highlight potential decoupling between the REE and the U-Pb isotopic systems. The emplacement of the protoliths of felsic and mafic HP rocks in the Lévézou Massif was estimated at ca. 470 Ma. HP metamorphism peaked at 21-23 kbar and 680-800°C at ~358 Ma. The subsequent fast exhumation reached 8-9.5 kbar and ~600°C at ~352 Ma, highlighting a very fast exhumation followed by a cooling rate of 50°C/Ma. Granites from the Lévézou massif display kyanite-garnet-muscovite-quartz pseudomorphs after cordierite that equilibrated at ca. 15-17 kbar and ~670°C. The major deformation in these granites is associated with the exhumation stage, locally accompanied by partial melting, and seems to occur in different episodes, from ~352 Ma to ~340 Ma. On a larger scale, these results are fully in line with the HP ages described in the Variscan Belt and allow to reconsider the tectonics in the French Massif Central.
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Caroline Lotout. Âge, durée et enregistrement du métamorphisme de haute pression dans le massif Central. Sciences de la Terre. Université Rennes 1, 2017. Français. ⟨NNT : 2017REN1S128⟩. ⟨tel-01810106⟩

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