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Serpentinites in an Alpine convergent setting: Effects of metamorphic grade and deformation on microstructures

^a Centre de Recherche en Matière Condensée et Nanosciences, CNRS, campus de Luminy, F-13288 Marseille cedex 09, France
^b present address: Institut de Minéralogie et de Physique des Milieux Condensés, CNRS UPMC IPGP, case 115, 4 place Jussieu, F-75252 Paris cedex 05, France
^c Laboratoire de Sciences de la Terre, CNRS UCB ENS-Lyon, bâtiment Géode, 2 rue Dubois, F-69622 Villeurbanne cedex, France
^d Laboratoire "Magmas et volcans", CNRS UBP OPGC, 5 rue Kessler, F-63038 Clermont-Ferrand cedex, France

^* Corresponding author, e-mail: auzende{at}impmc.jussieu.fr

Alpine antigorite serpentinites associated with eclogites were investigated to determine if they can be used as indicators of the tectono-metamorphic conditions during subduction and exhumation processes. The detailed petrology of serpentinites sampled in the Monviso massif (Western Alps, Italy) was combined with a transmission electron microscopy study. Alpine serpentinites display a degree of serpentinization close to 100%. Antigorite is the main mineral present, forming non-pseudomorphic textures in the various studied samples and exhibiting a homogeneous chemical composition, with limited cationic substitutions. Considering its oceanic origin, the Alpine serpentinite in the Monviso massif formed a lizardite + chrysotile assemblage that recrystallized under greenschist-facies conditions into poorly ordered antigorite, with a modulation wavelength showing significant variations at the crystal scale. Under blueschist-facies conditions, the modulation wavelength of antigorite becomes regular. Thus, periodic antigorites can be related to high-grade conditions, while poorly ordered antigorites characterize lower metamorphic grade. In the present study, we failed to observe any elimination of structural defects with increasing metamorphic grade. While around 50% of the antigorite crystals are highly ordered, it seems that this ordering is at least partly obliterated by retrogressive deformation. Antigorite displays strong evidence of deformation-sensitivity, and the observed microstructures can be directly related to the mechanical behaviour of serpentinites in subduction zones. We investigated the deformation-induced microstructures in serpentinites collected in the Erro-Tobbio eclogitic unit (Ligurian Alps, Italy), which appear to preserve prograde and retrograde structures formed during subduction. According to the microstructural evidence, shearing is accommodated by brittle and/or ductile deformation mechanisms. Collected samples were fractured at different scales (cm to nm) and have a well-developed schistosity characterized by a strong crystallographic fabric. With increasing metamorphic grade, the brittle behaviour gives way to pressure-solution, which persists up to eclogite-facies conditions. The common obliteration of high-grade microstructures in antigorite, as observed in the Monviso serpentinites, results from continuous recrystallization of this mineral during retrogressive deformation.

Key-words: serpentinite, antigorite, HP-LT metamorphism, microstructures, deformation, pressure solution.

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