Fluid pathways and high pressure metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)

International audienceThe Mt. Emilius klippe (Western Alps, Italy) corresponds to a segment of the stretched Adriatic continental margin metamorphosed in granulite-facies during Permian. This slice was subducted during the early Cenozoic Alpine subduction with the underlying eclogite-facies remnants of the Tethyan seafloor (Zermatt-Saas zone). Near the base of the Mt. Emilius massif, a shear zone presents eclogite-facies hydrofracture systems associated with deformation-induced re-equilibration of granulites during high pressure metamorphism. We herein focus on a pluri-hectometer-length domain of sheared mafic boudins hosted in the granulitic paragneiss. These mafic boudins exhibit garnetites and garnet veins, clinopyroxenites, clinozoisite and calcite veins. We report multiple events of fracture opening, brecciation, boudinage and parallelization of structures coevally with fluid-rock interaction, metasomatism and volume change. Our integrated petrological, micro-textural and geochemical investigations illustrate the multiplicity and the chemical variability of fluid sources during prograde to peak metamorphic evolution in the lawsonite-eclogite-facies field (at ~ 2.15-2.4 GPa, 500-550°C). The calcite veins crosscutting the garnetites have relatively low δ18OVSMOW values (c. +6.5 per mil) near those for marble layers (and nearby calc-silicates) embedded within the metasomatized granulites (+8 to +10 per mil). It is proposed that infiltration of externally-derived H2O-rich fluids derived from the plate interface flushed the marbles, promoting decarbonation followed by short-distance transport and re-precipitation along garnetite fractures. This study highlights the importance of inherited structural heterogeneities (such as mafic bodies or sills) in localizing deformation, draining fluids from the downgoing plate, and creating long-lasting mechanical instabilities during subduction zone deformation