Exploring fluid-rock interactions along the subduction interface - The record within (ultra)high-pressure metasediments

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Subduction interfaces are the boundary zones between the downgoing and the upper plate during progressive orogeny and are important sites of deformation, fluid flow and metamorphism. These processes are often coupled and significantly modify crustal and mantle rocks and therefore affect the rheological and mechanical behaviour of units in the vicinity of such plate interfaces, thus influencing interplate coupling and seismicity. In this project, we will use a multidisciplinary approach combining microstructural, textural, petrological and geochronological methods to decipher, quantify and develop a model for the interplay between the mobilization and migration of fluids, the formation of interface-parallel shear zones, and metamorphic transformations within rheologically weak quartz-rich metasedimentary lithologies along the blueschist- to eclogite-facies part of the subduction interface. We will employ the relatively novel technique of in-situ U-Pb geochronology via LA-ICP-MS on garnet to constrain the timing and duration of phases of fluid-rock interactions and fluid-assisted prograde metamorphism. Metasediments of (ultra)high-pressure metamorphic terranes formerly metamorphosed and deformed along an important Alpine subduction interface between continental margin units in the hanging wall and the downgoing oceanic slab and now exposed in the Internal Western Alps of Italy will serve as an example.