Relict meso and micro-structures in orogenic garnet peridotites as tracers of mantle dynamics and metasomatism at convergent plate margins

At subduction zones, a number of geologic processes are caused by influx in the supra-subduction mantle wedge of fluid phases released by the subducting plates. The distribution of fluids in such settings affects the mineralogical, chemical and structural transformation of rocks and, hence, the survival of relict minerals and structures of previous events. These features can be investigated by means of field-based studies of high and ultrahigh-pressure (HP-UHP) orogenic terrains that contain mantle wedge materials tectonically sampled by the subducting plates. Here we review two examples of garnet peridotites hosted in HP-UHP continental crust, which record different P-T stories: (i) shallow spinel-facies lithospheric mantle wedge down-dragged to depth during subduction and recrystallized to garnet + amphibole assemblages due to the infiltration of crust-derived fluids (Ulten Zone garnet peridotites, Eastern Alps, Italy); (ij) transition-zone mantle upwelled and accreted to cratonic roots, and involved in subduction-zone recrystallization at 200 km depth enhanced by crustal fluids (UHP garnet peridotites, Western Gneiss Region, Norway). Our textural and petrologic study shows that the water distribution controls development of the new assemblages and the inetasomatic imprints of these rocks, independently on the depth and degree of metamorphism. We conclude that mantle re-fertilization by crust-derived subduction fluids is an effective mechanism working on a 100-200 km depth range.