Geochemical evolution of melt/peridotite interaction at high pressure in subduction zones

The Borgo outcrop of the Monte Duria Area (Adula-Cima Lunga unit, Central Alps, Italy) is an excellent example of melt-peridotite interaction which occurred under a deformation regime at high pressure, that enabled the combination of porous and focused flow of eclogite-derived melts into garnet peridotites. Migmatised eclogites are in direct contact with retrogressed garnet peridotites and the contact is marked by a tremolitite layer, also occurring as boudins parallel to the garnet layering in the peridotites, derived from a garnet websterite precursor produced by the interaction between eclogite-derived melts with the peridotite at high pressure. LREE concentrations of tremolitite along a 120 m length profile, starting from the eclogite-peridotite contact to the inner part of the peridotite, show a progressive enrichment coupled with a peculiar fractionation. Numerical modelling assuming the eclogitic leucosome as the starting percolating melt reproduces the REE enrichment and LREE/HREE fractionation observed in tremolitite bulk rocks within the first 30 m. The comparison between the REE composition of the retrogressed garnet websterites along the profile and the result of our model suggests that reactive melt infiltration at high pressure is a plausible mechanism to modify the REE budget of mantle peridotites that lie on top of the subducting crustal slab.