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 peri-dotites, derived from a garnet websterite precursor produced by the interaction between eclogite-derived melts with the peridotite at high pressure. LREE concen-trations 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 com-parison between the REE composition of the retrogressed garnet websterites along the profile and the result of our model sug-gests 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.[GRAPHICS]