High pressure melting of eclogites and metasomatism of garnet peridotites from Monte Duria Area (Central Alps, N Italy): A proxy for melt-rock reaction during subduction

In the Monte Duria area (Adula-Cima Lunga unit, Central Alps, N Italy) garnet peridotites occur in direct contact with migmatised orthogneiss (Mt. Duria) and eclogites (Borgo). Both eclogites and ultramafic rocks share a common high pressure (HP) peak at 2.8 GPa and 750 degrees C and post-peak static equilibration at 0.8-1.0 GPa and 850 degrees C. Garnet peridotites show abundant amphibole, dolomite, phlogopite and orthopyroxene after olivine, suggesting that they experienced metasomatismby crust-derived agents enriched in SiO2, K2O, CO2 and H2O. Peridotites also display LREE fractionation (La-N/NdN= 2.4) related to LREE-rich amphibole and clinopyroxene grown in equilibriumwith garnet, indicating thatmetasomatismoccurred at HP conditions. At Borgo, retrogressed garnet peridotites showlowstrain domains characterised by garnet compositional layering, cut by a subsequent low-pressure (LP) chlorite foliation, in direct contactwithmigmatised eclogites. Kfs + Pl + Qz + Cpx interstitial pocket aggregates and Cpx + Kfs thin films around symplectites after omphacite parallel to the Zo + Omp + Grt foliation in the eclogites suggest that they underwent partial melting at HP.