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

The contact between garnet peridotites and eclogites is marked by a tremolitite layer. The same rock also occurs as layers within the peridotite lens, showing a boudinage parallel to the garnet layering of peridotites, flowing in the boudin necks. This clearly indicates that the tremolitite boudins formed when peridotiteswere in the garnet stability field. Tremolitites also show Phl + Tc + Chl + Tr pseudomorphs after garnet, both crystallised in a static regime postdating the boudins formation, suggesting that they derive from a garnet-bearing precursor. Tremolitites have Mg# N 0.90 and Al2O3 = 2.75 wt% pointing to ultramafic compositions but also show enrichments in SiO2, CaO, and LREE suggesting that they formed after the reaction between the eclogite-derived melt and the garnet peridotite at HP. To test this hypothesis, we performed a thermodynamic modelling at fixed P = 3 GPa and T = 750 degrees C to model the chemical interaction between the garnet peridotite and the eclogitederived melt. Our results show that this interaction produces an Opx + Cpx + Grt assemblage plus Amp + Phl, depending on the water activity in the melt, suggesting that tremolitites likely derive from a previous garnet websterite with amphibole and phlogopite.

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.