High pressure melting of eclogite: structural and geochemical evidence for multiple metasomatism of grt-peridotites from the Duria Area (Central Alps, N Italy)

The Duria Area is located in the southern part of the Adula Nappe complex on the north-eastern side of the Como Lake (N Italy). Garnet/chlorite peridotites outcrop within migmatitic gneiss or in direct contact with amphibole-bearing migmatites containing boudins of variably granulitised eclogites. The contact between mafic and ultramafic rocks is marked by a metasomatic rim composed by tremolite and dispersed round-shaped chlorite aggregates, which also occurs within the peridotite body. The occurrence of lobes and cusps structures and Qtz+Pl+Kf+Bt leucosomes in eclogites provide evidence for partial melting of these rocks. Petrography and mineral chemistry indicate that peridotites and associated eclogites experienced a HP metamorphic peak at ≈ 2.8 GPa and ≈ 730 ± 20 °C, followed by a granulitic overprint at 0.8-1.2 GPa and 850 °C during their exhumation path (Tumiati et al. 2018). The micostructural evidence of Qtz+Pl+Kf+Bt pockets in eclogites support the field evidence of a partial melting event. Eclogites are characterised by a HP assemblage formed by Grt+Ky+Omp+Kf, suggesting that a former phengite was completely consumed by the reaction Phe+Cpx+SiO2=Grt+Ky+Kfs+Melt. At this stage a clinopyroxenite reaction front probably results from a Ca-rich mafic silicate melt-peridotite interaction. Therefore, we suggest that tremolite-rich metasomatic rim probably represents a former grt- clinopyroxenite reaction front subsequently retrogressed at fluid-present conditions through the reaction Cpx+Opx+Grt+H2O=Tr+Chl. Bulk rock trace element analyses of peridotites show a strong fractionation in REE (La/NdN=2.4) likely indicating that these rocks record an interaction with a silicate melt. The metasomatic rim shows higher REE concentrations with a LREE enrichment and a slight Eu negative anomaly. This pattern broadly resembles the trace elements composition of pyroxenites from other localities of eastern Europe (i.e. Gföhl Moldanubian Nappe), interpreted as the product of infiltration of slab-derived melts in the overlying lithospheric wedge (Medaris et al., 2006). The trace elements patterns of peridotites, metasomatic rim, and associated eclogites show also an enrichment in fluid-mobile elements, such as Cs, Rb, Ba and K, and an high U/Th ratio suggesting that a subsequent fluid-assisted metasomatic event affected both mafic and ultramafic rocks under granulite or amphibolite facies conditions. The Monte Duria area represents a unique case study where mafic melt-peridotite interaction occurs at high pressure and relatively high temperature in the Adula Nappe complex. The melt-rock interaction recorded by the Duria peridotites could thus represent a proxy for the crust-to mantle mass transfer at great depths in “warm” subduction environments. Medaris, L.G., Beard, B.L. & Jelínek, E. (2006): Mantle-Derived, UHP Garnet Pyroxenite and Eclogite in the Moldanubian Gföhl Nappe, Bohemian Massif: A Geochemical Review, New P-T Determinations, and Tectonic Interpretation. Int. Geol. Rev., 48, 765-777. Tumiati, S., Zanchetta, S., Pellegrino, L., Ferrario, C., Casartelli, S. & Malaspina, N. (2018): Granulite facies overprint in garnet peridotites and kyanite eclogites of Monte Duria (Central Alps, Italy): clues from srilankite- and sapphirine- bearing symplectites. J. Petrol., doi:10.1093/petrology/egy021.