Multi-stage metasomatism in a mantle wedge and exhumation mélange: New insights from the peridotites of the Ulten Zone (Eastern Alps, Italy)

The peridotites from the Ulten Zone (UZ) in the Italian Eastern Alps derive from a mantle wedge, were incorporated in a crustal slab during continental subduction in the course of the Variscan orogeny, and subsequently exhumed in a crust-mantle mélange. The complex pre-Alpine metamorphic history is documented by a variety of mineral assemblages and microstructures. This PhD work aims to shed light on individual melt- and fluid-mediated metasomatic stages affecting the UZ peridotites and to contribute to the understanding of the element cycle during crust-mantle interaction in a continental collisional setting: The UZ peridotites were subjected to refertilization likely induced by interaction with rising metasomatic liquids from deeper parts of the subduction zone. Inclusions of dolomite in primary spinel formed in the high-temperature spinel-stability field from these liquids. At the slab-mantle interface, the peridotites experienced interaction with aqueous fluids released from the crustal slab. The formation of discrete dolomite grains in fine-grained garnet-amphibole peridotites indicates that the fluids carried a carbon-component. Zircon geochronology yields an age of ca. 333 Ma reflecting the time of peridotite entrapment into the crustal host rocks. Zircon grew from aqueous fluid/s released from crystallizing leucosome during emplacement of the peridotites into the crustal host rocks. During exhumation to crustal levels, the peridotites interacted with fluids sourced from different adjacent lithologies. This is mirrored by highly variable stable-isotope (carbon and oxygen) compositions of carbonates in UZ peridotites. Late-stage serpentinization was accompanied by formation of calcite-brucite intergrowths as a product of fluid-mediated dolomite breakdown according to the reaction CaMg(CO3)2 + H2O → CaCO3 + Mg(OH)2 + CO2. Total carbon concentrations in UZ peridotites suggest that mantle-wedge peridotites can store crust-derived carbon via various processes involving melt/fluid-rock interaction, and that carbon is ultimately mobilized to crustal levels during exhumation of an orogenic crust-peridotite association.