Evidence for multi-stage metasomatism of chlorite-amphibole peridotites (Ulten Zone, Italy): Constraints from trace element compositions of hydrous phases

Peridotites from the Ulten Zone (Upper Austroalpine Domain, Central-Eastern Alps, Italy) derive from a mantle wedge environment and record a complex metamorphic history. This study focuses on amphibole-spinel peridotites and chlorite-tremolite peridotites. Chlorite is generally closely intergrown with Cr-spinel, indicating that it grew at the expense of former Al-spinel. No garnet relics or chlorite pseudomorphs after garnet have been found. Moreover, amphibole and chlorite trace element patterns display no fractionation in HREE, suggesting that these chlorite peridotites never equilibrated in the garnet stability field. On the basis of textures, bulk rock and mineral major and trace element compositions three stages of metasomatism are documented in the mantle rocks. We propose that the earliest stage represents melt impregnation at plagioclase peridotite conditions and is unrelated to subduction. The metasomatism leading to spinel- and chlorite-amphibole peridotites is related to the progressive influx of a fluid with crustal signature, derived from neighbouring subducted continental crust as the mantle wedge peridotites approach the slab. The observation that garnet peridotites and chlorite peridotites, which never equilibrated in the garnet stability field, are hosted within the same gneisses, suggests that slices of mantle wedge peridotite with different P-T trajectories can be sampled by subducted and exhumed crust.