The Variscan garnet peridotites from the Eastern Alps ( Ulten Zone): records of subduction metasomatism in the mantle wedge

At subduction zones, influx in the mantle wedge of fluids released by subducting plates affects the properties and composition of large mantle domains. These effects are investigated through field-based studies of mantle peridotites hosted in high- and ultrahigh-pressure (HP-UHP) terrains. We review the case-study of the Ulten Zone (UZ) peridotite lenses inside the eclogitefacies Variscan continental crust (Eastern Alps). These rocks record transition from spinel- (1200 °C; 1.5 GPa) to garnet-facies (850 °C; ~2.5 GPa) and represent mantle wedge tectonically emplaced in the subducted Variscan continental slab. Stability of amphibole and locally dolomite in the garnet-peridotites indicates modal metasomatism by COH fluids. Radiometric constraints indicate that HP metamorphism of crust and garnet-bearing mantle rocks took place simultaneously at ~ 330 Ma. Significant enrichment in Cs, Ba, Pb, U of the UZ garnet (+ amphibole + dolomite) peridotites indicates that fluids were sourced by the associated crust and infiltrated the peridotites at HP, in a tectonic environment comparable to a slab-mantle wedge interface. Measurement of Fe3+ contents of garnet from UZ peridotites enabled to estimate oxygen fugacities (FMQ to FMQ +2) higher than those of garnet-peridotite xenoliths from sub-cratonic mantle, thereby indicating an oxidizing environment during fluid influx in the UZ mantle. Comparable features pertain to UHP mantle wedge rocks from Western Norway, suggesting that similar metasomatic processes potentially act on a large range of subduction depths. The UZ peridotites are thus relevant to assess the fluid-mediated crust-to-mantle element transfer during subduction, a major process for the chemical differentiation of the Earth.