P-T evolution of `crustal garnet peridotites and included pyroxenites from Nonsberg area (upper Austroalpine), NE Italy: from the wedge to the slab

Up-to-date thermobarometers have been applied to garnet-bearing peridotites and pyroxenites from Nonsberg area (upper Austroalpine, NE Italy), a remnant of the lower crust of the Central European Variscan belt. The Nonsberg ultramafic rocks record an unusual metamorphic history, from high-temperature, moderate-pressure, spinel-facies conditions to low-temperature, high-pressure, garnet-facies conditions. This peculiar P-T evolution is interpreted as a result of subduction processes, which involved mantle-wedge peridotites overlying a subducting continental slab. The following scenario is proposed based on our P-T estimates and available physical models for subduction zones. Relatively high-temperature (~1200°C, 1.31-1.58 GPa) mantle-wedge peridotites (spinel lherzolites) were intruded by hot (>1400°C) hydrous melts rising from deeper, inner portions of the wedge. Coarse-grained, garnet-free pyroxenites were segregated from these melts and formed veins in the host peridotites. Convection within the wedge induced by the downgoing slab caused the peridotites to flow towards the slab while cooling at essentially constant pressure. The peridotites were then dragged to greater depths at slightly decreasing, or nearly constant, temperature by the downward mantle flow near the wedge corner and near the wedge-slab interface and eventually emplaced into the underlying slab. Entrainment in the continental crust, either as tectonic slices or as sinking mantle blobs, was probably favored by density contrast between overlying mantle and underlying crust. Mantle flow and entrainment in the cold continental crust caused the peridotites to cool down to ~850°C before, or while, being subducted together with the slab to depths of about 90 km. During this T-decrease, Pincrease path, garnet formed at the expense of spinel and pyroxenes, giving rise to garnet ± spinel-bearing lherzolites and pyroxenites.