An Unusual Garnet Pyroxenite from the Granulitgebirge, Germany: Origin in the Transition Zone (>400 km depths) or in a Shallower Upper Mantle Region?

Meter-sized boudins of a garnet pyroxenite layer embedded in serpentinized garnet peridotite of the Granulitgebirge, Germany, were studied. This rock consisted originally of megacrysts. Due to exsolution, almost mm-wide lamellae of garnet and dominating clinopyroxene formed, running parallel over several centimeters. On the basis of electron microprobe mineral analyses and geothermobarometry, we determined P-T conditions of ~25 kbar and 1000°C for the exsolution process. Recrystallization of the rock triggered by deformation led to a partially equigranular fabric. This happened mainly close to the final metamorphic stage at 11 kbar and 900°C after the mantle fragment had been inserted into the lower portion of thickened continental crust during the Variscan orogeny. We determined trace-element concentrations of the minerals by LA-ICP-MS and bulk-rock compositions of the garnet pyroxenite with XRF and ICP-MS. The precursor megacrysts were either Al-rich clinopyroxenes formed at ~17 kbar and 1400°C or majoritic garnets from an upper portion of the transition zone (> 400 km depth). According to REE patterns ((Nd/Yb)N ~ 0.03) and concentrations of other minor and trace elements such as K, Sc, Ti, Ni, and Sr in the former megacrysts, we suggest a majoritic garnet as precursor mineral. Uplift of the rock, consisting only of garnet megacrysts (probably a restite or cumulate) might have occurred within a mantle plume facilitated by the presence of melt that interacted with the rock, leading to U-shaped REE patterns.