Variscan migmatites, eclogites and garnet-peridotites of the Ulten zone, Eastern Austroalpine system

In northwestern Trentino (Nonsberg-Ultental area, northeastern Italy) the strongly foliated high-grade gneisses, migmatites and the mafic and Spl/Grt-bearing peridotite lenses of the Ulten zone record metamorphic processes which could be related to the evolution history of the Central Europe Variscan belt. The high P-T conditions calculated for the gneisses (1.5 ± 0.5 GPa; 600 < T < 850°C, depending on the XH2O) are comparable to those calculated for the mafic rocks (1.4 GPa; 640 < T < 700°C) and for the Grt-peridotites (1.2 < P < 2.0 GPa; 630 < T < 814°C). These conditions should correspond to those under which the S1 deformation has developed the mylonitic foliation in the gneisses. The drop in P registered by the gneisses, mafic rocks and Grt-peridotites (down to P < 1.0 GPa; T < 900°C) and pointed out by a post-S1 migmatization, may be related to the relaxation and exhumation processes after thickening. The decompressional component of the exhumation history has induced a statical growth of Spl-pyroxene/Spl-Amph symplectites after Grt in the Grt-peridotites and of Cpx-Pl/Amph-Pl in the eclogites. Textures and T estimates obtained from primary pre-exsolution pyroxene porphyroclasts in Grt-peridotites suggest that these could come from a high-T mantle environment, possibly a mantle wedge overlying a subducting slab, and may have been incorporated as sliced fragments into the crust during the Variscan collision. Metasomatism in the Grt-peridotites seems to confirm this origin. This article demonstrates that the Ulten zone probably represents a deep fragment of the Variscan belt, slightly reworked during the Alpine orogeny. Hence, this poses the problem of the possible occurrence of Variscan remnants with a European affinity in the Austroalpine system (i.e., African plate).