Eclogitic metamorphism in the Alpine far-west: petrological constraints on the Banchetta-Rognosa tectonic unit (Val Troncea, Western Alps)

The Banchetta-Rognosa tectonic unit (BRU), covering an area of 10 km2 in the upper Chisone valley, consists of two successions referred to a continental margin (Monte Banchetta succession) and a proximal oceanic domain (Punta Rognosa succession) respectively. In both successions, Mesozoic meta-sedimentary covers discordantly lie on their basement. This paper presents new data on the lithostratigraphy and the metamorphic evolution of the continental basement of the Monte Banchetta succession. It comprises two meta-sedimentary sequences with minor metaintrusive bodies preserving their original lithostratigraphic configuration, despite the intense Alpine deformation and metamorphic re-equilibration. Phase equilibrium modeling points to a metamorphic eclogitic peak (D1 event) of 20-23 kbar and 440-500 °C, consistent among three different samples, analyzed from suitable lithologies. The exhumation P-T path is characterized by a first decompression of at least 10 kbar, leading to the development of the main regional foliation (i.e. tectono-metamorphic event D2). The subsequent exhumation stage (D3 event) is marked by a further decompression of almost 7-8 kbar associated with a signifcant temperature decrease (cooling down to 350-400 °C), implying a geothermal gradient compatible with a continental collision regime. These data infer for this unit higher peak P-T conditions than previously estimated with conventional thermobarometry. The comparison of our results with the peak P-T conditions registered by other neighboring tectonic units allows to interpret the BRU as one of the westernmost eclogite-facies unit in the Alps.