This contribution aims at filling this gap: the metamorphic peak conditions registered by the BRU continental basement are quantified using a petrologic approach based on phase equilibria modeling, and the metamorphic evolution of this poorly investigated sector of the Western Alps is constrained for the first time. However, at present, quantitative studies aimed at estimating the metamorphic peak conditions of the BRU are lacking. Ballèvre et al., 2020 Beltrando et al., 2010 Bousquet et al., 2008), the BRU is located in the external part of the belt, characterized by low temperature, high pressure (LT/HP) metamorphism with peak conditions at the blueschist-eclogite-facies transition. According to the available, large-scale, metamorphic maps of the Western Alps (e.g. This study focuses on the Banchetta-Rognosa tectonic unit (BRU Corno et al., 2019, 2021), which is a composite unit including continental and oceanic derived rocks, exposed between Chisonetto and Troncea valleys (tributary valleys of the upper Chisone valley, Italian Western Alps Figs. 1a) presently exhumed within the Liguria-Piemonte domain registering a metamorphic peak at the blueschist to eclogite-facies transition. Based on the current state of knowledge, the continental Acceglio-Col Longet nappe (Michard et al., 2003 Schwartz et al., 2000) and the Ambin-Vanoise massifs (Strzerzynski et al., 2012) are the westernmost units (Fig. Alpine peak P–T conditions range from eclogite-facies conditions in the inner part of the belt (Internal Crystalline Massifs, Sesia zone, Zermatt-Saas-like units), with locally coesite-eclogite-facies conditions (Lago di Cignana unit, Internal Piedmont Zone on the left side of the Aosta valley, and Brossasco-Isasca unit, southern Dora Maira massif) to lower greenschist-facies conditions in the more external part (Briançonnais-Houillere zone). 1a), show metamorphic peak conditions decreasing from east to west. The Western Alps, consisting of a stack of European and Adriatic continental units and Liguria-Piemonte oceanic units (Fig. Burov et al., 2014 and references therein). Defining the extension, zoning and P–T evolution of such subducted crustal units is of primary importance for interpreting the large-scale processes of continental subduction and exhumation in the orogenic systems (e.g. Bolin et al., 1995 Nakamura & Hirajima, 2000 Okay et al., 1989 Zhang et al., 1995) and Norway (e.g. Chopin et al., 1991 Compagnoni, 2003 Groppo et al., 2019), Himalayas (e.g. This is notably well-recognized for example in the Alps (e.g.

In collisional zones, segments of continental and oceanic crusts are subducted to great depths and then exhumed as high to ultra-high pressure (HP/UHP) units (Guillot et al., 2009). 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. These data infer for this unit higher peak P–T conditions than previously estimated with conventional thermobarometry. The subsequent exhumation stage (D3 event) is marked by a further decompression of almost 7–8 kbar associated with a significant temperature decrease (cooling down to 350–400 ☌), implying a geothermal gradient compatible with a continental collision regime.

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. Phase equilibrium modeling points to a metamorphic eclogitic peak (D1 event) of 20–23 kbar and 440–500 ☌, consistent among three different samples, analyzed from suitable lithologies. It comprises two meta-sedimentary sequences with minor meta-intrusive bodies preserving their original lithostratigraphic configuration, despite the intense Alpine deformation and metamorphic re-equilibration. This paper presents new data on the lithostratigraphy and the metamorphic evolution of the continental basement of the Monte Banchetta succession. In both successions, Mesozoic meta-sedimentary covers discordantly lie on their basement. The Banchetta-Rognosa tectonic unit (BRU), covering an area of 10 km 2 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.

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