Abstract
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 meta-intrusive 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 significant 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.
Highlights
In collisional zones, segments of continental and oceanic crusts are subducted to great depths and exhumed as high to ultra-high pressure (HP/UHP) units (Guillot et al, 2009)
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
The detailed lithostratigraphic, structural, petrographic and petrologic analysis of the poorly investigated basement rocks of the BRU allowed us to make some hypothesis about the nature of their protoliths and to reconstruct their metamorphic evolution
Summary
Segments of continental and oceanic crusts are subducted to great depths and exhumed as high to ultra-high pressure (HP/UHP) units (Guillot et al, 2009). This is notably well-recognized for example in the Alps Groppo et al, 2019), Himalayas Groppo et al, 2007, 2016; Lanari et al, 2013; Laskowski et al, 2016; O’Brien, 2019), China Defining the extension, zoning and P–T evolution of such subducted crustal units is of primary importance for interpreting the largescale processes of continental subduction and exhumation in the orogenic systems Defining the extension, zoning and P–T evolution of such subducted crustal units is of primary importance for interpreting the largescale processes of continental subduction and exhumation in the orogenic systems (e.g. Burov et al, 2014 and references therein)
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