The Piedmont Zone From the Western Alps (Italy): New Insights From High‐Pressure Ophiolitic Metasediments in the Soana Valley

Metaophiolites and metasediments of the Piedmont Zone are exposed in the Soana Valley, a poorly studied area of the Western Alps, located between the Europe-related Gran Paradiso massif and the Adria-related Sesia–Lanzo zone. Previous investigations reported the occurrence of only greenschist-facies metamorphic assemblages in the metasediments. We present new petrographic analyses and phase equilibria modelling of garnet-chloritoid- and garnet-lawsonite-choritoid-bearing mica schists from the Soana Valley. These mica schists preserve relicts of Alpine high-pressure mineral assemblages, testifying both a prograde and a decompressional evolution. Four main metamorphic stages are recognized from compositional zoning of garnet in a garnet-lawsonite-chloritoid-bearing mica schist: a subduction-related prograde stage at 17.0–18.5 kbar and 420°C–440°C (high-pressure blueschist-facies), a pressure peak at 19.0–21.5 kbar and 440°C–460°C (blueschist–eclogite-facies transition) and two decompressional stages at 17.5–18.5 kbar and 470°C–490°C (high-pressure blueschist-facies) and 14–15 kbar and 490°C–510°C (peak temperature). The peak pressure conditions of the Soana Valley metasediments are comparable (∆P ≤ 2–3 kbar) to those reported for some units of the external Western Alps located at structurally lower levels. Our results lead us to interpret the Piedmont Zone as a stack of tectonic slices, each one with its own metamorphic history, accreted together during subduction, rather than an assemblage of two juxtaposed belts characterized by distinct metamorphic histories, as traditionally reported.