HP tectono‐metamorphic evolution of the Internal Piedmont Zone in Susa Valley (Western Alps): New petrologic insight from garnet+chloritoid‐bearing micaschists and Fe–Ti metagabbro

Abstract

AbstractNew petrologic data from meta‐ophiolites exposed in a relatively poorly known sector of the Western Alps (i.e. Susa Valley, Internal Piedmont Zone) are reported. Garnet+chloritoid‐bearing phengitic micaschist and Fe–Ti metagabbro preserving eclogite facies assemblages have been investigated in detail to constrain the HP tectono‐metamorphic evolution of meta‐ophiolites in the study area. Microstructural investigations allowed to identify different assemblages related to different tectono‐metamorphic stages, whose pressure–temperature (P–T) conditions of the HP stages were constrained using the pseudosection modelling approach combined with multi‐equilibrium thermobarometry. Four main metamorphic stages have been recognized: (a) an Alpine peak‐P stage (M1a), which reached the HP–UHP eclogite facies boundary (P = 25–29 kbar, T = 460–510°C); (b) a slightly prograde decompression stage (M1b), still under eclogite facies conditions (P = 21–25 kbar, T = 500–530°C); (c) a greenschist facies re‐equilibration stage (M2); (d) a nearly isobaric late heating stage (M3), developed at the boundary between greenschist‐ and amphibolite facies conditions. The new P–T path inferred for the HP phases recorded by the studied meta‐ophiolites is compatible with that reported from other meta‐ophiolite units in the Western Alps, that is, the Zermatt–Saas ophiolite to the north and the Monviso ophiolite to the south, confirming that subduction of the Tethys oceanic lithosphere occurred at similar P–T regimes all along its length. The comparison with the tectonic evolution of the adjacent Dora Maira continental crustal unit suggest that the early exhumation of eclogitized oceanic crust likely occurred along the plate interface in the subduction channel, without the buoyancy contribution of continental crust.