Structural and metamorphic evolution of Val d'Ala eclogites (Western Alps - Lower Piemonte Zone)

Abstract

The ophiolites of the Piemonte Zone (PZ) are remnants of the Jurassic Alpine Tethys Ocean. Their transformation into eclogites attests to the deep subduction of the oceanic crust beneath the Adria plate during Alpine convergence. The Alpine evolution in the central part of the PZ is still poorly understood, and this work focuses on the structural and thermobarometric study of Val d'Ala eclogites, one of the Lanzo valleys south of the Gran Paradiso Massif. We investigated the petro-structural evolution of eclogites, analyzed their mineral chemistry and whole-rock composition, determined the protolith age using zircon geochronology, and estimated the Alpine P-T path. The protolith of the eclogites can be traced back to basalts with an N-MORB signature. Zircon geochronology indicates a protolith ²⁰⁶Pb/²³⁸U age of 158.5 $\pm$ 1.3 Ma, about 7 Myr older than the nearby FeTi gabbros. The first tectono-metamorphic stage (D1) developed under eclogite-facies conditions reaching up 2.5 GPa and 480–500 °C, indicating a thermal gradient of 6 °C/km during the Alpine evolution. The second stage (D2) involved the exhumation of the eclogites to 1–1.5 GPa at about 460 °C, under blueschist-facies conditions, while the final stages (D3/D4) occurred under greenschist-facies conditions at about 22 °C/km. Thus, the Val d'Ala eclogites evolved and were exhumed within a cold subduction zone during the burial of the Alpine Tethys Ocean, with final exhumation occurring during the continental collision between the European and Adria plates. The comparison with the other eclogites within the PZ suggests that all these ophiolites represent different oceanic lithospheric slices sampled at various depths and possibly times, recording different structural evolutions during their burial and exhumation in the mantle wedge of the same subduction zone.