On the occurrence of a Variscan eclogite in the Argentera-Mercantour Massif, Western Alps: Implications for the evolution of the southern Variscan belt

The newfound Bois de Sélasse eclogite in the eastern Argentera-Mercantour Massif (External Crystalline Massifs, Western Alps) is crucial for better constraining the tectonic evolution of the southern part of the Euro- pean Variscan belt. The whole-rock com- position of this eclogite aligns with that of a basaltic protolith with a normal mid-oceanic- ridge affinity, and U-Pb dating on igneous zircon cores reveals an emplacement age of 524 ± 5 Ma. The emplacement may have oc- curred either in the oceanic lithosphere to the north of the active Gondwana margin or within a back-arc basin during the sub- duction beneath Gondwana. Exceptionally preserved prehnite–pumpellyite to eclogite facies minerals provide evidence of prograde metamorphism along a standard oceanic subduction geotherm (≤10 °C/km). Peak eclogite facies conditions are constrained at 610–660 °C and 1.9–2.3 GPa by thermody- namic modeling combined with Ti-in-zircon and Zr-in-rutile thermometry. A minimum age for eclogite facies metamorphism is es- tablished at 339 ± 6 Ma by U-Pb dating on metamorphic zircon rims. The protolith of the Bois de Sélasse eclogite is indeed older than the Variscan oceans, but it was similarly affected by Variscan subduction. We discuss the implications of this new finding in the context of the European Variscan belt.