Reversed Metasomatism at the Slab-Mantle Interface in a Continental Subduction Channel: Geochemical Evidence from the Ultrahigh-Pressure Metamorphic Whiteschist in the Western Alps

It is well known that the mantle wedge metasomatized by fluids derived from the subducting slab serves as the source of arc magmas. However, it is uncertain whether the subducting crust would be metasomatized by fluids released from mantle wedge metasomatites. Now this is firstly illustrated by a detailed study of petrology and geochemistry for whiteschist from the Dora-Maira Massif in the Western Alps.Based on the whole-rock geochemistry and zirconology for the whiteschist and its country rock, it is concluded that the protolith of the whiteschist is a kind of S-type granites, providing a resolution to the long-standing controversy on the protolith nature of the whiteschist in this region. The δ¹⁸O values of metamorphic zircon in the whiteschist are significantly lower than those of magmatic zircon, suggesting that the protolith was metasomatized by low δ¹⁸O fluids before the peak UHP metamorphism. The whiteschist shows the highest δ²⁶Mg values up to 0.75‰ among high-T silicate rocks, which are much higher than those of the metagranite, suggesting that the metasomatic fluids have heavy Mg isotope compositions.Based on the Mg isotope systematics of major terrestrial silicate reservoirs, it is proposed that such fluids would probably originate from talc-rich serpentinites that were generated at forearc depths by hydration of the mantle wedge peridotite during prograde subduction of the Neotethyan oceanic slab. The mantle wedge serpentinites were then metastably carried by the subducting continental crust to subarc depths, where they underwent dehydration for reversed metasomatism of the deeply subducting continental crust at the slab-mantle interface in the continental subduction channel. The results provide not only the new idea for tracing fluid sources in the continental subduction channel, but also the first example that the deeply subducting continental crust underwent the reversed metasomatism by the fluids derived from dehydration of the mantle wedge metasomatite. This reversed metasomatism would greatly modify the geochemical composition of the deeply subducted continental crust, which has bearing on the origin of arc magmas rich in heavy Mg isotopes.