Multi-stage subduction-related metasomatism recorded in whiteschists from the Dora-Maira Massif, Western Alps

Abstract

Whiteschists from the Dora-Maira massif (Western Alps, Italy) are Mg and K-rich metasomatised granites which experienced ultra-high-pressure metamorphism and fluid–rock interaction during Alpine continental subduction. The sources and timing of fluid infiltration are a source of significant debate. In this study, we present boron (B) isotopes and other fluid-mobile trace-element (FME) concentrations in various generations of phengite from whiteschists and their country rock protoliths to investigate the sources and timing of metasomatic fluid influx. Reconstructed bulk rock concentrations based on modal data and mineral compositions indicate that significant amounts B and other FME were added to the rock during prograde metamorphism, but that this fluid influx postdates the main Mg metasomatic event. High B concentrations (150–350 $$\upmu$$ μ g/g) and light $${\delta^{\text{11}}}\text{B}$$ δ 11 B values (− 16 to −4 ‰) recorded in phengite point to a B-rich sediment-derived fluid as the main source of B in the whiteschists. Further redistribution of FME during metamorphism was associated with breakdown of hydrous minerals, such as talc, phlogopite, and ellenbergerite. The source of the Mg-rich fluids cannot be constrained based on the B data in phengite, since its signature was overprinted by the later main B metasomatic event. Rare tourmaline-bearing whiteschists record additional information about B processes. Tourmaline $${\delta^{\text{11}}}\text{B}$$ δ 11 B values (− 6 to +1 ‰) are in isotopic equilibrium with similar fluids to those recorded in most phengite, but phengites in tourmaline-bearing samples record anomalous B isotope compositions that reflect later redistribution of B. This study demonstrates the utility of in situ analyses in unravelling complex fluid–rock interaction histories, where whole-rock analyses make it difficult to distinguish between different stages of fluid–rock interaction. Polymetasomatism may result in decoupling of different isotopic systems, thus complicating their interpretation. The Dora-Maira whiteschists interacted with multiple generations of fluids during subduction and therefore may represent a long-lived fluid pathway.