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

Harris, Benedict J R; De Hoog, Jan C M; Halama, Ralf; Schertl, Hans-Peter; Chen, Yi-Xiang

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 µg/g) and light δ11B 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 δ11B values (-6 to +1 ‰) are in isotopic equilibrium with similar fluids to those recorded in most phengite, but phengites in tourmaline-bearing samples records 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.

• Journal Article Type: Article
• Acceptance Date: Sep 11, 2023
• Online Publication Date: Oct 16, 2023
• Publication Date: Nov 1, 2023
• Deposit Date: Sep 12, 2023
• Publicly Available Date: Oct 26, 2023
• Journal: Contributions to Mineralogy and Petrology
• Print ISSN: 0010-7999
• Publisher: Springer Verlag
• Peer Reviewed: Peer Reviewed
• Volume: 178
• Issue: 11
• Article Number: 76
• DOI: https://doi.org/10.1007/s00410-023-02058-0
• Keywords: Subduction metasomatism, Ultra-high-pressure metamorphism, Whiteschist, Fluid–rock interaction, Boron isotopes
• Publisher URL: https://link.springer.com/article/10.1007/s00410-023-02058-0