Fe–Mg isotope constraints on mantle processes and tectonic evolution of the Yilashan ophiolite, Tibet, China

The mantle processes and tectonic evolution of ophiolites in the Bangong–Nujiang suture zone remain debated, particularly regarding melt extraction, metasomatism, and chromitite formation in supra-subduction zone (SSZ) settings. Conventional tracers often fail to distinguish overprinting processes. Fe–Mg stable isotopes, which are sensitive to both partial melting and fluid/melt–rock interaction, offer a novel tool to fingerprint these processes. Here we present integrated Fe–Mg isotopic data, whole-rock geochemistry, and platinum-group element (PGE) systematics for harzburgites and chromitites from the Yilashan ophiolite (central Bangong–Nujiang suture zone) to address these issues. Harzburgites display near-chondritic δ⁵⁶Fe values (0.02–0.09‰) and limited δ²⁶Mg variability (–0.06 to 0.10‰), consistent with residues after high-degree melt extraction. Their depleted major element compositions, together with enrichment in large-ion lithophile elements and depletion in high-field-strength elements, require subsequent subduction-related metasomatism. In contrast, chromitites exhibit systematically lighter δ⁵⁶Fe values (down to –0.31‰) and a wide range of δ²⁶Mg values (–0.25 to 0.29‰), indicating pronounced isotopic heterogeneity.This variability is best explained by melt–rock interaction within focused melt pathways, consistent with the spatial association of chromitites with dunite channels, whereas alteration effects are not dominant but cannot be fully excluded. PGE systematics, including IPGE enrichment and distinct Pd/Ir–Pt/Pt* relationships, support chromitite formation from primitive boninitic melts, while harzburgite PGE patterns indicate melt infiltration or refertilization. Together, these data define a multi-stage evolutionary framework involving mantle depletion, subduction-related metasomatism, chromitite formation, and subsequent modification during cooling. Given current data limitations, this framework should be regarded as a working hypothesis requiring further testing.