Evolving mantle processes in a MOR–SSZ transition and their role in chromitite formation: A case study from the Qielihu ophiolite (Tibet)

The Qielihu ophiolite in the central Bangong–Nujiang Suture Zone (Tibet) exposes mantle peridotites and podiform chromitites formed during the transition from mid-ocean ridge (MOR) to supra-subduction zone (SSZ) settings. Mineral chemistry, whole-rock major and trace elements, platinum-group elements (PGE), and Mg-Fe isotopes are integrated to constrain mantle evolution and chromitite genesis. Spinel compositions record progressive melt extraction from lherzolite (<10%) to highly refractory harzburgite and dunite (>30%), consistent with whole-rock trends of increasing MgO and decreasing Al₂O₃–CaO. Harzburgites and dunites show enrichment in LREEs and large-ion lithophile elements, positive U anomalies, and depletion in high-field-strength elements, reflecting metasomatism by oxidized, subduction-derived fluids. Chromitites display IPGE-enriched and PPGE-depleted PGE patterns, together with coupled variations in δ⁵⁶Fe (−0.37 to −0.01‰) and δ²⁶Mg (−0.52 to +0.28‰) that are independent of spinel Cr#. These features indicate redox-sensitive melt–fluid interaction rather than melting degree as the primary control on isotopic fractionation. This study proposes a multistage evolution involving MOR-type melt extraction, subsequent SSZ fluid infiltration, and focused percolation of Cr-rich melts during subduction initiation. Chromitite crystallization and accumulation were promoted by localized increases in oxygen fugacity and enhanced melt–rock interaction.