Osmium and zinc isotope constraints on the origin of chromitites from the Yarlung-Zangbo ophiolites, Tibet, China

Abstract

Chromitites or chromite mineralization of varying degrees has been discovered in the various ophiolites along the east–west trending Yarlung-Zangbo Suture Zone (YZSZ) in Tibet, China. The high-Cr variety dominates the Yarlung-Zangbo chromitites, with rare high-Al chromitites reported in the Zedang, Dongbo, and Purang ophiolites. Using empirical equations, the calculated parental magmas that formed the high-Cr YZSZ chromitites are similar to boninitic melts. ¹⁸⁷Os/¹⁸⁸Os ratios of chromites from the YZSZ chromitites range from 0.12525 to 0.12933, lower than the proposed present-day ¹⁸⁷Os/¹⁸⁸Os values for the primitive upper mantle. The T_RD age variation of the YZSZ chromitites from late Neo-Proterozoic to early Triassic thus reflects that their parental magmas are derived from depleted mantle sources mixed with diachronous ancient mantle domains. The light Zn isotopic compositions of the YZSZ chromitites indicate that subducted materials (e.g., serpentinites and sediments) have contributed to the parental magma of the YZSZ chromitites. By compiling previously published data on mantle peridotites of the YZSZ ophiolites, we concluded that the YZSZ ophiolites may either have formed initially in an ultraslow-slow mid-ocean ridge environment and were then trapped in a supra-subduction zone environment, or have formed in an ultraslow-slow forearc spreading center in a supra-subduction zone environment. The Luobusa ophiolite hosting the largest chromite deposits is discriminated from the other ophiolites in the YZSZ by a thick dunitic transition zone. Previous theoretical modeling indicates that relative to olivine, only a small amount of cumulus chromites crystallize in cotectic volume ratios of around 100:1 to 100:2 of olivine to chromite, which means that large chromite bodies should always be accompanied by a significantly larger mass of dunites. Therefore, we concluded that a thick dunite transition zone or large masses of dunite of boninitic affinity is an indicator for chromitite prospecting in the future.