Petrogenesis and tectonic setting of the high-Al chromitites and associated peridotites in the Kudi ophiolite, NW Tibetan Plateau

The origin and tectonic setting of high-Al chromitites in ophiolites remain controversial. This study presents a comprehensive analysis of fresh high-Al chromitites and their host peridotites from the Kudi ophiolite, NW Tibetan Plateau. The peridotites consist of harzburgites and dunites. The harzburgites represent depleted mantle peridotites that have undergone ∼20% partial melting, while the dunites associated with chromitites are cumulates crystallized from magmas. Both have experienced infiltration by late-stage melts/fluids. A compositional profile of the chromitites reveals that olivine compositions, such as MgO, FeO, Mn, Co and Zn, show covariations with those of chromite. This relationship cannot be attributed solely to subsolidus elemental exchange between olivine and chromite; instead, it suggests that fluids played a crucial role in the formation of chromitites, in addition to melt–peridotite interaction. The estimated parental magma compositions in equilibrium with chromite and clinopyroxene suggest that the chromitites formed from island-arc tholeiitic magma, whereas dunites are cumulates of normal mid-ocean ridge basalt (N-MORB), medium-Ti basalt and island-arc tholeiitic magma. The compositional shift of the parental magma from N-MORB to island-arc basalt resembles lavas in the Izu–Bonin–Mariana forearc, suggesting that the Kudi chromitites probably formed in a sub-arc setting during subduction initiation.