Origin and tectonic architecture of the Dargai ophiolitic peridotites and chromitites: A geochemical perspective on platinum-group elements

Abstract

The Dargai ophiolites in northern Pakistan are characterized by extensive peridotite and chromitite exposures, however, their geochemical evolution remains debated. Here we investigate the mineral chemistry and platinum-group elements (PGEs) of the Dargai ophiolitic peridotites and associated chromitites to elucidate their genesis and tectonic evolution. Olivine, pyroxenes, and spinel compositions suggest a forearc setting, with Dargai peridotites representing refractory mantle residues formed through two stages of melting. Initial low-degree melting produced less depleted peridotites and high-Al chromitites with mid-ocean ridge affinity, followed by high-degree melting resulting in the formation of high-Cr chromitites and highly depleted peridotites in a supra-subduction zone. The depletion of Palladium and Platinum in high-Cr chromitites suggests that they were formed from sulfur-undersaturated melts, while enrichment in high-Al chromitites reflects that they were formed from sulfur-saturated parental melts. Melt impregnation after partial melting influenced the geochemical signatures, revealing interactions between peridotite and infiltrating melts. The geochemical evolution, combined with melt modeling, suggests the formation of MORB-like melts during proto-forearc spreading and the formation of boninitic melts during mature arc formation, explaining the co-occurrence of high-Al and high-Cr chromitites.