Depleted mantle beneath an oceanic island arc: New insights into peridotites of the RasKoh ophiolite, western Pakistan

Abstract

The RasKoh ophiolite of the Sorap Massif, western Pakistan, is a relatively little known body in the vast Alpine–Himalayan orogenic belt (AHOB). This paper focuses on the RasKoh peridotites to better understand the origin and evolution of the ophiolite. Field observations, confirmed by thin-section study, identified three types of peridotite; lherzolite, harzburgite, and dunite, which commonly have gradational boundaries. The mineral compositions of the peridotites (olivine, Cr-spinel, and pyroxene) suggest that these ultramafic rocks mostly represent refractory residues of mantle peridotites after ~12 to 25% partial melting and various degrees of melt-rock reaction. This interpretation is supported by variable contents of major elements and the chondrite-normalized rare earth element (REE) patterns. Enrichment of water-soluble elements, such as Rb, Ba, Sr, and light rare earth elements (LREE) also indicate infiltration of fluids into the depleted peridotites. Synthesizing available geological information with our new geochemical data, we suggest that the RasKoh ophiolite is a fragment of a former Triassic to Jurassic oceanic plate that became a mantle wedge above an intra-ocean subduction zone during the Cretaceous. Fluids from the subducted plate re-enriched the depleted peridotites of the RasKoh island arc. The ophiolite was emplaced during collision between the RasKoh island arc to the south and the Chagai continental arc to the north leading to exhumation of the re-enriched peridotites. This new model is much simpler than most previous ones and better explains the tectonic evolution of western Pakistan.