Petrogenesis of podiform chromitites in the Sapat Complex Ophiolite, northern Pakistan: constraints from mineralogy, geochemistry, and Re–Os isotopes

Abstract

The Sapat Ophiolitic Complex (Northern Pakistan) represents a well-preserved Neo-Tethys oceanic lithosphere remnant along the Main Mantle Thrust/Indus Suture Zone (MMT/ISZ). This study provides new mineralogical, geochemical, and Re–Os isotopic data on chromitites (disseminated, semi-massive, and massive) hosted in dunites. The associated Cr-spinel grains commonly contain base-metal sulfide (BMS), silicate, and platinum-group mineral (PGM) inclusions. Chromitites exhibit uniform Cr# [Cr/(Cr + Al)] (0.79–0.85) and variable Mg# [Mg/(Mg + Fe 2+ )] (0.52–0.70), consistent with boninitic parental magmas. The melt compositions calculated from spinel chemistry, yield (Al 2 O 3 ) melt values of 9.36 to 11.55 wt% and (FeO/MgO) melt ratios of 0.60 to 1.22. Trace element concentrations of the chromitites (e.g., Co: 206–544 ppm, Ga: 13–27 ppm, Ni: 441–1300 ppm, Sc: 2.42–5.55 ppm and V: 613–801 ppm) fall within the compositional ranges reported for chromitites derived from the mantle units of other well-characterized ophiolitic complexes. These geochemical features are indicative of an island arc tholeiitic melt exhibiting boninitic affinity. Such magmas were most likely in chemical symmetry with the podiform chromitites and the surrounding mantle peridotites from which they crystallized. Furthermore, the distribution patterns of minor and trace elements in the Sapat Complex chromites support a boninitic magma source. Field relationships, petrographic observations, and whole-rock and mineral chemistry collectively suggest that these chromitites were influenced by both melt-rock reaction and melt-melt interaction. These interactions facilitated the crystallization of high-Cr chromitites from boninitic melts enriched in Cr but depleted in Al and Ti. The iridium-group platinum-group elements (IPGEs) enriched signatures (average: 72.9–165.8 ppb), measured initial 187 Os/ 188 Os ratios (0.12161–0.12411) and low Re/Os ratios (0.01–0.29) are consistent with those of mantle-hosted chromitites worldwide, supporting crystallization from deep-mantle boninitic melts in a supra-subduction zone (SSZ) setting.