Petrological and Re-Os isotopic constraints on the origin of chromitites and peridotites of the late cretaceous Waziristan ophiolite, western Pakistan: implications for subduction initiation

The Waziristan Ophiolite Complex (WOC) occurs between the Afghan microplate in the west and the Indian plate in the east along the Main Waziristan Thrust in western Pakistan. A complete ophiolitic sequence, including residual lherzolites, harzburgites with foliated and discordant dunites, gabbroic rocks, dyke swarms, pillow and massive basaltic rocks, is present in the complex. Widespread podiform chromitites are usually enclosed by thin dunitic haloes in the complex. Chromian spinels in lherzolites-harzburgites are characterized by Cr# [= Cr/(Cr  +  Al)] values ranging from 0.19 to 0.60, and plot within or overlap the fields of abyssal to fore-arc peridotites, while chromian spinels in dunites and chromitites with Cr# values from 0.57 to 0.79 exhibit geochemical affinities similar to those of boninites. Trace element and REE characteristics, including the enrichment of light REE in the Waziristan Complex peridotites indicate the contribution of subduction-related fluids/melts, including boninitic mantle melts, validating a subduction initiation process in their genesis, and these features are similar to those observed in supra-subduction zone peridotites. Chromitites are characterized by enrichment of Os, Ir, Ru and Rh relative to Pt and Pd on a chondrite-normalized platinum group elements (PGE) diagram. 187Os/188Os isotopic ratios of the WOC are 0.12675 to 0.13011 and 0.11355 to 0.14421, coupled with 187Re/188Os ratios of 0.0112 to 0.0272 and 0.0026 to 1.1869, for the chromitites and peridotites, respectively. These data suggest that the peridotites were modified by melt metasomatism and/or Re addition and chromitites formed by crystallization from melts. The Os isotopic composition of the chromitites reflects the isotopic composition of the melt. The calculated oxygen fugacities (ƒO2) values of the studied samples are moderate to high, with an oxidation state of log fO2 units ranging from  − 0.17 to   +   0.99 for harzburgite,  +  0.37 to   +   1.63 for dunites and   +   0.52 to   +   2.84 for chromitites. The moderate to high oxidation state of the studied upper-mantle ophiolitic complexes suggests a boninitic source in the mantle wedge within an arc setting. In addition, the two-pyroxene thermometer presents mean equilibrium temperatures of 825°C, 841°C, 910°C and 910°C for harzburgite, lherzolite, dunite and pyroxenite samples, respectively. The spinel in lherzolites-harzburgites reflects the degree of depletion after tholeiitic melt extraction by proto-forearc spreading and infant arc generation, whereas spinel in dunites were affected by boninitic melt percolation and chromitites formed from boninitic melts during the mature proto-forearc stage, with a significant influence of slab-derived fluids at high ƒO2.