PGE and isotopic characteristics of Shergol and Suru Valley Ophiolites,Western Ladakh:Implications for supra-subduction tectonics along Indus Suture Zone

Present study reports the PGE-geochemistry of mantle peridotites and Nd-isotope geochemistry of arc related mafic rocks from the Indus Suture Zone(ISZ),western Ladakh.The total PGE concentration of the Shergol and Suru Valley peridotites(∑PGE=96-180 ppb)is much higher than that of the primitive mantle and global ophiolitic mantle peridotites.The studied peridotites show concave upward PGE-distribution patterns with higher palladium-group PGE/Iridium-group PGE ratios(i.e.,0.8-2.9)suggesting that the partial melting is not the sole factor responsible for the evolution of these peridotites.The observed PGE-distribution patterns are distinct from residual/refractory mantle peridotites,which have concave downward or flat PGE-distribution patterns.Relative enrichment of palladium-group PGE as well as other whole-rock incompatible elements(e.g.,LILE and LREE)and higher Pd/Ir ratio(1.1-5.9)reflects that these peridotites have experienced fluid/melt interaction in a supra-subduction zone(SSZ)tectonic setting.Also,the Shergol mafic intrusives and Dras mafic volcanics,associated with the studied peridotites,have high^(143)Nd/^(144)Nd ratios(i.e.,0.512908-0.513078 and 0.512901-0.512977,respectively)and positiveε_(Nd)(t)(calculated for t=140 Ma)values(i.e.,+5.3 to+8.6 and+5.1 to+6.6,respectively),indicating derivation from depleted mantle sources within an intra-oceanic arc setting,similar to Spongtang and Nidar ophiolites from other parts of Ladakh Himalaya.The transition from SSZ-type Shergol and Suru Valley peridotites to Early Cretaceous tholeiitic Shergol mafic intrusives followed by tholeiitic to calc-alkaline Dras mafic volcanics within the Neo-Tethys Ocean exhibit characteristics of subduction initiation mechanism analogous to the Izu-Bonin-Mariana arc system within western Pacific.