Influences of subduction derived fluids and melt in the genesis of Nidar ophiolite peridotites, Ladakh Himalaya, India: Evidence from mineralogy, PGE and Nd isotopic compositions

Abstract

The Nidar ophiolite is one of the well-preserved and almost complete ophiolite sections of the Neo-Tethyan oceanic lithosphere, obducted along the continental margin between the Indian and the Eurasian plate. This ophiolite sequence is mostly dominated by ultramafic rocks, consisting of forearc-related refractory, mainly harzburgite, dunite, and serpentinite, with minor intrusions of lherzolite, chromitites, and pyroxenites. In this present study, detailed mineralogical, whole rock geochemistry (major oxides, trace elements, PGE), and Nd isotopic composition of mantle-derived peridotites have been carried out to constrain the petrogenesis and melt evolution. These peridotites are depleted in nature due to the low modal composition of clinopyroxene, high forsterite content in olivine, and wide variation in Cr# and bulk rock chemistry, indicating variable degree of partial melting. The spoon-shaped rare earth element (REE) patterns indicate metasomatism by fluids derived from a subducting slab enriched in light REEs. Geochemical composition of the studied peridotites rocks is marked by high ratio of Al₂O₃/TiO₂, LILE-LREE enrichment, HFSE depletion, and spoon-shaped chondrite-normalized REE patterns and (La/Sm)_N > 1 and (Gd/Yb)_N < 1, indicates some involvement of boninitic mantle melts and validate a subduction initiation process. The total PGE of the peridotites (ΣPGE = 33–337 ppb) is much more enriched than that of the primitive mantle and other ophiolite peridotites. The PGE distribution displays a concave upward pattern with higher PPGE/IPGE ratios (i.e., 0.11–1.45), suggesting that partial melting is not the only process for the evolution of the Nidar ophiolite peridotites. Enrichment of PPGE and incompatible elements (like LREE) and higher Pd/Ir ratio (0.69–8.26) indicates that these peridotites have undergone fluid/melt interaction in a supra-subduction zone (SSZ) tectonic setting. PGE concentrations of these depleted harzburgites and dunites, formed by partial melting of cpx–harzburgites in an SSZ that produced the boninitic-like melt. The enrichment of incompatible elements like the PPGE is mainly due to the circulation of fluids in the subduction zone, which leads to the PGE fractionation in mantle peridotites. Also, these peridotites have ¹⁴³Nd/¹⁴⁴Nd ratios (0.51148–0.51262) and εNd(t) (t = 140 Ma) values (i.e., +0.97 to −21.3), indicating derivation from depleted mantle sources within an intra-oceanic arc setting. The geochemical behavior exhibited by the Nidar ophiolite peridotites suggests the evolution of a highly depleted fore-arc mantle wedge significantly modified by various fluids and melts during subduction. The mineralogical, geochemical, and Nd isotopic composition of these peridotites and dunites mutually depict the diverse mantle compositions, suggesting insights into the interactions between the oceanic crust and mantle as well as associated geochemical cycling in an SSZ environment.