Magmatic response to the subduction initiation of Early Cretaceous Nidar Ophiolite Complex, eastern Ladakh, NW Himalaya

Early Cretaceous Nidar Ophiolite Complex (NOC, eastern Ladakh) is associated with the north-dipping supra-subduction of the Neo-Tethyan Ocean along the Indus suture zone. The supra-subduction zone ophiolite formed in the forearc setting records the magmatic response to the subduction initiation, but the magmatic evolution in the NOC is poorly constrained. The low-Ti gabbros have low SiO₂ in whole-rock composition and high Mg# in clinopyroxene. They also record highly depleted magma In contrast, dolerites and basalts have relatively higher SiO₂ in whole-rock composition and lower Mg# in clinopyroxene, with flat REE patterns accompanied by fractional crystallization. Significant variation in Yb content relative to Tb/Yb ratio also supports fractional crystallization from gabbros to basalts. In Th/Yb-Nb/Yb diagram, all samples plot in the region from the MORB type to the island arc tholeiite. The Nd-Sr isotopes and high Ba/La ratio suggest that the NOC was originally derived from a single depleted mantle source similar to the MORB and was subsequently affected by hydrothermal alteration, resulting in greenschist- to lower amphibolite-facies overprint to form albite, actinolite, epidote and chlorite. Detrital zircon U-Pb ages from volcanic sediments associated with the NOC concentrated at ca. 136 Ma, representing the timing of the main magmatic phase in the NOC. Our data, combined with the geochronological and geochemical data in previous studies, suggest that the low-Ti, highly depleted magma in the NOC was firstly generated at extensional spreading in the upper plate during subduction initiation, and then changed to island arc tholeiite composition with the development of the subduction zone during Early Cretaceous.