Supra-subduction zone magmatism and extreme mantle depletion in the Early Cretaceous Nidar Ophiolite Complex, NW India

The Early Cretaceous Nidar Ophiolite Complex (NOC) in south Ladakh offers key insights into the geodynamic evolution of the Neo-Tethyan Ocean during subduction initiation. Although previously identified as a supra-subduction zone (SSZ) ophiolite, the tectonic connection between the SSZ setting and the former spreading center remains unclear. In this study, new mineral and whole-rock data, Nd-Sr isotopes, and detrital zircon ages from the NOC provide further constraints. The gabbros, with low SiO₂, exhibit ultra-depleted light rare earth elements (LREE) and pronounced negative Nb, Zr, and Ti anomalies, suggesting melting of a highly depleted Jurassic mantle lithosphere during subduction initiation. Dolerites and basalts, with higher SiO₂ and lower CaO, show nearly flat to slightly depleted LREE patterns, indicating normal mid-ocean ridge basalt-like compositions. Fractional crystallization played a key role in forming doleritic and basaltic melts, as indicated by plagioclase zoning and ɛNd (t) values (+7.8 to + 8.7). High-Mg andesites likely formed through second-stage melting of a metasomatized mantle wedge, with later magmatic recharge. Magmatic temperatures of 1065–1080 °C in clinopyroxenes and reverse zoning in plagioclase support this interpretation. The NOC underwent a regional hydrothermal alteration, forming secondary albite, actinolite, epidote, and chlorite. U–Pb zircon ages of 136.2 ± 1.6 Ma constrain the minimum age of subduction initiation. In the Th/Yb–Nb/Yb diagram, NOC samples range from the mantle array to island arc tholeiite, resembling the Spongtang ophiolite but differing from the calc-alkaline series of the Late Cretaceous Dras-Spong arc. The Cretaceous SSZ magmatism in south Ladakh likely reflects the lifecycle of SSZ ophiolites rather than separate subduction zones. The Nidar-Spongtang ophiolites and Dras-Spong arc record magmatic responses to subduction initiation and mature subduction, respectively, within the same island arc complex between 140–78 Ma.