[S-MP25] Oceanic and Continental Subduction Processes

[SMP25-01] Supra-subduction zone magmatism and extreme mantle depletion in the Early Cretaceous Nidar Ophiolite Complex, NW India

Akinori Sato¹, *Takeshi Imayama¹, Dripta Dutta¹, Yasuaki Kaneda², Shota Watanabe^2,3, Takeshi Hasegawa², Masayo Minami⁴, Yuki Wakasugi^4,5, Shigeyuki Wakaki^6,7, Keewook Yi⁸ (1.Institute of Frontier and Science Technology, Okayama University of Science, 2.Graduate School of Science and Engineering, Ibaraki University, 3.Mount Fuji Research Institute, Yamanashi Prefectural Government, 4.Institute for Space-Earth Environmental Research, Nagoya University, 5.Gangoji Institute for Cultural Properties, 6.Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, 7.National Museum of Japanese History, National Institutes for the Humanities, 8.Korea Basic Science Institute)

Keywords:Nidar Ophiolite Complex, subduction initiation, supra-subduction zone magmatism, extreme mantle depletion

The Early Cretaceous Nidar Ophiolite Complex (NOC) in south Ladakh provides important insights into the geodynamic evolution of the Neo-Tethyan Ocean in response to subduction initiation. Although previous studies have identified the NOC as a supra-subduction zone (SSZ) ophiolite, the tectonic relationship between the SSZ setting and the former spreading center remains poorly understood. Here, we present new data on mineral and whole-rock compositions, Nd-Sr isotopes of igneous rocks, and detrital zircon ages from the overlying volcanosedimentary rocks in the crustal section of the NOC. The gabbros, characterized by low SiO₂ content, display ultra-depleted light rare earth elements (LREE) patterns with pronounced negative anomalies in Nb, Zr, and Ti. These findings suggest the presence of an extremely depleted mantle source, likely associated with first partial melting of refractory Jurassic mantle lithosphere with pre-existing depletion signatures during subduction initiation. The dolerites and basalts are characterized by relatively high SiO₂ and low CaO, with nearly flat to slightly depleted LREE patterns indicating normal mid-ocean ridge basalt-like compositions. Normal zoning in plagioclase, similar ε_Nd (t) values (+7.83 to +8.67), and increasing Yb content relative to Ce/Yb and Tb/Yb ratios suggest that fractional crystallization was the primary process in the formation of doleritic and basaltic melts. High-Mg andesites likely formed through second partial melting of a metasomatized mantle wedge, coupled with later magmatic recharge into the earlier basaltic magmatism. This interpretation is supported by magmatic temperatures of 1065–1080 °C in clinopyroxenes and the reverse zoning patterns in plagioclase. The NOC underwent a regional hydrothermal alteration, resulting in the formation of secondary albite, actinolite, epidote, and chlorite. The timing of subduction initiation in the NOC is constrained by concordia zircon U-Pb ages of 136.2±1.6 Ma from the volcanosedimentary rock. In the Th/Yb-Nb/Yb diagram, the Early Cretaceous NOC samples plot within fields ranging from the mantle array to island arc tholeiite, similar to the Spongtang ophiolitic mélange but contrasting with the calc-alkaline series of the Late Cretaceous Dras arc. We propose that the compositional variation and temporal relationships of Cretaceous SSZ magmatism in southern Ladakh reflect the lifecycle of SSZ ophiolites, as observed in other SSZ ophiolites, rather than the results developed in the two-separate subduction zones as previously reported. The Nidar-Spongtang ophiolites and the Dras arc primarily record magmatic responses to subduction initiation and mature subduction, respectively, within the same island arc complex during 140-78 Ma.