[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

佐藤 成修¹、*今山 武志¹、Dutta Dripta¹、金田 泰明²、渡部 将太^2,3、長谷川 健²、南 雅代⁴、若杉 勇輝^4,5、若木 重行^6,7、Yi Keewook⁸ (1.岡山理科大学フロンティア理工学研究所、2.茨城大学理学部地球環境コース、3.山梨県富士山科学研究所、4.名古屋大学宇宙地球環境研究所、5.公益財団法人 元興寺文化財研究所、6.国立研究開発法人 海洋研究開発機構 高知コア研究所、7.国立歴史民俗博物館、8.韓国基礎科学研究所)

キーワード：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.