Subduction initiation of oceanic lithosphere is the key scientific problem of the plate tectonics on Earth. When, where, and how an oceanic subduction began remained obscure because of little geological records. Here, we report a newly documented ophiolitic sequence, including ultra-slow spreading ridge fragments (V1) and associated fore-arc lavas (V2) units in the Bangong-Nujiang suture zone, north-central Tibetan Plateau. The V1 units are controlled by detachment fault with peridotite, gabbro and pillow basalt. Peridotite exhibits uniform characteristics of abyssal peridotite, representing residues after low-degree melting of asthenospheric mantle. Mafic rocks display geochemical features of normal mid-ocean ridge basalt (N-MORB). These features, combined with zircon U-Pb dating, indicate that formation of the oceanic lithosphere occurred along an ultra-slow spreading ridge around 177–170 Ma in this area. The V2 units occur as a complete sequence from bottom to top of fore-arc basalt (FAB), basaltic andesite, boninite, and high-Mg arc-related rocks with formation ages of 160–150 Ma, which directly extruded and overlay on the V1 units. This sequence is identical to those of the Izu-Bonin-Mariana (IBM) fore-arc, and represent magmatic response to subduction initiation (SI). Thus, we propose that the Beila ophiolite preserved an entire process of oceanic transition from ultra-slow spreading oceanic ridge to the SI. Subduction initiation in the Beila area was induced by southward subduction transition after the closure of the northern branch of the Bangong-Nujiang Meso-Tethys Ocean and subsequent continental collision. This study provides direct geological evidence for delineating the geodynamic mechanism of an oceanic initial subduction.

中文翻译：

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超慢速扩张脊附近的诱发俯冲起始：以青藏高原中北部贝拉蛇绿岩为例


大洋岩石圈俯冲起始是地球板块构造的关键科学问题。由于地质记录很少，大洋俯冲何时、何地以及如何开始仍然不清楚。在这里，我们报告了一个新记录的蛇绿岩序列，包括青藏高原中北部班公-怒江缝合带的超慢扩张山脊碎片（V1）和相关的弧前熔岩（V2）单元。 V1单元受橄榄岩、辉长岩和枕状玄武岩的滑脱断层控制。橄榄岩表现出均匀的深海橄榄岩特征，代表软流圈地幔低度熔融后的残留物。镁铁质岩石显示出正常洋中脊玄武岩（N-MORB）的地球化学特征。这些特征与锆石 U-Pb 测年相结合，表明海洋岩石圈的形成发生在该地区大约 177-170 Ma 的超慢扩张脊上。 V2单元从下到上为弧前玄武岩(FAB)、玄武安山岩、博尼岩和高镁弧相关岩石的完整层序，形成年龄为160~150 Ma，直接挤压并叠加在弧前玄武岩上。 V1 单位。该序列与伊豆-博宁-马里亚纳 (IBM) 弧前序列相同，代表对俯冲起始 (SI) 的岩浆反应。因此，我们认为贝拉蛇绿岩保存了从超慢速扩张洋脊到SI的整个海洋过渡过程。贝拉地区俯冲起始是班公湖-怒江中-特提斯洋北支闭合后向南俯冲转变以及随后的大陆碰撞诱发的。这项研究为描述海洋初始俯冲的地球动力学机制提供了直接的地质证据。