A slab break-off model for mafic–ultramafic igneous complexes in the East Kunlun Orogenic Belt, northern Tibet: insights from early Palaeozoic accretion related to post-collisional magmatism

The East Kunlun Orogenic Belt (EKOB) in northern Tibet provides an important record of the amalgamation of the Wanbaogou oceanic basalt plateau and the Qaidam Block. Here we report geochemical, geochronological, and Hf isotopic data for newly identified late Silurian–Early Devonian mafic–ultramafic igneous complexes from the EKOB at the northern margin of the Tibetan Plateau. These complexes are dominantly composed of gabbro and pyroxenite rocks. Three complexes yield zircon U–Pb ages of 398.8 ± 1.8, 420.2 ± 1.2, and 413.4 ± 0.78 Ma. The ε_Hf(t) values of zircons range from +0.8 to +3.3 with T_DM1 ages of 897 to 998 Ma. Modelling of the geochemical data indicates that these igneous complexes have a hybrid origin, involving depleted mantle fluids derived from a previous subduction event and crustal materials. The geochemical and geochronological data suggest that these complexes formed in a post-collisional setting linked to break-off of a subducted oceanic slab, which occurred after the Wanbaogou oceanic basalt plateau amalgamated with the Qaidam Block in the late Silurian–Early Devonian.

藏北东昆仑造山带（East Kunlun Orogenic Belt, EKOB）为万宝沟大洋玄武岩高原与柴达木地块的拼合过程提供了重要研究记录，具有极高的科学价值。本文报道了采自青藏高原北缘东昆仑造山带的新识别出的晚志留世-早泥盆世镁铁质-超镁铁质火成岩杂岩的地球化学、地质年代学与Hf同位素数据。该类杂岩主要由辉长岩与辉石岩组成。三个杂岩样品的锆石U-Pb定年结果分别为398.8±1.8 Ma、420.2±1.2 Ma及413.4±0.78 Ma。锆石的ε_Hf(t)值介于+0.8至+3.3之间，其Hf同位素单阶段模式年龄（T_DM1）为897 Ma至998 Ma。地球化学数据模拟结果显示，该火成岩杂岩具有混合成因，其物质来源包含前期俯冲事件形成的亏损地幔流体与地壳物质。综合地球化学与地质年代学数据可知，该杂岩形成于碰撞后构造背景，与俯冲洋板片拆沉作用相关，该拆沉事件发生于晚志留世-早泥盆世万宝沟大洋玄武岩高原与柴达木地块拼合之后。