The possibility that the Proto-Tethys Ocean may have undergone intra-oceanic subduction during ocean closure remains poorly constrained due to a lack of geological evidence for a mature intra-oceanic arc. Here we present new geochemical and geochronological data for potential arc-related volcanic rocks adjacent to the accretionary complex and forearc basin in the North Qaidam collisional belt, northern Tibetan Plateau. The volcanic rocks are dominated by foliated basalt, andesite, tuff, and minor dacite with zircon U-Pb ages ranging from 517 to 497 Ma. They show distinctive geochemical characteristics and can be subdivided into three groups: island-arc intermediate-basic volcanic rocks, back-arc basin basalts (BABB), and dacites with intra-oceanic arc affinity. The island-arc volcanic rocks have variable ε_Nd(t) values (+1.6 to +7.5) that decrease northward and were generated by partial melting of depleted mantle wedge modified by hydrous fluid and sediment melt. The BABBs have high ε_Nd(t) values (+5.3 to +6.6) and formed through the melting of MORB-like mantle, whereas the nearby dacites have positive ε_Nd(t) values (+1.9 to +3.6) similar to the surrounding island-arc volcanic rocks and were derived from partial melting of intra-oceanic arc crust as a result of BABB underplating. Integrated analysis of the spatial-temporal distribution of these volcanic rocks and the reconstructed intra-oceanic arc-trench system confirms the existence of the earliest Phanerozoic intra-oceanic arc formed in response to north-directed intra-oceanic subduction. This unrecognized subduction of the Proto-Tethys Ocean in the North Qaidam belt initiated at ca. 530 Ma, matured ca. 520 Ma, and terminated by ca. 480 Ma.

中文翻译：

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青藏高原北部最早的显生宙内洋弧的识别揭示了原特提斯洋内的俯冲作用

由于缺乏成熟的洋内弧的地质证据，原特提斯洋在海洋闭合期间可能经历洋内俯冲的可能性仍然很少受到限制。在这里，我们提供了青藏高原北部柴北碰撞带的增生杂岩和弧前盆地附近潜在与弧相关的火山岩的新地球化学和年代学数据。火山岩以叶状玄武岩、安山岩、凝灰岩和少量英安岩为主，锆石 U-Pb 年龄为 517 至 497 Ma。它们表现出独特的地球化学特征，可分为三类：岛弧中基性火山岩、弧后盆地玄武岩（BABB）和具有洋弧亲和性的英安岩。岛弧火山岩具有可变的ε _Nd (t)值（+1.6至+7.5），该值向北减小，是由含水流体和沉积物熔体改造的贫化地幔楔部分熔融而产生的。 BABB 具有较高的 ε _Nd (t) 值（+5.3 至 +6.6），是由类 MORB 地幔熔融形成的，而附近的英安岩具有正 ε _Nd (t) 值（+1.9 至 +3.6），与周围的岛弧火山岩，是由于 BABB 底侵作用导致洋内弧地壳部分熔融而形成的。综合分析这些火山岩的时空分布和重建的洋内弧海沟系统，证实了最早的显生宙洋内弧的存在，这是由向北的洋内俯冲作用形成的。柴达木北部带原特提斯洋的这种未被识别的俯冲始于大约 1997 年。 530 Ma，成熟约。 520 Ma，并终止于约。 480马。