Subduction Within the Proto-Tethys Ocean Revealed by Recognition of the Earliest Phanerozoic Intra-Oceanic Arc, Northern Tibetan Plateau

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 epsilon 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 epsilon Nd(t) values (+5.3 to +6.6) and formed through the melting of MORB-like mantle, whereas the nearby dacites have positive epsilon 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. Closure of an ocean basin is achieved through subduction of oceanic lithosphere at a continental margin or in an intra-oceanic setting. Identification and structure of subduction-related magmatic rocks are critical for fully understanding the complex processes associated with ocean closure and the paleotectonic framework. Geological mapping and geochemical analyses of the Tanjianshan Group in the North Qaidam collisional belt, northern Tibetan Plateau were undertaken. Results demonstrate the presence of relics of the earliest Phanerozoic intra-oceanic arc and an associated complete arc-trench system, generated by previously unrecognized intra-oceanic subduction far from microcontinents that are now adjacent. This study reveals the entire evolution of a missing intra-oceanic subduction system within the Proto-Tethys Ocean, which started ca. 530 Ma, matured ca. 520 Ma and ended ca. 480 Ma. Relics of the earliest Phanerozoic intra-oceanic arc discovered in North Qaidam collisional belt Proto-Tethys Ocean in North Qaidam belt experienced north-directed intra-oceanic subduction Intra-oceanic subduction started at ca. 530 Ma and lasted until ca. 480 Ma