Early Carboniferous to Middle Triassic Subduction-Accretion Records of the South Tianshan Ocean: Evidence from the Jingbulake metasedimentary rocks and mafic schists

The final closure timing of the South Tianshan ocean has been controversial. This study focuses on the Jingbulake mafic-ultramafic complex in the eastern segment of the northern margin of the Southwestern Tianshan, Xinjiang. Our fieldwork in the southwestern part of the complex shows that, besides mafic-ultramafic rocks, it contains gabbro, granitic gneiss, amphibolite, and schist. Systematic field mapping and cross-section observations were conducted, which show that the various rock units are in tectonic contact. An integrated analysis of zircon U-Pb geochronology, whole-rock geochemistry, and Sr-Nd-Pb isotopes on collected metasedimentary rocks and metabasites was performed. Our new results show that the youngest detrital zircon age from the metasedimentary rock (sample 25JB-05) is 242±2 Ma, indicating its depositional age is no earlier than the Middle Triassic. The youngest zircon ages from the metabasites (samples 25JB-13 and 25JB-15) are 293±4 Ma and 351±4 Ma, respectively. These ages reveal that the protoliths of these metabasite blocks, originally mafic/ultramafic rocks, were formed during the Early Permian and Early Carboniferous, respectively. Geochemically, the metabasites belong to the tholeiitic series and exhibit characteristics of enriched mid-ocean ridge basalt (E-MORB), along with depleted Nd isotopic compositions (εNd(t) = +3.34 to +3.72). Together with their Pb isotopic signatures, these features confirm that their protoliths are fragments of the South Tianshan Ocean ophiolite. Our data identify a record of oceanic subduction-accretion from the Early Carboniferous to the Middle Triassic in the southwestern Jingbulake area. This supports the conclusion that the closure of the Paleo-South Tianshan Ocean occurred later than the Middle Triassic and that the termination of accretionary orogenesis in the Central Asian Orogenic Belt likely persisted until the Middle-Late Triassic across the entire region.