Fu D et al.,EPSL-2023

Back-arc basins are pivotal sites for the generation and consumption of oceanic lithosphere in mature intra-oceanic convergent plate margins. However, the processes and mechanisms by which back-arc oceanic basins generate and terminate remain enigmatic. Here we document the life cycle of an extinct Paleozoic back-arc from an ophiolite-accretionary complex in the Proto-Tethyan North Qilian orogenic belt, northeastern Tibet, which records early spreading and subsequent closure through two episodes of subduction. Zircon U-Pb ages of gabbro and plagiogranite indicate two episodes of oceanic crust formation at ca. 480 Ma and ca. 452-448 Ma. Low-Ti gabbro, basalt, and plagiogranite (ca. 480 Ma) from the accretionary complex have back-arc geochemical signatures, whereas ultramafic rocks, gabbro, basalt, and plagiogranite suite from the ophiolite sheet (ca. 452-448 Ma) exhibit forearc characteristics. These data, together with recent structural and metamorphic evidence, indicate that the ophiolitic rocks were derived from both back-arc and forearc settings, and record the magmatic response to early spreading of the North Qilian back-arc basin and subsequent subduction initiation. Early subduction and slab rollback of the North Qilian oceanic lithosphere led to formation of the North Qilian arc and spreading of the back-arc, whereas later subduction initiation of the back-arc oceanic crust resulted in its termination and closure. These findings indicate that the subduction of back-arc oceanic crust is a primary mechanism in the closure of back-arc basins and thereby highlights the complex evolutionary tempo of back-arc basins in Earth history.