The tempo of back-arc basin evolution: Insights from the early Paleozoic Proto-Tethyan North Qilian orogenic belt, northeastern Tibet

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.