Tectonic evolution and linkage of the South and North Qilian oceans (northeastern Tibet) during the early Paleozoic

The Qilian orogen in northeastern Tibet preserves early Paleozoic ophiolitic mélanges, high-pressure/low-temperature metamorphic rocks, and magmatic arc products, making it a key region for understanding Proto-Tethyan ocean-basin evolution and orogenic processes. However, disagreement persists regarding the paleo-oceanic configuration of the Qilian Ocean, including the subduction polarity, number of ocean basins, and dynamics of ocean closure in the region. Here, we present new zircon U-Pb ages, whole-rock geochemistry, zircon Hf isotopic data, and compiled datasets to reconstruct the early Paleozoic tectonic evolution of the Qilian orogen. Our results show that the South, Central, and North Qilian belts record genetically linked but temporally asynchronous histories. The South Qilian belt experienced intra-oceanic subduction and arc magmatism, followed by continental-margin arc development and continental collision, culminating in a voluminous magmatic flare-up at ca. 450–440 Ma. The North Qilian belt preserves a more complex arc and backarc basin system, with sequential ocean-basin expansion, subduction, local backarc formation, collision, and slab-breakoff-driven upper-plate modification. The North Qilian backarc basin reached its collision-related magmatic peak at ca. 425 Ma, indicating that its final closure occurred later than that of the principal South and North Qilian oceanic systems. Prior to ca. 460 Ma, the Central Qilian belt experienced subduction-arc magmatism, with zircon crystallization age patterns and εHf(t) patterns comparable to those of the North Qilian belt. After ca. 460 Ma, the Central Qilian belt experienced collisional and post-collisional tectonism similar to the South Qilian. This suggests that the Central Qilian was likely a continental block responding to interactions with the South and North Qilian belts along both its margins. Integrated geophysical, geochronological, geochemical, and thermal data indicate that the Qilian orogen developed through a segmented, diachronous, and bidirectional subduction-collision system. This system included the South Qilian Ocean, which was divided by an intra-oceanic arc into northern and southern branches, the bidirectionally subducting North Qilian Ocean, and the subsequently formed North Qilian backarc basin. Two main collision stages are recognized: diachronous closure of the northern and southern South Qilian branches broadly coeval with closure of the principal North Qilian Ocean, followed by final closure of the rollback-generated North Qilian backarc basin and amalgamation with the Alxa block. This model accounts for the distribution of ophiolites, high-pressure metamorphic belts, accretionary complexes, forearc–backarc basin records, and variations in ages, isotopic signatures, thermal regimes, and magmatic assemblages across the South, Central, and North Qilian belts. More broadly, this reconstruction provides a process-based framework for evaluating how diachronous ocean closure, continental collision, and episodic crustal growth are recorded during Wilson-cycle evolution in composite orogens.