Petrogenesis and tectonic implications of I-type and A-type granitoids in the Dachadaban area, North Qilian Orogenic Belt

The North Qilian Orogenic Belt is a key region for understanding the evolution of the Paleo-Asian Ocean and continental accretion. The Dachadaban area preserves a complete ophiolite suite and coeval granitoid intrusions. Zircon and apatite U–Pb geochronology, whole-rock geochemistry, and Sr–Nd isotopic analyses were conducted on intermediate–felsic intrusions from this area. Zircon and apatite ages of 512–504 Ma (Late Cambrian) record a regional magmatic pulse. The rocks are metaluminous to weakly peraluminous, high-K calc-alkaline, enriched in LILEs (Rb, Th) and depleted in HFSEs (Nb, Ta, Ti), with right-inclined REE patterns and moderate negative Eu anomalies, typical of island-arc granitoids. Initial 87 Sr/ 86 Sr ratios (0.7205–0.7286), ε Nd (t) values (−9.7 to −6.4), and T DM2 ages (2019–1751 Ma) indicate magmas derived mainly from partial melting of Paleoproterozoic crust with ca. 20%–30% mantle input. Zircon Ti-thermometry indicates crystallization temperatures ranging from approximately 692 °C–826 °C, accompanied by a shift in oxygen fugacity from relatively oxidized to reduced conditions and a concomitant decrease in water content. Apatite and whole-rock geochemical characteristics collectively indicate an evolution from I-type to A-type granitoids. Tectonic discrimination suggests that this magmatism occurred in a back-arc extensional setting driven by slab rollback during waning subduction, rather than in a post-collisional environment. Earlier-formed I-type granites underwent dehydration melting under thermal perturbation and decompression, producing late-stage A 2 -type granites, marking a transition of the North Qilian Orogen from convergence to extension at ca. 512–503 Ma.