Petrogenesis and Tectonic Implications of Late Cretaceous Granites in the Yidun Terrane

The Yidun Terrane, located in the eastern Sanjiang Tethys domain of southwestern China, records extensive Late Cretaceous magmatism, yet its geodynamic setting remains controversial. Zircon U–Pb dating yields a crystallization age of 103.37 ± 0.51 Ma. The rocks are characterized by high SiO2 contents (73.9–78.9 wt.%), ferroan compositions, predominantly high-K calc-alkaline and weakly peraluminous affinities, elevated 10,000 × Ga/Al values (2.81–3.29), and low P2O5 contents (0.03–0.04 wt.%). Integrated mineralogical and whole-rock geochemical evidence indicates an A-type affinity, while incompatible-element geochemical characteristics further suggest an A2 affinity. Their highly evolved compositions and depletion in Eu, Sr, Ba, and P are consistent with substantial fractional crystallization, although source composition and melting conditions may also have contributed to these features. Zircon εHf(t) values range from −1.1 to +0.5, with two-stage Hf model ages of 1.14–1.24 Ga, whereas whole-rock εNd(t) values range from −5.2 to −4.9, with two-stage Nd model ages of 1.30–1.32 Ga, indicating that the magma was derived predominantly from Mesoproterozoic crust with only limited juvenile input. Regional comparisons reveal substantial spatial differences among the Late Cretaceous plutons, but the available dataset is insufficient to establish a continuously sampled north–south evolutionary trend. Overall, the Queershan pluton most likely formed in a post-collisional extensional setting following the Lhasa–Qiangtang collision, and this process may have been further enhanced by slab break-off of the Bangong–Nujiang Ocean. In contrast, the younger magmatism in the southern Yidun Terrane is more readily reconciled with rollback-related back-arc extension associated with continued northward subduction of the Yarlung Zangbo Neo-Tethys.