Petrogenesis and Tectonic Setting of Late Permian Granitoids in the East Kunlun Orogenic Belt, NW China: Constraints from Petrology, Geochemistry and Zircon U-Pb-Lu-Hf Isotopes

Permian magmatic rocks are extensively distributed in the East Kunlun Orogenic Belt (EKOB), yet controversies persist regarding the petrogenesis of granitoid rocks and the tectonic evolution of the Buqingshan-A’nyemaqing Ocean (BAO), which is a part of the Paleo-Tethys. This study addresses these debates through petrological analyses, whole-rock geochemistry and zircon U-Pb-Lu-Hf isotopic investigations of newly identified granitoids in the EKOB. Monzogranite (MG) and quartz porphyry (QP) yield weighted mean ages of 254.7 ± 1.1 Ma and 254.3 ± 1.1 Ma, respectively. Geochemically, the MG shows metaluminous to weakly peraluminous low-K calc-alkaline I-type affinity, characterized by high SiO2 and low K2O, MgO and FeOT contents, as well as marked enrichment in light rare earth elements (LREEs), but depletion in Eu, Ba, Sr, P and Ti anomalies. In contrast, the QP exhibits a peraluminous high-K calc-alkaline I-type affinity, displaying high SiO2 but low Na2O and P2O5 contents. It is enriched in LREEs and Rb but displays negative Nb, Sr, P and Ti anomalies. Zircon εHf(t) values range from −1.6 to 2.6 for MG and −4.4 to 1.5 for QP. We suggest that both MG and QP were derived from the partial melting of juvenile mafic lower crust, and that MG underwent a high degree of fractional crystallization. A synthesis of multiscale geological evidence allows us to propose a five-stage tectonic evolution for the BAO in the EKOB: (1) oceanic basin initiation before ca. 345 Ma; (2) incipient northward subduction commencing at ca. 278 Ma; (3) slab rollback stage (263–240 Ma); (4) syn-collisional compression (240–230 Ma); (5) post-collisional extension (230–195 Ma).