Petrogenesis of the early Cretaceous Jilong granodiorite porphyry in the Bangong Co-Nujiang metallogenic belt, Xizang, China: Constraints from zircon U-Pb geochronology, Hf isotopes, and whole-rock geochemistry

Various Early Cretaceous metallogenic intrusions, such as Jilong, Longrong, and Shuangchahe, develop in the Longrong region, western section of the Bangong Co-Nujiang metallogenic belt. The Jilong intrusion lies in the middle of the Longrong region, consisting of biotite granodiorite porphyry and biotite granodiorite. LA-ICP-MS zircon U-Pb dating results show that the zircon U-Pb age of the granodiorite porphyry is (116.4±0.7) Ma (MSWD = 1.07), suggesting that the Jilong intrusion formed during the Early Cretaceous epoch. The whole-rock geochemical characteristics of the granodiorite porphyry exhibit high contents of SiO₂ (63.66%～65.24%), Al₂O₃ (14.23%～16.21%), and MgO (2.32%～2.53%), with low contents of P₂O₅ (0.24%～0.26%) and TiO₂ (0.53%～0.61%). The rocks have a Rittmann index (σ₄₃) ranging from 0.86 to 1.47, an aluminum saturation index (A/CNK) from 0.98 to 1.12, and a coefficient of differentiation (DI) from 63 to 68. They are enriched in large ion lithophile elements (LILEs) such as Rb and K, depleted in high field strength elements (HFSEs) such as Nb, Ta, and Ti. They are also depleted in Ba relative to Rb and Th, with a negative Eu anomaly (δEu = 0.37～0.58). Generally, the granodiorite porphyry is a calc-alkaline arc I-type granitoid with typical island arc magmatite characteristics. It exhibits heterogeneous zircon Hf isotopic compositions (ε_Hf(t)= －15.7～－6.8) with a two-stage Hf model age (t_DM2) ranging from 2.2 Ga to 1.6 Ga. Both the whole-rock zircon saturation temperatures (T_Zr = 782～790℃) and the zircon Ti saturation temperatures (T_Ti = 603～772℃) are low, with Mg# values ranging from 49.38 to 55.96, presenting characteristics of crust-mantle mixing. It is concluded that Jilong porphyry is the product of mixing felsic magma which forms through the partial melting of the Paleoproterozoic crust, with a small amount of mantle-derived magma under the metasomatism of subduction-related fluids. The inversion of the northward subduction slabs of Bangong Co-Nujiang Tethys in the Early Cretaceous epoch (120～110 Ma) may be the primary dynamic mechanism.