Petrogenesis of the Late Indosinian adakitic granites in East Kunlun and their constraints on the collision process

The collision process of East Kunlun orogenic belt is still a subject of debate. The discovery of Triassic adakitic granites in the Xiaonanchuan area of East Kunlun provides an opportunity to address this issue. The Moshigou and Bentoushan granitic intrusions are exposed in the Xiaonanchuan area. These granites primarily consist of quartz, plagioclase, potassium feldspar, and biotite. They formed during 209-208 Ma and 201-200 Ma, respectively. They have high SiO₂ and Al₂O₃ and are rich in sodium. They also have high Sr content (398×10^-6~613×10^-6) and Sr/Y ratios (50~97) and are depleted in heavy rare earth elements without Eu anomalies, exhibiting typical geochemical charaterisitics of adakitic rocks. The Moshigou granite have negative whole-rock ε_Nd(t) (-3.60~-3.34) and variable zircon ε_Hf(t) (-1.3~+5.9), indicating their derivation from the partial melting of the thickened lower crust. The Bentoushan granite have negative whole-rock ε_Nd(t) (-1.65~-1.55) and positive zircon ε_Hf(t) (+3.4~+7.3), suggesting their origin from meta-basic rocks-dominated thickened lower crust with an eclogite residue. Comprehensive analysis shows that the initial collision of East Kunlun orogenic belt occurred during early Late Triassic. Subsequently, slab break-off occurred, leading to decompressional melting of the mantle and inducing partial melting of crust and formation of adakitic granites. This geological event also triggered the formation of skarn Fe deposits, polymetallic ores, and gold deposits. After late Late Triassic, the East Kunlun orogen transitioned into post-collision stage characterized by lithospheric extension and mantle upwelling, giving rise to mantle-derived magmatic rocks, A-type granites, and adakitic granites. Ultimately, the orogenesis of East Kunlun concluded after 198 Ma.