Petrogenesis and tectonic setting of the Late Permian A-type granitic dyke swarm in Ulungur, East Junggar

A large number of Late Paleozoic granites occur in the East Junggar on the southwest margin of the Central Asian Orogenic belt, providing a good chance to reveal the crustal evolution of the Central Asian Orogenic Belt. In this study, a series of Late Paleozoic granitic dykes striking vertically to the suture zone were recognized in Akejila, northwest of Ulungur of East Junggar. Their timing of emplacement and petrogenesis are critical to understand the tectono-crustal evolution of the study area. SHRIMP zircon U-Pb dating yielded a Late Permian age (266±2 Ma). Geochemical analyses indicate that they are characterized by relatively high concentration of silica (SiO₂=75.66%~76.69%) and alkali (Na₂O+K₂O=8.67%~9.16%), but low calcium (CaO =0.06%~0.14%) and magnesia (MgO =0.04%~0.06%)content. The obvious negative Eu anomaly (δEu=0.16~0.18), relative depletion of Ba, Sr, P and Ti, and significant enrichment of Nb, Zr, Th, Ta and Hf, as well as high Ga/Al value (>2.6) characterize the weakly peraluminous A1 type granites. The positive ε_Nd(t) value (+4.7) and a young model age (655 Ma) suggest that these dykes are mainly derived from the juvenile mantle. Therefore, the comprehensive analysis indicates that the dykes were formed by the partial melting of the juvenile lower crust caused by the upwelling of the mantle in a post-collision setting.