The Eastern Tianshan area with numerous Late Paleozoic granitoids constitutes the southwestern part of the CAOB. The intrusive tectonic setting of these granitoids has been a highly debated issue. The Dananhu Aketag granitic pluton located on the south margin of the Eastern Tianshan Mountains is composed mainly of porphyry K-feldspar granite. SHRIMP zircon U-Pb dating yielded a ²⁰⁶Pb/²³⁸U age of 306.5±2.8 Ma(MSWD=0.53), which is interpreted as the formation age of the pluton. All samples from the pluton are characterized by high values of SiO₂, K₂O, Na₂O and low values of CaO, MgO, P₂O₅, TiO₂, suggesting weakly peraluminous rocks (A/CNK=1.01～1.03, A/NK=1.09～1.12). The samples exhibit a relative enrichment of LREE as well as negative anomalies of Eu. In the primitive mantle (PM) normalized trace element patterns, all of these rocks are enriched in large ion lithophile elements (LILEs, Cs, Rb, Th, K) and strongly depleted in high field strength elements (HFSEs, Nb, Ta, P, Ti) in comparison with the primitive mantle. They have high Ga/Al ratios and belong to A2-type granite. These characteristics suggest that the Aketag pluton was generated by the partial melting of mafic lower crust in a post-collisional setting. An important implication is that the ocean in the Eastern Tianshan area was closed before Permian.