Derived from the deep lithospheric source and emplaced at shallow crustal levels, alkali- rich intrusive rocks are regarded as a window for deciphering the nature of the mantle. An alkaline composite pluton composed of predominant syenite and subordinate intermediate- mafic rocks is exposed in the Kuoktagh area, the transition region between the eastern Tianshan Belt and the north margin of Tarim craton. New LA- ICP- MS zircon ages range from ~227 to ~224 Ma, indicating Indosinian emplacement. Samples from the pluton are enriched in total alkali (Na2O+K2O=7. 93%~12. 28%), Al (Al2O3=15. 62%~18. 67%), and poor in Mg (MgO=0. 12%~4. 01%), Ti (TiO2=0. 14%~1. 63%). They exhibit a metaluminous affinity (A/CNK=0. 78~1. 00) and belong to high- K calc- alkaline to shoshonitic series. In terms of trace element patterns, the studied rocks are characterized by the enrichment in high- field- strength element (HFSE), such as Nb, Ta, Th, U, Zr, and Hf, and depletion in Sr, P, and Ti. The studied samples possess strong enrichment in light rare earth elements (LREE) relative to heavy rare earth elements (HREE), with variable Eu anomalies (δEu=0. 12~1. 72). Samples from the pluton show variable but overall “ancient”（enriched）whole- rock Sr- Nd and zircon Hf isotopic results （（87Sr/86Sr）i=0. 70489~0. 70581, εNd（t）=14. 56~6. 74, εHf（t）=11. 48~3. 06）, and have zircon δ18O values (5. 49‰~6. 87‰) slightly higher than that of the mantle (5. 3‰±0. 6‰). Considering petrological and geochemical features of the studied pluton, combined with other geological evidence, we suggest that the studied alkaline rocks are of A1- type granite and were formed in an intra- plate extensional setting. Taking into account the characteristics of major elements, trace elements and Sr- Nd- Hf- O isotopes, we propose that the composite pluton is the product of assimilation and fractional crystallization (AFC), and the primary magma was derived from an enriched lithosphere mantle. North margin of Tarim craton, eastern Tianshan Belt and Beishan region were likely together in an intra- plate, anorogenic tectonic framework. The temporospatial distribution of A- type granites in these regions might record the closure history of the Paleo- Asian Ocean from west to east via a ‘scissor- like’ model.