The Dongsu granites from the Solonker suture zone are characterized by relatively high silicon (SiO₂=69.76%~75.58%), high alkali and potassium (K₂O+Na₂O=6.99%~9.16%, K₂O/Na₂O=0.70~1.69), high TFeO/MgO (3.43~11.26), and low MgO (0.08%~0.50%). Geochemical analyses show that the granites have relatively high LREE, obvious negative anomalies of Ba, Sr, Nb, Eu, and concentration of incompatible elements (Rb, Th, Ta). Major and trace elements indicate that the Dongsu pluton is of aluminous A-type granite. By LA-ICP-MS zircon U-Pb dating, a weighted mean ²⁰⁶Pb/²³⁸U age of 221.5±0.81 Ma (MSWD=0.57) is considered to be the formation age of the Dongsu granites which are the products of Late Triassic magmatic activities. Combined with geochemical information and regional geological data, it is concluded that the Dongsu granites were formed by partial melting of crust materials (meta-greywacke) at the stage of post-orogenic extension. Its intrusion marked the end of the evolution of the Paleo-Asian Ocean and the terminal of the collision of the Solonker suture zone in the Late Triassic.