Abstract: The Chengwan granite is located in Tongbai area, South Qinling. The lithology is mainly granulite (biotite) syenite granite. Lithochemical analysis shows that it has high SiO₂, high alkali, poor MgO and low TiO₂, and A/CNK=0.99~1.40, with an average of 1.17, thus belonging to the high-K calc-alkaline series. Chondrite-normalized REE patterns of the Chenwang granite are characterized by right-inclined "seagull"with Eu anomalies (δEu=0.11~0.81, 0.46 on average). The rock is characterized by enrichment of HFSE (Zr, Th, Hf), Rb, K, and strong depletion of Ba, Sr, P and Ti. The values of 10000Ga/Al range from 2.84 to 3.63. Zircon saturation temperatures range from 817~879℃. All these features suggest that the Chengwan granite belongs to A2-type granite. LA-MC-ICP-MS U-Pb dating yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 793.0±4.1Ma for the zircon from syenogranite, indicating that the Chengwan granite was formed in the late Neoproterozoic Qingbaikouan period rather than previously considered Early Paleozoic. Its formation was related to the breakup of Rodinia supercontinent, and was formed in the early stage of breakup. The South Qinling still belonged to the northern margin of the Yangtze continental margin in Tongbai area, globally belonging to continental cleavage at the middle to late stage of the Neoproterozoic (about 825Ma to 611Ma).