Objectives:The widely exposed Chengjiang Formation on the western margin of the Yangtze Block represents a sedimentary rock series closely related to the evolution of the Rodinia supercontinent. Its formation time limit and tectonic background have significant implications for the Neoproterozoic basin evolution in South China and the reconstruction of ancient continents. Results:Through field investigations, two layers of volcanic interbeds were discovered at the top of the Chengjiang Formation in the Luquan area of Yunnan Province. The lithology is gray-white rhyolitic crystal tuff. LA-ICP-MS zircon U-Pb dating of the tuff yielded a weighted average age of 735.6±5.2 Ma (n=24, MSWD=0.14), marking the first acquisition of the top boundary age of the Chengjiang Formation in Yunnan, thus constraining the depositional time limit of the Chengjiang Formation to 812-735.6 Ma. Rock geochemical and Lu-Hf isotopic studies reveal that the crystal tuff is significantly enriched in large ion lithophile elements (LILE) such as Rb, Th, and K, and depleted in high field strength elements (HFSE) such as Ba, Sr, P, Ti, Nb, and Ta. The rare earth element distribution curve exhibits a "swallowtail" pattern, consistent with the characteristics of the low-Sr, high-Yb Nanling-type granite. Its formation originated from 5%~10% partial melting of Mesoproterozoic early (TDM2: 1932~1265 Ma, with an average value of 1704.3 Ma) lower crustal argillaceous rocks, with minor mantle material involvement. Conclusions:It formed in an extensional tectonic setting, related to the breakup of the Rodinia supercontinent in the South China Block, and represents significant petrological and magmatic records of the Nanhua rift.