The Tong'an Formation is part of the folded basement in the southwestern margin of Yangtze Craton and further subdivided into five lithological units. LA-ICP-MS U-Pb dating of detrital zircons from tuffaceous siltstone (sample 100620-1) and magmatic zircons from gabbro-diorite dyke (sample 100620-2-1 and 100620-2-2) in this study was used to provide insight into the geochronology of the Tong'an Formation. Two major age populations of 1.8~1.9 Ga and 2.28~2.38 Ga of the detrital zircons from the third lithological unit suggest that sediments of the Tong'an Formation are of diverse provenances. Three grains of the youngest detrital zircons from the tuffaceous siltstone yielded a mean 207Pb/206Pb age of 1806 Ma. Zircon 207Pb/206Pb age of 1513±13 Ma (sample 100620-2-1) and intercept up age of 1531±18 Ma (sample 100620-2-2) have been obtained from gabbro-diorite dyke intruding the third unit. Thus, the geochronology of the Tong'an Formation was estimated between ~1.8 Ga to 1.5 Ga based upon the newly obtained U-Pb age result. The zircon ages of 1513 Ma and 1531 Ma from gabbro-diorite dyke are consistent with the previous data of 1503±17 Ma from volcanic tuff of the Heishan Formation in the Dongchuan Group, which suggests that gabbro-diorite dyke intruding the Tong'an Formation and volcanic tuff from Heishan Formation in the Dongchuan Group belongs to the same magmatic cycle. The third lighological unit of the Tong'an Formation is proposed to be correlated with the Heishan Formation of the Dongchuan Group in Yunnan Province.