Abstract: Abstract: The Tongan Formation is one of the most ancient basement strata on the western margin of Yangtze block; nevertheless, accurate stratigraphic diagenetic age of this formation has had no related reports yet, with only the intrusion age of ~1.5 Ga from zircon of gabbro-diorite dyke and the oldest deposition time of ~1.8 Ga from detrital zircons. Based on petrographic research and using tuff as the study object, the authors dated the volcanic zircons from meta-basic volcanics tuff of the lower first period and meta-tuff of the bottom fourth period of the Tongan Formation by using LA-ICP-MS zircon situ U-Pb dating method. As a result,, the authors respectively acquired the 207Pb/206Pb age of 1833Ma for tuffaceous basic volcanic rock and 1508±15Ma for meta-volcanic tuff. Some conclusions have been reached: (1) ~1.8Ga represents the depositional age of the lower first period, which defines the oldest deposits of the Tongan Formation, consistent with the Dongchuan Group and Dahongshan Group; (2) the depositional age of the bottom fourth period is ~1.5Ga, in accord with the ages of the third period of the Tongan Formation and the Heishan Formation of the Dongchuan Group, as shown by data obtained by previous researchers; (3) the authors further define the Tongan Formation, which should be under the Huili Group, in accordance with the Dongchuan, Hekou and Dahongshan groups, as a heteropic product.