It is difficult to date pyroclastic rocks,for almost all the dating methods,due to the multiple sources during their formation.40Ar/39Ar incremental heating results on groundmass selected from the samples show that the age spectra are meaningless geologically.However,singe crystal total fusions of CO2 lasing on the sanidine separates could yield rational 40Ar/39Ar results and distinguish their sources in this study.Timing on three formations of the Moshishan Group,after avoiding the exotic and altered grains by lasing on the single sanidine separate,was reported in this paper.The lowermost portion of the Chawan Formation gives an age of 113.7±0.3 Ma;the lower part of the Xishantou Formation was formed 116.4±0.4 Ma ago and the bottom of the Gaowu Formation took its shape at 118.4±0.4 Ma.These new ages are much younger than the previous ones,suggesting that these thick volcanic formations had been formed in very short durations. It is difficult to date for pyroclastic rocks, for almost all the dating methods, due to the multiple sources during their formation. 40Ar / 39Ar incremental heating results on groundmass selected from the samples show that the age spectra are meaningless geologically. Host, singe crystal total fusions of CO2 lasing on the sanidine separates could yield rational 40Ar / 39Ar results and distinguish their sources in this study. Timing on three formations of the Moshishan Group, after avoiding the exotic and altered grains by lasing on the single sanidine separate, was reported in this paper.The lowermost portion of the Chawan Formation gives an age of 113.7 ± 0.3 Ma; the lower part of the Xishantou Formation was 116.4 ± 0.4 Ma ago and the bottom of the Gaowu Formation took its shape at 118.4 ± 0.4 Ma.These new ages are much younger than the previous ones, suggesting that these thick volcanic formations had been formed in very short durations.