Lasing on pyroclastic rocks: A case study of 40Ar/39Ar dating on Moshishan Group,eastern Zhejiang Province

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. Supported by National Natural Science Foundation of China (Grant Nos. 40221402, 40673048 and 90714007)