Laser 40Ar/39Ar chronology of the Mesozoic volcanic rocks from Longjiang basin in northern Da Hinggan Mountains

There has long been controversy concerning the stratigraphic division and the implications of Longjiang and Guanghua Formations, which constitute the main units in Longjiang basin, northern Da Hinggan Mountains. According to field observation of the established profiles of these two formations and neighboring areas, the volcanic rocks of the three Formations(Longjiang Formation, Guanghua Formation, Ganhe Formation) distributed extensively in Longjiang basin in northern Da Hinggan Mountains were studied on the basis of the laser 40Ar/39Ar dating method. The results show that the ages of these three formations are 125.1Ma±1.5Ma~117.0Ma±9.0Ma, 125.4Ma±1.8Ma~121.6Ma±1.4Ma, and 123.1Ma±1.1Ma~82.0Ma±4.0Ma, respectively. The age of Early Late-Cretaceous is most likely to have a good agreement with that of Gushanzheng Formation distributed rarely in northern Da Hinggan Mountains. In combination of the field observation with dating results, the present Longjiang and Guanghua Formations should be treated totally as the originally-named Longjiang Formation on the ground of having experienced simultaneously evolution process under the condition of the same volcanic cycle, and the name of Guanghua Formation previously separated from the upper layer of originally-named Longjiang Formation should be abandoned. An analysis of the characteristics of the volcanic-sedimentary strata shows that Longjiang basin has a good stratigraphic correlation with its northern neighboring Dayangshu basin in the same geological evolution process, indicating that there probably exists an oil-and-gas prospective area in Longjiang basin.