The evidence for the existence of Early Cretaceous continental rift in West Qinling orogenic belt: the significance of the newly built Maixiushan Group in Tongren area of Qinghai Province

The only Early Cretaceous continental volcanic strata in Qinghai Province are distributed in Maixiushan area between Tongren County and Zeku County in Huangnan Prefecture of Qinghai Province and lie in Zeku subunit of West Qinling stratigraphic unit. Based on profile study and regional stratigraphic correlation, the authors newly named this suite of strata Early Cretaceous Maixiushan Group （K1M）. The purplish red-grayish purple intermediate-basic volcanic rocks intercalated with sandy conglomerate in its lower part constitute the newly-built Duohemao Formation（K1d），which is a suite of typical continental fissure eruption alkaline basalt and continental red bed formation. The purplish red sandy conglomerate assemblage in the upper part of the Maixiushan Group remains called Wanxiu Formation（K1w），which is a suite of alluvial fan facies continental molasses formation. The two parts are obviously in conformable contact. According to the angular unconformity between Maixiushan Group and its overlying and underlying strata, the isotopic ages of volcanic rocks of Duohemao Formation and the insects, estherites and plant fossils in Wanxiu Formation, the authors hold that the Maixiushan Group was formed in Early Cretaceous. Distributed along the nearly NS-trending tensional faulting belt, the Maixiushan Group is of the intracontinental rift facies sedimentation which was produced in Early Cretaceous intracontinental environment and withered away in the same environment, and the formation of the Maixiushan Group indicated that the the West Qinling area entered into a regional extension stage in Early Cretaceous period.