Constraint on the late Mesozoic regional angular unconformity in West Liaoning--North Hebei by LA-ICP-MS dating

The angular unconformity between the Tuchengzi Formation and Zhangjiakou Formation in the Chengde area is commonly regarded as the direct evidence of boundary between Jurassic and Cretaceous system, and the evidence of tectonic transformation interfaces in Mesozoic Era in the West Liaoning-North Hebei. Moreover, the lower boundary age of the Zhangjiakou Formation in the Chengde was to be thought about 135 Myr in the past. Based on detailed study and systematic sampling in Yixian-Beipiao, Lingyuan, Chengde and Luanping areas in the field, the authors obtained the following isotopic ages by means of LA-ICP-MS dating: In the Lingyuan area, the Zhangjiakou Formation and the Yixian Formation are angular unconformable; the age range of the Zhangjiakou Formation volcanic rock is from 133 Myr to 130 Myr (seven samples), and that of the Yixian Formation volcanic rock is from 126 Myr to 121 Myr (eleven samples). In the Luotuoshan of Chengde, the Tuchengzi Formation and the Zhangjiakou Formation are angular unconformable; the age at the lower Zhangjiakou Formation is about 124 Myr (one sample), which reflects that the age range of the lower Zhangjiakou Formation is from 129 Myr to 124 Myr combined with the former study. In the Luanping basin, there is a largest sediment gap in upper Mesozoic, whose lower boundary age is confined in 131 Myr to 130 Myr. In the Yixian-Beipiao area, the second member of the Yixian Formation, making up more than 3/4 thickness of the Yixian Formation, was formed in 126 Myr to 119 Myr, and the age of the Yixian Formation at the bottom is about 132 Myr. Moreover, the upper boundary age of the Tuchengzi Formation is about 136Myr in the West Liaoning-North Hebei region. Based on the above isotopic data combined with the geological characteristics in the field in the West Liaoning--North Hebei region, it reflects that there was a widespread tectonic event in the period of 136 Myr to 126 Myr (mainly in 130-126Myr) in the West Liaoning-North Hebei, and it formed the late Mesozoic regional angular unconformity. Moreover, the lower boundary ages of the late Mesozoic regional angular unconformity in different areas are different, and the tectonic action and creature assemblage are different at the top and bottom layers of the regional angular unconformity. Those imply that the Late Mesozoic regional angular unconformity is a regional interface of tectonic transformation in Mesozoic Era in the West Liaoning-North Hebei.