LA-ICP-MS dating of Zhangjiakou Formation volcanic rocks in the Zhangjiakou region and its geological significance

The geological cross section of the Zhangjiakou Formation is originally established in the Zhangjiakou region. From the dating of four samples from typical cross sections of the Zhangjiakou Formation in the Zhangjiakou region, the age range is found to be mainly from 143.0±3.7 Myr to 136.1±1.4 Myr, which almost equals the sum of the age range of the Zhangjiakou Formation (136-135 Myr) and that of the Tuchengzi Formation (143-136 Myr) in the Luanping region, and the Zhangjiakou and Tuchengzi Formations in Zhang- jiakou-Luanping of North Hebei are found to be conformable. The evidence above indicates that the Zhangjiakou and Tuchengzi For- mations, which are distributed in Zhangjiakou-Luanping, were formed under the same geological setting. Volcanic activity was earlier and longer (143-136 Myr) in the western region (Zhangjiakou) than that in the eastern region (Luanping). The "Zhangjiakou Forma- tion" in Chengde and Pingquan formed in the period in which the Yixian Formation in West Liaoning formed, so should be renamed the Yixian Formation.