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Zircon and beddeleyite U-Pb precision dating of basic rock sills intruding Xiamaling Formation, North China

LI Huai-kun, LU Song-nian,LI Hui-min, SUN Li-xin, XIANG Zhen-qun, GENG Jian-zhen, ZHOU Hong-ying Tianjin Institute of Geology and Mineral Resources, Tianjin 300170, China  
Diabase sills are wide-spread in the shales of the Xiamaling Formation to the north of the Malanyu uplift of the North China Craton. Time-dating minerals zircon and baddeleyite, separated from the diabase, have been dated by SHRIMP and ID-TIMS U-Pb isotopic methods. The emplacement age of the diabase 1320±6 Ma has been obtained, which indicates that the upper boundary age of the Xiamaling Formation must be younger than 1320 Ma. Based on recent dating achievements the Xiamaling Formaiton can be rather precisely put in the time span of 1400~1350 Ma. The Mesoproterozoic stratigraphic sequence of the Yanliao aulacogen in the North China Craton is collated and a new subdivision is proposed in this paper. In the new proposal the Changcheng System includes the Changzhougou, Chuanlinggou, Tuanshanzi and Dahongyu formations in ascending order with time span of 1800~1600 Ma; the Jixian System includes the Gaoyuzhuang, Yangzhuang, Wumishan, Hongshuizhuang and Tieling formations with time span of 1600~ 1400 Ma; and the 1400~1000 Ma part of the Mesoproterozoic strata above the Jixian System is undefined and much further work is needed.
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