The re-examination of the age and stratigraphic subdivision of the Hutuo Group in the Wutai Mountains area, North China Craton: Evidences from geology and zircon U-Pb geochronology.
五台地区滹沱群时代与地层划分新认识:地质学与锆石年代学证据

In order to constrain the age of the Hutuo Group, five meta-sandstone samples are collected from Sijizhuang Formation of Doucun Subgroup, Wenshan Formation of Dongye Subgroup, and Xiheli Formation of Guojiazhai Subgroup, respectively, in Hutuo Group, Wutai Mountains area, North China Craton. Zircons from these samples have been analyzed with the technique of La-MC-ICPMS. The results show that most of the 207Pb/206Pb ages of 2 sandstones from Sijizhuang Formation mainly concentrate at two peaks, ~2.5Ga and 2.1~2.2Ga, respectively. The provenances of the~2.5Ga detrital zircons are derived from the Neoarchean Wutai Group and contemporaneous granitoids in the Wutai Mountains area. 19 grains of the 2.1~2.2Ga detrital zircons have the weighted mean 207Pb/206Pb age of 2134±5Ma. So the maximal deposition age of the Sijizhuang Formation is limited to 2134Ma or less. Combining the formation age of the volcanic rocks of 2140±10Ma and thick layer of conglomerates at the bottom of the Sijizhuang Formation, we deduce that the Hutuo Group deposited at ~2.2Ga, middle Paleoproterozoic. The 207Pb/206Pb ages of the detrital zircons from the bottom of the Wenshan Formation mainly distribute at 2050~2122Ma. Among these detrital zircons, 64 grains with younger ages produce the weighted mean age of 2068±3Ma, indicating the maximal deposition time of the Dongye Subgroup at ca. 2070Ma. Considering the volcanic rocks of 2087±9Ma in the Qingshicun Formation at the top of the Doucun Subgroup, we confer that the Dongye Subgroup deposited at ca. 2070Ma. The youngest zircon age from the metasandstone in Guojiazhai Subgroup gives the 207Pb/206Pb age of 1958±10Ma, constraining the Guojiazhai Subgroup deposited at the late or end of Paleoproterozoic. The North China Craton was finally cratonized at the end of Paleoproterozoic. As the Guojiazhai Subgroup unconformably overlies the Dongye Subgroup, the two stratigraphic units have recorded distinct geological features and may deposit at different geological stages. Therefore, we suggest that the Guojiazhai Subgroup should be separated from the Hutuo Group and re-named as the Guojiazhai Group which initially deposited at the age of 1.9~1.8Ga, and the substantial sedimentation completed during to post-cratonization process of the North China Craton.