摘要:
本文从五台地区滹沱群豆村亚群四集庄组、东冶亚群纹山组和郭家寨亚群西河里组地层中共采集了5件浅变质砂岩样品,并对其进行了La-MC-ICPMS锆石U-Pb年龄测定。分析结果显示,四集庄组2件砂岩样品碎屑锆石207Pb/206Pb年龄主要集中于~2.5Ga和2.1~2.2Ga两个峰值,其中~2.5Ga碎屑锆石来自新太古代五台群和五台地区花岗质杂岩;2.1~2.2Ga碎屑锆石获得207Pb/206Pb加权平均年龄2134±5Ma,限定了四集庄组砂岩沉积下限为2134Ma。结合四集庄组火山岩形成时代(2140±10Ma)和四集庄组底部发育厚层砾岩,我们认为滹沱群初始形成时代为~2.2Ga,即早元古代中期。东冶亚群纹山组底部砂岩中碎屑锆石207Pb/206Pb年龄主要集中于2050~2122Ma之间,其中64粒相对年轻的锆石获得207Pb/206Pb加权平均年龄2068±3Ma,代表了东冶亚群形成时代下限为2070Ma左右。综合豆村亚群青石村组火山岩形成时代2087±9Ma,我们认为东冶亚群初始形成于2070Ma左右。郭家寨亚群中最年轻碎屑锆石207Pb/206Pb年龄为1958±10Ma,表明郭家寨亚群开始沉积时代小于1.95Ga,为早元古代晚期/末期。区域上,早元古代末期是华北最终克拉通阶段,而郭家寨亚群与东冶亚群呈明显的角度不整合接触关系,两者记录了明显不同的地质过程。因此,我们建议郭家寨亚群应从滹沱群中解体出来并独立命名为郭家寨群,且郭家寨群可能沉积于华北克拉通化过程中/之后,开始沉积的时代为1.9~1.8Ga。
Abstract:
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.