摘要:
滹沱群是华北最典型的古元古界地层之一,限定其时代对建立早期地层年代格架和探讨华北早前寒武纪地质演化过程意义重大。本文对五台山地区蒋村镇附近的滹沱群地层进行了详细研究,从与蒋村砾岩中石英斑岩砾石相同的基岩中选取了2件石英斑岩样品,并获得其锆石 U-Pb 年龄结果分别为(2166±17) Ma和(2138±17) Ma。据此,限定蒋村地区碎屑沉积岩时代应小于2138 Ma。结合四集庄组底部发育厚层的砾岩和区域内2.2~2.1 Ga岩浆事件,我们进一步认为滹沱群底界年龄为~2.2 Ga。由于滹沱群豆村亚群和东冶亚群经历了同期褶皱变形作用,而侵入于蒋村四集庄组未变形正长花岗岩脉的锆石 U-Pb 年龄结果为(1816±18) Ma。因此,本文限定豆村亚群、东冶亚群的时代为2.2~1.82 Ga。石英斑岩中锆石Lu-Hf同位素分析结果为176Lu/177Hf 为0.000717~0.002239,176Hf/177Hf 为0.281399~0.281563,其176Hf/177Hfi为0.281315~0.281496和εHf(t)为–3.37~4.11,变化范围较大,初步认为其源自于新太古代早期上地壳的部分熔融并有地幔物质添加。
Abstract:
The Hutuo Group is one of the classic Paleoproterozoic strata in North China, and its age is important for building the stratigraphy framework and investigating the early Precambrian evolution process in the North China Craton. In this paper, we probed into the stratigraphy of Hutuo Group at Jiangcun, Wutai Mountains area, and obtained the zircon U-Pb ages of (2166±17) Ma and (2138±17) Ma from two quartz porphyries, which are identical with the ages of quartz porphyry pebbles in conglomerates of the Hutuo Group. Therefore, we infer that the clastic rocks in Jiangcun were deposited after 2138 Ma. Considering the distribution of a large amount of conglomerates at the base of Sijizhuang Formation and the existence of 2.2~2.1 Ga magmatism in Wutai Mountains area, we further deduce that the initial deposition of Hutuo Group took place at ca. 2.2 Ga. In combination with the fact that Doucun and Dongye Subgroups underwent the similar deformation, and undeformed syenogranite vein of 1816±18Ma intruded into the Sijizhuang Formation, we hold that the deposition age of Doucun and Dongye Subgroup should be 2.2~1.82 Ga. A sample of quartz porphyry yielded the zircon 176Lu/177Hf of 0.000717~0.002239 and176Hf/177Hf of 0.281399~0.281563. Based on the large variation of 176Hf/177Hfi andεHf(t) in zircons, we propose that the quartz porphyry mainly originated from the upper crust with the addition of the depleted mantle.
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