华北地块北缘上石炭统—中三叠统碎屑锆石研究及其地质意义

    A study of detrital zircons of Late Carboniferous-Middle Triassic strata in the northern margin of North China block and its geological implication

    • 摘要: 利用碎屑锆石U-Pb年龄和Hf同位素对平泉地区晚石炭世—中三叠世地层进行物源分析,重塑内蒙古隆起的隆升历史,进而探讨古亚洲洋的闭合过程。5个样品的LA-ICP-MS碎屑锆石U-Pb年龄由5个年龄峰组成:峰1中晚二叠世(270~250Ma)、峰2晚石炭世—早二叠世(325~290Ma)、峰3泥盆纪(400~360Ma)、峰4古元古代(1900~1700Ma)和峰5新太古代-古元古代早期(2600~2400Ma)。其中,本溪组碎屑锆石主要由峰3、峰4和峰5组成,山西组、孙家沟组和刘家沟组则主要由峰2和峰4组成,下石盒子组由峰1、峰4和峰5组成。5个样品具有相同的Hf同位素特征,显生宙锆石的εHf(T)在-3.2~-25.5之间,与内蒙古隆起的晚古生代侵入岩体完全相同。碎屑锆石U-Pb年龄和Hf同位素特征显示物源主要来自内蒙古隆起,不同层位的物源变化表明内蒙古隆起曾存在2次重要的隆升事件:第一期发生在360~312Ma之间,第二期发生在276~258Ma期间。第一次隆升奠定了盆-山的基本构造格局,第二次隆升之后古隆起进入稳定剥蚀期,为盆地提供持续稳定的物源,盆地内部发育了一套粗碎屑沉积物,可以推断在276Ma左右古亚洲洋已经关闭。

       

      Abstract: In order to interpret the uplift history of Inner Mongolia Paleo-uplift and understand the tectonic evolution of Paleo-Asian Ocean, the authors conducted U-Pb dating and Hf isotope analysis of detrital zircons from distinct levels of Late Carboniferous-Middle Triassic strata in Pingquan. U-Pb ages of detrital zircons from five samples fall into five groups, i.e., Group 1 of Middle-Late Permian (270~250Ma), Group 2 of Late Carboniferous-Early Permian (325~290Ma), Group 3 of Devonian (400~360Ma), Group 4 of Paleoproterozoic (1900~1700Ma) and Group 5 of Neoarchean-Paleoproterozoic (2600~2400Ma). Among these five samples, age spectra of Benxi Formation are composed of Group 3, Group 4 and Group 5, age spectra of Shanxi Formation, Sunjiagou Formation and Liujiagou Formation mainly consist of Group 2 and Group 4, and age spectra of Lower Shihezi Formation are composed of Group 1, Group 4 and Group 5. In addition, zircons from the five samples have the same Hf isotope characteristics. εHf(T) values of Phanerozoic zircons range from -3.2 to -25.5, consistent with the values of Late Paleozoic plutons in Inner Mongolia Paleo-uplift. The zircon age constituents and Hf isotopes suggest a provenance from Inner Mongolia Paleo-uplift, and the zircon age variation through different levels implies two critical uplifting events, which occurred during 360~312Ma and 276~258Ma respectively. 360~312Ma uplift built up the tectonic framework of the North China basin and provided sediments for the basin. After 276~258Ma uplift event, Inner Mongolia Paleo-uplift was steadily denuded and thus supplied a great amount of coarse-grained sediments to the sedimentary basin which had unchanged U-Pb age spectra of detrital zircons, suggesting that the Paleo-Asian Ocean might have been closed before 276Ma.

       

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