鄂尔多斯盆地南缘赵老峪剖面奥陶系凝灰岩 锆石SHRIMP U-Pb定年及其地质意义

鄂尔多斯盆地南缘奥陶系赵老峪组的时代一直是依据古生物化石组合确定的,但基于笔石带其时代为晚奥陶世,而依据牙形石组合则为中奥陶世或者中奥陶世晚期,存在明显的差异,缺少高质量同位素年龄的约束。本文选择富平赵老峪剖面赵老峪组凝灰岩夹层进行精确的SHRIMP锆石U-Pb同位素测年,获得了453.2±6.9 Ma年龄结果,将此前认为的中、晚奥陶世沉积严格地确定为晚奥陶世。结合区域地质资料讨论,认为鄂尔多斯南缘上奥陶统凝灰岩层的形成与古秦岭洋壳向北的俯冲有关,可能记录了与这个过程伴生的火山喷发事件。该同位素年龄表明这里的奥陶纪地层在华北克拉通是最年轻的,故它不仅准确地确定了地层的形成时代,而且对华北克拉通早古生代地质演化史的认识有着重要的意义。

Confirmation of Ordovician Sediments in South Margin of Ordos Basin by SHRIMP U-Pb Zircon Dating of Volcanic Tuff Interlayers and Its Significance

The age of the Ordovician Zhaolaoyu Formation has been determined by fossil assemblage, and different ages have been suggested according to this method. Some researchers thought the Zhaolaoyu Formation was formed during the Late Ordovician according to recognition of the Graptolite zone, whereas others thought it was formed during the Middle Ordovician or late Middle Ordovician according to the Conodont assemblage. In this paper, SHRIMP U Pb zircon age of 453.2 ± 6.9 Ma has been obtained from the volcanic tuff interlayer in the Zhaolaoyu Formation, the Zhaolaoyu section of the Fuping area. Considering the sample was collected under the fossil zone in the Zhaolaoyu section, this age indicated that the Zhaolaoyu Formation developed in the Fuping area belongs to Late Ordovician. Furthermore, the geological implications of this zircon age are discussed in combination with regional data. The form of volcanic tuff interlayer in the Zhaolaoyu Formation may be best interpreted as the responses to the northward subduction of ancient Qinling oceanic basin. The isotope chronological data show that the Zhaolaoyu Formation on southern margin of the Ordos Basin is the youngest in North China Craton, so, these data not only accurately indicate the age of the strata but also have great significance for understanding of the geological evolution of the Early Paleozoic in the whole North China Craton.