内蒙古银额盆地哈日凹陷下白垩统碎屑锆石U-Pb测年及其物源意义*

前人认为银额盆地下白垩统物源主要来自凹陷周缘的造山带或凸起区,但对于下白垩统的物源具体属于何套地层、确切源区位置等问题,并未开展过相关研究。笔者以哈日凹陷为例,通过对该凹陷下白垩统8个沉积岩样品开展地球化学和碎屑锆石U-Pb年代学研究,以期解决以上问题。研究结果表明: (1)碎屑锆石按年龄可以分为三叠纪锆石、二叠纪锆石、石炭纪锆石和古老锆石4类,对应的年龄分别为$237.9\pm3.2\pm-238.3\pm8.4$ Ma, $255.0\pm9.7-285.9\pm4.3$ Ma, $307.0\pm23.0-330.0\pm3.0$ Ma and $434.0\pm4.0-2584.0\pm14.0$ Ma 。(2)微量元素和锆石年龄组成特征表明,下白垩统以二叠系和石炭系物源供给为主,包括二叠纪和石炭纪侵入岩和沉积岩。(3)锆石年龄分布指示哈日凹陷下白垩统物源具有多源区的特征,凹陷西北部的洪格尔吉山、南部的宗乃山和东部的切刀山对其均有物源贡献。(4)3个物源区对巴音戈壁组和苏红图组的物源贡献率具有一定的差异,如宗乃山地区对苏红图组有较大的物源贡献率,这种差异可能由沉积物迁移距离、源区地质体剥蚀速率、构造运动等原因所致。下白垩统物源特征的确定,对于恢复哈日凹陷早白垩世原型盆地、建立早白垩世沉积古地理模型等均具有重要的意义。

Lower Cretaceous detrital zircon U-Pb dating and its provenance significance in Hari sag,Yingen-Ejinaqi Basin,Inner Mongolia

It is generally recognized that the source of Lower Cretaceous in Yingen-Ejinaqi Basin is from orogenic belt or uplift around the sags,but some queries about provenance were still unanswered,such as the specific formations,exact source area ,etc. In this paper,taking Hari Sag as an example,based on geochemical test analysis and zircon U-Pb dating of 8 drilling rock samplings,the study of geochemical characteristics and detrital zircon U-Pb chronology were carried out. The results showed that: (1)Zircon could be divided into 4 types by age,they are Triassic zircons,Permian zircons,Carboniferous zircons and ancient zircons,and the corresponding age were $237.9\pm3.2\pm-238.3\pm8.4$ Ma, $255.0\pm9.7-285.9\pm4.3$ Ma, $307.0\pm23.0-330.0\pm3.0$ Ma and $434.0\pm4.0-2584.0\pm14.0$ Ma respectively. (2)The characteristics of trace elements and the zircon ages indicated that the sedimentary source of the Lower Cretaceous in Hari Sag was dominated by the Permian and Carboniferous provenance,including pluton and sedimentary strata. (3)The age distribution of detrital zircon indicated that there were multiple provenance in the Lower Cretaceous of the Hari Sag,such as Honggeeji mountain in the northwest,Zongnai mountain in the south,and the Qiedao mountain in the east of the sag. (4)There were different contribution of these provenance to Bayingebi Formation and Suhongtu Formation,like Zongnai mountain was the biggest contributor to Suhongtu Formation. This contribution difference might be due to the transport distance of sediments,denudation rate of geological bodies,structural movements and so on. The determination of the provenance of the Lower Cretaceous was of great significance for the restoration of the Early Cretaceous Prototype Basin,the establishment of the Early Cretaceous sedimentary paleogeographic model and many more.