陕西耀县上奥陶统桃曲坡组下部钾质斑脱岩U-Pb年龄的首次报道  

First Report on Zircon U-Pb Geochronology of K-bentonites from the Lower Part of the Upper Ordovician Taoqupo Formation in Yaoxian,Shaanxi

作　　者：王振涛[1] 周洪瑞[2] 王训练[2] 景秀春[2] 马伯永[3] 

机构地区：[1]中国地质科学院矿产资源研究所国土资源部盐湖资源与环境重点实验室,北京100037 [2]中国地质大学(北京)地球科学与资源学院,北京100083 [3]中国地质调查局,北京100037

出　　处：《地质学报》2018年第2期330-340,共11页Acta Geologica Sinica

基　　金：国家重点研发计划(编号2017YFC0602704);国家重大基础研究计划"973项目"(编号2011CB403001)资助的成果

摘　　要：华北克拉通上奥陶统的发现和地质时代的确定一直颇受重视。前人通过对华北奥陶系顶部地层中牙形石和笔石记录的核实,将耀县桃曲坡组的顶限定在笔石Diplacanthograptus spiniferus带结束的位置(凯迪阶下部),该位置代表了华北奥陶系的最高层位。而《中国地层表》(2014)将桃曲坡组的底置于艾家山阶和钱塘江阶分界处附近,后者略高于上述笔石生物带结束的位置,可见分歧是明显的,亟需来自绝对年龄资料的约束。笔者在耀县桃曲坡组下部发现了一层斑脱岩,地球化学数据揭示了其富钾质特征(K2O>Na2O),K2O含量(3.81%)接近鄂尔多斯盆地南缘奥陶系17个斑脱岩样品的平均值(4.75%)。锆石LA-ICP-MS U-Pb测年表明其就位年龄为453.7±3.3 Ma(MSWD=0.25),考虑到采样层位接近桃曲坡组的底界,认为桃曲坡组的底可置于454 Ma(桑比阶晚期)。这与前人在周缘地区上奥陶统斑脱岩获得的年龄在误差范围内一致,表明本文获得的年龄是可信的。因此,《中国地层表》(2014)中桃曲坡组的底界需要适当下移。Discovery of theUpper Ordovician in the North China Craton and determination of its geological time has received considerable concerns. By verifying the previous records of conodont and graptolite in the top part of the Ordovician carbonate sequences at different localities, this study defined top of the Taoqupo Formation in Yaoxian County within the closure position （449.73 Ma, the early Katian） of graptolite Diplacanthograptus spiniferus Biozone. This position represents the highest sequence of the Orvocician in North China. However, The Stratigraphic Chart of China （2014） placed the top of the Taoqupo Formation around the division line between Qiantangjiangian and Hirnantian Stages （445.6 Ma）, with the latter slightly higher than the end position of the graptolite biozone. The difference is evident and needs absolute age data. This study found one layer of bentonite at the lower part （about 5m to the bottom） of the Taoqupo Formation in Yaoxian. Geochemical data of bentonite samples reveal that they are potassium-rich feature （K2O〉Na2O）, with K2O content of 3.81% close to the average value （4.75%） of 17 Ordovician bentonite samples from the southern margin of the Ordos Basin. Zircon LA-ICP-MS U-Pb dating shows that the bentonite bed emplaced at 453.7±3.3 Ma （MSWD = 0.25）. Considering sampling sequences and relatively quick sedimentary rate of the limestone, it can be concluded that the bottom of the Taoqupo Formation is at 454 Ma （late Sandbian Stage）. The age obtained in this paper is consistent with the previously obtained data, with deviation within the error range, indicating that the age in this paper is credible. Therefore, the bottom boundary of the Taoqupo Formation in The Stratigraphic Chart of China （2014） should shift downward appropriately.

关 键 词：华北克拉通 奥陶系 桃曲坡组 钾质斑脱岩 锆石 LA-ICP-MS 

分 类 号：P534.42[天文地球—古生物学与地层学] P588.2[天文地球—地质学]