地质学报2018，Vol.92Issue(2)：330-340,11.
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
摘要
Abstract
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-MSKey words
North China Craton/Ordovician/Taoqupo Formation/K-bentonite/zircon/LA-ICP-MS引用本文复制引用
王振涛,周洪瑞,王训练,景秀春,马伯永..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[J].地质学报,2018,92(2):330-340,11.基金项目
本文为国家重点研发计划(编号2017YFC0602704)和国家重大基础研究计划“973项目”(编号2011CB403001)资助的成果. （编号2017YFC0602704）
