内蒙古北山地区盘陀山一带长城纪古硐井群碎屑锆石LA-ICP-MS U-Pb年龄及其地质意义

长城纪古硐井群作为内蒙古北山地区古老褶皱基底之上的第一个沉积盖层,尚未获得可靠的年龄资料,前人只是根据其产出的层位和岩石组合特征与燕山地区的长城系对比后,将其置于中元古代长城纪。对古硐井群变质长石石英砂岩中碎屑锆石进行了LA-ICP-MS U-Pb测年,结果显示,锆石~(207)Pb/~(206)Pb年龄数据90%介于1.3~2.2Ga之间,其余年龄介于2.4~3.2Ga之间,峰期在1.6~2.0Ga之间,最年轻的碎屑锆石~(207)Pb/~(206)Pb年龄为1393±50Ma。通过对比碎屑锆石的年龄频谱和周围古地块的形成时代,碎屑锆石的U-Pb年龄限制了古硐井群的沉积时代,应为中元古代蓟县纪,古硐井群的沉积物可能来自北山-阿拉善和塔里木-敦煌地块。

LA-ICP-MS U-Pb geochronology of detrital zircons from the Gudongjing Complex-Group in Pantuoshan area of Beishan region within Inner Mongolia during Changchengian Period and its geological significance

Changchengian Period Gudongjing Complex-Group serves as the first sedimentary cover on the ancient folded base-ment in Beishan region of Inner Mongolia, but without reliable dating data. Previous researchers assigned it to Mesoproterozoic Changchengian Period only on the basis of the contrast with the Changchengian system of the Yanshan area in such aspects as for-mation layers and rock association characteristics. In this paper, the LA-ICP-MS zircon U-Pb method was used for the determi-nation of the age of the detrital zircon in metamorphic quartz sandstone of the Gudongjing Complex-Group, and the test results show that the 90% ages of zircon ²⁰⁷Pb/²⁰⁶Pb data are 1.3~2.2Ga, the rest of ages are 2.4~3.2Ga, peak period ages are 1.6~2.0Ga, and the youngest ²⁰⁷Pb/²⁰⁶Pb age of detrital zircon is 1393±50Ma. Based on comparing the age spectrum of the detrital zircon with the formation age of the surrounding massif, the dertital zircon limits the sedimentary age of the Gudongjing Complex-Group as the Mesoproterozoic Jixianian Period. The sediments of the Gudongjing Complex-Group might have come from Beishan-Alxa and Tarim-Dunhuang blocks.