内蒙古巴林左旗二叠系碎屑锆石LA-ICP-MS U-Pb年龄及构造意义

LA-ICP-MS U-Pb Geochronology of the Permian Detrital Zircons in Baarin Left Banner of Inner Mongolia and Its Tectonic Significance

内蒙古巴林左旗碧流台地区分布一套碎屑岩,以砂板岩组合为特征,时代原为早二叠世。对该套地层所取砂岩样品进行LA-ICP-MS U-Pb年代学研究,对取自长石岩屑砂岩样品中100颗碎屑锆石进行测试分析,结果显示,100颗锆石的谐和年龄主要集中分布在4个年龄区间,分别为253~298Ma,309~362Ma,428~595Ma,851~1526Ma,最年轻的一组锆石年龄为253~298Ma,存在266Ma的锆石峰值年龄。266Ma的年龄限定了地层的沉积时限,即地层应在晚二叠世沉积,且砂岩的碎屑锆石年龄特征和地层岩性组合与区域上发育的林西组可以对比,故将该套地层划为林西组。年龄数据普值特征反映出,该地层具有多物源供给的特点,但物源主体来自东北各地块,未见华北板块的物源区年龄信息,表明华北板块与西伯利亚板块在晚二叠世并未完全碰撞闭合,253~298Ma的锆石年龄与西拉木伦缝合带附近与板块俯冲、碰撞相关的岩浆事件相吻合,表明两大板块的最终碰撞拼合时间应晚于266Ma。

A set of clastic rocks in the Biliutai area of Baarin Left Banner of Inner Mongolia,characterized by sand and slate assemblages,was considered as the Early Permian deposits previously.The 100 detrital zircons from the feldspar lithic sandstone of the strata were dated by LA-ICP-MS U-Pb method.The result shows that the age of the detrital zircons can be mainly divided into four groups：253~298Ma,309~362Ma,428~595Ma,851~1526Ma,of which,the 253~298Ma age data show a peak age of 266 Ma.The age of 266 Ma limits the sedimentary period of the strata,which should be the Late Permain deposits.Moreover,the age characteristics of the detrital zircons and the lithologic associations of the strata can be compared with those of Linxi Formation in a regional scale,so the strata should be assigned to the Linxi Formation.The characteristic of the age spectra of the detrital zircons reflects that there were multiple provenances forming the strata,and the provenances were mainly from northeast blocks rather than North China plate,which indicates that North China plate had not yet collided with Siberia plate to close up completely in the Late Permian.And the 253~298Ma age coincided with that of the magmatic event which was related to plate subductions and collisions near Xar Moron River suture zone,and that indicates that the final closing time of the two plates was later than 266 Ma.