大兴安岭北段扎兰屯地区林西组碎屑锆石U-Pb年代学特征及其地质意义

DETRITAL ZIRCON U-Pb CHRONOLOGY OF THE LINXI FORMATION IN ZHALANTUN AREA,NORTHERN DAXINGANLING MOUNTAINS:Geological Implications

内蒙古扎兰屯地区林西组以砂板岩组合为重要特征,形成时代为晚二叠世.对扎兰屯地区林西组粉砂质泥质板岩、粉砂岩2个样品进行了LA-ICP-MS锆石U-Pb测年.结果显示,150个谐和—近谐和年龄主要分布于6个年龄区间:259.7~281.9 Ma(n=18),加权平均年龄为273.3 Ma;291.3~311.5 Ma(n=110),加权平均年龄为298.1 Ma;319.2~334.5 Ma(n=10),加权平均年龄为327.0 Ma;340.3~375.2 Ma(n=9),加权平均年龄为356 Ma;399.7~400.7 Ma(n=2),加权平均年龄为400.2 Ma;728.5~765.4 Ma(n=2),加权平均年龄为750.0 Ma,上述6组年龄峰值与区域上的岩浆事件基本吻和.林西组主要物质供给区为研究区内早二叠世侵入岩及大石寨组火山岩,占碎屑物总含量的85%,少部分来自于晚石炭世之前的古生代侵入岩及扎兰屯群等地质体.林西组与宝力高庙组分别记录了晚古生代250~270 Ma及310~325 Ma之间发生的两次重要的区域隆升事件.

The Late Permian Linxi Formation in Zhalantun area, Inner Mongolia, is characterized by the assemblage of sandstone and slate. The LA-ICP-MS zircon U-Pb dating results of two samples, silty argillaceous slate and sihstone, show that 150 age data are mainly distributed in six ranges, namely 259.7-281.9 Ma （n=18）, 291.3-311.5 Ma （n= 110）, 319.2-334.5 Ma （n=10）, 340.3-375.2 Ma （n=9）, 399.7-400.7 Ma （n=2） and 728.5-765.4 Ma （n=2） with the weighted average ages of 273.3, 298.1, 327.0, 356, 400.2 and 750.0 Ma, respectively. The above six groups of age peaks are basically consistent with the time of regional magmatic activities. The Early Permian intrusive rocks and volcanic rocks from Dashizhai Formation make up 85% of total clastic materials for the sedimentary rocks of Linxi Formation, while the Paleozoic intrusive rocks （earlier than Late Carboniferous） and geologic bodies like Zhalantun Group offer a few elastics as well. Two important regional uplift events occurred in Late Paleozoie 250-270 and 310- 325 Ma are recorded respectively by the Linxi and Baoligaomiao formations.