青藏高原祁漫塔格地区早古生代火山岩岩石构造组合和LA-ICP-MS锆石U-Pb年龄

摘要

A study of the Early Paleozoic volcanic rocks in Qimantag area reveals that they were formed in the Middle-Late Ordovi-cian period. The volcanic rocks can be assigned to four volcanic rock belts, i.e., Gonglugou - luanshigou belt, Kuangou - Xi-aolangyashan belt, Bayan Gol-Suhait belt and Naling Gol belt. The first volcanic rock belt belongs to a combination of Nb-enriched basalts characterized by high content of Nb (7.38×10-6-10.1×10-6), TiO2 (1.56%-2.24%) and P2O5 (0.24%-0.34%). The combinations of the volcanic rocks in the second belt are complicated, the SiO2 mass fractions of these rocks are concentrated on two end members, and these rocks can be further divided into two sub-combinations of basalt+basatic andesite and dacite+lithoidite. U-Pb LA-ICP-MS analyses of zircons in basalt and lithoidite yielded the data of 440.2±2.4Ma and 450.3±1.2Ma, respectively. The content of TiO2 (1.1%) and K2O(0.14%) is low while that of Na2O(2.17%) is high for the rocks of the tholeiite combinations in the third belt. Although the content of TiO2 (1.64%) is relatively high, the tholeiite combinations in the fourth belt are very similar to those in the third belt.%祁漫塔格地区早古生代海相火山岩发育,初步确定该地区早古生代海相火山岩的形成时代为中晚奥陶世,可划分出4个火山岩带,分别为公路沟-乱石沟带、宽沟-小狼牙山带、巴音郭勒-苏海图带和那陵格勒河带.公路沟-乱石沟带火山岩为富铌玄武岩组合,具有相对高的Nb(7.4×10-6～10.1×10-6)、TiO2( 1.56％～2.24％)和P2Os(0.24％～0.34％)含量.宽沟-小狼牙山带火山岩岩石类型复杂,SiO2质量分数集中分布于2个端元,可进一步分为玄武岩+玄武安山岩组合和英安岩+流纹岩组合,玄武岩中获得了440.2Ma±2.4Ma的LA-ICP-MS锆石U-Pb年龄值,流纹岩中获得了450.3Ma±1.2Ma的年龄值.巴音郭勒-苏海图带为拉斑玄武岩组合,具低钛(1.1％)、富钠(2.17％,)、低钾(0.14％)的特点.那陵格勒河带火山岩也为拉斑玄武岩组合,特征与巴音郭勒-苏海图带相似,但具有更高的Ti含量(1.64％).这3类岩石组合均代表了弧后裂陷环境中形成的火成岩岩石构造组合.