西准噶尔萨吾尔山一带吉木乃组流纹岩LA-ICP-MS锆石U-Pb年代学及地质意义

吉木乃组是西准萨吾尔山地区地质时代久存争议的地层,也是火山岩占地层总厚度比例最高的火山-沉积地层。采用同位素年龄标定火山岩准确的地质时代,是解决本组时代分歧的有效手段。吉木乃组顶部新发现的流纹岩中获得(304.1±2.5)Ma(n=15,MSWD=1.07)的LA-ICP-MS锆石U-Pb年龄,确认成岩时代为晚石炭世晚期(大致相当于卡西莫夫期)。结合前人在本组下部层位采集的植物化石,限定吉木乃组时代为晚石炭世(大致为巴什基尔期卡西莫夫期)。流纹岩同位素定年准确约束了吉木乃组地质时代上限,为区域地层格架的建立和地层对比提供了可靠的时代依据。

LA-ICP-MS zircon U-Pb Dating for Rhyolite of Jimunai Formation in the Saur Mountain,Western Junggar,Xinjiang,and its Geological Implications

The geological age of the Jimunai Formation has long been controversial in the Saur Mountain of West Junggar. It is also a volcanic-sedimentary stratum with the highest proportion of volcanic rocks to the total thickness of strata, and thus,it is an effective method to resolve the age divergences of the Jimunai Formation using accurate geological age of the volcanic rocks. LA-ICP-MS dating was used to determine the zircons from the newly discovered rhyolite of the upper Jimunai Formation,which yielded zircon U-Pb age of 304.1±2.5 Ma (n=15,MSWD=1.07),corresponding to the end of the Late Carboniferous. In view of predecessors′ fossil information,we suggest that the formation age of Jimunai Formation is Bashkirian to Kasimovian of Late Carboniferous.Isotopic dating of rhyolite accurately constrained the minimum age of the Jimunai Formation,and provided new chronological evidence for the establishment of regional stratigraphic framework and stratigraphic correlation.