西准噶尔哈拉阿拉特组火山岩LAICPMS锆石U-Pb年龄

哈拉阿拉特组是西准噶尔哈拉阿拉特山一带分布最广, 研究程度最低, 时代认识分歧最大, 火山岩在该区全部组级地层中所占比例最高, 且层位相对较新的海相火山-沉积建造。笔者在该区进行1:5万区域地质调查中, 对哈拉阿拉特组中下部层位辉石安山岩与流纹岩进行LA-ICP-MS测年, 获得锆石U-Pb年龄分别为(306.9±5.5)Ma(MSWD=1.8)与(304.5±3.1)Ma(MSWD=0.63), 二者误差范围内一致, 综合所测锆石的CL图像特征, 指示其代表了哈拉阿拉特组的形成时代。结合前人化石资料将哈拉阿拉特组时代确定为晚石炭世, 认为其中下部层位时代不老于306.9Ma为宜。这一成果准确约束了区内海相火山地层的时代, 并为研究西准噶尔构造带的火山岩浆作用及其构造演化提供了新的年代学证据。

Zircon LA-ICP-MS U-Pb Dating of Volcanics in Hala'alate Formation of Western Junggar

Hala'alate formation is widely distributed in Hala'alate Mountain. Few researches have been conducted on this formation and there is great disparity in its age identification. The proportion of volcanic rocks in the area is the highest of all the group-level strata, and the position is relatively new marine volcano and sedimentary formation. LA-ICP-MS dating was used to determine the zircons from pyroxene andesite and rhyolite of the Hala'alate formation by 1:50000 regional geological survey, which yielded zircon U-Pb age of (306.9±5.5) Ma(MSWD=0.39) and (304.5±3.1) Ma(MSWD=0.63) respectively, both were consistent within the error range. Combining with the CL image features of zircons, the formation age of Hala'alate formation was signified. In view of predecessors' fossil information, which confirmed that the forming age of Hala'alate formation was late Carboniferous, the authors considered that the age of lower strata was no earlier than 306.9 Ma. The findings accurately constrained the period of marine volcanic strata in this area, and provided new chronological evidence for volcanic magmatism and tectonic evolution of western Junggar tectonic belt.