内蒙古科右前旗大石寨组火山岩锆石LA—ICP—MSU—Pb年龄及其形成背景

对大石寨组火山岩的形成时代和形成背景的认识存在较大的分歧。对采自建组剖面底部英安岩样品中的锆石进行阴极发光照相、微区原位LA—ICP—MS微量元素分析和U-Pb测年。锆石U—Pb同位素分析结果显示，锆石发育典型的岩浆震荡环带，高Th／U（0．51～1．57），轻稀土元素亏损，重稀土元素富集，具有强烈的正Ce异常和强烈的负Eu异常等特征，表明锆石均属于岩浆成因。22个分析点获得的谐和年龄为（314±1）Ma（MSWD=0．88），代表了大石寨组火山岩的形成时代。结合前人的资料，认为大石寨组火山岩为裂陷早期的产物，而晚古生代的沉积岩石组合为在这一基础上发展起来的具有裂陷盆地性质的沉积组合。

LA-ICP-MS zircon U-Pb age of the volcanic rocks from the Dashizhai Formation in Keyouqianqi, Inner Mongolia, China and its tectonic setting.

Timing and tectonic setting of volcanic rocks from Dashizhai Formation remain controversial. A combined study of cathodoluminescence images （CL）, in situ LA-ICP-MS U-Pb isotope dating and trace element analysis were carried out for zircons from the Dashizhai Formation dacite. The zircon LA-ICP-MS U-Pb analysis shows that the zircons are of typical magmatic origin with oscillatory zoning, higher concentrations of Th, U, and REE. Th/U ratios of zircons range from 0.51 to 1.57. Chondrite-nor- realized REE were loss of light rare earth elements and significant enrichment of heavpl;are earth elements, zircons have a strong positive Ce anomaly and negative Eu anomalies strongly, which shows these zircons are rrfagmatic in origin. 22 points give a weighted aw erage age （313±2）Ma （MSWD=2.8）. These new data suggnst that the protolith age of Dianshizhai Formation was at about 300 Ma. Together with geochemical data from previous works, it is suggested that the Dashizhai Formation volcanic rocks were formed in the rift basin, and the Late Paleozoic sedimentary rocks, including Zhesi Formation and Linxi Formation, were developed on this rift basin.