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-normalized REE were loss of light rare earth elements and significant enrichment of heavy rare earth elements. zircons have a strong positive Ce anomaly and negative Eu anomalies strongly, which shows these zircons are magmatic in origin. 22 points give a weighted average age (313±2)Ma(MSWD=2.8). These new data suggust 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.