LA-ICP-MS zircon dating from the Lajimiao mafic complex in the Qinling orogenic belt
秦岭拉鸡庙镁铁质岩体锆石LA-ICP-MS年代学研究

The Lajimiao mafic complex, located on the south limb of the North Qinling orogenic belt, consists mainly of gabbro (80%)and norite-gabbro (15%),with minor diorite.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 Lajimiao diorite.Cathodoluminescence images show two kinds of zircon grains, prismatic with clear magmatic zonations and round with complicated internal structure, some of which have no magmatic zonation.All together 28 U-Pb analyses have been taken on cores and overgrowths of 26 zircon grains.Two groups of ages were obtained, with weighted average 206Pb/238U ages of 973±60Ma for round zircons, and 422±7Ma for prismatic ones, respectively.All analyses show high concentrations of Th, U, and HREE, with positive Ce anomaly and negative Eu anomaly.The high Th/U ratios indicate these zircons are magmatic in origin.The younger group (422±7Ma) represents the formation age of the Lajimiao gabbro complex, which is probably relative to Paleozoic collision of the North China Block with the South China Block or a micro-block separated from it, which led to the closure of the Qinling Ocean.The older group (973±60Ma) is likely to be inherited zircon, formed in magmatic event(s) during the assemblage of Rodinia supercontinent.Given that no typical North China craton ages were detected, we speculate that the North Qinling was closer to the South China Block than the North China Block in the Proterozoic.