LA-ICP-MS zircon U-Pb dating of the Mesoproterozoic granitic gneisses at Changningyi of Zhangjiachuan area on the eastern edge of the Qilian Orogenic belt

The newly identified Mesoproterozoic granitic gneisses are located in Paleoproterozic Longshan Group at the eastern end of Qilian Orogenic belt. In order to accurately determine the formation age of granitic gneisses from Changningyi, the authors carried out single grain zircon micro area U-Pb isotope dating by means of Laser Ablation Inductively Coupled Plasma Mass Spectrometry. The Th/U ratios of zircons from granitic gneisses are between 0.50 and 2.09, and CL images show that the internal zircon has developed typical oscillatory girdle structure of magma growth, which belongs to the product of magma crystallization. And the original rock crystallization age of granitic gneiss is 1765Ma±57Ma, suggesting that granite rock was produced in early Mesoproterozoic period, and was probably formed in an post-orogenic environment, under which North China Block was experiencing the Lüliang movement. In addition, from the captured inherited magma zircon, the authors obtained the inherited ages of 1.95Ga and 2.20Ga. In combination with regional geological data and on the basis of a comprehensive study and analysis, the authors found that Qilian block remained a southwestern part of North China Block in Mesoproterozoic, for they had the same crystalline basement and structural properties. Besides, they both experienced the post orogenic stage and intracontinental stretch cracking stage after Lüliang movement. So they were probably closely related to the polymerization and cracking of Columbia supercontinent.