祁连造山带东端张家川地区长宁驿中元古代花岗质片麻岩LA-ICP-MS锆石U-Pb年龄及其构造意义

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

在祁连造山带东端古元古界陇山岩群中新识别出中元古代长宁驿花岗质片麻岩。为了精确确定长宁驿花岗质片麻岩的形成时代,对其进行了激光等离子体质谱LA-ICP-MS单颗粒锆石微区U-Pb同位素测年,花岗质片麻岩中锆石Th/U比值介于0.50～2.09,阴极发光图像显示锆石内部发育典型的岩浆生长震荡环带结构,属于岩浆结晶的产物。测得花岗质片麻岩的原岩岩浆结晶年龄为1765Ma±57Ma,表明该花岗质片麻岩原岩的形成时代为中元古代早期,可能形成于华北板块经吕梁运动后的后造山环境中,同时还从捕获的继承性岩浆锆石中得到了时代为1.95Ga、2.20Ga左右的继承年龄。结合区域地质资料,综合分析研究后认为,在中元古代祁连地块仍为华北板块西南缘的一部分,两者之间具有相同的结晶基底和构造属性,它们共同经历了吕梁运动后的后造山和陆内伸展裂解阶段,很可能与Columbia超大陆的聚合和裂解相关。

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 crystaUization. And the original rock crystallization age of granitic gneiss is 1765Ma-57Ma, suggesting that granite rock was produced in early Mesoproterozoic period, andwas probably formed in an post-orogenic environment, under which North China Block was experiencing the Liiliang 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. Be- sides, they both experienced the post orogenic stage and intracontinental stretch cracking stage after Liiliang movement. So they were probably closely related to the polymerization and cracking of Columbia supercontinent.