大兴安岭扎兰屯地区前寒武纪变质岩系年龄及其构造意义

Chronological study and tectonic significance of Precambrian metamorphic rocks in Zhalantun area of Da Hinggan Mountains

大兴安岭扎兰屯地区出露较多的前寒武纪变质岩系,包括先前认为是古元古代的兴华渡口群、新元古代的佳疙瘩组和新元古代—早寒武世的倭勒根岩群,但一直缺少精确的年代学依据。通过LA-ICP-MS锆石U-Pb同位素测年,对这3个变质岩群的原岩时代进行了厘定。结果显示,兴华渡口群中绿泥石白云母片岩的锆石^(206)Pb/^(238)U年龄加权平均值为520.1±4.3Ma(n=11,MSWD=1.6),佳疙瘩组中长英质糜棱岩锆石^(206)Pb/^(238)U年龄加权平均值为512.0±2.9Ma,倭勒根岩群中绿泥绢云片岩最年轻锆石^(206)Pb/^(238)U年龄加权平均值为491.7±11.9Ma,年龄最密集区加权平均值为516.7±4.5Ma。锆石图像特征及Th/U值均显示岩浆型锆石特征。综上所述,扎兰屯地区出露的兴华渡口群、佳疙瘩组和倭勒根群形成时代均为早古生代早期,而非先前认为的"前寒武纪"。同时,上述年龄谱系表明,该地区在480~500Ma和500~530Ma存在2期强烈的岩浆活动,应与东北地区晚泛非期岩浆-变质事件有关。

A lot of Precambrian metamorphic rocks are exposed in the Zhalantun area of Da Hinggan Mountains, which include Paleoproterzoic Xinghuadukou Group, Neoproterozoic Jiageda Formation and Neoproterozoic-Early Cambrian Wolegen Group, however, accurate chronological evidence has been lacking. Based on the LA-ICP-MS zircon U-Pb dating, the authors determined the original rock age of these three metamorphic groups. The results show that zircons from the chlorite muscovite schist in the Hinghua Dukou Group has a weighted average 206Pb/238U age of 520.1±4.3Ma, the weighted average 206Pb/238U ages of zircons from the Changliang mylonites in the Jiapigang Formation are 512.0±2.9Ma, the weighted mean 206Pb/238U age of the youngest zircons of the greenstone sericite schist in the Injalagenian rock mass is 491.7 ± 11.9Ma, and the weighted average 206Pb/238U age of most intensive areas is 516.7± 4.5Ma. The features of zircon images and Th/U values exhibit magmatic zircon characteristics. The above data indicate that the Xinghuadukou Group, Jiageda Formation and Wolegen Group were formed in Early Paleozoic rather than previously thought ＂Precambrian＂. At the same time, the above age pedigree indicates that there are two strong magmatic events in 480 - 500Ma and 500 - 530Ma in this area, which were related to the late Pan African metamorphic magmatic events in Northeast China.