大兴安岭北部新林地区倭勒根群大网子组锆石LA-ICP-MSU-Pb年龄及其地质意义

摘要

大兴安岭位于中亚造山带的东段，自北向南划分为额尔古纳地块、兴安地块和松嫩地块。倭勒根群主体分布于额尔古纳地块，前人将其归属为新元古代早寒武世。对新林地区倭勒根群大网子组的上部变火山岩段和下部变沉积岩段进行了锆石LA ICP MS U Pb定年。测试结果显示：上部变火山岩的形成年龄为（430．7±4．1）Ma，属早志留世；下部变沉积岩中碎屑锆石的最小峰值年龄为（480．1±2．9） Ma ，指示其沉积时间不早于早奥陶世。综合文献资料确定：新林地区倭勒根群浅变质岩系是一套时间跨距从寒武纪到早志留世的岩石地层组合，而非新元古代早寒武世；新林蛇绿岩的构造侵位时间不早于早奥陶世；新林地区的大网子组、兴隆沟地区的早奥陶世沉积与多宝山伊尔施早奥陶世火山弧构成了大兴安岭北部地区的早奥陶世弧盆体系。%The Great Xing’an Range ,located within the east part of the Central Asian orogenic belt (CAOB) ,is composed of E'erguna ,Xing’an and Songnen blocks ,from north to south .The weakly metamorphosed rock series of the Wolegen Group are mainly distributed in the northern Xing ’an block , and were previously regarded as the Neoproterozoic-Early Cambrian strata ,in spite of rare reliable geological and geochronological evidence .In this paper ,we report the LA ICP MS zircon U Pb ages of the Dawangzi Formation in Wolegen Group .The Dawangzi Formation consists of two lithologic units :the upper unit characterized by low-grade metamorphic volcanic rocks and the lower composed of low-grade metamorphic sedimentary rocks .The zircon dating results show that the meta-volcanic rock of the upper unit was formed in the Early Silurian period ,its age is (430 .7 ± 4 .1) Ma .The meta-sedimentary rock of the lower unit was formed not earlier than the Early Ordovician period as attested by the peak age of (480 .1 ± 2 .9) Ma of the youngest population of zircons .The existing data show that the low-grade metamorphic rocks of the Wolegen Group are a stratum combination ,which formed during the Cambrian to the Early Silurian instead of Neoproterozoic-Early Cambrian period . The zircon geochronology in the rock from the Dawangzi Formation shows that the tectonic emplacement of the Xinlin ophiolites were not older than the Early Ordovician period .The Ordovician clastic sedimentary rocks in Xinlin and Xinglong area were coeval with the Ordovician Duobaoshan volcanic arc volcanism , which indicates that there was an arc-basin system in the Northern Great Xing’an Range during the Early Ordovician period .