吉林省塔东群锆石U-Pb定年、微量元素研究及其地质意义

U-Pb Dating and Trance Elements Composition of Tadong Group from Jilin Province and Their Geological Implications

吉林省塔东群以含大型磁铁矿床而著称,塔东铁矿是吉黑成矿省内最大的火山喷流沉积-变质改造型铁矿.由于塔东群地质特征复杂,多种地质作用复合叠加,使其形成时代归属一直存在争议.通过LA-ICP-MS锆石定年,确定了吉林省塔东群黑云斜长片麻岩和磁铁透辉斜长变粒岩中锆石年代.黑云斜长片麻岩中锆石呈长柱状,Th/U值为0.17~0.65,206Pb/238U加权平均年龄为517.6±2.7Ma,代表了其原岩火山岩形成的年龄.磁铁透辉斜长变粒岩中共有两组锆石年龄,其中一组短柱状,Th/U值介于0.78~1.00,加权平均年龄为249.8±4.0Ma,代表后期热事件年龄;另一组锆石半自形短柱状晶形,异常高的U、Th,Th最高含量可达17 422μg/g,^(206)Pb/^(238)U加权平均年龄为518.3±3.8Ma,表示热液活动年龄,显示该区518.3Ma左右经历了一次热液作用,可能与该区的岩浆作用以及塔东铁矿的成矿有关.据此,塔东群内塔东铁矿可能存在一期早寒武世(±520Ma)的成矿作用.

Tadong Group of Jilin Province is known for large-scale magnetite ore deposit,and Tadong iron deposit is the largest submarine volcano plume-metamorphic-reformation iron deposit in the Jilin-Heilongjiang metallogenic province.Complicated of geological characteristics and geological processes made the ownership of Tadong Group long been controversial.In this paper,the U-Pb zircon ages of the biotite plagioclase gneiss and magnetite diopside plagioclase leptynite are obtained by LA-ICP-MS.Zircons that obtained from biotite plagioclase gneiss are prismatic,and Th/U range between 0.17 and 0.65.U-Pb dating of these zircons record 206Pb/238 U weighted mean age of 517.6±2.7Ma,indicating the formation age of the vocanic.In the magnetite diopside plagioclase leptynite,there are two kinds of zircons with two age groups,respectively.The first kind of zircons are short columns,and Th/U range between 0.78 and 1.00.U-Pb dating of these zircons record 206Pb/238 U weighted mean age of249.8±4.0Ma,indicating the age of thermal disturbance.Another kind of zircons are subhedral short prismatic,and have extremely high U,Th contents,the highest Th contents is 17 422μg/g,respectively.U-Pb dating of these zircons record ^（206）Pb/^（238）U weighted mean age of 518.3±3.8Ma,attributing to the age of hydrothermal event.It is indicated that there is a hydrothermal process at 518.3Ma or so,which might be related to magmatism.Iron mineralization of Tadong iron deposit of Jilin Province is most likely related with this hydrothermal event.Therefore,one of mineralizations happened in the Early CambrianPeriod（±520Ma）in Tadong iron deposit.