湖北随州大狼山群片岩中碎屑锆石的U-Pb年龄及其意义

LA-ICP-MS zircon U-Pb dating of schist from the Dalangshan Group in Suizhou City,Hubei Province,and its implications

湖北随州大狼山群片岩样品中锆石LA-ICP-MSU-Pb年龄结果表明,锆石属于碎屑锆石,未受到后期绿片岩相变质作用的明显改造,锆石年龄代表了其原岩的年龄,是不同构造-热事件的响应。本区锆石的U-Pb年龄多集中在新元古代早中期(884～758Ma),可分为4组,反映南秦岭地区在该时期至少经历了4次构造-热事件,可能与Rodinia超大陆的聚合与裂解过程相对应。其中1颗锆石还给出了2500Ma的年龄,反映在南秦岭地区可能发生过新太古代末期至古元古代早期的构造-热事件。通过地层和锆石年代学的限定,认为大狼山群的成岩时代应为早震旦世。

Located in the southern margin of South Qinling belt,the Dalangshan Group is a series of volcanic-sedimentary rocks and exposed well in Xinyangdain area of Suizhou City,Hubei Province.To find out the diagenetic age of the Dalangshan Group and provide credible time information on the materials in ＂Qinling microplate＂,the authors carried out a detailed fieldwork on Dalangshan Group and collected representative schist samples to study petrography and LA-ICP-MS zircon U-Pb geochronology.The schist mainly consists of quartz （60%～65%）,muscovite （30%～35%） and plagioclase （～3%）.The cathodoluminescence （CL） images reveal that most zircons from the schist are euhedral with oscillatory internal zoning,but a few grains display no zoning/weak zoning suggesting metamorphic origin.LA-ICP-MS zircon analyses show that 2 in 30 spots of these zircons have relatively high content of Th （820×10-6,888×10-6） and U （1 024.47×10-6,1 146.97×10-6）,but others have low content of Th （25×10-6～481×10-6） and U （34.60×10-6～752.10×10-6）.Except for six points with Th/U ratios ranging from 0.22～0.39,Th/U ratios all range from 0.42 to 1.41.Most zircon U-Pb ages from the schist are between early and middle Neoproterozoic （884～758 Ma） and could be divided into four groups,indicating that at least four tecto-thermal events occurred in the south Qinling region during this period.The first group of 6 spots yield 206Pb/238U ages between 758 Ma to 769 Ma with a 206Pb/238U weighted average age of 765±6 Ma （MSWD=0.31）;the second group of 8 spots yield 206Pb/238U ages between 781 Ma to 802 Ma with a 206Pb/238U weighted average age of 791±6 Ma （MSWD=1.4）,suggesting that both of them may be relevant to the breakup of Rodinia supercontinent.The third group of 9 spots yield 206Pb/238U ages between 818 Ma to 840 Ma with a 206Pb/238U weighted average age of 831±6 Ma （MSWD=1.2）;the fourth group of 4 spots yield 206Pb/238U ages between 870 Ma to 884 Ma with a 206Pb/238U weighted average age of 879±9 Ma （MSWD=0.52）,implying that both of them may be relevant to the convergence of Rodinia supercontinent.The scattering of LA-ICP-MS zircon U-Pb ages suggest that the zircons from the schist of Dalangshan Group are detrital zircons and have not been obviously influenced by later greenschist-facies metamorphism.All of the four groups of zircon ages are concentrated in the Neoproterozoic,indicating that the formation of the schist in the Dalangshan Group should not be earlier than Neoproterozoic.A spot with 2 500 Ma age implies a possible tecto-thermal event in the South Qinling region during late Neoarchean to early Paleoproterozoic period.The LA-ICP-MS zircon U-Pb ages of schist in combination with the stratigraphy suggest that the Dalangshan Group was formed in early Sinian.