东天山大白石头南新元古代片麻状花岗岩锆石U-Pb年代学、岩石地球化学及地质意义

Zircon U-Pb Age,Geochemistry and Tectonic Implications of Neoproterozoic Granite from South of Dabaishitou,East Tianshan

对东天山大白石头南片麻状花岗岩的研究,可以为新元古代早期Rodinia超大陆汇聚事件提供约束.在野外调查和岩相学研究的基础上,对该片麻状花岗岩开展了LA-ICP-MS锆石U-Pb定年、全岩地球化学和锆石原位Hf同位素分析.研究显示,片麻状花岗岩LA-ICP-MS锆石U-Pb年龄为922.7±7.9Ma.岩石富SiO2(70.04%~71.60%)、碱(Na2O+K2O=5.93%~6.58%)、Al2O3(13.88%~14.91%)和低MgO(1.13%~1.29%).岩石Al2O3/TiO2(25~27)小于100,CaO/Na2O(0.7)大于0.3,K2O/Na2O(1.6~2.2)大于1.0,富集大离子亲石元素Rb、Th、K及La等,亏损Ba、Ta、Nb和Sr等,(La/Yb)N(7.29~8.11)小于10,δEu小于0.5,均显示出S型花岗岩特征.锆石εHf(t)均为正值(3.22678~13.72746),二阶段模式年龄为920~1598Ma,表明新元古代花岗岩形成于大陆边缘构造环境.综合已有研究结果,可以推断天山地区出露的该期构造岩浆事件可能对应于新元古代的罗迪尼亚(Rodinia)超大陆汇聚事件.

Researches on the south of Dabaishitou gneissic granite can provide constraints on early Neoproterozoic Rodinia supercontinent convergence in the East Tianshan.In this paper,zircon LA-ICP-MS U-Pb dating,in-situ Hf isotope and whole-rock geochemical composition analyses are presented for the gneissic granite.The results show that the LA-ICP-MS zircon U-Pb isotopic dating on the gneissic granite yields a mean U-Pb age of 922.7±7.9 Ma,which approximately represents the intrusive age of the granite.Geochemical characteristics of major elements of the granite exhibit high SiO2(70.04%-71.60%),Na2 O+K2 O(5.93%-6.58%)and Al2O3(13.88%-14.91%)contents,and low MgO(1.13%-1.29%)contents.Al2O3/TiO2(25-27)ratios are less than 100,while CaO/Na2 O(0.7)and K2 O/Na2 O(1.6-2.2)ratios are more than 0.3 and 1.0,respectively.The gneissic granite is characterized by relativeenrichment in Rb,Th,K,La,and depletion in Ba,Ta,Nb and Sr.All the samples are slightly enriched in LREE with(La/Yb)Nratios ranging from 7.29 to 8.11(less than 10),and all the gneissic granite show clear negative Eu abnormalies(δEu<0.5).These characteristics are similar to values of typical S-type granite.Additionally,zirconεHf(t)values fall in the range of 3.226 78-13.727 46,with the two stage mode age from 920-1 598 Ma.Based on the chemical characteristics and Hf isotopic compositions,the Neoproterozoic gneissic granite was likely generated in a continental margin tectonic setting.With consideration of the age information on the ancient terranes from several areas,we suggest that the Tianshan ancient blocks probably formed a part of Rodinia during early Neoproterozoic period.