云开地区变质沉积岩碎屑锆石U-Pb年龄、Lu-Hf同位素特征及其地质意义  

U-Pb age and Lu-Hf isotope of detrital zircons from the meta-sedimentary rocks in the Yunkai region and their geological significance

作　　者：韩坤英[1] 许可娟 高林志[1] 丁孝忠[1] 任留东[1] 刘燕学[1] 庞健峰[1] HAN KunYing XU KeJuan GAO LinZhi DING XiaoZhong REN LiuDong LIU YanXue PANG JianFeng(Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China)

机构地区：[1]中国地质科学院地质研究所

出　　处：《岩石学报》2017年第9期2939-2956,共18页Acta Petrologica Sinica

基　　金：中国地质调查局项目(1212011120115;DD20160345);科技基础性工作专项(2012FY120100)联合资助

摘　　要：华夏陆块西南部云开地块变质杂岩主要由天堂山岩群和云开群以及古生代花岗质岩石组成。天堂山岩群主要由片麻岩、变粒岩、石英岩等岩石组成,变质程度较深;云开群主要由片岩、板岩、千枚岩、变质砂岩等岩石组成。应用LA-ICP-MS U-Pb微区定年和Lu-Hf同位素分析以及全岩地球化学分析方法,本文对天堂山岩群和云开群变质沉积岩进行了研究。在阴极发光图像中,变质沉积岩的锆石显示岩浆震荡环带,但普遍遭受重结晶改造及存在变质增生边。由于变质增生边普遍较窄,U-Pb定年和Hf同位素分析几乎都在具岩浆环带和重结晶结构的成分域上完成的。具岩浆环带和重结晶结构的成分域的Th/U比值普遍大于0.1。年龄变化从>3000~500Ma,但主要集中在600~1200Ma之间,主峰值为960Ma左右。碎屑锆石的ε_(Hf)(t)值在-25.6~15.4之间,两阶段Hf模式年龄在972~4496Ma之间。全岩地球化学分析显示LREE富集,HREE亏损,具有较明显的负Eu异常(δEu=0.55~0.71),天堂山岩群与云开群REE配分模式一致,反映源区物质相似,主要来源于花岗质地壳物质。结合前人研究成果,可得出如下结论:天堂山岩群与云开群形成时代没有明显差别,虽然它们在变质变形方面存在较明显区别;天堂山岩群和云开群沉积时代为古生代早期-新元古代;格林威尔期岩浆岩为主要物源区,新元古代时期华夏陆块处于格林威尔造山带内与之相邻;华夏陆块存在太古宙古老陆壳。The Yunkai metamorphic complex in the southwestern Cathaysia block is mainly composed of the Tiantangshan Group, the Yunkai Group and the Paleozoic granitic rocks. The Tiantangshan Group is mainly composed of high grade metamorphosed gneisses, leptynites and quartzites. The Yunkai Group mainly consists of schists, slates, phyllites and metamorphosed sandstones. This study conducted LA-ICP-MS U-Pb and Lu-Hf isotopic analysis and whole-rock geochemical analysis on the metamorphosed sedimentary rocks of the Tiantangshan and Yunkai groups. Zircons from the metamorphosed sedimentary rocks show magmatic oscillating zoning and are also subjected to recrystallization with presence of metamorphic rims in the cathodoluminescence images. U-Pb dating and Hf isotopic analysis are almost performed on the magmatic and reerystallized domains due to the very thin metamorphic rims. Th/U ratios of the magmatic and recrystallized domains are generally greater than 0. 1 and their ages range from 3000Ma to 500Ma, but mainly in the range of 600 ~ 1200Ma with the main peak at about 960Ma, 8Hf（t） values of the detrital zircons are between -25.6 and ＋ 15.4 with two-stage Hf model ages of 972 -4496Ma~ Whole rock geochemical analysis show LREE enrichments and HREE depletions with obviously negative Eu anomalies （ 8Eu = 0. 55 ~ 0. 71 ）. Similar REE distribution patterns of the Tiantangshan and Yunkai groups suggest similar source rocks of granites. Combined with the previous studies, we concluded that there is no obvious difference on the formation age of the Tianshenshan and Yunkai groups, although there are obvious differences in their metamorphism and deformation. The depositional age of the two groups is the Early Paleozoic-Neoproterozoie and their major provenances are Grenville-aged magmatic rocks. In the Neoproterozoic, the Cathaysia block is adjacent to the Greenville orogenic belt and had an Archean continental crust.

关 键 词：云开地区 碎屑锆石 年龄 LU-HF同位素 天堂山岩群 云开群 

分 类 号：P597[天文地球—地球化学]