华南埃迪卡拉系底部钾质斑脱岩的岩石地球化学特征及其地质意义  

Petrologic and geochemical characteristics of K-bentonites from the basal Ediacaran in Yangtze Platform,South China and their geological significance

作　　者：万斌[1,2] 关成国[1,2] 周传明[1] 孟凡巍[1] 庞科[1,2] 唐卿[1,2] 饶馨[1,2] 

机构地区：[1]现代古生物学和地层学国家重点实验室中国科学院南京地质古生物研究所,南京210008 [2]中国科学院大学,北京130039

出　　处：《岩石学报》2013年第12期4373-4386,共14页Acta Petrologica Sinica

基　　金：中国科学院重点部署项目(KZZD-EW-02);国家自然科学基金重点项目(41130209)、国家自然科学基金面上项目(41173051)和国家自然科学基金古生物学与地层学特殊学科点(J1210006)联合资助

摘　　要：我国华南扬子地台不同地区(湖北宜昌、安徽休宁)埃迪卡拉系底部的盖帽碳酸盐岩中均保存有浅色粘土岩层。岩石薄片和扫描电镜观察,以及X射线衍射分析表明,这些粘土岩层以伊利石和伊/蒙混层矿物为主,同时含有港湾状熔蚀石英、高温透长石及岩浆锆石等斑晶矿物。X荧光光谱仪和ICP-MS等离子质谱仪等地球化学元素分析表明,这些粘土岩层具有相似的元素组成和配分模式,以高K2O、低TiO2,富含Nb、Ta、Zr、Hf、Th、Y等亲石元素为特征,Al2O3/TiO2、Zr/Hf和Ti/Th值均指示了酸性火山灰的性质,同时稀土元素配分模式与花岗岩较为相似。岩石学和地球化学特征表明,这些浅色粘土岩层是火山喷发事件形成的钾质斑脱岩。通过两处剖面的对比,可以将安徽休宁地区的钾质斑脱岩年龄限制在大约635Ma左右。岩浆判别图指示了这些钾质斑脱岩的原始岩浆为高钾亚碱性流纹英安岩;依据微量元素特征和构造环境判别图,初步认为原始岩浆形成于大陆弧环境。这些钾质斑脱岩在区域上的分布,表明我国扬子地台埃迪卡拉纪早期发生了广泛的火山活动,"雪球地球"事件的结束可能是导致这期以钾质斑脱岩为代表的火山喷发事件的诱因。Light-coloured claystone layers occur in the basal Ediacaran cap carbonate in Yichang, Hubei Province and in Xiuning, Anhui Province of Yangtze Platform, South China. Petrologic studies including thin section and SEM observations, as well as XRD analysis, indicate that the claystones mainly consist of illite and mixed-layer illite-smectite, and some phenocryst minerals, such as vericular quartz, glassy feldspar and magmatic zircon. Geochemical analysis using XRE and ICP-MS indicates that claystones from Yichang and Xiuning have similar elemental composition and distribution patterns, which are characterized by high K2O and low TiO2, and rich in lithophile elements Nb, Ta, Zr, Hf, Th, and Y. Al2O3/TiO2, Zr/Hf and Ti/Th values suggest that they are acidic volcanic ash in origin, and REE distribution patterns resemble that of granite. Both petrologic and geochemical data indicate that the claystones from the cap carbonates in South China are K-bentonites, which can be used for stratigraphic correlation. The ages of the K-bentonites from Xiuning area are ~635Ma by correlating with Yichang section. Application of various chemical discrimination diagrams and characteristics of trace elements implie that these K-bentonites were possibly derived from high K subalkaline rhyodacite formed in continental arc environment provisory. The regional distribution of these K-bentonites in the Yangtze Platform suggests a widespread volcanic event at the beginning of the Ediacaran, which may have been triggered by sudden meltdown of the Snowball Earth.

关 键 词：埃迪卡拉系 钾质斑脱岩 盖帽碳酸盐岩 华南 

分 类 号：P588.142[天文地球—岩石学] P588.22[天文地球—地质学]