安徽巢湖平顶山中二叠统孤峰组硅质岩的地球化学特征及成因  

Geochemistry and Origin of Siliceous Rocks from the Gufeng Formation of Middle Permian in the Pingdingshan Area,Chaohu Region,Anhui Province

作　　者：杨水源[1] 姚静[1] 

机构地区：[1]南京大学地球科学系,南京210093

出　　处：《高校地质学报》2008年第1期39-48,共10页Geological Journal of China Universities

基　　金：南京大学地球科学系地质学理科人才基地基金资助项目

摘　　要：安徽巢湖平顶山地区中二叠统孤峰组的层状硅质岩成因一直存在不同的认识。对其详细的野外地质和岩相学观察,及硅质岩常量元素、稀土元素和氧同位素地球化学分析结果表明,这些硅质岩在Al-Fe-Mn三角图、SiO2-(K2O+Na2O),SiO2-Al2O3,SiO2-MgO,Al2O3-(K2O+Na2O)双变量图解中都落入了生物成因的硅质岩范围内;K2O/Na2O平均值为1.32,δCe具有轻微的负异常,轻稀土富集,这些特征显示研究区硅质岩应为海水生物沉积成因。硅质岩的ΣREE值在19.87～63.95μg/g之间,平均值为44.59μg/g;δCe范围在0.72～0.97,平均值为0.84;Ceanom范围在-0.174～-0.011,平均值为-0.088;(La/Yb)N值在0.44～1.98之间,平均值为1.01;(La/Ce)N值在1.04～1.3之间,平均值为1.16,这些地球化学指标指示了研究区硅质岩沉积于大陆边缘的缺氧的水体环境中。研究发现,与硅质岩互层的泥质岩中存在火山毛,预示该区硅质岩的硅可能有部分来源于火山物质。The bedded siliceous rocks of this study occur in the Gufeng Formation of the Middle Permian in the Pingdingshan area, Chaohu region, Anhui Province. The origin and depositional environments for the siliceous rocks have been debated for a long time. In this paper, we report the detailed field and petrographic observation, Combined with the data of major elements, rare earth elements and oxygen isotopic compositions of these rocks, our results show that in the Al-Fe-Mn ternary, and the SiO2-（K2O＋Na2O）, SiO2-Al2O3, SiO2-MgO, Al2O3-（K2O＋Na2O） binary discriminant diagrams, these rocks fall in the biochemically- deposited area. These siliceous rocks show an average K2O/Na2O ratio of 1.32, slightly negative Ce anomaly, and a comparatively enrichment of LREE, which indicate a biochemically-deposited origin for these rocks. The siliceous rocks have the total REE contents of 19.87-63.95μg/g, with an average of 44.59μg/g. Their δCe values are 0.72-0.97, with an average of 0.84, and their Ce values are -0.174--0.011, with an average of -0.088. The average （La/Yb）N and （La/Ce）N values are 1.01 and 1.16, respectively. All these geochemical characteristics suggest that these siliceous rocks were deposited in a continental margin setting where the sea water during deposition was under an anoxic condition. We found volcanic hairs in the mudstones interbedded with the siliceous rocks, which indicates that some of the silicon in the siliceous rocks might come from the volcanic rocks.

关 键 词：层状硅质岩 地球化学 成因 中二叠统 孤峰组 巢湖 

分 类 号：P595[天文地球—地球化学] P588.2[天文地球—地质学]