宣城水东地区中二叠统硅质角砾岩地球化学特征及其成因环境研究

Geochemical Characteristics of Siliceous Rocks in the Middle Permian Gufeng Formation at Shuidong Area,Xuancheng City and Its Implication for Genesis and Sedimentary Environment

为研究长江中下游成矿带东部宣城水东地区中二叠统孤峰组中发育的一套硅质岩特别是其中的硅质角砾岩的成因,在详细的野外地质调查和室内岩石学研究基础上,对取自该套硅质岩中的7件硅质角砾岩、7件块状硅质岩和1件中薄层硅质岩样品进行了主量元素、微量元素和稀土元素地球化学分析。结果表明,硅质角砾岩和硅质岩的地球化学特征相似,其SiO_2含量高（93.70%-99.24%）、Al/（Al＋Fe＋Mn）值较高、轻稀土略富集、无明显铈异常和铕异常,显示非热液成因的特征。硅质岩的地球化学特征指示其主要为正常海水沉积成因,形成于靠近大陆边缘的沉积环境,受一定的陆源碎屑输入影响。硅质角砾岩的形成可能与江南断裂带活动影响有关,后期的海底热液对硅质岩进行叠加或动力学改造,并在局部形成角砾状构造。

Chert,particularly siliceous breccia,are well developed in middle Permian Gufeng Formation at Shuidong town,Xuancheng city,Middle-Lower Yangtze Metallogenic Belt. However,their origins and depositional environments remain controversial. Based on extensive field observations and petrography study of the siliceous breccia and chert,we analyzed major and trace elements of fifteen samples which include seven siliceous breccia,seven massive cherts and one thin-layered chert. These siliceous rocks have high SiO_2（ 93. 70%– 99. 24%） and Al/（ Al＋Fe＋Mn） ratios,and are characterized by slight enrichment of LREE without significant Ce and Eu anomalies,indicating that these rocks were bio-genetic and formed in an open continental margin environment with minor terrigenous clastic input. Meanwhile,the Jiangnan fault belt may play an important role in the formation of such chert. It could provide the passway for hydrothermal fluids that not only change the composition of chert but also trigger the formation of siliceous breccia.