北京周边二叠系山西组泥岩地球化学特征及其地质意义

Geochemical characteristics of Permian Shanxi Formation mudstones in Beijing and their geological significance

对采自北京周边的二叠系山西组露头及钻井泥岩样品进行了岩石学和地球化学研究,结果表明:岩石主要由粘土矿物和石英组成,兼有少量碳酸盐岩和长石;岩石中碳酸盐岩含量的高低与CaO、MgO含量有很好的对应关系;SiO2、Na2O和K2O亏损,TiO2和Fe2O3T富集,反映了基性物源的存在;微量元素特征比值显示源区母岩为非单一物源,稀土元素总量较高,轻稀土元素富集,重稀土元素平坦,铕大部分具明显负异常、少部分微弱正异常,Ce基本无异常,显示为多物源,各样品稀土元素配分模式与大陆上地壳一致,显示了沉积物具有同源性。源岩应为来自于华北地块北缘的大陆上地壳的沉积岩、花岗岩和碱性玄武岩的混合。二叠系山西组泥岩沉积于覆水较深的还原环境,源区构造背景为大陆岛弧与活动大陆边缘。

The petrological and geochemical characteristics of Permian Shanxi Formation mudstones in Beijing were investigated to determine the provenance,paleoenvironment and tectonic setting of the rocks,which are mainly composed of clay minerals and quartz,with carbonate and feldspar as minor components.The carbonate content of the rocks has good correspondence with content of CaO and MgO.The SiO2,Na2O and K2O are depleted whereas Al2O3,TiO2 and Fe2O3T are enriched,reflecting a basic source.The mudstones show homogeneous REE content,relative enrichment of LREE and depletion of HREE,with most obviously negative Eu anomalies,less or no Eu positive anomalies,and almost no Ce anomalies.The REE distribution patterns show that they have the same source rocks composed of sedimentary rock,granite and alkaline basalt from the upper continental crust of the northern margin of North China,as is also shown by the ratios of trace elements.They were deposited in a reducing environment with relatively deep water.The tectonic settings of the source area are continental island arc and active continental margin.