北京地区中元古界下马岭组页岩地球化学特征及其地质意义

对采自北京地区中元古界下马岭组的页岩样品,进行了地球化学研究,结果表明:主量元素Al2O3、Ca O和Na2O含量明显偏低,K2O和Mn O含量稍微偏低,Si O2、Ti O2及Fe2O3T含量同上地壳基本一致。微量元素及其比值特征显示源岩为混合源岩。稀土元素总量较高,轻稀土富集、重稀土平坦,铕明显负异常、铈弱负异常,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。源岩为来自于内蒙古隆起的花岗岩类、闪长岩类及基性岩的混合。微量元素比值及铈弱负异常的特征表明中元古界下马岭组页岩为覆水较深的还原环境沉积,源区构造背景为大陆岛弧。

Geochemical characteristics of Mesoproterozoic Xiamaling Formation shales in Beijing and their geological significance

Based on analyses of the sampled shales of field outcrops and well cuttings, the authors studied the geochemical characteristics of the shales of Mesoproterozoic Xiamaling Formation in Beijing. Al₂O₃, Na₂O and K₂O are depleted whereas SiO₂, TiO₂ and Fe₂O₃^T are relatively enriched. The shales show homogeneous REE content, relatively enrichment of LREE and depletion of HREE,obvious negative Eu anomaly, and slightly negative Ce anomaly. The REE distribution patterns from different shale samples showing an affinity reveal that they have the same source rocks including granitic, dioritoid and mafic rocks from the upper continental crust of Inner Mongolia uplift, and the ratios of trace elements show these features too. They were deposited in a reducing environment with relatively deep water. The tectonic setting of the source area seems to have been a continental island arc.