贺兰山中段奥陶系米钵山组砂岩地球化学及构造背景分析

本文利用奥陶系米钵山组砂岩地球化学分析,结合区域地质研究,探讨贺兰山中、晚奥陶世的构造环境。贺兰山中段奥陶系米钵山组砂岩的地球化学研究表明,砂岩的SiO2平均含量为81.3%;Al2O3/SiO2值0.07～0.11,平均值为0.08;K2O/Na2O值变化较大,最大60.7,一般介于4.79～7.81;Fe2O3+MgO含量较低,介于2.1%～2.81%。砂岩微量元素Nb丰度及V/(V+Ni)与Ce/La、Sr/Ba值均较高,说明砂岩沉积于湿热、还原、低盐度环境,具有大陆型沉积特征。砂岩稀土元素富集,含量在116×10-6～195×10-6之间,平均值为158×10-6;δEu为0.52～0.58,具显著的负铕异常。这些数据指示了米钵山组具有重力流快速堆积的特征和大量陆源补给,浊流沉积作用是重力流携带陆源物质的主要途径。通过多种构造环境判别图解分析,显示物源区地质构造具有被动大陆边缘性质。

Analysis on geochemistry and tectonic setting of sandstones from the Ordovician Miboshan Formation in the middle section of Helan Mountain

Using the geochemical analysis of sandstones from the Ordovician Miboshan Formation and combining with the regional geology of the Helan Mountain,this paper approaches the tectonic setting of the Helan Mountain during the Middle-Late Ordovician.The geochemical study of the sandstones from the middle section of the Helan Mountain suggests that the sandstones have average SiO2 content of 81.3%, Al2O3/SiO2=0.07～0.11(average,0.08),K2O/Na2O=4.79～7.81,low Fe2O3+MgO contents of 2.1%～2.81%.The sandstones have relatively higher contents of Nb and higher ratios of V/(V+Ni),Ce/La and Sr/Ba,indicating a humid tropical,reducing and low salinity environment during the deposition of the sandstones,suggesting characteristics of the continent-type sedimentary origin.The sandstones are enriched in rare earth elements,with total REE contents of 116～195 ppm(average,158 ppm),and prominently negative Eu anomalies of 0.52～0.58.All the geochemical data indicate that the Miboshan Formation shows characteristics of the quick accumulation of gravity current and large terrigenous supply,the turbidity sedimentation is the main way to carry terrigenous materials.The analysis of various tectonic setting discrimination diagrams shows that the geological structure of provenance exhibits a nature of passive continental margin.