贺兰山中南段奥陶系米钵山组的沉积环境与构造背景分析

Element Geochemistry and Depositional Setting of Ordovician Miboshan Formation in Central-Southern Helan Mountain

贺兰山中南段奥陶系米钵山组的岩石地球化学研究表明,其砂岩SiO2平均含量为75.18%;Al2O3为6.26%～14.36%,变化范围不大,但含量较高;TiO2/MnO值为3.05～58.00,变化范围较大。K2O/Na2O值0.63～9.13。页岩SiO2含量在68.77%～77.50%之间,TiO2/MnO值与Al2O3/MnO值都比较高,分别为21.30～51.00及大于450.00。Na2O含量偏低,部分样品<0.01%,K2O含量偏高(3.05%～4.03%),TiO2与CaO含量接近正常水平。砂岩稀土元素总量较高,在144.80×10-6～170.57×10-6之间,LREE/HREE值为9.68～16.49,δEu为1.01～1.31,具弱的正铕异常,无Ce负异常或负异常不明显;砂岩微量元素Ni、V、Cu、Pb、Ba特别是Cr的丰度大大高于上地壳丰度。Sr及成矿元素Zn、Ga低于地壳丰度,V/(V+Ni)与Ce/La值均较高,这些数据说明本区沉积于还原、湿热环境,具有大陆型沉积特征。通过多种相关图解,均反映其物源形成于大陆岛弧和活动大陆边缘带的构造背景。

The Ordovician Miboshan Formation widely distributed in the Helan Mountain is comprised of a series of detrital rocks interbedded with argillaceous rock and carbonate rock, with the development of flysch bedding and typical turbidite Bouma sequence. The average SiO2 and A1203 content of sandstone is 75.18% and 6.26%- 14.36%, respectively, and TiO2/MnO and K20/Na20 ratios range respectively from 3.05 to 58.00 and from 0.63 to 9.13. In addition, the content of SiO/in shale ranges from 68.77% to 77.50%, the TiO2/MnO ratio is high, ranging from 21.30 to 51.00, while the A1203/MnO ratio is higher than 450.00. The content of Na20 is relatively low, being lower than 0.01 in some samples. The content of K20 is relatively high, ranging from 3.05 to 4.03. The content of TiO2 and CaO is close to the normal level. The total content of REE in sandstone is relatively high, ranging from 144.80×10^-6 to 170.5×10^-6. The LREE/HREE ratio is 9.68-16.49, and 6 Eu is 1.01-1.31, exhibiting weak positive Eu anomaly. There exists no or only very weak negative Ce anomaly. The abundances of such trace elements in sandstone as Ni, V, Cu, Pb, Ba and, especially, Cr are much higher than their Clarke values in the Earth＇s crust. The abundances of Sr and ore-forming elements like Zn and Ga are lower than their Clarke values in the Earth＇s crust. The V/（V＋Ni） ratio and Ce/La ratio are also high. All these data show that the sedimentation of this area occurred, in a reducing, moist and hot environment, characterized by continental deposition. A variety of related diagrams suggest that the source material was formed in continental island arc belts and active continental margins.