鄂尔多斯盆地陇东地区延长组物源区构造属性研究

摘要

Tectonic setting and orientation of the provenance area of the Yanchang Fm remain unclear. To understand the tectonic property of the provenance area of this formation, this paper describes the sandstone petrologic characteristics, geochemical characteristics and distribution patterns of rare earth elements of Chang8 to Chang6 Member in Longdong area, Ordos Basin to determine the tectonic settings and diversity of provenance rocks in different areas. The results show that source rocks during Chang8 to Chang6 sedimentary period mainly came from volcanic arc source and transitional continental source associated with fold-thrusting, but source rocks of Chang7 and Chang6 in Huanxian and Yanwu, western basin were different from Chang8, part of which came from stable craton continental source. The REE abundance indicates that REE model was characterized by enrichment in light REE, depletion in heavy REE, and negative Eu anomaly. The provenance was involved with granitic crust materials which were characterized by containing more felsic rocks. Comparing samples from the northeast, northwest, southwest, and south Longdong area with various lithologies from ancient land surrounding the basin, this study shows that REE distribution patterns generally had the right-leaning characteristics and were different from each other. Also, samples from different areas could match well with different lithologies from ancient land surrounding the basin, reflecting the differences in source rocks and tectonic features.%鄂尔多斯盆地延长组物源区的构造属性和物源方向一直是研究热点，但存在不确定性。为加深对这一问题的理解，文中选择陇东地区三叠系延长组长8—长6沉积期的砂岩，通过对岩矿特征以及稀土元素地球化学特征及配分模式分析，研究了物源区的母岩构造属性及不同地区的母岩差异性。结果表明陇东地区延长组长8—长6沉积期母岩主要来自火山弧物源区及与褶皱逆冲相关的过渡大陆物源区，但是研究区西部环县、演武地区长7—长6沉积期的部分物源还有来自稳定克拉通的陆块物源区，展示出差异性。砂岩稀土元素丰度分析发现，轻稀土元素相对富集，重稀土元素相对亏损，Eu元素负异常，反映源区主要与花岗质地壳物质有关，以含较多的长英质岩性为特征。将陇东地区东北部、西北部、西南部、南部的样品与盆地周缘古陆各类岩石的稀土元素配分模式进行了对比，发现稀土元素配分模式既普遍具有右倾的特征，又具有一定的差异性，各地区样品稀土元素配分模式均能与盆地周缘古陆匹配良好，反映了母岩来源与构造属性的差异性。