鄂尔多斯盆地陇东地区二叠系山西组—石盒子组母岩类型和构造背景

通过岩石学及地球化学分析方法,对鄂尔多斯盆地陇东地区二叠系山西组1段—石盒子组8段碎屑岩物源区的母岩类型和构造属性进行分析。研究结果表明: 山1段和盒8段的砂岩碎屑组成相似,具有基本相同的物源区组成,它们在构造背景、物源及物源特征上具有很好的继承性;砂岩碎屑颗粒镜下特征及石英阴极发光颜色显示母岩主要为变质岩和火成岩;轻矿物特征反映物源与再循环造山带源区中的碰撞缝合线及褶皱—逆掩带物源区密切相关;研究区与北秦岭地区秦岭群和宽坪群斜长石种类一致;构造环境判别图解反映物源区长期受被动大陆边缘物源影响,到晚古生代后期有向主动大陆边缘转换的趋势;源岩判别图解显示物源区主要是长英质岩石;山1段、盒8段沉积岩的REE配分模式均呈右倾型且基本相互平行,与北秦岭地区秦岭群、宽坪群变质岩的稀土元素配分模式相似;锆石U-Pb同位素定年结果显示物源主要来自元古代,有少部分来自晚古生代。综合分析认为研究区主要受被动大陆边缘物源影响,其次为主动大陆边缘物源,母岩主要为北秦岭下元古界秦岭群的片麻岩、石英岩等和中、上元古界宽坪群的绿泥石片岩、阳起石片岩,其次为北秦岭造山带晚古生代岩浆岩。

Parent rock types and tectonic setting of the Permian Shanxi and Shihezi Formations in Longdong area,Ordos Basin

Based on petrological and provenance analysis,this paper discusses tectonic property and provenance of the Member 1 of Shanxi Formation and Member 8 of Shihezi Formation of Permian in Longdong Area,Ordos Basin. The results suggest that detrital mode compositions of these two members are similar,which were formed from the same parent rocks and tectonic setting. Microscopic characteristics and CL image of quartz show that the parent rocks are metamorphic rocks and igneous rocks;Light mineral characteristics are associated with collisional suture and fold-thrust belt sources. The plagioclase An and EPMA analysis indicate that the study area and the north Qinling orogenic belt have similar plagioclase type. Tectonic discrimination plots indicate that the provenance was mainly affected by passive continental margin source until the end of Late Paleozoic,then under the influence of active continental margin. Parent rock discrimination plots suggest that the provenance was dominated by felsic rocks. The REE distribution patterns of the sedimentary rock in Shan 1 and He 8 Members are consistently declining to the right,and are very similar to those of Qinling Group and Kuanping Group in the north Qinling orogenic belt. The dating of U-Pb isotope in zircon reveal that they can be divided into two groups according to their ages,which mostly dated from Proterozoic and some from Late Paleozoic. It is concluded that the provenance mainly affected by passive continental margin,others are active continental margin sources. Furthermore,the parent rocks of Shan 1 and He 8 Members are mainly from gneiss and quartzite of Qinling Group and chlorite schist and actinolite schist of Kuanping Group in the north Qinling orogenic belt,a few from igneous rocks of late Paleozoic era in the north Qinling orogenic belt.