内蒙古阿拉善北部杭乌拉地区早古生代硅质岩地球化学特征及其构造意义

内蒙古阿拉善北部杭乌拉地区位于中亚造山带的中段南缘,处于西伯利亚板块、塔里木板块和华北板块之间,是研究中亚造山带和古亚洲洋演化的重要区域。文中以杭乌拉地区硅质岩地球化学特征为主要研究内容,结合前人研究结果,对阿拉善北部地区早古生代构造背景进行分析。杭乌拉地区下古生界西双鹰山组和班定陶勒盖组硅质岩镜下可见黏土、陆源粉砂和放射虫等,表现为明显的沉积成因特征。硅质岩地球化学特征一致性较差,其中Fe、Mn、Al等主量元素、Sc/Th值、(La/Ce)_n值和δCe值反映出硅质岩为大陆边缘沉积,(La/Yb)_n值反映硅质岩接近于远洋沉积背景,δEu 值表现为无热液作用影响,V、V/Y、Ti/V值多接近于洋中脊和大洋盆地硅质岩特征,U/Th、Ba/Sr值也属于热水成因硅质岩特征。硅质岩稀土元素配分曲线呈平坦状,无明显的右倾(大陆边缘轻稀土元素富集配分模式)和左倾(开放洋盆重稀土元素富集配分模式)特征。研究区早古生代地质记录相对较少,但是鉴于该地区晚古生代复杂的构造演化特征及其对早古生代古地理格局的继承,并结合上述硅质岩地球化学特征,认为研究区在早古生代为一个多岛洋环境。

Geochemical characteristics and its tectonic significance of the Early Paleozoic siliceous rocks in Hangwula area of northern Alxa,Inner Mongolia

Hangwula area of the northern Alxa in Inner Mongolia,located on the southern margin of the middle Central Asian Orogenic Belt(CAOB)and dominated by the Siberian Plate in its north,the Tarim Plate in its west and the North China Plate in its south,is one of the most typical areas for researching tectonic evolution of the CAOB and the Paleo-Asian Ocean. In this article,geochemical characteristics of the Early Paleozoic siliceous rocks were mainly studied to re-establish the tectonic evolution of the northern Alxa in Early Paleozoic combining with the previous results. The Early Paleozoic siliceous rocks of the Xishuangyingshan Formation and the Bandingtaolegai Formation in the Hangwula area were identified to contain clay,terrigenous silt and radiolarians under the microscopic analysis,which reveals that these siliceous rocks were of the distinct sedimentary origin. However,these Early Paleozoic siliceous rocks showed poor uniformity on their geochemical characteristics: The discrimination diagrams of major elements like Fe,Mn,Al and the ratio analyses of rare earth elements(REEs)like Sc/Th,(La/Ce)_n,and δCe indicate that the siliceous rocks belong to the stable continental margin deposits;but(La/Yb)_n value reflects a pelagic sedimentary background. δEu value shows that the origin process of the siliceous rocks was without hydrothermal participation. The ratio analyses of trace elements like V/Y,Ti/V conform to the ocean ridge or the ocean basin setting,but the U/Th and Ba/Sr radios accord with the hydrothermal feature. REE distribution patterns of the Early Paleozoic siliceous rocks are relatively flat with positive europium anomaly but without steep right-dipping LREE-rich trend or left-dipping HREE-rich trend,suggesting that their generation cannot be linked to classic continental margin or ocean basin. Although the Lower Paleozoic sedimentary records of the Hangwula area were relatively scarce,in view of the complex tectonic evolution characteristics of the Upper Paleozoic and its palaeogeographical inheritance,and combined with the above geochemical characteristics of the siliceous rocks,it can be considered that the Hangwula area of northern Alxa was in an archipelagic ocean environment during the Early Paleozoic.