Geochemical characteristics and its tectonic significance of the Early Paleozoic siliceous rocks in Hangwula area of northern Alxa,Inner Mongolia
Zheng Xiaoming1, 2, Yin Haiquan3, Gao Lei2, Wang Shengyu4, Wang Yankai5, Zhou Hongrui2, Zhang Weijie2
1 China Non-ferrous Metals Resource Geological Survey,Beijing 100012; 2 School of Earth Sciences and Resources,China University of Geosciences(Beijing),Beijing 100083; 3 First Crust Monitoring and Application Center,CEA,Tianjin 300180; 4 Beijing Institute of Geology,Beijing 100120; 5 Hebei Institute of Regional Geological and Mineral Resource Survey,Langfang 065000,Hebei;
Abstract：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.
Zheng Xiaoming,Yin Haiquan,Gao Lei et al. Geochemical characteristics and its tectonic significance of the Early Paleozoic siliceous rocks in Hangwula area of northern Alxa,Inner Mongolia[J]. JOPC, 2017, 19(3): 491-502.
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