Chemical Composition of Paleozoic Sandstones in Southwest Jiangxi Province and Its Implication for Tectonics
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摘要: 本文通过对赣西南早古生代砂岩进行全岩主量元素和稀土元素的分析,经与世界各地已知板块构造背景的砂和砂岩进行比较,结合华南大地构造研究的最新成果,提出研究区砂岩的物源区是沉积岩和结晶岩类,其板块构造环境属于被动大陆边缘。Abstract: In southern China there distributes widespreadly a thick suite of early Paleozoic flysch deposites extending from northern Guangdong.western Fujian.western Zhejiang.southern Jiangxi to south eastern Hunan, which make up an important geological component of southern China. The flysch, however, has been seldom studied with regard to sedimentology and there is few knowledge about the tectonic settings untill now. Paleozoic sandstones in southwestern Jiangxi are discussed petrologically and geochemically in the paper in order to identify their provenance and tectonic settings. The early Paleozoic deposits include the Cambrian and Ordovician and lack of the Silurian. The stratigraphic sequences are clear in the areas of Yongxin -Taihe and Congyi - Dayu. The Cambian is divided into three groups, i. e. Niujiaohe Group (L. Cam. ),Gaotan Group (M. Cam. )and Shuishi Group (U. Cam. ) and the Ordovician divided into five formations, Jueshangou (L. Ord. ). Qixiling (L. Ord. ). Longxi (M.Ord. ). Hanjiang (M.Ord. )and Shikou Formation (U. Ord. ). The sandstones are chiefly graywackes and the major detrital composition of it are quartz, feldspar and lith clact. The quartz content is very high (65 - 80%) and most of them are monocrystalline ones. Cathodolu-minescenece analysis of the quartz grain shows that most of the quartz are brown and less violet. Higher SiO2 content (79.9%). Lower Al2O3/SiO2 ratio (0. 13), higher Al2O3/(CaO + Na2O) (9. 94) ratio (9. 94) and higher K2O/Na2O ratio (1. 9) are the major elemental features of the sandstones. REE analysis of the sandstone manifests higher total REE content (167ppm), enreched LREE, higher (La/Yb). value (12. 51) and negative Eu abnormally. Compare the chemical composition of sandstones with those in the known tectonic settings.it shows that the former were formed in a passive continental margin and their provenance was sedimentary rocks and crystalline rocks including gneiss, schist and granitoids etc. There are some different ideas on the tectonics of southeastern China. Flysch in southwestern Jiangxi locates between Yangtze Plate and Cathaysian Plate tectonically. To the west of flysch belt is the littoral and neritic deposit facies area in the Yangtze Plate margin and to the east of it is Longquan - Jianou melange zone in northwestern margin of Cathaysian Plate. According to the plate tectonic pattern of southeast China.it can be inferred that the study area belongs to the passive margin of Yangtze Plate and the provenance are the basement and the cover of the plate.
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