藏南古堆地区三叠系涅如组含放射虫硅质岩地球化学特征及沉积环境分析

Study on geochemical characteristics and sedimentary environment of containing radiolaria chert of Triassi Nieru Formation in Gudui area of southern Tibet

通过野外地质调查和系统的岩石化学、主量元素、微量元素以及稀土元素等研究,探讨了藏南地区三叠系涅如组含放射虫硅质岩的地球化学特征及其沉积环境。研究区的硅质岩呈黑色、灰黑色或灰褐色薄层状或透镜状产出,具有较低的SiO_2含量,对研究区的硅质岩的主量元素运用了Fe-Mn-Al三角成因判别图、Al/(Al+Fe+Mn)比值、K_2O/Na_2O比值、Si/(Si+Al+Fe)比值、100×(Al_2O_3/SiO_2)-100×(Fe_2O_3/SiO_2)、Al_2O_3/(100-SiO_2)-Fe_2O_3/(100-SiO_2)及Al_2O_3/(Al_2O_3+Fe_2O_3)-Fe_2O_3/TiO_2图解等,显示硅质岩为具有生物成因的非热水或弱热水的大陆边缘沉积环境;硅质岩的稀土元素ΣREE变化较大,通过对硅质岩的稀土元素北美页岩标准化和球粒陨石标准模式图对比、Al_2O_3/(Al_2O_3+Fe_2O_3)-LaN/CeN图解及特征指数Ce/Ce*和LaN/CeN分析研究,显示硅质岩为大陆边缘、大陆边缘—深海的过渡环境;对硅质岩的微量元素运用了U/Th及Ni/Co比值,显示了非热水沉积物的环境特征。综合研究分析岩石化学、主量元素、微量元素以及稀土元素等因素及生物特征,可以得出研究区的硅质岩具有生物特征的非热水的大陆边缘沉积环境。

The geochemical characteristics and sedimentary environment of containing radioiaria chert of Triassi Nieru Formation in the Southern Tibet are analyzed on the basis of the geological field survey and systematic study of major elements, trace elements and rare earth elements. The chert in the studying area is black as thin bedded or lenticular shape,in which the content of SiO2 is lower. With Fe - Mn - Al triangle genesis discrimination diagram,Al/（Al,Fe and Mn）ratio,K2O/Na2O ratio,Si/（Si 4- Al ＋ Fe）ratio,100 × （Al2O3/ SiO2） - 100 × （Fe2O3/ SiO2）,Al2O3/（100-SiO2） - Fe2O3/（100- SiO2） and Al2O3/（Al2O3＋ Fe2O3） - Fe2O3/ TiO2 diagrams in major elements of the chert,they indicate that the chert formation environment is the sedimentary environment of continental margin with a biological genesis of not hot water or weak hot water. The variation of ZREE of cherts is larger. With the comparison of REE pattern models of the cherts of the north American shale standard and chondrite standard, Al2O3 / （Al2O3 ＋ Fe2O3 ） - LaN/CeN diagrams and feature parameters of Ce/Ce^＊and LaN/CeN ,it is proposed that formation environment of chert is the transition of continental margin and continental margin-deep sea environment. U/Th and Ni/Co ratio of trace elements of the chert show the not-hot water environment characteristics of the sediment. With Comprehensive study and analysis of rock chemistry,major elements,trace elements and rare earth elements and biology feature,it is concluded that the formation environment of cherts in the study area is the sedimentary environment of continental margin with biological characteristics of the not-hot water.