川西南盐津地区志留系龙马溪组页岩地球化学特征及地质意义*

In order to find out the element geochemical characteristics of Silurian Longmaxi Formation shale in Yanjin area,southwestern Sichuan Basin,as well as the material source and structural background of shale formation,the petromineralogy and geochemistry of 19 shale samples from Niuzhai section of Yanjin area were tested and analyzed. Combined with the previous research results and adjacent areas,the rare earth elements were mainly applied to discuss the sedimentary environment, the accumulation rate,material source and tectonic setting of the Longmaxi Formation shale in the study area. The results show that the mineral composition of the Longmaxi Formation in Niuzhai section of Yanjin is mainly quartz,carbonate minerals and clay minerals. The mineral triangle diagram shows that the Longmaxi Formation shale is mainly a combination of clay shale and mixed shale. The normalized partition pattern of rare earth elements is characterized by Ce deficit and weak Eu enrichment. The δCe values,Ni/Co and V/Cr values of the samples indicate that the paleo-water depth increases suddenly in the early stage and becomes shallow gradually in the later stage. Oxygen content gradually changes from anoxic reduction state in the early stage to weak oxidation state in the later stage. La_N/Yb_N values show that the shale deposition rate increases firstly and then decreases under the influence of sea level and terrigenous debris input. Provenance analysis shows that the source rocks area are mainly felsic igneous rocks and intermediate-acid geological bodies of granitoids. The La-Th-Sc diagram and tectonic setting characteristics show that the main tectonic setting of the source rocks is passive continental margin. Combined with previous studies and tectonic setting,it is inferred that the main source of the Longmaxi Formation shale is formed in the Passive continental margin of hypoxia,and the provenance is mainly from the Kang-Dian ancient land to the west of the study area,followed by the Central Guizhou uplift.     

四川盆地东南缘中上寒武统白云岩稀土元素特征及成因意义——以贵州毕节长树田剖面为例

以野外剖面测量及沉积相分析为基础,结合室内薄片鉴定,分析了贵州毕节长树田中上寒武统白云岩稀土元素特征,认为该区白云岩普遍具有交代残余结构和ΣREE较高的特点,且以下部含灰泥质白云岩ΣREE含量最高。该区白云岩下部REE高含量主要来自陆源碎屑注入和粘土矿物的吸附作用,而白云岩整体ΣREE含量较高,与沉积时高咸度卤水在早期埋藏作用中渗透回流交代有着密切的关系。δEu和δCe相比,δCe值较大,反映出弱氧化和高碱度沉积环境,Ce具弱富集特征,与其沉积时处于局限台地环境,水体较浅,易于氧化,且水体较咸,处于碱性沉积环境,沉积了石膏、岩盐等盐类矿物等地质事实相吻合。