滇东南弗拉斯期放射虫硅质岩地球化学特征及沉积环境探讨

Discussion on Geochemical Characteristics and Sedimentary Environment of the Frasnian Radiolarian Chert in Southeastern Yunnan

在滇东南麻栗坡地区广泛分布的榴江组薄层状硅质岩中,发现包含放射虫动物群Helenifore robustum和Helenifore laticlavium的重要分子,地质时代为晚泥盆世弗拉斯皁期。硅质岩S iO2含量在80.2%~95.72%之间,A l2O3含量集中在0.32%~3.83%之间,平均为2.07%,S i/A l值集中在20.94~62.16之间,表明硅质岩含有高的陆源泥质沉积物;MnO/TiO2值（除两件样品稍大）大多数在0.03~0.41之间。常量元素A l/（A l＋Fe＋Mn）值在0.57~0.64之间（6P-B5为0.34）,A l/（A l＋Fe）值（除6P-B5为0.34）在0.58~0.71之间,S i/（S i＋A l＋Fe）值均大于0.9,反映硅质岩主要受生物作用和陆源的制约,而热液活动的影响较弱;稀土元素Ce/Ce＊为0.62~1.37,个别具有弱的Ce/Ce＊负异常。LaN/YbN值为0.47~1.74,平均为1.21。LaN/CeN值为0.75~1.70,平均为1.11。具有大陆边缘硅质岩的地球化学特征,少数具有远洋深水盆地的性质;微量元素V含量多数集中在14.5~30.9μg/g,显示靠近大陆边缘硅质岩特征,少数高达75.3μg/g和218μg/g,可能与热水作用密切相关;U/Th值集中在0.27~0.40,平均为0.86,另外有三件样品大于1.0,表明硅质岩主体为正常的生物成因,局部热液影响较强。放射虫硅质岩、浊积岩建造以及硅质岩地球化学特征研究表明,滇东南地区在弗拉斯期可能处于大陆边缘裂谷构造环境。

Writers found some important fossil of Helenifore robustum and Helenifore laticlavium radiolarian fauna from the bedded chert of the Liujiang Formation,which distributed abroad in southeastern Yunnan province,so the chert is Frasnian of Late Devonian.The chert has SiO2 80.2%~95.72%,Al2O3 0.32%~3.83%,average is 2.07%,Si/Al ratio is 20.94~62.16,which indicates that the chert is riched in terrigenous muddy deposit.Most of Mn/TiO2 ratios of the chert are 0.03~0.41,but sample 6P-B5 is 0.34.All Si/（Si＋Al＋Fe） ratios are greater than 0.9,whichsuggests that the chert formation is mainly controlled by biological action and terrigenous deposit,and the hydrothermal activity is feebleness.The Ce/Ce＊ ratios are 0.62~1.37,there is Ce/Ce＊ negative individually.LaN/YbN ratios are 0.47~1.74,average value is 1.21.LaN/CeN ratios are 0.75~1.70,average value is 1.11.The chert has geochemical characteristics of continental edge chert,minority has characteristics of abyss.The V contents are mainly 14.5~30.9μg/g,showing the characteristics of continental edge chert;and a few are as high as 75.3μg/g and 218μg/g,might be related to hydrothermal action.U/Th ratios are mainly between 0.27 and 0.40,average is 0.86,three samples have high U/Th ratios greater than 1.0,indicating that the chert is mainly natural biogenetic,parts are strongly affected by hydrothermal action.The radiolarian bedded chert,its geochemical characteristics and turbidite formation indicate that southeastern Yunnan area is a continental marginal rift basin at Frasnian,Late Devonian.