湘西花垣地区奥陶-志留系沉积岩稀土元素地球化学特征及地质意义

REE Geochemical Characteristics of Ordovician-Silurian Sedimentary Rocks in the Huayuan Area,Hunan Province and Their Geological Significances

为探讨湖南花垣地区奥陶-志留纪的沉积环境,对该区各地层稀土元素特征进行了研究。结果显示,各地层样品总体呈轻稀土"右倾"重稀土"平坦状",稀土总量与Al_2O_3含量呈正相关、与CaO含量呈负相关或无明显相关性。奥陶系地层轻重稀土比值为7.25~9.23(均值为8.39),LaN/YbN为1.23~1.42,均值为1.33,Eu负异常,δCe为0.85~0.91(均值为0.88);志留系地层轻重稀土比值为7.30~9.07,均值为8.37,LaN/YbN为1.11~1.51,均值为1.29,Eu负异常,δCe为0.92~0.94(均值为0.93)。结合前人研究成果认为,在奥陶纪-志留纪阶段,研究区沉积物主要是来自雪峰山隆起的陆源碎屑物质,沉积环境为弱还原性浅海环境,海平面呈下降趋势,并发生了沉积相变,这可能导致志留纪的沉积速率高于奥陶纪,且在奥陶纪末-志留纪初水体酸度下降。

In order to discuss the sedimentation environment of Ordovician-Silurian strata in the Huayuan area,Hunan Province,a comparative study on characteristics of REE in different strata has been carried out. The results show that there are general LREE right-declined HREE flat REE patterns for most of samples. The ω（ Σ REE） contents of samples are correlated positively with Al_2O_3 contents but negatively or irregularly correlated with CaO contents. Samples of Ordovician strata have ΣLREE/ΣHREE ratios of 7. 25-9. 23,with a mean of 8. 39,LaN/YbNratios of 1. 23-1. 42,with a mean of 1. 33,negative Eu anomaly,and δCe values of 0. 85-0. 91,with a mean of 0. 88. Samples of Silurian strata haveΣLREE/ΣHREE ratios of 7. 30-9. 07,with a mean of 8. 37,LaN/YbNratios of 1. 11-1. 51,with a mean of 1. 29,negative Eu anomaly,and δCe values of 0. 92-0. 94,with a mean of 0. 93.Combining with results of previous researches and this study,it is believed that the Ordovician to Silurian sediments were mainly sourced from terrigenous detrital materials resulted from the Xuefengshan uplift,and were deposited in a weak reduced shallow marine environment with the decline of sea levels and the variation of sedimentary facies from Ordovician to Silurian periods. This may induced the higher sedimentation rate of Silurian sediments than that of the Ordovician sediments and the decrease of acidity of the water at the end of Ordovician-the beginning of Silurian.