摘要
湘中南地区奥陶系由“细碎屑岩-硅质岩系-粗碎屑岩”构成,三者厚度变化具有明显的规律性：厚度等值线的展布逐渐趋于北东方向,厚度最大区域向南东方向迁移.区内岭口剖面烟溪组硅质岩SiO2含量（89.08%-94.32%）和Al/（Al＋Fe＋Mn）值（0.52-0.79）较高,具有轻稀土略富集、无明显铈异常和铕异常的特点;大桥剖面烟溪组硅质岩SiO2含量高（91.74%-95.14%）,Al/（Al＋Fe＋Mn）值为0.34-0.56,具有轻稀土富集、无明显铕异常和间歇性铈负异常、Y/Ho比值低（20.65±1.63）的特点.硅质岩地球化学特征及图解说明其主要为正常海相生物成因,形成于开阔的大陆边缘背景.对比邻近地区相应层位数据发现,湘中南及其邻区中-晚奥陶世硅质岩成因与沉积背景相似,指示其形成于统一盆地中,结合地层等厚度图分析认为,盆地经历了被动大陆边缘-前陆盆地的转换,硅质岩系可能是前陆盆地初始阶段的产物,在其展布范围内无明显热液影响,暗示造成华夏地块抬升的地球动力学来源可能还在该套硅质岩系展布范围的更南部或东南部.
Ordovician strata in central- southern Hunan consist of'finer clastic-chert- bearing series-coarser clastic rock', andtheir thickness varies regularly. Spatially, the thickness contours are distributed northeastwards nd meanwhile, the thickest tends to move southeastwards. In the study area, chert samples of Yanxi Formation in Lingkou section show high SiO_2values(89.08 %-94.32%) and Al/(Al+Fe+Mn) ratios(0.52-0.79), and are characterized by slight enrichment of LREE and no obvious cerium and europium anomalies. Samples of equivalent formation in Daqiao section shows high SiO_2values(91.74 %-95.14%), and the Al/(Al+Fe + Mn) ratios range from 0.34 to 0.56, with characteristics of LREE enrichment, intermittently negative cerium anomaly and no europium anomaly, and low Y/Ho ratios(20.65 ± 1.63). Geochemical characteristics and discrimination diagrams of middle- upper Ordovician cherts in central-southern Hunan indicate that these rocks were bio-genetic and formed in an open continental margin.A correlation with equivalent formation in adjacent areas shows that these cherts have similar origins and sedimentary environments,which indicates that they formed in the same basin. Combined with the analysis of the iso-thickness diagram, it could be suggested that the basin in central-southern Hunan experienced the change from passive margin to foreland basin during Ordovician, and the cherts probably resulted from this transition. In addition, there is no obvious hydrothermal activity, suggesting the geodynamics that caused the uplift of Cathaysian block might have been located to the further southern or southeastern extension range of the cherts series in central-southern Hunan.
出处
《中国地质》
CAS
CSCD
北大核心
2016年第3期936-952,共17页
Geology in China
基金
中国地质调查局地质矿产调查评价专项(12120114039301
12120113063600
1212011120117)项目资助~~
关键词
硅质岩
地球化学
奥陶纪
盆地演化
华南
cherts
geochemistry
Ordovician
basin evolution
South China
