湘中桃江中奥陶统黑色岩系岩石学地球化学及成因

桃江中奥陶统黑色岩系主要由黑色页岩等七类岩石组成,主要矿物为伊利石、绿泥石、玉髓等。该岩系经历了强的晚期成岩作用。碳同位素(δ~(13)C=-9.35～-20.13‰)等特征表明硫酸盐还原作用形成了富~(12)C的成岩碳酸盐矿物。黑色岩系富Mn、Ba等元素,B-V图解表明它们为海相,V/Cr值表明其形成于还原环境。湘中中奥陶统黑色岩系的空间分布、几何形态等表明其形成与氧最小带有关。

PETROLOGY, GEOCHEMISTRY AND GENESIS OF THE MIDDLE ORDOVIClAN BLACK SHALE SERIES IN TAOJIANG, THE CENTRAL REGION OF HUNAN PROVINCE

In the central region of Hunan province, Middle Ordovician includes Hule and Modaoxi Formations which mainly consist of black shale series. In Taojiang, the black shale series is mainly made up of mudiocks which can be divided into seven types, such as black shale, black siliceous shale, etc. According to microscope study, X-ray diffraction and infrared (?)pectra, the black shale series is mainly composed of illite, chlorite and chalcedony. The black shale series is (?)ich in carbonate minerals, chalcedony, pyrite and graptolites. The pyrites are mainly expressed as framblidal, microspherulitic, automorhpic granular and allotriomorphic granular ones. The black shale series possesses dark color and microlamination which show it was formed when the water body is quiet, anoxic and lack of disturbance of benthos. In teims of C. I. and I(002)/(001) diagram, opening index of illite, the characteristics of organic matter and rock phyrolysis, we can conclude that the black shale series in Taojiang experienced strong diagenesis. Based on the carbon isotope of carbonate minerals in black shale series, we guess that in the early stage of diagenesis, because of bacterial (?)ulphate reduction, framboidal pyrite, microspherulitic pyrite, ~(12)C-rich cabon dioxide and diagenetic carbonate minerals wete formed. The black shale series in Taojiang is rich in manganese, balium etc.. The B and V values indicate that it was formed in ocean. Xhe V/Cr values mean that it was formed in anoxic environment. The black shale series is poorer in organic carbon comparing with the black shale series in the southern China. The Ordovician black shale series in the South Sea of China is onl distributed zonally in marginal relatively shallow water or the central region of Hunan Province. It is parallel to Jiangnan islands and their scope is related to transgression and regression. Its forming is related to the O_2-minimum zone.