中国南方灯影峡期海洋碳酸盐岩原始δ^(13)C和δ^(18)O组成及海水温度

PRIMARY δ^(13)C ANDδ^(18)O VALUES IN MARINE CARBONATES AND THE SEA WATER TEMPERATURE OF DENGYINGXIA AGE IN SOUTH CHINA

中国南方灯影峡期(晚前寒武纪)是白云岩广泛发育的海洋碳酸盐沉积时期,在灯影组中部发育从海水直接沉积、沉淀的原生白云岩,目前仍保留其原始组构特征。从40个原生白云石(岩)中测得:泥晶白云石的C值为3.64‰,O值为-1.17‰(n=6);藻白云岩的C值为3.52‰,O值为-1.86‰(n=15);海水纤状白云石胶结物C值为2.90‰,O值为-2.65‰(n=8);海水刃状白云石胶结物的C值为2.96‰,O值为-2.41‰(n=8);泥晶纹层和海水纤状白云石胶结物的C值为2.79‰,O值为-3.13‰。40个岩样的C平均值为3.25‰±0.44‰,O平均值为-2.12‰±0.98‰(均以PDB标准)。对于灯影峡期海相白云岩的原始C和O值,不采用所有样品的平均值,而是采用原生白云石沉积物与海水白云石胶结物C值和O值两个图示分布区重叠部分的最重同位素值,即:C值为4.43‰(PDB标准),O值为-0.62‰(PDB标准),将其作为灯影峡期海洋碳酸盐岩的原始同位素组成。对海水原生白云石胶结物包裹体盐度进行了测定,海水O计算值为2.90‰(SMOW标准),用原始O值计算的原生白云石形成时的海水温度为40.8℃。这说明中国南方灯影峡期的海洋为炎热的较高的海水温度环境。

The Dengyingxia Stage (Late Precambrian) in South China is a marine carbonate sedimentary stage with extensively developed dolostones. The primary dolostones precipitated directly from sea water are developed in the middle unit of Dengying Formation and their primary texture are reserved well. The δ13C and δ18O values have been determined from 40 samples of primary dolomites (dolostones): δ13C=3.64‰ and δ18O= -1.17‰ (n=6) for micrite dolomites; δ13C = 3.52‰ and δ18O = - 1.86‰ (n = 15) for algae dolostones; δ13C=2.90‰ and δ18O= -2.65‰ (n=8) for fibrous dolomite cements; δ13C = 2.96‰ and δ18O = -2.41‰ (n =8) for bladed dolomite cements; δ^(13) C=2.79‰ and δ^(18) O= - 3. 13‰ (n = 3) for micrite lamella and fibrous dolomite cements. The average δ13C and δ18O values of all 40 samples are 3.25‰ ±0.44‰ and -2.12‰ ± 0. 98‰ respectively (PDB standard). The heaviest δ13C and δ18O values in overlapping area of the values of the primary dolomite sediments and the cements in diagram, δ13C= 4.43‰ (PDB) and δ18O= -0.62‰ (PDB), has been determined as the primary carbon and oxygen isotope composition of the Dengyingxia marine carbonate. The δ^(18) O value of sea water of Dengyingxia Age is 2. 90‰ (SMOW) calculated based on the salinity of the inclusion in primary dolomite cement. The temperature of sea water at that time is 40.8℃ calculated on the basis of the primary δ^(18) O value. It indicates that hot climate and high temperature existed in Dengyingxia Age in South China.