鄂尔多斯盆地奥陶系碳酸盐岩成岩相碳、氧稳定同位素特征

Carbon and oxygen stable isotopic features of diagenetic facies of Ordovician carbonate rocks in Ordos Basin

王起琮 张阳 肖玲

Wang Qicong Zhang Yang Xiao Ling

根据岩石薄片及阴极发光图像,研究了鄂尔多斯盆地奥陶系碳酸盐岩成岩相类型及岩相特征。据碳、氧稳定同位素(δC, δO)分析数据,研究了各类成岩相的地球化学特征。奥陶系海相碳酸盐岩台地在早古生代处于近地表成岩域,划分为正常海水、蒸发海水、早期大气淡水及热海水4类成岩环境及成岩相。正常海水、热海水及蒸发海水成岩相的δO和δC值普遍较高,随生物碎屑含量增加δO及δC值趋于减小;早期淡水成岩相的δO和δC值普遍较低。晚古生代,奥陶系顶部进入表生期成岩域,据目前残存地层的岩石类型,表生期成岩域可划分为峰峰组的灰岩风化成岩相和马家沟组的云岩风化成岩相,其δO和δC值明显低于早期淡水成岩相的值。晚古生代末期,奥陶系进入3 000~4 000 m的中-深埋藏成岩域,划分为封闭系统及开放系统成岩相。封闭系统成岩相的δO和δC值主要继承了其原岩的δO和δC值特征;而在开放系统中,由于同位素高温分馏作用,其成岩相的δO和δC值则大幅度降低。研究结果表明,碳酸盐岩岩相特征及碳、氧稳定同位素组成是追溯碳酸盐岩成岩环境及识别和划分成岩相的重要指标。

Based on thin section and cathodoluminescence imaging,this paper discussed the types of diagenetic facies and the characteristics of lithofacies of Ordovician carbonate rocks in Ordos Basin.In combination with carbon and oxygen stable isotopic characteristics,we also studied the geochemical characteristics of each diagenetic facies.During the early Paleozoic,the Ordovician carbonate platform was in a near-surface diagenetic tract and can be divided into four diagenetic environments and facies including normal seawater,evaporative seawater,early meteoric freshwater and hot seawater according to the diagenetic medium.The δO and δC of diagenetic facies of normal seawater,hot seawater and evaporative seawater are generally high,but tend to decrease along with the increasing bioclastic content.In contrast,the δO and δC of the early freshwater diagenetic facies,are generally low.During the late Paleozoic,the top of the Ordovician was in the epidiagenetic tract.According to the rock types of the current residual strata,the epidiagenetic tract can be divided into limestone weathering diagenetic facies in the Fengfeng Formation and dolomite weathering diagenetic facies in the Majiagou Formation.Their δO and δC values are significantly lower than that of the early freshwater diagenetic facies.At the end of the Late Paleozoic,the Ordovician Formation was in the moderate-deep burial diagenetic tract at a depth of 3 000-4 000 m,which can be divided into a closed system diagenetic facies and an open system diagenetic facies.The δO and δC values of the closed system diagenetic facies are similar with their original rocks.In contrast,the δO and δC values of the open system diagenetic facies are much lower than their original rocks because of the isotope fractionation effect in the hot environment.These results indicate that the petrographic characteristics and the δO and δC va-lues of carbonate rocks are important indicators for identification of the diagenetic environment and classification of diagenetic facies of carbonate rocks.

10.11743/ogg20130511