鄂尔多斯盆地马家沟组中组合马五_5亚段白云岩特征及成因机理

通过对鄂尔多斯盆地马家沟组五段5亚段白云岩的岩石学和地球化学研究表明,主要岩性有泥晶白云岩、泥粉晶白云岩和粉晶白云岩等三类。泥晶白云岩类为泥晶结构,阴极发光为暗红光,白云石有序度低;Na、K和Sr值较高;δ~(13)C平均-1.23‰,δ~(18)O平均-7.59‰,锶同位素平均值0.709 81,形成于闭塞局限、盐度相对较高的环境,由准同生白云石化作用所形成,并发育少量晶间微孔。泥粉晶白云岩类为泥晶—细粉晶结构,不发光或发极暗的红光,白云石有序度0.75;常量元素、微量元素大幅降低;δ~(13)C和δ~(18)O的平均值分别是-0.43‰和-7.29‰,锶同位素平均值0.709 36,由准同生后回流渗透白云石化作用所形成,并发育晶间孔。粉晶白云岩不发光或发极暗红光,白云石有序度0.81;常量元素、微量元素有明显的突变;δ~(13)C和δ~(18)O平均值分别为-0.65‰和-6.46‰,锶同位素平均值0.709 67,由埋藏白云石化作用所形成,并发育晶间扩溶孔或溶蚀孔。

Characteristics and Genesis of Ordovician Majiagou Submember-55 Dolostone in Middle Assemblage,Ordos Basin

Based on petrological and geochemical analysis for core samples from the Middle Ordovician Majiagou submember-55 dolostones in Ordos basin, three main types of dolostones are recognized, including micritic dolostone, siltcrystal micritic dolostone and siltcrystal dolostone. The characteristics of them are described as follows:(a) The micritic dolostone is formed in euxinic-restricted and high-salinity environment by penecontemporaneous dolomitization, in which a small number of intercrystal mircopores develop. It is characteristic of micrite texture, garnet light in cathoduluinescence observation, lower degree of order and high content of Na, K and Sr elements. The average values of δ13C and δ18O are-1.23‰and-7.59‰respectively, and the ratio of 87Sr/86Sr is 0.70981. (b) Siltcrystal micritic dolostone is formed by post-penecontemporaneous reflux seepage dolomitization, in which intercrystal pores develop. It is characteristic of siltcrystal-fine micrite texture, dirtier, no or very dark light in cathoduluinescence, 0.75 of degree of order, significantly decreasing of the content of major and trace elements. The average values ofδ13C and δ18O are -0.43‰ and -7.29‰, and the ratio of 87Sr/86Sr is 0.70936. (c) Siltcrystal dolostone, in which intergranular dissolution pores and dissolution pores develop, shows the same feature as siltcrystal-micrite dolostone in cathoduluinescence but the degree of order is 0.81. The content of major and trace elements shows apparently abrupt change. The average values of δ13C and δ18O are -0.65‰ and -6.46‰ and the ratio of 87Sr/86Sr is 0.70967. It is indicated that siltcrystal dolostone is formed by burial dolomitization.