鄂西南下三叠统嘉陵江组二段白云岩化机制

ORIGIN OF DOLOMITE IN THE SECOND MEMBER OF JIANLINGJIANG FORMATION OF LOWER TRIASSIC,SOUTHWESTERN HUBEI PROVINCE,CHINA

鄂西南恩施地区下三叠统嘉陵江组为碳酸盐岩沉积,发育石灰岩与白云岩的互层,通过野外观察和室内分析表明,嘉二段主要发育两种类型的白云岩,即泥晶-粉晶白云岩和砂屑白云岩。在沉积环境和岩石学特征研究的基础上,结合微量元素、碳氧同位素和阴极发光等技术,对白云岩的形成机理进行了分析。泥晶-粉晶白云岩发育有鸟眼构造、窗格构造和石膏假晶等暴露沉积特征,有序度低,白云石发红色光、紫红色光,与准同生白云岩化作用有关;砂屑白云岩有序度高、Mn和Fe含量高、Na含量和Mn/Sr小,具有较负的δ18 O值和较正的δ13 C值,形成于受大气淡水影响的氧化环境,砂屑发紫红色光,与混合水白云岩化作用有关;同时,部分砂屑白云岩的Sr含量异常也揭示了其叠加了埋藏白云岩化作用,白云岩化流体来自其上覆和/或下伏泥晶-粉晶白云岩中的富Mg2+卤水。

The Jialingjiang Formation of Lower Triassic in the southwestern Hubei Province consists primarily of carbonate rocks characterized by limestone interbedded with dolomite： According to field outcrop investigation and laboratory analysis, the dolomite has been divided into micritic dolomite and doloarenite respectively. Based on the study of petrological characteristics, combined with the research of cathodoluminescence and geochemical properties of trace elements, carbon and oxygen isotopes, the genesis of dolomitization in the Jialingjiang Formation is thoroughly investigated. It is considered that the genesis of micritic dolomite is related to penecontemporaneous dolomitization characterized by birdseyes, fenestral fabrics, gypsum pseudomorph,lower ordering degree and red or purplish red cathodeluminescenee. While the genesis of doloarenite is related to mixed water dolomitization characterized by higher ordering degree,lower value of Na and Mn/Sr, higher value of Mn and Fe, relatively negative δ^18O value, positive δ^13C value and purplish red cathodeluminescence. Meanwhile, the abnormity of Sr reveals that burial dolomitization is the succession of mixed water dolomitization,and the dolomitizing fluids come from the Mg^2＋- riched brine of micritic dolomite.