平行连晶状白云石的发现及其成因探讨

THE DISCOVERY OF PARALLEL GROUP DOLOMITE AND A STUDY OF IT'S GENESIS

四川盆地下三叠统嘉陵江组第四段地层中,广泛发育着一种呈单向延伸的复品白云石,作者在扫描电镜下首次发现这是一种尚未有人报道过的白云石平行连晶体,其连生方向与每个白云石单晶的三次对称轴L^3相垂直。文中详细地研究了这种白云石的矿物学特征、赋存岩相和生成时间,认为它的发育环境为与盐化泻湖毗邻的碳酸盐浅滩,共成因与泻湖底郎Mg/Ca及CO_3/Ca比值高的卤水经由浅滩向海回流,从而引起白云石晶体周期性快速生长有关。

<ABSTRACT> A kind of compound crystal dolomite, which emerges in one-way elongation, occurs widely in the dolostone in the fourth Member of Jialingjiang Formation ( Lower Triassic ) in Sichuan Basin. With a scanning electron microscope, the authors discovered that it is a kind of parallel group dolomite. So far, this kind of dolomite has not heen reported in literature. The growth direction of this dolomite is vertical to uc' axis of each dolomite single crystal. The dolomite has relatively high content of MgCO3 gram molecule chemically and ordered degree in structure. It can occur in seven lithofacies, but tends to assemble in the dolostones rich in grains ( such as oolitic dolostone and bioclast pack-dolostone ). The forming time of this dolomite was in its early diagenetic stage. It might grow freely in the rocks ( or sediments ) rich in effective pore, but not in pseudocrystal after some acicular minerals ( e. g. gypsum or aragonite) as considered by some researchers. The developed environment of this dolomite was a carbonate shoal adjecent to a salt lagoon. The genesis of the dolomite was in relation to reflux brine, of which the Mg / Ca and CO3 / Ca ratios were fairly high in the bottom of lagoon, reflowing through the shoal toward sea and resulting inperiodic and rapid growth of dolomite crystal.