08 第7 白云岩和白云化作用(1).ppt

第七章碳酸盐岩岩石学第六节白云岩的类型与成因(Classification and genesis of Dolomite)
第六节白云岩的类型与成因
主要由白云石(>50%)组成的沉积碳酸盐岩,叫白云岩。
白云岩的分类
1、根据成因:原生白云岩、次生白云岩
2、根据白云化程度:
1)有残余结构:残余鲕粒白云岩、残余生屑白云岩等
2)无残余结构:巨晶、粗晶、中晶、细晶、粉晶、微晶白云岩。
第六节白云岩的类型与成因
一、白云岩的分类
对石灰岩的分类原则基本适用于白云岩。
①只需将石灰岩结构分类表中的“石灰岩”改为“白云岩”,“灰泥”改为“云泥”
②白云岩中晶粒结构发育,并且晶粒较粗,多呈自形或半自形晶,集合体常呈砂糖状
③白云岩中交代结构发育
Euhedral, ferroan dolomite , replacement of a limestone. Note the consistently more ferroan cores (darker stain) and less ferroan rims. There is extensive solid solution along the line from pure dolomite to ferrodolomite. Ferroan dolomites and ankerite thus mon precipitates.
An example of largely euhedral dolomite where the crystals have dark cores and limpid rims. This is an mon fabric in dolomites. The cloudy cores have been interpreted to reflect mixing zone conditions in which metastable, inclusion-rich dolomite formed. A shift to more marine conditions led to precipitation of the more inclusion-free, limpid dolomite outer zones that may, in part, be cements.
Replacement dolomite crystals, with cloudy ferroan cores and clear, less ferroan rims, in a wackestone sediment. Although most of the dolomite crystals are euhedral, they have considerable variation in crystal sizes (polymodal). Many of these crystals formed by overgrowth and replacement of fossil fragments (the largest is a replacement of an echinoid fragment).
A dolomitization front in a micritic limestone — dolomitization plete on the right side and sparse on the left. In this case, the presence of permeable burrows led to highly heterogeneous replacement patterns as dolomitizing fl uids moved preferentially through the burrows and altered material within, and directly adjacent to, those fluid conduits.
A stained example showing euhedral dolomite crystals in which the cloudy, inclusion-rich cores were dissolved and the resultant pore spaces were fi lled by calcite cement (red). Note the multicrystalline nature of the calcite fi lls within many of the rhombic dolomolds.
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