鄂尔多斯盆地奥陶系马家沟群块状白云岩的包体研究

Inclusion Study of Massive Dolostones of Ordovician Majiagou Group in Ordos Basin

鄂尔多斯盆地奥陶系马家沟群的块状白云岩，按晶粒结构可分为细砂糖状白云岩（晶粒大小为０．０１—０．１ｍｍ）和粗砂糖状白云岩（晶粒大小为０．０５—０．２５ｍｍ）。包体分析为块状白云岩的成因研究提供了最直接的信息。细砂糖状白云岩的主体白云石中液态包体不发育，但孔隙充填方解石中的包体发育，由２４个孔隙充填方解石中的包体测出的均一温度为４９—７４℃，测算的包体形成深度的最大值为１７２ｍ。由此可以推断，其主体白云石应形成于埋深小于１７２ｍ的浅埋藏环境。粗砂糖状白云岩的主体白云石中液态包体发育，激光拉曼光谱分析测得的包体成分中普遍含有甲烷（ＣＨ４）等有机成分，说明这类包体形成于深埋藏的有机质干气演化阶段。所测的包体均一温度的最低值为１０４℃（未经压力校正），由此测算的包体形成深度的最小值为２６００ｍ。由此可见，粗砂糖状白云岩形成于埋深不小于２６００ｍ的深埋藏热水白云化环境。

Massive dolostones of the Ordovician Majiagou Group can be grouped into fine saccharoidal dolostones and coarse saccharoidal dolostones. An analysis of inclusions offers the most direct information for the study of origin of the massive dolostones. Inclusions are poorly developed in the host dolomites of fine asccharoidal dolostones, but are well developed in pore filling calcite crystals. The homogenization temperatures of these inclusions (24 samples) range from 49℃ to 74℃,and the maximum depth of inclusion formation calculated from such temperatures is 172m. It can thus be inferred that the host dolomites of fine saccharoidal dolostones must have been formed in shallow burial environments with the depth of less than 172m. Inclusions are well developed in the host dolomites of coarse saccharoidal dolostones. Analytical data of Raman Spectra show that these inclusions are commonly methane bearing ones, suggesting that they must have been formed in deep burial environments. The lowest homogenization temperature of these inclusions is 104℃(with no pressure correction). and the least depth for inclusion formation corresponding with such a temperature is about 2600m. It is hence believed that the coarse saccharoidal dolostones must have been formed in deep burial environments with hot water dolomitization.