Genesis of the Massive Ordovician Dolostones in the Ordos Basin, North China: Evidence from Inclusions

Massive dolostones are well developed and can be classified into fine saccharoidal dolostones and coarse saccharoidal dolostones in the Ordovician of the Ordos Basin, North China. An analysis of the inclusion reveals information for the study of the massive dolostones' genesis. The majority of thin sections show that the inclusions are poorly developed in the host crystals of the fine dolomites, but are well-developed in the pore-filling calcite crystals. The homogenization temperatures of the inclusions range from 49°C to 74°C, and the maximum depth of the inclusion formation calculated from such temperatures is about 172m. Thus the host crystals of the fine saccharoidal dolostones have been formed in shallow burial environments with a depth of about 172m. On the other hand, the inclusions are well -developed in the host crystals of the coarse saccharoidal dolostones. Analytical data of Raman spectra show that, among the inclusions, there is a great deal of methane and other organic material, suggesting that the inclusions must have been formed during the dry gas production stage of the organic material evolution in deep burial environments. The lowest homogenization temperature of the inclusion is 104°C (with pressure correction), and the least depth for the inclusion formation corresponding to such a temperature is about 2600m. Hence, it is believed that the coarse saccharoidal dolostones must have been formed in deep burial environments with hot liquid dolomitization. Therefore, the inclusion analyses bring out the conclusion that there are at least two types of dolomitization in the massive dolostones of the Ordovician of the Ordos basin, one of shallow burial with warm liquid, and the other deep with hot liquid.