摘要
The carbonates in the Middle Ordovician Ma_5～5submember of the Majiagou Formation in the northern Ordos Basin are partially to completely dolomitized.Two types of replacive dolomite are distinguished：（1） type 1dolomite,which is primarily characterized by microcrystalline（〈30 urn）,euhedral to subhedral dolomite crystals,and is generally laminated and associated with gypsumbearing microcrystalline dolomite,and（2） type 2 dolomite,which is composed primarily of finely crystalline（30-100 urn）,regular crystal plane,euhedral to subhedral dolomite.The type 2 dolomite crystals are truncated by stylolites,indicating that the type 2 dolomite most likely predated or developed simultaneously with the formation of the stylolites.Stratigraphic,petrographic,and geochemical data indicate that the type 1 dolomite formed from near-surface,low-temperature,and slightly evaporated seawater and that the dolomitizing fluids may have been driven by density differences and elevation-related hydraulic head.The absence of massive depositional evaporites in the dolomitized intervals indicates that dolomitization was driven by the reflux of slightly evaporated seawater.The δ～（18）O values（-7.5 to-6.1 ‰） of type1 dolomite are slightly lower than those of seawaterderived dolomite,suggesting that the dolomite may be related to the recrystallization of dolomite at higher temperatures during burial.The type 2 dolomite has lowerδ～（18）O values（-8.5 to-6.7 ‰） and Sr～（2＋） concentration and slightly higher Na～＋,Fe～（2＋）,and Mn～（2＋） concentrations and～（87）Sr/～（86）Sr ratios（0.709188-0.709485） than type 1 dolomite,suggesting that the type 2 dolomite precipitated from modified seawater and dolomitic fluids in pore water and that it developed at slightly higher temperatures as a result of shallow burial.
The carbonates in the Middle Ordovician Ma_5～5submember of the Majiagou Formation in the northern Ordos Basin are partially to completely dolomitized.Two types of replacive dolomite are distinguished：（1） type 1dolomite,which is primarily characterized by microcrystalline（〈30 urn）,euhedral to subhedral dolomite crystals,and is generally laminated and associated with gypsumbearing microcrystalline dolomite,and（2） type 2 dolomite,which is composed primarily of finely crystalline（30-100 urn）,regular crystal plane,euhedral to subhedral dolomite.The type 2 dolomite crystals are truncated by stylolites,indicating that the type 2 dolomite most likely predated or developed simultaneously with the formation of the stylolites.Stratigraphic,petrographic,and geochemical data indicate that the type 1 dolomite formed from near-surface,low-temperature,and slightly evaporated seawater and that the dolomitizing fluids may have been driven by density differences and elevation-related hydraulic head.The absence of massive depositional evaporites in the dolomitized intervals indicates that dolomitization was driven by the reflux of slightly evaporated seawater.The δ～（18）O values（-7.5 to-6.1 ‰） of type1 dolomite are slightly lower than those of seawaterderived dolomite,suggesting that the dolomite may be related to the recrystallization of dolomite at higher temperatures during burial.The type 2 dolomite has lowerδ～（18）O values（-8.5 to-6.7 ‰） and Sr～（2＋） concentration and slightly higher Na～＋,Fe～（2＋）,and Mn～（2＋） concentrations and～（87）Sr/～（86）Sr ratios（0.709188-0.709485） than type 1 dolomite,suggesting that the type 2 dolomite precipitated from modified seawater and dolomitic fluids in pore water and that it developed at slightly higher temperatures as a result of shallow burial.
基金
supported by the Major National Science and Technology Projects of China (Grant No. 2011ZX05045)
Sinopec (Grant No. 34550000-13-FW0403-0010)
