Origin of dolomite in the Middle Ordovician peritidal platform carbonates in the northern Ordos Basin, western China

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.