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

The carbonates in the Middle Ordovician Ma(5)(5) submember of the Majiagou Formation in the northern Ordos Basin are partially to completely dolomitized. Two types of replacive dolomite are distinguished: (1) type 1 dolomite, which is primarily characterized by microcrystalline (<30 mu m), euhedral to subhedral dolomite crystals, and is generally laminated and associated with gypsum-bearing microcrystalline dolomite, and (2) type 2 dolomite, which is composed primarily of finely crystalline (30-100 mu m), 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 delta O-18 values (-7.5 to -6.1 parts per thousand) of type 1 dolomite are slightly lower than those of seawater-derived dolomite, suggesting that the dolomite may be related to the recrystallization of dolomite at higher temperatures during burial. The type 2 dolomite has lower delta O-18 values (-8.5 to -6.7 parts per thousand) and Sr2+ concentration and slightly higher Na+, Fe2+, and Mn2+ concentrations and Sr-87/Sr-86 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.