Early dolomitisation of the Lower-Middle Ordovician cyclic carbonates in northern Tarim Basin, NW China

High-frequency metre-scale cycles are present within the Lower-Middle Ordovician carbonate successions in northern Tarim Basin, NW China. These metre-scale cycles were variably dolomitised from top to bottom. Three types of replacive dolomites were recognised, including dololaminite（very finely to finely crystalline, planar-s to nonplanar-a dolomite;type-1）, patterned dolomite（finely crystalline, planar-s dolomite; type-2）, and mottled dolomite（finely to medium crystalline,nonplanar-a（s） dolomite; type-3）. Petrographic evidence indicate these dolomites were primarily deposited in supratidal to restricted subtidal environments, and formed in near-surface to shallow burial realms. Geochemically, all types of dolomites have similar δ^13C and ^87Sr/^86 Sr ratios comparable to calcite precipitated in equilibrium with the Early-Middle Ordovician seawater. These geochemical attributes indicate that these dolomites were genetically associated and likely formed from connate seawater-derived brines. Of these, type-1 dolomite has δ^18O values（.4.97‰ to.4.04‰ VPDB） slightly higher than those of normal seawater dolomite of the Early-Middle Ordovician age. Considering the absence of associated evaporites within type-1 dolomite, its parental fluids were likely represented by slightly evaporated（i.e., mesosaline to penesaline） seawater with salinity below that of gypsum precipitation. More depleted δ^18O values（.7.74‰ to.5.20‰ VPDB） of type-2 dolomite and its stratigraphic position below type-1 dolomite indicate the generation of this dolomite from mesosaline to penesaline brines at higher temperatures in near-surface to shallow burial domains. Type-3 dolomite yields the most depleted δ^18O values（–9.30‰to –7.28‰ VPDB）, pointing to that it was most likely formed from coeval seawater-derived brines at highest temperatures in a shallow burial setting. There is a downward decreasing trend in δ^18O values from type-1 through type-2 to type-3 dolomites, and in abundance of dolomites, indicating that the dolomitising fluids probably migrated downward from above and persisted into shallow burial conditions.