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- Title
Early dolomitisation of the Lower-Middle Ordovician cyclic carbonates in northern Tarim Basin, NW China.
- Authors
Guo, Chuan; Chen, DaiZhao; Dong, ShaoFeng; Qian, YiXiong; Liu, CunGe
- Abstract
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 C and Sr/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 O 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 O 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 O 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 O 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.
- Subjects
TARIM Basin (China); DOLOMITE; ORDOVICIAN stratigraphic geology; CARBONATES in soils; GEOCHEMISTRY
- Publication
SCIENCE CHINA Earth Sciences, 2017, Vol 60, Issue 7, p1283
- ISSN
1674-7313
- Publication type
Article
- DOI
10.1007/s11430-017-9056-1