Dolomitization Facilitated by Clay Minerals on Mixed Siliciclastic-Carbonate Shoals of Carboniferous Age in the Tarim Basin, China: Constraints on Element Mobility and Isotope Geochemistry

In the western Tarim Basin, Carboniferous granular dolostones deposited on a carbonate platform contain a small amount of terrigenous materials of sand-size fraction, agglomerated clay minerals, or similar phases. However, the role of terrigenous materials on dolomitization is still unclear. The aim of this study was to reveal the dolomitization mechanism. The granular dolomites have small crystal size, earthy yellow color, and fabric-retentive texture, with relatively good order. These features indicate dolomites precipitated during early diagenesis. The ratio of rare earth elements (RREs) abundance of the stable isotopes 87Sr/86Sr relative to Post-Archean Australian Shale (PAAS) normalized patterns was used to study the source of the dolomitizing fluids. The composition of REEs is characterized by heavy rare earth (HREE) enrichment (average NdSN/YbSN = 0.83). There is a positive (La/La*)SN anomaly and slightly positive (Gd/Gd*)SN and (Y/Y*)SN anomaly; δ18O of seawater in fractionation equilibrium with granular dolostones was from −2.8‰ to 1.7‰ PDB, implying the dolomitizing fluid was contemporary, slightly evaporated seawater. The granular dolostones on the relatively thick shoals were subject to subaerial exposure before pervasive dolomitization, with evidence that the input of detrital kaolinite predated the formation of dolomites. Higher 87Sr/86Sr values and ∑REE in granular dolostones than the values in equivalent limestones indicate that dolomitization was related to terrigenous materials. Within the terrigenous materials, the negative-charged clay minerals may have catalyzed the dolomitization, resulting in dramatically decreased induction time for precipitation of proto-dolomites. A greater amount of terrigenous materials occurred on the shoals at the sea level fall, resulting from enhanced river entrenchment and downcutting. As a result, after subaerial exposure, the penesaline water flow through the limy allochems sediments lead to dolomitization, with the catalysis of illite on relatively thick shoals.