Origin of dolomitization in the Lower Cretaceous Bayanhua Group dolomitized rocks, Bayindulan Sag, Erlian Basin

The Bayindulan Sag situated in Erlian Basin, northeast China, is characterized by pervasive dolomitization in Upper A4 Member to Lower T1 Member of Bayanhua Group. These stratum in the Lower Cretaceous Bayanhua Group were deposited in a lacustrine environments. The main objective of this study is to identify the origin of dolomitization and distribution model of dolomitized rocks. Based on petrographic examination, mud-crystal dolomites (<0.03 mm), silt-crystal dolomites (0.03–0.1 mm) and fine-crystal dolomites (0.1–0.25 mm) have been observed, and the former two types were predominant. Meanwhile, only one type of dolomite have been defined: replacive dolomite. A variety of dolomite textures have also been observed, the major ones of them are scattered, blocky, joined and annular textures. These dolomites have strongly depleted oxygen isotopic values from–17.48‰ to–6.96‰ with the average of–10.48‰ (PDB), carbon isotopic values from 0.24‰ to 8.37‰ with the average of 5.39‰ (PDB). The obvious depletion of oxygen isotopic was presented as a result of the increase in temperature with progressive burial, and the high carbon isotopic content of dolomites maybe result from the involvement of CO 2 in the dolomitization process. These CO 2 were generated by bio-methanation which commonly took place in moderately deep burial environments. The Sr values from 62.4 ppm to 262 ppm as well as the Sr/Ba ratios from 0.24 to 0.76 in Tengyi Formation mudstones indicate the freshwater lacustrine sedimentary environments. Taking the petrographic and geochemical data into consideration, the conclusion that the dolomites have formed in a freshwater environments during deep burial depths at temperatures of at least 60 °C can be drawn. Petrographic observations and mineralogy analyses indicate that dolomitization is closely associated with tuffaceous materials, the Mg needed for the dolomitization is provided by dissolution of tuffaceous materials and dolomitization is most probably driven by tuffaceous materials alteration at elevated temperatures.