Geochemistry of the dolomitic rocks from the Permian Lucaogou Formation in the Jimusar depression, Junggar Basin, Xinjiang

The dolomitic rocks from the Permian Lucaogou Formation in the Jimusar depression, Junggar Basin, Xinjiang mainly consist of micritic dolostone, tuffaceous dolostone and dolomitic tuff. In the present paper, the highlights are concentrated in the geochemical signatures, provenance and sedimentary environment of the dolomitic rocks widespread in the study area. These dolomitic rocks have P contents ranging from 262×10^-6 to 17283×10^-6 (with an average of 427.70×10^-6, very close to that of North America Shale Composite), c(Mg)/c(Ca) ratios of 0.03-1.44, Th/U ratios of 0.58-2.00, V/(V+Ni) ratios of 0.60-0.89, V/Cr ratios of 1.25-6.38, V/Sc ratios of 3.65-16.19, and Sr/Ba ratios of 0.55-8.25. The analytical results outlined above suggest that the dolomitic rocks in the Lucaogou Formation were formed in the salt-water reducing lake environments with abundant organisms, and magnesium ions are inferred mainly to be originated from deep-seated magmatic hydrothermal solutions. The carbon and oxygen isotopic data also demonstrate that the Lucaogou Formation was laid down in the continental salt-water lake environments with higher temperatures. There is a tendency of gradual increase of temperatures and gradual decrease of salinities from the early to later stages of the deposition of the Lucaogou Formation. However, there is no evidence in favour of salinity increase in the highly-evaporated lake water due to temperature increase. The abnormal changes of the above-mentioned palaeotemperatures and palaeosalinities based on carbon and oxygen isotopic data of the carbonate minerals may be caused by the input processes of allochthonous fresh water in the later stages of the deposition of the Lucaogou Formation.