MICROBIAL DOLOSTONES RELATED TO HYDROTHERMAL-VOLCANIC ACTIVITIES IN PERMIAN LACUSTRINE ENVIRONMENT, SANTANGHU BASIN, NW CHINA

The "Dolomite Problem" has been a hot and confusing research for more than 200 years. Laminated dolostones occuring in the Santanghu Basin, NW China, may provide a new research target. The Permian Lucaogou Fm. shows that the basin was a starved and deep lacustrine intracontinental rift basin containing mainly hydroclastic and carbonate sedimentations. 556 Samples from 30 cores were studied using microscope, CL, and SEM microscopy, microprobe and XRD analyses. Element composition, Sr, C and O isotopic compositions were analyzed. Four main features were observed. 1) The dolostones occur as fine mm-laminae, which are intercalated within tuffaceous shale and composed mainly of dolomite (70% on average), quartz (19.4%), alkaline feldspars (5.1%) and little analcime and pyrite. 2) The dolomites have a uniform grain size mostly less than 5 micron, mainly anhedral to subhedral, low degree of order. Special nm-size spheres and filamentary minerals were observed on the surface and in intercrystalline pores in some samples. 3) δ¹³C_PDB (5.2-9.9‰, 6.9‰ on average) may suggest the forming of dolomites may be affected by generation of bacterial participating methane. δ¹³O_PDB (-1 to -17.4‰, -7.5‰ on average), and the values of dolostones rich with subhedral dolomite are more negative than anhedral ones, indicating recrystallization by hydrothermal fluids. 4) Whole-rock strontium isotope ratios (0.70466 to 0.70538, 0.70514 on average) indicate that the lake water may be mixed with mantle-derived fluids. The dolostone is interpreted as a rare type of microbial dolostones related to hydrothermal-volcanic activities for four reasons: 1) The crystal shape, isotopic value and low degree of order are similar to those of modern microbial dolomite. 2) Repetitive volcanic-hydrothermal activities may have increased the temperature of lake floor unevenly, which broke through the dynamic barrier temperature (~ 50 ^oC) for dolomite precipitation. 3) Hydrothermal fluids formed by groundwater reaction with mantle-originated magma and wall rocks provided abundant Mg²⁺, Ca²⁺ and CO^2- for dolomite formation. 4) Thermophilic microbes near hydrothermal vents may have multiplied rapidly, creating a suitable chemical condition. The results shed lights into the environmental conditions in ancient hydrothermal and volcanic rift basins.