GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 200-3
Presentation Time: 9:00 AM-6:30 PM


JIAO, Xin1, LIU, Yiqun2, YANG, Wan3, ZHAO, Minru1 and LI, Zhexuan1, (1)Department of Geology, Northwest University, No.229, North Taibai Street, Xi'an, Shaanxi, 710069, China, (2)Department of Geology, Northwest University, No.229 Taibai North Road Xi’an, Shaanxi, 710069, China, (3)Geology and Geophysics Program, Missouri University of Science and Technology, Rolla, MO 65409

The origin of dolomite has been a mystery for many years. Here we propose a possible primary origin of dolomite related to microbial and hydrothermal activities. The dolomite occurs in middle-Permian Lucaogou Fm., Santanghu Basin, NW China, which is an intracontinental rift basin. The formation contains lacustrine black fine-grained sedimentary rocks, including mainly tuffaceous mudstones and dolostones, which are the main source rocks in NW China. The dolostones are mm-laminated, and intercalated with tuffaceous shale and composed mainly of dolomite (77% on average), quartz (14%), alkaline feldspars (3%), and little analcimes and pyrites. Dolomite crystals are commonly less than 3 μm in size, mainly anhedral to subhedral with a low degree of stoichiometry. The silicate mineral grains are angular, shard-like, bedding parallel, and show normal grading in dolomite-dominated laminae, indicating a volcanic origin during deposition and precipitation. Unique nanoscale filamentary minerals and build-up structures were observed at intercrystalline pores of dolomite and laminae of dolostones in some samples, respectively. And δ13CPDB (5.2‰ to 9.9‰, 6.9‰ on average) of dolomite suggests the formation of dolomites was affected by the generation of bacterial participating methane. δ13OPDB (-1‰ to -17.4‰, -7.5‰ on average) values of dolostones composed of subhedral dolomite are more negative than those of anhedral ones, which indicate recrystallization by hydrothermal activities. The whole-rock strontium isotope values (0.70462 to 0.70525, 0.70501 on average) and δ26Mg (-0.89 to -0.24, -0.52 on average) values indicate that the sedimentary fluids may be derived from mantle. The dolostone is a rare type of microbially-influenced dolostone related to hydrothermal and volcanic activities. Repetitive volcanic-hydrothermal activities raised the temperature unevenly on the lake floor, which broke the threshold dynamic barrier temperature (~50℃). Hydrothermal fluids which reacted with mantle-originated magma and wall rocks provided abundant Mg2+. Thermophilic microbes near the hydrothermal vents multiplied rapidly, generating a favorable chemical condition for dolomite precipitation.