Qing LI^1,2, Zaixing JIANG¹, Wenxuan HU³ & Xuelian YOU^4
1School of Energy Resources, China University of Geosciences, Beijing 100083, PR China, e-mail: tsinglee2659@gmail.com
²Department of Geosciences, The Pennsylvania State University, University Park, PA 16802, USA,
e-mail: qul15@psu.edu
³State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, PR China, e-mail: huwx@nju.edu.cn
⁴School of Ocean Sciences, China University of Geosciences, Beijing 100083, PR China, e-mail: youxuelian007@

ORIGIN OF DOLOMITE IN THE MIDDLE TRIASSIC ZHOUCHONGCUN FORMATION, CENTRAL LOWER YANGTZE REGION, SOUTHEAST CHINA

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The Middle Triassic Zhouchongcun Formation (correlated to the Anisian stage by global standards) in the Lower Yangtze region is mainly composed of limestones that have been partially to completely dolomitized. However, the origin of these dolomites and the nature of dolomitization fluid have not been well documented. The Geshan section, a typical section dealing with the Middle Triassic dolomite of the Lower Yangtze region, was chosen as the research object in this study. The dolomite in the Zhouchongcun Formation from Geshan section commonly displays planar-e texture and locally planar-s texture with crystal sizes up to 80μm. These dolomites commonly contain residual calcites inclusions in their crystals and the development of dolomite is associated with stylolites, indicating that this dolomite formed by replacement and the dolomitization occurred at burial depth of more than 500m. The dolomites have a narrow range of δ¹³C values (from 5.4‰ to 7.2‰, V-PDB), which are partly overlapped with the theoretical δ¹³C value of seawater dolomite and show slightly enrichment. The narrow range and positive δ¹³C values suggest that the influence of fluid with isotopically light δ¹³C was negligible in the dolomitization. The large overlap of ⁸⁷Sr/⁸⁶Sr ratios of the dolomites with the coeval Triassic seawater suggests that the dolomitizing fluids may have derived from modified Triassic seawater preserved in formations. In comparison with Triassic seawater dolomite, the Geshan dolomite has depleted of δ¹⁸O values (-8.4‰ to -6.5‰, V-PDB) and low Sr but high Fe, Mn concentration and Ce/Ce* ratios, suggesting that dolomitization occurred in an anoxic subsurface environment with increasing temperature during compaction at shallow to intermediated burial. The Ce/Ce* ratio and ƩREE+Y contents of Zhouchongcun carbonate increase with depth, reflecting that the environment becoming more anoxic and having higher temperature with the deeper burial depth. Based on the burial history of the Lower Yangtze region, the dolomitization most likely occurred during the Late Triassic period.