ORIGIN OF DOLOMITE IN THE MIDDLE TRIASSIC ZHOUCHONGCUN FORMATION, CENTRAL LOWER YANGTZE REGION, SOUTHEAST CHINA
Qing LI1,2, Zaixing JIANG1, Wenxuan HU3 & Xuelian YOU4
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@
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Abstract
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.
Keywords:
- Dolomite
- isotope
- trace
- elements
- REE
- seawater
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© 2015 by the author(s). Licensee CJEES, Carpathian Association of Environment and Earth Sciences. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
How to cite
Qing LI, Zaixing JIANG, Wenxuan HU & Xuelian YOU (2015). ORIGIN OF DOLOMITE IN THE MIDDLE TRIASSIC ZHOUCHONGCUN FORMATION, CENTRAL LOWER YANGTZE REGION, SOUTHEAST CHINA
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