The Biyang Depression, lying in the eastern Nanxiang Basin, has a dustpan-shape with faults developed to the southwest and southeast. The Paleogene Hetaoyuan Formation in this depression hosts one of the oldest soda-d...The Biyang Depression, lying in the eastern Nanxiang Basin, has a dustpan-shape with faults developed to the southwest and southeast. The Paleogene Hetaoyuan Formation in this depression hosts one of the oldest soda-deposits, the Anpeng deposit. These sodium carbonate-bearing strata consist of lutites, muddy dolomites, and bedded soda-deposits, which are dominated by nahcolite with little trona, mirabilite, and halite. In the top Unit 3 and lower Unit 2 of Hetaoyuan Formation where the sodium carbonates are concentrated, dolomitic shale, muddy carbonate, and sodium carbonate can be separated based on their distinct A1203, TiO〉 Na20, MgO＋CaO, and LOI （loss on ignition） contents. By using Na20/（MgO＋CaO） ratio as an indicator for the relative concentration of Na carbonates to Mg-Ca carbonates, the muddy dolomites can be classified into two groups. One group with Na20/（MgO＋CaO）〈0.6 exhibits a positive correlation between this ratio and AI203＋TiO2 content, an proxy for ter- rigenous clastic input and the other group with Na20/（MgO＋CaO）〉0.6 presents a negative correlation. As the clastic propor- tion decreases, the former indicates that it tends to form pure Mg-Ca carbonates reflecting evaporation less than or equal to re- plenishment for the lake water in a relative humid climate, and the latter reveals a trend to form sodium carbonates with lake water＇s evaporation more than replenishment in a relatively dry climate. From muddy dolomite to sodium carbonate samples, with Na20/（MgO＋CaO） ratio increasing, the A1 normalized Zr, Ti, La, K, Rb, and Ba tend to be enriched and the post-Archean Average Shale （PAAS） normalized positive Eu anomaly becomes more obvious. Integrated with the high B contents and the report of searlesite in the bedded sodium carbonates and the regional extensional tectonic regime, the above geochemical characteristics were interpreted to signify hydrothermal contribution to forming these soda-deposits through the deep fault systems of basin margins.展开更多
基金supported by National Natural Science Foundation of China(Grant Nos.40672080,40921062,41302083)Fundamental Research Funds for the Central Universities,China University of Geosciences(Wuhan)(CUG120807)
文摘The Biyang Depression, lying in the eastern Nanxiang Basin, has a dustpan-shape with faults developed to the southwest and southeast. The Paleogene Hetaoyuan Formation in this depression hosts one of the oldest soda-deposits, the Anpeng deposit. These sodium carbonate-bearing strata consist of lutites, muddy dolomites, and bedded soda-deposits, which are dominated by nahcolite with little trona, mirabilite, and halite. In the top Unit 3 and lower Unit 2 of Hetaoyuan Formation where the sodium carbonates are concentrated, dolomitic shale, muddy carbonate, and sodium carbonate can be separated based on their distinct A1203, TiO〉 Na20, MgO＋CaO, and LOI （loss on ignition） contents. By using Na20/（MgO＋CaO） ratio as an indicator for the relative concentration of Na carbonates to Mg-Ca carbonates, the muddy dolomites can be classified into two groups. One group with Na20/（MgO＋CaO）〈0.6 exhibits a positive correlation between this ratio and AI203＋TiO2 content, an proxy for ter- rigenous clastic input and the other group with Na20/（MgO＋CaO）〉0.6 presents a negative correlation. As the clastic propor- tion decreases, the former indicates that it tends to form pure Mg-Ca carbonates reflecting evaporation less than or equal to re- plenishment for the lake water in a relative humid climate, and the latter reveals a trend to form sodium carbonates with lake water＇s evaporation more than replenishment in a relatively dry climate. From muddy dolomite to sodium carbonate samples, with Na20/（MgO＋CaO） ratio increasing, the A1 normalized Zr, Ti, La, K, Rb, and Ba tend to be enriched and the post-Archean Average Shale （PAAS） normalized positive Eu anomaly becomes more obvious. Integrated with the high B contents and the report of searlesite in the bedded sodium carbonates and the regional extensional tectonic regime, the above geochemical characteristics were interpreted to signify hydrothermal contribution to forming these soda-deposits through the deep fault systems of basin margins.
