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Trace elements and stable isotopic geochemistry of an Early Cambrian chert-phosphorite unit from the lower Yurtus Formation of the Sugetbrak section in the Tarim Basin |
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| Authors: | ChunYan Yao DongSheng Ma HaiFeng Ding XiaoYong Zhang Hui Huang |
| Institution: | 1. School of Earth Sciences and Engineering, State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing, 210093, China 2. Nanjing Center, China Geological Survey, Nanjing, 210016, China 3. Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing, 100094, China 4. Jiangsu Environment Monitoring Center, Nanjing, 210036, China 5. Guangdong Provincial Key Laboratory of Geological Processes and Mineral Resource Exploration, Department of Earth Sciences, Sun Yat-sen University, Guangzhou, 510275, China |
| Abstract: | A chert-phosphorite unit from the Sugetbrak section in the Tarim Basin was analyzed for rare earth elements (Ce, Eu), redox sensitive proxies, and carbon isotopic compositions (δ13Ccarb and δ 13Corg) in the lower Yurtus Formation of the Early Cambrian period. Redox sensitive element ratios (Th/U, V/Cr, Ni/Co, and V/Sc) were employed to determine the palaeoenvironmental conditions during this period. The ratios indicated that the depositional environment of the chert-phosphorite-black shale unit ranged from suboxic to anoxic. Negative Ce and positive Eu anomalies in the chert-phosphorite assemblages of the studied Yurtus Formation indicated the existence of a redox-stratified ocean, similar to that of South China. Overturn or upward expansion of the deep water-mass probably reached the shallow marine zone after the formation of the Yurtus phosphorites. The characteristics of the negative Ce anomaly may be due to phosphoritic inheritance from the Ce-depleted signature of the overlying water column. Subsequent hydrothermal inputs and reduced detrital supplies of the deep water caused by the upwelling affected certain redox sensitive elements in the sedimentary basin. δ 13Ccarb and δ13Corg negative excursions in the Yurtus chert-phosphorite unit may be related to a transgression phase when episodic basinal upwelling moved 12C- and P-rich waters from the pelagic basin floor to the continental shelf. Although carbon isotopic compositions in the Yurtus chert-phosphorite assemblages may have suffered from diagenetic alteration, they can be used to probe diagenetic conditions. Multi-proxy geochemical studies indicated that the δ 13Ccarb values of the Yurtus chert-phosphorite assemblages might be considered reflections of a predominantly suboxic environment that was subsequently affected by hydrothermal inputs due to the upwelling. |
| Keywords: | redox sensitive element ratios carbon isotope sedimentary environment Early Cambrian Tarim Basin |
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