Geochemical Constraints on the Hydrothermal Chert of the Kuhfeng Formation in the Middle Permian in the Lower Yangtze and Its Significance
[Objective]The organic-enriched black bedded cherts in the Kuhfeng Formation were deposited in the Lower Yangtze during the Middle Permian.The hydrothermal cherts may be the key part for the relationship analysis between chert deposits and the extra-ordinarily high organic matter enrichment.This would be the basic theories for the shale gas exploration in this area.[Methods]This work uses major,minor,and rare-earth elements,combined with the petrology,to analyze the origin of chert and their tectonic setting for the Kuhfeng Formation in the Yeshan of Tongling and Pingdingshan of Chaohu,Anhui province.[Results]The black bedded cherts in the Chaohu area con-tain abundant siliceous sponge spicules and radiolarians.The cinerous cherts in the Kuhfeng Formation in the Tongling area contain extremely rare siliceous fossils and consist of micro-quartz.Hydrothermal proxies,such as Al/(Al+Fe+Mn),Eu/Eu*,Al-Fe-Mn diagram,and LuN/LaN,indicate a hydrothermal and biotic origin for the chert in the Kuhfeng Formation in the Tongling and Chaohu areas,respectively.The La-Th-Sc diagram and crossplot between La/Sc and Ti/Zr indicate oceanic island arc and active continental margin origins for the Tongling and Chaohu areas.[Conclusions]The chert in the Kuhfeng Formation in the Tongling area was primary deposited by hydrothermal ac-tivity.The silicic acid was brought by a hydrothermal vent near the slope environments.The black bedded cherts in the Lower Yangtze area are biotic in origin,but the source of silicon was related to the hydrothermal activity.The or-ganic matter enrichment in the Kuhfeng Formation in the Lower Yangtze area may be related to the phytoplankton flourishment induced by the nutrient element input from hydrothermal activity.
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