The Discovery Of K-bentonite At Early Nanhuan Period In South Jiangxi Area And Its Tectonic Significance

The Early Nanhua Shangshi Formation in South Jiangxi area(in western Catheaysia block)contains several clay beds of milk white and kelly color.The results of systematic geochemical and chronological study showed that all of them had the properties of potassium rich(K2O>3.5%)which belonged to the sequence of the K-bentonites.LA-ICP MS U-Pb dating on zircons has yielded three sets of ages:(756.0±7.5)Ma.(774.3±8.5)Ma and(774.1±8.8)Ma.All of them are the result of the magmatic activity of the early Nanhua.Analysis on trace elements and geochemical composition of the bentonites suggests that their source rock is likely derived from island-arc volcanism.Zircon Hf isotope composition have ?Hf(t)values of-20.1?5.8 and two-stage Hf-isotope model ages of 1306?3030Ma.It indicates input of mantle material during the magma evolution process.Combining with tectonic setting and time limitation indicated that there are volcanic arcs in the subduction of the oceanic crust in 774Ma-756Ma,which further proves that the Catheaysia and Yangtze block have not been in collision in the early Nanhua period.