Zircon U-Pb Geochronology And Genesis Of K-bentonite At The Paleozoic-Mesozoic Key Stratigraphic Boundaries Of South China

Paleozoic-Mesozoic stratums are well developed in South China, and a number of K-bentonite layers have been recognized near the boundaries of Ordovician-Silurian and Permian-Triassic in the Yangtze Block at South China, where the K-bentonite was produced by sedimentation and alteration of volcanic ashes in marine environments. In this paper, we mainly studied the K-bentonite layers near the following key stratigraphic boundaries:(1) K-bentonite layers exposed near the Ordovician-Silurian boundary in the Upper Ordovician Wufeng Formation, which located at Huanghuachang, Yichang, Hubei province. We focused on the study of the first and the eighth K-bentonite layers in Wufeng Formation. (2) K-bentonite layers exposed near the Ordovician-Silurian boundary in the Upper Ordovician Wufeng Formation, which located at Nanbazi, Tongzi, in Guizhou province. We focused on the study of the eighth K-bentonite layer in Wufeng Formation. (3) K-bentonite layers exposed near the Permian-Triassic boundary, which located at Daxiakou, Xingshan, in Hubei province. We focused on the study of the fifth, the ninth and the tenth K-bentonite layers in Daye Formation. We mainly studied the clay minerals and detrital minerals in the bentonite. By zircon U-Pb dating and zircon Hf isotopes test, and combined with whole rock geochemistry, we obtained the precise isotopic ages of the rocks, and could provide clear evidence for the causes of altered volcanic ash, then restore the original rocks of the bentonite and discuss their possible material sources.We have proved that there are many K-bentonite layers near the boundaries of Ordovician-Silurian and Permian-Triassic at South China. Where the bentonite is compose of illite/montmorillonite mixing clay minerals, besides, volcanogenic minerals likeβquartz, zircon, apatite, sanidine and sedimentogenic minerals in later period like pyrite, gypsum, barite were found.Zircon U-Pb chronology reveals the boundary ages of our studied K-bentonite layers.445±2Ma for the boundary of Linxiang Formation and Wufeng Formation,444±2Ma for the eighth K-bentonite layer in Wufeng Formation, in YingChang, Hubei province.449±9Ma for the eighth K-bentonite layer in Wufeng Formation, Nanbazi profile, Tongzi in Guizhou province. 251.1±1.0Ma for the boundary of Permian-Triassic, and 249.4±0.8Ma and 252±3Ma for two layers exposed within Triassic Daye Formation, within Daxiakou profile, Xingshan.Geochemical characteristics indicate that these rocks are potassium bentonite rocks, original rocks are mostly acidic volcanic rocks, which is corresponding to the discovery of (3 quartz among the detrital minerals. K-bentonite in South China Ordovician-Silurian boundary shows typical characteristics of arc volcanics, while K-bentonite near Permian-Triassic boundary within Daxiakou profile, at Xingshan, in Hubei, shows Intraplate volcanics characteristics.After detailed studies of bentonite geochemistry, zircon U-Pb geochronology and Hf isotopic test, we suggest that the source rocks of bentonite near the Ordovician-Silurian boundary in South China were came from the volcanic activities of Qinling area, in the northern margin of Yangtze Block. The bentonite near the Permian-Triassic boundary at South China is closely related with Siberian plume activities.