Geochemical characteristics and genesis of T/P boundary clay and event clay in Da-fang aera, Guizhou Province

The Permian and Triassic strata are widely distributed over Dafang area, Guizhou Province. Clay at the boundary between Permian and Triassic was discovered which is 12~14cm in thickness. There are 3~4 beds of volcanic event clay above the boundary clay which are 5~18cm in thickness. K₂O ang MgO are relatively high in boundary clay between Permian and Triassic, event clay and volcanic event clay lie at the bottom of Songzikan Member. Their REE characteristics are also similar to each other. Boundary clay and event clay are both related to volcanic activity, as shown by Eu deficiency. Eu deficiency of boundary clay is more obvious than that of event clay, and some of this clay also show Ce deficiency. Tectonic background of boundary clay and event clay is related to felsic volcanic ash deposit in active continental margin, which is different from things of mud of Shabaowan Member and Jiujitan Member whose source area was feldspar arenite. Most of the trace elements in boundary clay are similar to those in event clay except Th, Y, Nb, Cd, Ga, In and Ta which are much higher than in volcanic event clay. It is thus inferred that there might have existed a little material from cosmic event in boundary clay.