黔北大方地区二叠系—三叠系界线粘土及事件粘土地球化学特征及成因

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

黔北大方地区二叠纪及三叠纪地层发育,在二叠系—三叠系界线处发现12~14cm厚的界线粘土,在界线粘土层之上还发现3~4层5~18cm厚的火山事件粘土层。二叠系—三叠系界线粘土、事件粘土及松子坎段底部火山事件粘土岩（绿豆岩）均具有高的K_2O和MgO含量;二叠系—三叠系界线粘土的稀土元素特征与关岭组松子坎段底部绿豆岩相似,与沙堡湾段事件粘土岩的稀土元素特征也十分相似,Eu异常较明显,说明界线粘土及事件粘土成因均与火山活动有密切关系。界线粘土比事件粘土具更明显的Eu异常,部分界线粘土还具有Ce异常。二叠系—三叠界线粘土、事件粘土及关岭组绿豆岩的源区均属于活动大陆边缘的长英质火山岩区,为酸性的长英质火山岩喷发（火山灰）沉积的产物,而沙堡湾段及九级滩段正常沉积泥岩的源区属于被动大陆边缘的长石砂岩岩区。界线粘土中,大部分微量元素与事件粘土及关岭组绿豆岩中对应微量元素的含量相近,但其中的Th、Y、Nb、Cd、Ga、In、Ta明显高于与火山活动有关的事件粘土及绿豆岩,推测事件粘土与少量宇宙事件外来物质的加入有关。

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.