THE STUDY OF THE CHEMICAL COMPOSITION AND EVOLUTION LAW OF THE MESOZOIC VOLCANIC ROCKS IN ZHEJIANG

Mesozoic volcanic rocks are widely distributed in Zhejiang. The volcanic activity started in the late Triassic, reached its culmination in the late Jurassic, and weakened greatly in the Cretaceous. The intensity of volcanic activity and the composition of the volcanic rocks vary markedly from place to place, therefore five zones and eighteen areas of volcanic activity may be distinguished. The types of volcanic rocks are relatively complex, but on the whole the main type is ignimbrites. The volcanic lavas are mainly rhyolite, while those of other compositions account for only a small proportion. The volcanic rocks on the coasts of Zhejiang, Fujian and Guangdong are linked together in one contiguous territory. They constitute an important component part of the system of volcanic rock belts of East Asia. Especially the compositions and several values of the volcanic rocks of Zhejiang and linjian are very close, but contrast strongly with those of R.A. Daly. This imicates that the volcanic rocks of Zhejiang and Fujian are characterized by the same source and origin. The evolution of the composition of the volcanic rocks shows a marked regularity in time and space. In terms of the time sequence of their formation, four magmatic cycles may be distinguished, and the general direction of the evolution is towards being acid and more alkaline. Laterally, i.e. from the coasts to the interior, the same trend of evolution is exhibited, but is not linear. Besides, there also exists a trend of longitudinal evolution parallel to the strike of the coastline. From southwest to northeast, the acidity of the volcanic rocks decreases gradually, while the alkali metals and degree of oxidation increase gradually.