CHARACTERISTICS OF IGNIMBRITES IN ZHEJIANG PROVINCE

After the study of the characteristics of ignimbrites both in China and abroad is reviewed, the author summarizes them in twelve aspects, i. e. distri- bution in time and space, cyclicity, modes of occurrence, geomorphology, litho- logy, rock types, facies changes, textures and structures, fragment shapes, criteria for detailed division and distinction from similar rocks, in the light of his knowledge gained through more than twenty years of practical work in Zhejiang province. In terms of time, the ignimbrites almost all occur in the Yanshanian cycle except for a small amount of intermediate ignimbrites which occur in the Shuangxiwu Group of the pre-Sinian Shengong cycle. In terms of spatial distribution, they are rare in northwestern Zhejiang, present in minor amount in central Zhejiang, and most widespread and thick along the coasts; besides, from northwestern to southeastern Zhejiang, the scale of volcanic eruption is increased obviously and the thickness of the rocks has a conspicuous tendency to increase. In the aspect of the cycic feature, the volcanic activity in Zheji- ang may involve three stages of magmatic activity, i. e. the geosynclinal, platform and continental-margin stages and six cycles of volcanic activity, such as the Shengongian, Jinningian and Yanshanian cycles. In the stage of geosynclinal development, marine volcanic eruption was predominant; in the stage of platform development, only a small amount of pyroclastic sedimentary rocks were formed; in the stage of continentalmargin motion, magmatic acti- vity was very intense, and volcanic activity reached a climax, thus giving rise to voluminous ignimbttes. As regards the modes of occurrence, they are domi- nated by fissure, central and paternoster （multicrater）eruptions, and loca- lly there also occur intrusions. Geomorphologically, the sost important feature is the development of columnar joints, which are highly resistant to erosion, thereby often forming steep cliffs. Lithologically, acid and intermediate-acid rocks predominate, while intermediate rocks are less; along the coasts there appear rarely occurring alkaline and slightly alkaline ignimbrites. The evolu- tion of the rocks is characterized by the presence of intermediate-acid rocks in the early stage, acid rocks in the middle stage and alkaline rocks in the late stage. Lithologically, the ignimbrites may usually change into lavas of the same composition; weakly welded rocks may change into tuffs. Textures of plastic deformation and pseudorhyolitic structures are another important feature of acid and intermediate ignimbrites as compared with their analogu- es. Plastic stretching of pyroclasts, mainly vitric fragments of plastic deforma- tion, torn-up fragments, other than some undeformed rigid fragments. Pseu- dorhyolitic structures are formed by parallel arrangement and overlapping of vitric fragments of plastic deformation, torn-up fragments; they resemble rhyolitic structures in appearance and so the name. The fragments making up the ignimbrites include rock fragments, crystal fragments, vitric fragments of plastic deformation, torn-up fragments of plastic deformation, plastic rock fra- gments and dusty fragments. The first two kinds of fragment are generally regarded as being formed by brittle fracturing of a rigid body. About this view, there are few disputes. On the other hand, as to the origins of the last four kinds of fragments, divergent views exist because their origins are comp- lex; so the paper devotes a lot of space to dealing with them. The ignimbri- tes may be subdivided into three subspecies, the strongly plastically deformed, intermediately plastically deformed and weakly plastically deformed ignimbri- tes. The two end members may grade into lavas and tuffs of the same comp- osition. The paper devotes certain space to discussing their diagnostic features and distinguishing criteria.