NOTES ON THE METAMORPHIC SERIES AND METAMORPHIC: BELTS OF VARIOUS METAMORPHIC EPOCHS OF CHINA AND RELATED PROBLEMS

Archean and early Proterozoic metamorphic series are chiefly distributed within the limit of the Sino-Korean platform. Those of middle and late proterozoic ages are mostly found in the eastern and western border regions of the Yangzi paraplatform, along the periphery of the Talimu （Tarim） platform, in the regions stretching from Yinshan to Tianshan, and part of the northern slope of Qinling, central Qilianshan, Kunlunshan and Altunshan. Metamorphic belts of Palaeozoic and later ages on the main have formed progressively outward from the areas of older metamorphics just mentioned （Fig. 1）. The temperature attained during the various metamorphic epochs has a general tendency to decrease with the decreasing of geological age. For example, Archean metamorphism was in fact mostly formed under moderate pressure, high to moderate temperature conditions rather evenly distributed over extensive areas. Proterozoic and later ones varied rather greatly inP-T conditions, thus giving rise to greater variation in types of metamorphism, with the gradual prevalence of medium-to lowpressure green schist facies in many regions, and even with the appearence of Cenozoic laumontite-prehnite-pumpellyite facies rocks. The extent of metamorphism also decreases with the decreasing of geological age. Thus the Archean metamorphism extends over extensive areas,and Proterozoic and later metamorphics are usually found in belts which differ in width. Similarly, Archean regional migmatization gives place to later marginal migmatization and the fracture zone type. While the degree of metamorphism of Archean and early Proterozoic metamorphics are often controlled by depth, i.e., older formations are of higher grade of metamorphism as compared with the younger ones of the same region, that of the metamorphic series of middle Proterozoic and later ages is chiefly more influenced by tectonic factors and heat flows. The superposition of metamorphic events, as reflected in the petrographic characters of the rocks concerned and also their geological settings, may due to the overprinting of those of different metamorphic epochs or of consequent metamorphic stages or phases of the same epoch. The former is exemplified by the occurrence of an earlier Archean epoch around 3 b.y. and a later one of 2.5 b. y. in the northeastern part of Hebei province, serving as an example of polymetamorphism, and the latter, by the confirmation of four successive phases of the Yenshanian （middle to late Mesozoic） metamorphic epoch to the southwest of Beijing, forming integral phases of a metamorphic cycle. The occurrence of glaucophane schist localities or terranes are mainly located in four belts or regions:1, TianshanNe Mongol belt;2, Qilian-Qinling-Dabie mountain ranges; 3,Yaluzangbu Jiang-Lancang Jiang-Ailaoshan region; 4,Yuli belt of Taiwan province. Palaeozoic paired metamorphic belts or their traces are distributed chiefly in the northern marginal mobile belt of the Sino-Korean and Tilimu massifs, and the mobile belt in between them and the Yangzi and the Chaidamu massifs on the south. Those of Mesozoic and Cenozoic ages are found mainly in the proximity of the boundary zones between the Eurasian lithospheric plate and the pacific or the Indian. Altogether eleven metamorphic epochs have been found in China, i.e., two in Archean, four in Proterozoic, two in Palaeozoic, two in Mesozoic and one in Cenozoic.