Abstract:

The Precambrian crystalline rock series in the Yadong area is previously called “the Nyalam Group”. However, it can be divided into two parts, which are angularly discordant with each other, and they are different in the rock association, metamorphism and deformation characteristics, and formation age. The lower rock series is established as a new formation named the Yadong Rock Group, whose rock association is the following: the lower part consists of  many kinds of gneisses intercalated with a minor amount of fine-grained quartzite and biotite schist ; the middle part consists of many kinds of migmatites; the upper consists of gneisses intercalated with a minor amount of leptynite, quartzite, and schist.The Yadong Rock Group contains deep-dervied enclaves such as pyroxenite, hornblendite, and granulite, which are never found in the Nyalam Group.In the light of the metamorphic and tectonic analyses, the Yadong Rock Group underwent the metamorphism from amphibolite facies to upper amphibolite facies, and many stages of plastic deformation of deep tectonic level, as is different from the Nyalam Group.The argument that the Nyalam Group represents the whole Precambrian strata in the Himalaya area is worth discussing.

Key words: Precambrian crystalline rock series, Yadong Rock Group, Yadong area, southern Tibet