藏南亚东地区前寒武纪亚东岩群的建立及其特征

The Foundation of the Precambrian Yadong Rock Group and Its Characteristics in the Yadong Area, Southern Tibet

藏南亚东地区前寒武纪结晶岩系以往统称为“聂拉木群”。本次研究发现该结晶岩系可划分为上、下两部分,二者之间原始接触关系为角度不整合。下部结晶岩系为本次新建的“亚东岩群”,其岩石组合为:下部为多种片麻岩夹少量细粒石英岩和黑云片岩;中部为多种混合岩;上部为多种片麻岩夹少量变粒岩、石英岩和片岩。不同部位的片麻岩和混合岩中含辉石岩、高压麻粒岩等“暗色”包体。研究表明,亚东岩群为角闪岩相─高角闪岩相变质作用的产物,经历了多期深部构造层次的韧性变形,与“聂拉木群”显著不同。因此,认为“聂拉木群”代表喜马拉雅地区全部“前寒武系”是值得商榷的。

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.