Precambrian crystalline basement in the Nyalam region,southern Xizang

The Precambrian crystalline basement arranged in the Nyalam-Qomolangma zone,southern Xizang is believed to be the most important metamorphic belt in the Himalayan orogen.In terms of lithologic association,metamorphism and regional correlation,in combination with a SHRIMP zircon U-Pb age of 686 Ma for the Rouqiecun Group Complex and a whole-rock Rb-Sr age of 796±103 Ma for the biotite-plagioclase granulite,the authors contend that their protoliths are all traced back to the Neoproterozoic,i.e.to the Sinian in age.The formerly Sinian-Cambrian Rouqiecun Group should be redefined as the Precambrian metamorphic strata.Actually,no Cambrian strata crop out in the Nyalam region.In this respect,the Precambrian strata are divided into the Nyalam and Rouqiecun Group Complexes.The former is subdivided into four tectonic-lithostratigraphic units including the Youyiqiao Formation Complex,Quxiang Formation Complex,Kangshanqiao Migmatites and Jiangdong Formation Complex,and the latter into two tectonic-stratigraphic units including Zhaxizong Migmatites and Tajiling Formation Complex.