U-PB, RB-SR ISOTOPIC CHRONOLOGY OF JINNING GRANITES IN THE SONGPAN-GANZE OROGENIC BELT AND ITS TECTONIC SIGNIFICANCES

A series of Jinning granites are located in the eastern margin of the main part of the Songpan-Ganze Orogenic Belt. The research is focused on Gezong and Donggu granites in the Danba area From the viewpoint of isotopic chronology, geochemistry and tectonic geology, their formatio n ages, origins and geneses of source materials, and the influences of Himalayan tectonic thermal events are discussed. It is suggested that they were the products of Jinning tectono-magmatism. The source mater ials were the poorly matured volcano-sedimentary rocks formed in the m iddle-late period of the middld-Proterozoic They represent in fact the basement of the old Yangtze Block. The late Proterozoic orogenic belt of the entire western margin of the Yangtze Block, including the Danb a area, should be the key part of the old Yangtze Block.