Abstract: Twomica granites are exposed in the Gangdise backarc fault uplift area and are discontinually distributed along Serirong-Baga fault as an E-W trending tring. Their contents of SiO₂, K₂O, Al₂O₃ are higher, but the contents of CaO are lower, A/CNK>1.1 and the content of corundum exceeds 2.1% by CIPW standard mineral calculation. All those show these granites are the peraluminous granites with the rheomorphic remelting characteristics, which formed during the syncollision circumstances.The results of SHRIMP zircon U-Pb dating reveal that the inherited zircon in twomica granitic rocks gave a few Archean and a large number of ages between 1 120 Ma and 1 489 Ma, which represent the age of the main source region of the zircon. Those ages are essentially in agreement with the protolith age of the metamorphic Nianqingtanggu Group, which indicate their close affinity. The rheomorphic remelting age of the twomica granites is 139 Ma. Combined with the regional data, it is suggested that the twomica granites were produced due to the crustal thickening which was caused by the Neotethys subducting underneath the Gangdise microblock and by its backarc oceanic basin closing in the same time.

Key words: SHRIMP U-Pb dating, two mica granite, Neotethys, Gangdise belt, Baga area