High-pressure metamorphic belt in Qiangtang, Qinghai-Tibet Plateau, and its tectonic significance.

The Qiangtang high-pressure (HP) metamorphic belt is distributed for ~500 km from Hongjishan eastward through Pianshishan, Lanling, Jiaomuchagari and Naruo to Caiduo Caka east of Shuanghu, tectonically on the southern side of the Lungmu Co-Shuanghu suture zone. Eastward the belt extends to north of Baqên and in the Gyitang area of Qamdo and connects with the Lancangjiang blueschist belt to form a HP metamorphic belt extending discontinuously nearly 2000 km. The Qiangtang HP metamorphic belt is composed mainly of blueschist and eclogite. Isotopic dating of glaucophane and phengite from blueschist yielded ages of 223 to 215 Ma and the metamorphic ages of eclogite range from 243 to 217 Ma. The metamorphic temperatures and pressures of blueschist are 410-460℃ and 0.67-0.75 GPa and those of eclogite are less than 500℃ and 1.56-2.35 GPa. The dating results of HP metamorphic rocks suggest that the final collision of the Lungmu Co-Shuanghu suture zone occurred in the early stage of the Late Triassic, and this HP belt is an important record of the convergence between Gondwana and Eurasia.