High-Pressure Eclogite-Blueschist Metamorphic Belt and Closure of Paleo-Tethys Ocean in Central Qiangtang, Qinghai-Tibet Plateau

The high-pressure metamorphic belt (HPMB) of eclogite-blueschist in Central Qiangtang ((sic)) lies in the Longmu Co ((sic))-Shuanghu ((sic)) suture zone. To the west, the HPMB extends 500 km from Hongjishan ((sic)) to Caiduochaka cast of Shuanghu; to the cast it extends to Baqing ((sic)) and Jitang ((sic)) in Qamdo ((sic)), and then bends southward to Yunnan ((sic)) Province. Including the Lancangjiang ((sic)) blueschist belt, the entire HPMB is about 2 000 km long. In Central Qiangtang, the belt is mainly composed of blueschist and eclogite, whereas in West Yunnan it contains only blueschist. The Baqing-Jitang segment is dominated by garnet phengite schist. 40Ar-39Ar dating of glaucophane and phengite from the blueschists yielded plateau ages ranging from 223 to 215 Ma, whereas SHRIMP U-Pb dating of zircon from the eclogites gives metamorphic ages of 243-217 Ma. The calculated metamorphic conditions for the blueschists are 410-460 degrees C and 0.67-0.75 GPa, and for the eclogites, <500 degrees C and 1.56-2.35 GPa. The metamorphic ages suggest that the Longmu Co-Shuanghu suture closed in the Late Triassic. The region south of the Longmu Co-Shuanghu-Lancang suture consists of the pan-African basement overlain by Gondwana sedimentary and meta-sedimentary rocks, whereas the region north of the suture is dominated by the Jinning ((sic)) basement and Yangtze sedimentary and metasedimentary rocks. The Qiangtang HPMB marks the closure of the paleo-Tethys Ocean.