Abstract:

 Within Qiangtang basin, northern Tibet, a metamorphic belt divides the Qiangtang basin into the northern and southern blocks and was designated as the central Qiangtang metamorphic belt. Although blueschist and eclogite were distinguished within this belt, the tectonic evolution of this metamorphic belt still remains largely debatable. One interpretation is that the central Qiangtang metamorphic belt marks an insitu palaeoTethyan suture zone between northern Qiangtang terrane of Cathaysian affinity and southern Qiangtang terrane of Gondwana affinity. The alternative argument suggests that the central Qiangtang metamorphic belt was underthrust southward beneath the Qiangtang terrane along the Jinsha suture zone about 200 km in the north. Therefore, the central Qiangtang metamorphic belt did not mark an insitu suture zone within Qiangtang terrane. Here we investigate the garnetbearing blueschists from Rongma area of central Qiangtang in northern Tibet, where is critical to understanding the tectonic evolution of the central Qiangtang metamorphic belt. The mineral assemblage in these rocks is characterized by porphyroblastic garnet set within a matrix of finegrained amphibole, white mica, epidote, chlorite, albite and quartz with accessory rutile, titanite and apatite. The Mn content of the garnet porphyroblasts decreases from core to rim, whereas the Fe and Mg contents show an increasing trend, respectively. Finegrained rhombshaped inclusions of paragonite and epidote in association with quartz occurring within the core of the garnet porphyroblasts are interpreted as pseudomorphs after lawsonite. Large grains of epidote with inclusions of Na amphibole, rutile and quartz occur in the matrix in contact with the rims of the garnet grains. In the matrix, we identified two stages of Na amphibole rimmed by NaCa amphibole and albite. Retrograde chlorite is rimmed by finegrained biotite. On the basis of microstructural observations and pseudosection modelling, we trace the pT path for the Rongma garnet blueschist from 2.0 GPa and 470 ℃ (lawsonitestability field) to about 17 GPa and 530—540 ℃ (epidote eclogite field), marking an initial increase in temperature and decrease in pressure. This stage is followed by a decrease of pressure through the blueschist facies down to pT conditions of about 0.6 GPa and 530 ℃. In combination with previous work including the available isotopic age data, the pT path obtained in the present study suggests deep subduction of palaeoTethyan oceanic crust between southern and northern Qiangtang blocks, supporting the model that the blueschist belt defines the location of the palaeoTethyan suture zone within northern Tibet. Since the transformation of lawsonite to epidote is known to release water along the subduction channel, our results also provide clues to a better understanding of the subductionrelated Late Permian to early Triassic andesitic magmas in northern Tibet. Furthermore, TO pseudosection has revealed that the epidote was formed during high O fugacity, implying that the new formation of fluid was also of high O fugacity. The free fluid with high O fugacity moving upward triggered off the partial melting of mantle wedge to form high oxidation magmas. This implies that gold rich porphyry copper deposits occurred within Qiangtang terrane.

Key words: Qiangtang terrane, garnet blueschist, pseudosection, high oxidation fluids