western Dabashan Tectonic belt and its significances

An Eopaleozoic cumulate, located in the Guoganjianian Mountain (Mt.) area of the Longmucuo-Shuanghu suture zone in the middle part of the Qiangtang of the northern Tibet, consists of the pyroxene-peridotite, cumulative pyroxenite, cumulative gabbro and plagioclase. According to authors’ research on the mineralogy and chronology of zircons in the cumulative gabbro, there are three kinds of zircons on the interior configuration and their Th, U amounts and ratios of Th/U open out that zircons are crystaled from an identical magma system. The SHRIMP U-Pb dating from same sample gives a concordant age of 461±7Ma (MSWD=1.3) and 431.7±6.9Ma (MSWD=0.54) which respectively indicated the early (i.e. Late Darriwilian stage of Middle Ordovician epoch) and late (i.e. Middle Telychian stage of Early Silurian epoch) magmasim during the Proto-and Paleo-Tethys expanding in the central region of the Qinghai-Tibet Plaoteau. The Eopaleozoic cumulate characterized by the MORB is considered not only as the representative of Eopaleozoic ophiolite remained in the Longmucuo-shuanghu suture zone but also, up to now, as the pioneer remanet of the Proto-and Paleo-Tethys with well-founded age in the mid-southern Qinghai-Tibet Plaoteau. Based on results from the confirmable Eopaleozoic ophiolite and its chronology restriction in the middle part of the Qiangtang, we suggest that the Proto-and Paleo-Tethys along the Longmucuo-shuanghu suture zone was leastways formed in the Middle Ordovician-Early Silurian and kept an evolvement until Triassic. Furthermore, we can infer that the Longmucuo-shuanghu suture zone in the central Qiangtang, Paleozoic accreting wedge and Mesozoic basin of the southern Qiangtang and Bangonghu-Nujiang suture zone on the southern margin of the southern Qiangtang constitute the huge suture zone, which was characterized by the final disappearance of the Tethys in the Qinghai-Tibet Plaoteau. The southerly subduction of the Proto-and Paleo-Tethys in Late Paleozoic induced the Carboniferous-Permian arc volcanic rocks, Permian granodiorites and subducing eclogites along the Gangdise belt.