The high-pressure metamorphic belt （HPMB） of eclogite-blueschist in Central Qiangtang （羌塘） lies in the Longmu Co （龙木错）-Shuanghu （双湖） suture zone. To the west, the HPMB extends 500 km from Hongjishan （红脊山） to Caiduochaka （才多茶卡）, east of Shuanghu; to the east it extends to Baqing （巴青） and Jitang （吉塘） in Qamdo （昌都）, and then bends southward to Yunnan （云南） Province. Including the Lancangjiang （澜沧江） 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 ℃ and 0.67-0.75 GPa, and for the eclogites, 〈500℃ 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 Condwana sedimentary and metasedimentary rocks, whereas the region north of the suture is dominated by the Jinning （晋宁） basement and Yangtze sedimentary and metasedimentary rocks. The Qiangtang HPMB marks the closure of the paleo-Tethys Ocean.
Bulk-rock major and trace elements and zircon U-Pb data are reported for Permian metamafic rocks from Guoganjianan, in the western part of Longmuco-Shuanghu-Lancang suture zone. These results offer new insights into mantle source characteristics and geodynamic setting of Permian ophiolitic fragments. U-Pb isotopic dating using SHRIMP II method reveals that the metamafic rocks were formed at 274.7± 3.9 and 279.8± 3.6 Ma. The metamafic rocks mostly show N-MORB-typed rare earth element patterns and are enrichment in large-ion lithophile elements, indicating that they are probably derived from partial melting of a depleted mantle in a back-arc basin. Our new data, together with recent studied results on Paleozoic ophiolitic fragments suggest the Paleo-Tethys Ocean in Central Qiangtang opened at Cambrian and widened between Ordovician and Devonian. Northward subduction started in the Late Devonian–Early Carboniferous and a back-arc basin developed during Permian.
Taoxinghu metamorphic peridotite is a firstly reported mantle sequence of ophiolite since Longmuco-Shuanghu-Lancangjiang suture zone （LSLSZ） was proposed, and it is also an important discovered for ophiolite studying in central Qiangtang. Based on detailed analyses of whole-rock geochemistry of Taoxinghu metamorphic peridotites and contrast to metamorphic peridotites in typical ophiolites worldwide, the paper investigates their petrogenesis and geological implication. The petrologic results show that the protolith of Taoxinghu metamorphic perdotites have the mineral assemblage and texture characteristic of mantle peridotite. Most metamorphic peridotites hav near global abyssal peridotites major elements contents, while the few is similar to SSZ-type peridotites. They exhibit typically U-shaped REE patterns, characterized by slight enrichment of LREE and HREE relative to MREE and a low fractionated LREE to HREE segment. Trace elements contents are low and all samples are strong enrichment in Cs, U, Pb, weak enrichment in Ba and depletion in Th, but negative Nb anomalies are only observed in few samples. That suggests Taoxinghu metamorphic peridotites have depletion mantle and suprasubduction affinities. A two-stage evolution history is considered： Taoxinghu metamorphic peridotites originated as the residue from melting at a ridge with 7%-20% degree of fraction melting and were subsequently modified by interaction with mafic melt and aqueous fluid within mantle wedge on subducted zone. Combined with previous studies, we preliminarily propose Taoxinghu metamorphic peridotites may be the Products of initial rifting of palo-Tethys, forming at middle Ordivician-upper Cambrian, and they may be the direct evidences for spreading of palo-Tethys.
