Studies on the Blueschist Belt in the Shuanghu Region, Central Northern Tibet and Its Tectonic Implications

The blueschist blocks extend intermittently more than 300 km, from the eastern Qagelela town, 13 km west of Shuanghu, Xiyaergang, central northern Tibet, through Naro, Jiaomuri, Gangtang Co, Guoganjianian Shan to western Gangmar Co-Lungmu Co. Most of the blueschist blocks are newly discovered by Li Cai et al. in recent years, which are associated with ophiolite melange and olistostromes, except those of Qagelela and Guoganjianian Shan.The protoliths of the blueschist are mainly of the Late Carboniferous sedimentary-volcanic rock series,belong to the products of the margin of the Gondwana land, including basic volcanic rock, argillaceous sandstone,sandy-argillaceous limestone etc., and a few basic lamprophyres.The H.P/T minerals mainly consist of crossite, next magnesio-riebeckite and minor winchite.The mineral assemblages are as follows: (1) Epidote + chlorite + crossite + albite; (2) Sericite + chlorite +magnesio-riebeckite (+winchite) + feldspar + quartz; (3) Calcite + crossite + epidote; (4) Chlorite + actinolite +winchite (±clinopyroxene).Estimated metamorphic pressure of the blueschist varies from 0.62~0.75 GPa and metamorphic temperature from 410~460℃, corresponding to the transitional facies between blueschist and greenschist.The 40Ar/39Ar isotope dating of crossite is 222.5±3.7 Ma, showing the metamorphic age ranging from the Late Paleozoic to Early Mesozoic.The high pressure metamorphic zone associated with ophiolitic melange being distributed along Lungmu Co-Gangmar Co-Guoganjianian Shan- Shuanghu-Lancang River may be as a tectonic symbol of the subduction of an "initial limited oceanic basin" at the northern side of the Gondwana land, which is taken as a boundary between the Gondwana- and the Eurasian land.