Ophiolitic Melanges in the Yarlung-Tsangpo "Big Bend" Canyon, SE Tibet

The geology of the Tsangpo's "Big Bend" canyon in southeasternmost Tibet is quite significant to collisional tectonics, but this rugged region has been little studied. New 1:250 000 scale mapping since 1999 has improved our knowledge of the area and revealed four major units: (a) Namche Barwa group. Plagioclase gneisses with amphibole-containing high-P granulite lenses, exposed in the central area of the canyon, and representing the lower Indian plate. (b) Gangdese group. Plagioclase-gneiss, amphibolite, marble, quartzite intruded by granites, exposed surrounding the Namche Barwa group, representing the lower Eurasian plate. (3) Ophiolitic melange of the Indus-Yarlung Tsangpo (IYT) suture. The former occurs roughly along the Tsangpo "Big Bend" canyon with good exposures at Pangxin, Jiaresa and between Pailong and Zhaqu; it separates the Namche Barwa and Gangdese groups. (4) Ophiolitic melange of the Jiali--Pailong Tsangpo (JPT) suture. The mafic rock assemblage of IYT includes radiolarian chert, basaltic pillow lava, diabase sills, cumulate complex and serpentinized peridotite. The ophiolitic zone has been metamorphosed and highly deformed to form a melange containing various blocks and matrix, including blocks of metamorphosed mafic and ultramafics, quartzite and mica quartz schist; marble, and both Namche Barwa and Gangdese gneiss. The petrochemistry and trace elements of IYT rocks show similar characteristics to other ophiolite suites in Tibet and western Sichuan. HREE and Cr, Co, Ni are strongly depleted in upper-mantle peridotite, which is slightly enriched in LREE and Rb, Sr, Ba, Nb, Ta. Meta-basalt and diabase are enriched in LREE with (La/Yb) ratios of 7.29 to 1.28, very different from MORB. Compared with MORB, IYT meta-basalts and diabases are enriched in lithophile elements and are depleted in transition elements, suggesting that the IYT ophiolite suite originated from a small oceanic basin. The initial magma formed in this environment could be LREE and lithophile element-enriched and might have been contaminated by more of these elements during the intrusion. Along the Jiali-Pailong fault zone near high mountain ridges, ultramafic and mafic blocks occur enclosed or intruded by Jurassic to Triassic granite batholiths. Blocks of silicate, marble, and hornstone also occur. Field and lab studies suggest that these blocks may be a remnant of an old ophiolite zone. Rb-Sr dating on mafic rocks suggests a rough age of 215+/-63Ma. Stratigraphic, petrologic, and tectonic studies suggest that the JPT zone originated from an inner-arc basin. Oceanic crust formed in late Triassic and subducted from south to north from late Triassic to Jurassic. Closing of this ocean and orogenesis started in middle Jurassic accompanied by granite intrusions. The ophiolitic melange developed during arc-to-arc collision. Our data support a new hypothesis that the Neo-Tethys ocean appeared between Paleozoic and early Cretaceous and closed along the Nujiang suture.