PETROLOGICAL CHARACTERISTICS AND ORIGINAL SETTINGS OF THE YARLUNG TSANGPO OPHIOLITIC MLANGE IN NAMCHE BARWA,SE TIBET
Geng Quanru1, Pan Guitang2, Zheng Lailin2, Sun Zhiming2, Ou Chunsheng3, Dong Han3
1. Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074; 2. Chengdu Institute of Geology and Mineral Resources, Ministry of Land and Resources, Chengdu 610082; 3. Geological Survey, Bureau of Geology and Mineral Resources of Gansu Province, Lanzhou 730050
Recently, geologic mapping with a scale of 1:250000 and a series of special works on the regional geology of the Eastern Himalayan Syntaxis clarified the spatial distribution, petrologic composition and structures of the Indus-Yarlung Tsangpo suture in SE Tibet, where was also known for the world-deepest canyon. It was a typical ophiolitic mélange roughly along the canyon with width of 2～10 km. The major components could be divided into rock blocks and matrix. The blocks were basically from the ophiolite suite including ultramafic mafic rocks, diabase, gabbro, quartz schist, marble and exotic gneiss blocks from the Namche Barwa Formation, which could be longer than 50 km or smaller than 0.5 m. Matrix was composed of strongly deformed mica schists and green schists. The petrological studies showed that the metamorphic basalts were formed in varied settings: near island arc in back-arc basin, fore-arc rift and inner ocean volcanic arc. Some meta-basalts from the eastern margin of the curved ophiolitic belt (Jiaresa to Pangxin) had much higher contents of Si and Mg and lower HFSE and Ti than typical MORB, which were similar to boninite and indicated their original tectonic setting in fore arc accretionary wedge. More REE and trace element analysis suggested that the meta basalts near Yigongbai and Maniweng could originated on island arc and the region between Pailong-Zhaqu-Baila and Gandai could be back arc basin by meta basalt petrologic analyzing. It was concluded that the ophiolite mélange in this belt was composed of "fragments" formed in different environments: fore arc accretionary wedge, island arc, back arc basin and even oceanic island, which was a typical SSZ-type ophiolite. The isotopic dating indicated that the oceanic crust could be formed about 200 Ma ago, much earlier than that of previously idea (110～120 Ma), which was further supported by geological survey in the adjacent regions.
Geng Quanru,Pan Guitang,Zheng Lailin et al. PETROLOGICAL CHARACTERISTICS AND ORIGINAL SETTINGS OF THE YARLUNG TSANGPO OPHIOLITIC MLANGE IN NAMCHE BARWA,SE TIBET[J]. Chinese Journal of Geology, 2004, (3): 388-406.