藏东南雅鲁藏布蛇绿混杂岩带的物质组成及形成环境

PETROLOGICAL CHARACTERISTICS AND ORIGINAL SETTINGS OF THE YARLUNG TSANGPO OPHIOLITIC MLANGE IN NAMCHE BARWA,SE TIBET

雅鲁藏布蛇绿混杂岩带位于藏东南南迦巴瓦峰地区 (喜马拉雅东构造结 ) ,呈弧形大致沿雅鲁藏布江分布 ,出露宽度 2～ 1 0km。带内岩石变质、变形强烈 ,按产状可分为两大类 :基质和岩块 (片 )。岩块 (片 )包括蛇绿岩套中的超镁铁岩、辉绿岩墙和辉长岩 ,石英 (片 )岩 ,大理岩和两侧老基底片麻岩等。岩块 (片 )大小不一 ,大的延伸可 >5 0km ,小的仅约 0 .5m ;基质是塑性变形十分强烈的绢云母石英片岩、二云母石英片岩、绿片岩等岩石组合。变质玄武岩岩石地球化学特征表明 ,该蛇绿混杂岩带可能由形成环境不同的“碎片”组成 ,包括弧前扩张带、岛弧、弧后盆地及洋岛等环境 ,是典型的消减带环境的蛇绿岩 (SSZ)。初步的同位素年代学测试结果 ,说明蛇绿岩可能在 2 0 0Ma前形成 ,比原认为距今 1 1 0～ 1 2 0Ma要早得多 。

Recently, geologic mapping with a scale of 1∶250 000 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.