西藏西部与班公湖特提斯洋盆俯冲相关的火成岩年代学和地球化学

对青藏高原西北部班公湖缝合带开展了野外地质调查，初步查明区内缝合带至少包含日土和狮泉河-改则两条蛇绿岩带。在两条蛇绿岩带北侧发现各有两期岛弧型岩浆岩发育，且形成时间严格对应。岩石地球化学分析表明，班公湖缝合带岛弧型岩浆岩的共同特征是富集大离子不相容元素Rb、Th、K和Pb；强烈亏损高场强元素Nb、Ta和Ti；Ba在微量元素蛛网图中总是相对亏损，这些特征说明班公湖地区存在两条俯冲带。从演化序列看，俯冲初期岩石属中钾钙碱性系列，之后岛弧岩浆作用向高钾钙碱性系列演变。锆石U—PbLA—ICPMS定年结果表明，北面的日土俯冲带洋壳俯冲从辉长岩墙开始，时代为（165．5±1．9）Ma（MSWD=1．16），在159Ma时岛弧岩浆作用规模增大，形成小型的花岗岩基；南面的狮泉河-改则俯冲带一开始俯冲（（166．4±2．0）Ma，MSWD=3．0）就有较大规模的石英闪长岩体侵入，之后岩浆作用减弱，到159．4Ma时只有一些小体积的花岗斑岩和闪长玢岩侵入。根据岛弧岩浆作用规模，认为班公湖中特提斯洋盆的俯冲一开始是以狮泉河俯冲带为主，之后狮泉河俯冲带的俯冲作用逐渐减弱。到晚侏罗世初（159Ma）北面的日土俯冲带成为洋壳俯冲的主体。鉴于两条岛弧火成岩带在空间配置上都位于由基性-超基性岩构成的蛇绿岩带北侧，地球化学上显示陆缘弧特征，因此，认为班公湖中特提斯洋盆应该是在中侏罗世晚期（约166Ma）沿日土和狮泉河两条俯冲带同时向北俯冲，构造属性上可能不是一个统一的大洋，而是包含了多个局限性洋盆。

Geochronology and geochemistry of igneous rocks related to the subduction of the Tethys oceanic plate along the Bangong Lake arc zone,the western Tibetan Plateau

Field geological investigation along Rutog-Shiquanhe and Duobuza-G(e)rz(e) across the Bangong Lake suture,the western Tibetan plateau suggests that the suture zone contains at least two subduction belts with two generations of arc magmatic rocks in the north side of either Rutog ophiolite or Shiquanhe-G(e)rz(e) ophiolite zones.Geochemical analyses indicate that the arc magmatic rocks of the Bangong Lake suture possess common signatures as follows:high enrichment of large lithophile elements,Rb,Th,K and Pb;strong depletion of high field strength elements,Nb,Ta and Ti and light depletion of Ba in the N-MORB normalized trace element distribution diagrams.According to magmatic evolution sequence,the arc magmatic rocks formed in the initial subduction stage have medium-K ealc-alkaline features,but the latter magmatic rocks became high-K calc-alkaline series.Trace element Sr has higher enrichment and Ba stronger depletion in the initial stage than in the latter stage.Zircon U-Pb LA-ICPMS dating results indicate that the oceanic crustal subduction along the Rutog ophiolite belt to north began with gabbro dyke at (165.5±1.9) Ma (MSWD = 1.16), and at (159.8±1.4) Ma (MSWD =2. 1) the arc magmatism became much stronger with formation of small granite batholith. Whereas along the Shiquanhe-G(e)rz(e) ophiolite belt to south in the initial stage ( ( 166.4±2.0) Ma, MSWD = 3.8) of oceanic crustal subduction there occurred quartz diorite intrusion with relatively large magma volume. Hereafter the arc magmatism became weak and at (159.19±1.9) Ma (MSWD = 3.5) was presented only by small-in-volume dykes of granite porphyry and diorite porphyrite. If we accept the arc magma volume as a scale of the arc magmatism strength it can be concluded that the subduction of the Bangong Lake middle Tethys oceanic basins occurred mainly along the Shiquanhe ophiolite belt at its initial stage, but henceforth here subduction strenth was reduced, and to the beginning of the late Cretaceous Epoch the subduction along the Rutog ophiolite belt became dominant.Taking in consideration that the two arc igneous rock belts all are located in north sides of the two ophilitic belts composed of basic-ultrabasic rocks, we propose that the Bangong Lake Tethys oceanic basin subducted northwards simultaneously at late Jurrasic Epoch (166 Ma) along two subduction belts of Rutog and Shiquanhe.Tectonically it may be composed of a series of limited basins.