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《Geochimica》 2009-06
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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

QU Xiao-ming1,WANG Rui-jiang1,XIN Hong-bo1,ZHAO Yuan-yi1 and FAN Xing-tao2 1.Institute of Mineral Resources,Chinese Academy of Geological Sciences,Beijing 100037,China;2.Analysis Center of Chinese Academy of Geological Sciences,Beijing 100037,China  
Field geological investigation along Rutog-Shiquanhe and Duobuza-Gêrzêacross 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-Gêrzêophiolite 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 calc-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-Gêrzêophiolite 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.
【Fund】: 国家科技支撑计划项目(2006BAB01A05);; 中国地质调查局青藏专项(1212010818097)
【CateGory Index】: P597.3
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