西藏雅鲁藏布江缝合带西段南北亚带蛇绿岩的成因探讨

Genesis and characteristics of the western part of the Yarlung Zangbo ophiolites,Tibet

雅鲁藏布江蛇绿岩带自萨嘎以西分为达巴-休古嘎布（南亚带）和萨嘎-达机翁（北亚带）两个亚带,但两者的关系和构造环境还存在争论。本文在实测剖面的基础上,对比南北亚带蛇绿岩产出和岩石组成,以及对比基性岩脉和围岩方辉橄榄岩的地球化学和年代学特征,探讨两个带的关系和构造环境。北带错不扎、加纳崩和巴尔基性岩脉具有高Si、Al,低K、Ti、P特征,属钙碱性玄武质成分,锆石U-Pb年龄为125~128Ma;南亚带东波、普兰、休古嘎布基性岩具有高Mg、Ti,低Si、K特征,为低钾拉斑玄武质成分,锆石U-Pb年龄为120~130Ma。南北带基性岩具有类似产状和年代学,球粒陨石标准化曲线与NMORB一致;N-MORB标准化蛛网图中显示Nb、Ta、Ti负异常,指示洋内俯冲带弧前或弧后环境。此外,两带基性岩围岩方辉橄榄岩中橄榄石、辉石、尖晶石具有类似的矿物成分特征,地球化学成分兼有弧前和深海地幔橄榄岩的特点,指示南北亚带蛇绿岩形成于俯冲带弧前环境。结合区域对比,南亚带蛇绿岩不发育沟-弧-盆体系,具有从北到南的构造侵位特征,南北亚带之间的仲巴微地体具有特提斯喜马拉雅的亲缘性,指示南北蛇绿岩亚带可能为相同大洋岩石圈的不同残余。

The tectonic setting and relationship between the Daba-Xiugugabu（ Southern belt） and Dajiweng-Saga（ Northern belt）ophiolitic sub-belts divided from Saga in the western part of the Yarlung Zangbo suture zone are the subject of debate. This paper imposes some restrictions on the genesis and relationship between the southern and northern sub-belts based on the construction of geological sections across from the Gangdise island arc,Southern sub-belt,Zhada-Zhongba terrane,Northern sub-belt and Tethys Himalayan formation,comparative analyses of rocks association,geochronology and geochemistry of the mafic dikes and the mineral compositions of their wall rock harzburgites in two sub-belts. The mafic dikes from the Cuobuzha,Jianabeng and Baer ophiolites in the northern belt are characterized by high Si,Al contents and low Ti,K,P contents. They plot as members of the calc-alkaline basalt series and in-situ analyses of the zircon grains yielded ages of 125 ~ 128 Ma. However,the mafic dikes from the Dongbo,Purang and Xiugugabu ophiolites in the southern belt are characterized by high Mg,Ti contents and low Si,K contents,so belonging to a low potassium tholeiite series,and the zircon grains yielded ages 120 ~ 130 Ma. The Southern and Northern ophiolite sub-belts are interpreted as having formed in a similr fore-arc tectonic setting of the supra-subduction zone,because both mafic dikes share similar occurrences,the zircon dating gives overlapping ages（ 120 ~ 130Ma）,show N-MORB type REE patterns normalized by chondrite and noticeable Nb,Ta and Ti negative anomalies in N-MORB normalized spider diagrams,which indicate that they have a fore-arc basin or back arc basin affinity in a supra-subduction environment. The wall rocks harzburgites of mafic dikes have geochemical compositions of both fore-arc and abyssal peridotites. Combined with the absence of any island arc,but showing a southward tectonic emplacement direction in the southern belt,detrital zircon geochronology and Hf isotope detain Permian clastic rocks suggest that,during the Paleozoic period,the Zhada-Zhongba terrane was a part of the Qiangtang-Greater India-Tethyan Himalaya system. Those lines of evidence indicate that both ophiolitic sub-belts probably represent the same Neo-Tethys ocean lithosphere in the same tectonic setting.