雅鲁藏布蛇绿岩——事实与臆想  

Yarlung Zangbo ophiolite: A critical updated view

作　　者：吴福元[1] 刘传周[1] 张亮亮[1] 张畅[1] 王建刚[1] 纪伟强[1] 刘小驰[1] 

机构地区：[1]中国科学院地质与地球物理研究所岩石圈演化国家重点实验室,北京100029

出　　处：《岩石学报》2014年第2期293-325,共33页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41130313);中国科学院战略性先导科技专项(B类)项目(XDB03010200)联合资助

摘　　要：位于西藏南部的雅鲁藏布蛇绿岩在我国研究程度最高,在国际上也有较高的知名度。该蛇绿岩东西延伸约2000km,代表了印度和亚洲之间消失的新特提斯洋,是确定上述两大板块间缝合线存在的重要岩石学标志。本文根据作者近几年的野外考察,结合前人发表的资料发现,该蛇绿岩有如下方面的重要特点。（1）各蛇绿岩剖面均发育大规模的橄榄岩体,超镁铁岩的分布远远大于镁铁质岩石。这些超镁铁岩体尽管在岩性上以方辉橄榄岩为主,但出现大量二辉橄榄岩;（2）镁铁质堆晶辉长岩不发育;（3）不存在席状辉绿岩墙群,取而代之的是顺层侵入在橄榄岩中的辉长岩-辉绿岩岩席。部分情况下,辉绿岩还侵入到玄武岩之中;（4）蛇绿岩上部发育有一定厚度的玄武岩,但玄武岩与橄榄岩之间经常被辉绿岩席所占据,部分情况下玄武岩与橄榄岩直接接触。（5）地幔橄榄岩与镁铁质岩石在Sr-Nd同位素和形成时代上存在显著差别;（6）辉长岩与辉绿岩形成在120-130Ma的狭窄时间段内,并具有类似亏损地幔的地球化学特点。上述资料表明,雅鲁藏布蛇绿岩中的超镁铁岩和镁铁质岩形成于不同时代,且在成因上不具任何联系。根据这些资料,本文提出,该区蛇绿岩的地幔橄榄岩可能为大陆岩石圈地幔。早白垩世期间,北侧亚洲大陆南缘位置的岩石圈由于拉张而使深部岩石圈地幔物质向上剥露。随着岩石圈拆离和减薄的不断进行,软流圈地幔发生减压熔融,形成目前见到的玄武岩和辉长-辉绿岩席。在拉张作用的高峰期,早期亏损的大陆岩石圈地幔在经历交代作用后发生部分熔融形成少量玻安质熔体。因此,雅鲁藏布蛇绿岩并不能代表新特提斯大洋,它与经典的蛇绿岩定义相差甚远。考虑镁铁质岩石发育有限的特点,雅鲁藏布蛇绿岩代表了一种超慢速扩张的洋盆形成环境,其扩张速率甚The Yarlung Zangbo ophiolite belt,located in southern Tibet,is one of the extensively studied ophiolites in China,and is internationally renowned as well. Distributed in east-west direction with a distance of ~ 2000km,this ophiolite had been used as an important petrological indicator to constrain the suture zone between India and Asia,and represents the vanished Neo-Tethyan Ocean between them. According to previous studies and our recent field investigations,however,this ophiolite belt is characterized by the following lines of features：（ 1） Mantle peridotites are predominated over mafic rocks; mantle peridotites mainly consist of harzburgites, but with a significant amount of lherzolites;（ 2） Cumulative gabbro is usually absent although it was identified in three localities with a maximum thickness less than 1000m;（ 3） Sheeted dyke is absent,but with gabbroic-doleritic sills intruding into the peridotites. In some cases,dolerites occur as dykes in basalts;（ 4） Generally,basalts are directly overlying the peridotites,although the interface between them is mostly occupied by doleritic sills;（ 5） Peridotites and mafic rocks are significant different in terms of formation age and Sr-Nd isotopic compositions;（ 6） Gabbros and dolerites were emplaced during ca. 120 ~ 130Ma with a limited temporal interval across the whole belt,and were derived from the depleted asthenospheric mantle. These lines of evidence suggest that the ultramafic and mafic rocks were formed in separated stages without any genetic connection. Therefore,we proposed that the mantle peridotites outcropped in the Yarlung Zangbo ophiolite belt are probably sub-continental lithospheric mantle beneath the Asian plate. During the Early Cretaceous,extension of the leading edge of the Asian continent resulted in the exhumation of the subcontinental lithospheric mantle,and the formation of an oceanic basin. Exhumation and thinning of the lithosphere resulted in upwelling and melting of the asthenosphere,which led to the eruption of

关 键 词：超慢速扩张 新特提斯洋 蛇绿岩 雅鲁藏布 西藏 

分 类 号：P588.1[天文地球—岩石学]