雅鲁藏布蛇绿岩——事实与臆想
Yarlung Zangbo ophiolite：A critical updated view
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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，其扩张速率甚至慢于目前广为人知的西Alps地区。根据全球蛇绿岩的情况，该蛇绿岩可被定义为日喀则型，是目前超慢速扩张洋盆的端元代表。Abstract: 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 basalt and intrusion of gabbro and dolerite. During the maximum extension, partial melting of the metasomatized refractory lithsopheric mantle gave rise to some amount of boninitic melts. Therefore, the Yarlung Zangbo ophiolite is much different from the ideal ophiolite section defined by the Penrose conference, and it could not be considered as a remnant of the Neo-Tethyan ocean between the Indian and Asian continents. Considering that the mafic rocks were limitedly developed in the area, we propose that the Yarlung Zangbo ophiolites represent an oceanic lithosphere formed at an ultraslow spreading center with a spreading rate much lower than that in western Alps. Therefore, the Yarlung Zangbo can be defined as an end-member of the ultraslow spreading setting; we term it as the Xigaze type since it is typically developed around Xigaze area in the central segment of this ophiolite belt.
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Key words:
- Ultraslow spreading /
- Neo-Tethyan ocean /
- Ophiolite /
- Yarlung Zangbo /
- Tibet
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