摘要
分布在雅鲁藏布江蛇绿岩带北部的早白垩世中晚期恰布林组(K_1^(2-3))由一套含有大量基性-起基性岩和放射虫硅质岩砾石的杂色砂砾岩组成,其物源区主要为当时北缘一套现今未知的蛇绿质混杂岩。这套蛇绿岩极可能是晚侏罗世至早白垩世特提斯洋壳早期俯冲的产物,可称为雅鲁藏布江古蛇绿岩,可能已剥蚀殆尽或其残余体尚未发现。现今被人们所熟知的雅鲁藏布江蛇绿岩则为晚白垩世特提斯洋壳后期俯冲的产物。
The middle-late Early Cretaceous Qiabulin Formation (K12-3) , which is distributed along the north margin of the Yarlung Zangbo ophiolite of southern Tibet, is characterized by a sequence of mottled sandstone and conglomerate containing abundant basic-ultrabasic and radiolarian chert pebbles. It has undergone strong structural deformation and occurs as a tectonic slice between the Tertiary Qiuwu Formation (E2) molasse to the north and the Cretaceous Sangzugang Formation (K13) limestone (west) or the Ngamring Formation (K13- K2) flysch deposits (east) to the south. The age of the Qiabulin Formation should be restricted to the middle-late Early Cretaceous according to the data of gastropod fossils found in the sediments and radiolarian fossils found in the chert pebbles of the conglomerate combined with the geological setting of the region.The Qiabulin Formation sandstone contains abundant feldspar, lithic fragments and strained quartz, of which quartziferous materials account for 37% - 70% and show the features of proximal rapid accumulation. The Qm-F-Lt and Qp-Lv-Ls diagrams of sandstones mainly show sub-duction complex sources. The Deri Member pebbles, including harzburgite, gabbro, basalt, pla-giogranite, plagioclase-hornblende schist and radiolarian cherts, are the components of an ophiolite suite. Composition analysis shows that the ophiolite contains mafic cumulating rocks and that the REE patterns of relevant samples are of the flattype, which apparently differ from the patterns of the Yarlung Zangbo LREE-depleted ophiolite and the LREE-enriched Bangong - Nu-jiang ophiolite. The radiolarian fossil assemblage in the chert pebbles of the Giabulin Formation conglomerate, moreover, is quite different form that of the chert in the Yarlung Zangbo ophiolite. From the facts discussed above, it is deduced that what mainly provided the materials for the Qiabulin Formation is an unkown ophiolitic melange on its north margin. The ophiolite, which is named the Yarlung Zangbo palaeo-ophiolite here, is the result of early subduction and emplacement of Tethys oceanic crust from Late Jurassic to Early Cretaceous, and so far it could have been eroded away or its remnants have not been discovered. The nowadays well-known Yarlung Zangbo ophiolite is the product of late subduction of the Tethys during the Late Cretaceous.
出处
《地质学报》
EI
CAS
CSCD
北大核心
1999年第1期7-14,共8页
Acta Geologica Sinica
基金
国家杰出青年科学基金(编号49625203)
关键词
雅鲁藏布江
古蛇绿岩
早白垩世
恰布林组
西藏
Yarlung Zangbo paleo-ophiolite
Early Cretaceous
Giabulin Formation
Tibet
