【摘要】 <正> 近年来,对雅鲁藏布蛇绿岩带的研究引起了国内外学者的广泛兴趣,因为它不仅对了解中生代以来特提斯海的演变,而且对阐明青藏高原地质发展历史都是十分重要的。 虽然,这条带内大规模超基性岩体的分布早已被发现,有些岩体已作过很好的研究。但是,整个蛇绿岩组合的综合性研究还刚开始。最近一些作者根据野外路线调查,对这条蛇绿岩带作了初步的论述。但由于缺乏对蛇绿岩比较系统的填图工作,关于它的分布、组成、层序、形成、侵位及其演化历史还有许多重要情况不十分清楚。为此,我们选择了该带中蛇绿岩出露良好的地区,即日喀则地区白朗—吉定之间,在东西长90公更多还原

【Abstract】 The ophiolite of Xigaze Prefecture is occurs in the middle section of the Yarlung Zangbo ophiolite belt.It extends in a narrow belt with about 10 km in width and 90 km in length,and has a near B-W trend,which changes into N-E direction at its east end.Two large thrust faults run on the southern and northern boundaries of this belt and the ophiolite distributed along these faults has a contact with the Triassie epi-metamorphic rocks and late Cretaceous flysch (the Xigaze Group) respectively.The ophiolite occurs as a series of imbricated thrusting slices which consist -of various units of rocks and were clearly dismembered by strong thrusting- However,on some sections the incomplete sequences can yet be observed.From field observation and comparison of various sections,the original strati-graphic sequences maybe reconstructed.They are in ascending order as follows:(1) ultramafic plutonic rock,the serpentinized harzburgitic rock as a representative (2) sheeted mafic dykes and sills complex,the swarm of sheeted diabasic dykes as a representative;(3) submarine erupted pillow basaltic and spilitic lavas;(4) pelagic sediments including red radiolarian chert and ophiolitic sedimentary rocks.Comparing with the typical sections of ophiolites in the world,the ophiolitic assemblage in.the prefecture studied is characterized by the absent of cumulites of ultramafic and mafic plutonic rocks and poor development of layered gabbro.The swarm of sheeted dykes and volcanic rocks have a small thickness and are in short higher silica rocks such as diorite,trodhjemite and keratophyrites.All of these indicate a weak differentiation of magma.According to the chemical composition the sheeted dykes and pillow_lavas can be attributed to oceanic tholeiitic rocks.Therefore,the ophiolites mentioned above may be generated in the central part of a rapidly spreading mid-oceanic ridge.The geological age of radiolaria in red chert and ophiolitic sediments that rest on the top of the volcanic rocks is from Late Jurassic to Middle Cretaceous.And the ophiolitic complex is unconfonnablely overlain by Oligocene-Miocene conglomeratic bed,and was subjected to two strong thrustings that took place before and after the deposition of the conglomeratic bed respectively.Prom these evidences,the authors believe that the ophiolite under investigation may be formed in the position of rapidly spreading ridge of the Mesozoic Tethys at a time span from Jurassic to Cretaceous,and later then it horizontally removed northward to the Yarlung Zangbo trench zone.In Early Tertiary the ophiolite as stated was subjected to emplacement and erosion near the trench zone.From Miocene on,the lifted ophiolite underwent a structural reformation under the strong compressive stress in S-N direction and received present occurrence更多还原