YAMDROK MELANGE,SOUTH TIBET |
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| Authors: | Liu Jianbing |
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| Institution: | Liu Jianbing 1,J.C.Aitchison 1,Badengzhu 2,A.M.Davis 1,S.V.Ziabrev 1,Luo Hui 1,I.McDermid 1 |
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| Abstract: | Yamdrok melange occurs south of, and parallel to, the Yarlung—Zangbo ophiolites, extending several hundreds of kilometres with a width of several to tens of kilometres. Areas near Baisa and Rilang in the Gyangze district were chosen for detailed investigation in this study. Three months of field mapping (1∶1000 and 1∶50000) has been followed by laboratory investigation to extract radiolarians from cherty blocks and matrix material. Laboratory work is continuing..Field investigations in the Baisa area near Gyangze indicate the presence of three melange facies:broken formation, matrix\|rich facies, and block\|in\|matrix melange. Broken formation is characterized by disruption of layering by means of boudinage and pinching\|and\|swelling and dispersal of blocks within the finer\|grained shales due to layer\|parallel extension. Broken formation occurs mostly as dispersed but more\|or\|less traceable lenses within a foliated matrix. A transition from broken formation to typical block\|in\|matrix melange is observed in the field. Further disruption of broken formation leads to the formation of typical block\|in\|matrix melange either, by later shearing, or by suspected mud diapirism. Matrix\|rich facies is characterized by a dominance of shale matrix containing small granules of sandstone and other lithologies. This facies commonly is subject to later deformation, with disruption of the primary foliation into sigmoidal structures. Block\|in\|matrix facies is the most common melange facies and is characterized by blocks of different sizes, shapes and lithologies either encased in, or floating on, relatively finer\|grained arenaceous\|argillaceous matrix. Blocks range in size from several centimeters to several hundreds of meters, and have various shapes from phacoidal, elongate, to irregular. The blocks are mainly composed of varicolored cherts, greywacke and limestone as well as igneous rocks including serpentinite and basalt breccia. The matrix is mainly composed of dark argillaceous shales and siliceous shales, and partly of yellowish green greywacke. The injection or intrusion of mud matrix into blocks is quite common in this melange facies. |
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| Keywords: | Yamdrok melange South Tibet melange facies broken formation matrix\|rich facies block\|in\|matrix Radiolaria |
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