The Maidui tectonic mélange zone and metamorphic ultrabasic rocks on the northern margin of the Coqen Basin,Xizang

The Maidui tectonic mélange zone located on the southern side of the Bangong-Nujiang suture zone and northern margin of the Coqen Basin, Xizang is a newly discovered melange zone within the Shiquanhe-Namco tectonic-ophiolitic mélange zone and consists of the slices of flysch,island-arc basic and intermediately basic volcanic rocks, exotic limestone,siliceous rocks,metamorphic ultrabasic rocks and diabase,which are oriented nearly EW and exhibit multiphase deformation.The metamorphic ultrabasic rocks have undergone the metasomatism and alteration from serpentinization through silicification to carbonatization,and plastic and brittle deformation,and are characterized by highly variable petrochemical compositions,higher MgO,Cr,Co and Ni contents and higher m/f ratios,and typical of the Alpine ultramafic rocks as part of residual mantle The Maidui tectonic mélange zone constitutes an important element of the Shiquanhe-Mangguoco-Namco-Lhari tectonic-ophiolitic mélange zone,and was formed in the Early and Middle Jurassic small back-arc and / or inter-arc oceanic basins associated with the southward subduction of the Bangong-Nujiang oceanic crust.