Yarlung Zangbo ophiolites (Southern Tibet) revisited:: Geodynamic implications from the mineral record

We present mineral chemistry data and petrological evidence from the Yarlung Zangbo suture zone ophiolites (Southern Tibet) suggesting that they represent a collage of heterogeneous massifs. Mantle sections in these ophiolites consist of harzburgite and lherzolite cut by several generations of gabbroic to diabasic intrusions, all affected by high-temperature deformation. Pyroxenitic bands are parallel to the mantle foliation. Crustal plutonic sections, consisting of dunite, wehrlite and gabbro, are thin or absent and have been observed only in the Dazhuqu massif Plagioclase is an additional phase associated with crustal peridotites. The mineral chemistry of silicate minerals and spinel in the mantle and crustal rocks varies widely and is believed to reflect complex melt percolation and reaction. The massifs record polybaric exhumation steps from at least 50 km depth to the plagioclase stability field. Pyroxene has re-equilibrated compositions from 1200 degrees C down to medium-grade metamorphic conditions. The mantle peridotites are interpreted as the residues of 10-40% partial melting of a fertile lherzolitic source. High Cr number, low TiO(2) content and relatively high Fe(3+) number of spinels suggest that the ophiolitic massifs were generated in a suprasubduction zone (arc or back-arc) environment.