Geochemistry and Sr-Nd-Pb isotopic composition of mafic rocks in the western part of Yarlung Zangbo suture zone：Evidence for intra-oceanic supra-subduction within the Neo-Tethys

Abstract：Ophiolites that discontinuously crop out along the Yarlung-Zangbo suture zone (YZSZ) are remnants of Neo-Tethyan ocean lithosphere emplaced during subduction of the Indian plate beneath Eurasia. The Purang massif is one of the largest ophiolites in the western part of the suture with exposed area of about 650 km2 and consists dominantly of harzburgites, minor Cpx- harzburgites and dunites as well as a series of uppermost crustal rocks (siliceous limestone, siliceous shale, chert, basaltic lavas, and pyroclastic rocks), without cumulate rocks and pillow lava. In the northwestern part of the massif, peridotites were intruded by diabase dykes and sills. The diabases show N-MORB type REE patterns with (143Nd/144Nd)t being 0.512904~ 0.512909 and εNd(t) +8.6~+8.7, and are characterized by LILE (mainly K, Sr, Rb) enrichment and noticeable Nb, Th and U negative anomalies. They have a supra-subduction affinity and were formed in a back-arc basin setting. A comparison with other YZSZ ophiolites suggests that various intra-oceanic supra-subduction events within the Neo-Tethys began in different periods. The oceanic crust of the eastern part of YZSZ was formed in about Middle Jurassic in the back-arc basin, while those in central and western segments were formed in about Early Cretaceous above the SSZ setting.