Adakitic rocks from slab melt-modified mantle sources in the continental collision zone of southern Tibet

Major-trace element and Sr-Nd-Pb isotopic data are presented for newly discovered adakitic rocks in the western Gangdese belt, southern Tibet. The Miocene (26-10 Ma) adakitic rocks from the southern Tibetan continental collision zones exhibit distinct differentiation trends typical of arc magmas. These rocks display geochemical affinities similar to those of Cretaceous (136-80 Ma) adakitic rocks derived from the partial melting of subducted Neotethyan slab in southern Tibet. The whole rock geochemical and isotope characteristics of the post-collision adakitic rocks reveal that their magmas likely originated from an upper mantle region previously metasomatized by slab melts during the Cretaceous subduction event. An interesting observation is that the E-W trending belt of adakitic rocks along the Yarlung Tsangpo suture zone occupies a fore-arc position, reflecting a geotectonic setting compatible with the genesis of adakitic magmas. The widespread occurrences of Cretaceous adakitic rocks in this region are interpreted to testify to the former location of the adakite-metasomatized mantle. Our favoured interpretation is that the spatial isotopic variation in the post-collision adakitic rocks is mostly linked to a westward increase in sediment input in the Tibetan mantle region. Under the framework of our paleo-subduction model, a slab break-off event that initiated at around 25 Ma would have allowed an asthenospheric upwelling beneath southern Tibet, which was instrumental in generating the post-collision adakitic magmatism in southern Tibet. (C) 2010 Elsevier B.V. All rights reserved.