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

Abstract

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.