Petrogenesis of Early Paleozoic diorites and mafic–intermediate dykes from the eastern Qilian orogen, NE Tibetan Plateau: implication for lithospheric processes

This paper presents an integrated study of petrology, geochronology, and geochemical and Sr–Nd–Hf isotopic compositions for the Jishishan medium- and fine-grained diorites and the Bamishan gabbroic and quartz dioritic dykes from the eastern Qilian orogen, to address their petrogenesis and the lithospheric evolution of the Qilian orogen. Laser ablation inductively coupled plasma mass spectrometry U–Pb zircon analyses show that they formed at c. 472–432 Ma. Geochemical and isotopic data suggest that the Jishishan diorites were derived from amphibole-bearing garnet-facies lithospheric mantle metasomatized by slab-derived fluids, whereas the Bamishan gabbroic dyke was derived from phlogopite-bearing garnet-facies lithospheric mantle enriched by sediment-derived melts. The Bamishan quartz dioritic dykes are high-potassic calc-alkaline and were predominantly derived from mafic lower crust. Based on available data, we suggest that the Jishishan diorites were generated during northward subduction of the South Qilian oceanic slab, whereas the Bamishan mafic–intermediate dykes formed in a post-collisional setting related to slab break-off. Moreover, we propose an integrated model for the Early Paleozoic tectonomagmatic evolution of the eastern Central Qilian, in which fluid-induced mantle metasomatism and melting characterized the oceanic subduction stage, whereas melting of varied sources (including subducted oceanic crust and overlying sediments, sediment melt-modified lithospheric mantle and the continental crust) was involved at the post-collisional stage.