Interaction between oceanic slab and metasomatized mantle wedge: Constraints from sodic lavas from the Qilian Orogen, NW China

Generation of arc volcanic rocks is a complicated process, potentially involving some or all of oceanic slab subduction, mantle wedge metasomatism, and melting of both the subducted slab and the mantle wedge. Interaction between melts of different sources plays important roles in the subduction factory and the formation of magmatic arcs and is still controversial. To elucidate the mantle-melts interaction process, we present a study of the sodic adakite-like lavas (445-444 Ma) and basalt-basaltic andesite (basaltic) enclaves (similar to 453 Ma) in the early Paleozoic intra-oceanic arc system from the South Qilian Accretionary Belt, Qilian Orogen. Basaltic enclaves in the Muli adakite-like lavas have high Mg# (62-69) high Cr and Ni, and radiogenic Sr and unradiogenic Nd isotopic compositions (I-sr = 0.706327-0.706497; epsilon(Nd)(t) = -3.2 similar to - 2.0). They are considered to represent primitive magmas derived from a highly metasomatized mantle wedge. The adakite-like lavas are dacitic with adakite-like compositions, enriched Sr-Nd isotopic compositions (I-sr = 0.705597-0.706747; epsilon(Nd)(t) = -1.7 similar to + 0.4) and significantly variable zircon Hf isotopic compositions (epsilon(Hf)(t)= +0.9 similar to + 17.8), most likely a hybrid magma produced by mixing primitive basaltic melts of metasomatized mantle wedge and melts of subducted oceanic crust. The Muli adakite-like lavas and basaltic enclaves occurred in an intra-oceanic island arc setting with an unusually hot thermal structure, which induced melting of subducted slab (including sediments and mafic oceanic crust) and metasomatized mantle wedge. The geodynamic mechanism associated with slab melting is plausibly either or both of the juvenile nature of South Qilian Oceanic Crust, and/or the heating of a slab edge by upwelling hot asthenospheric mantle during subduction initiation. (C) 2019 Elsevier B.V. All rights reserved.