Chemical geodynamics of mafic magmatism above subduction zones

•Mafic magmatism above oceanic and continental subduction zones proceeds in two-stage processes.•Crustal signatures are transferred by liquid phases into the mantle source of mafic igneous rocks.•The fractionation of melt-mobile trace elements is significant at the slab-mantle interface.•Metasomatism is the physicochemical mechanism for the geochemical transfer in subduction zones.•The melting of rocks is dictated by the thermal structure of subduction zones at different depths.