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Based on the updated results of experimental petrology and phase equilibria modelling and combined with the available thermal structure models of subduction zones, this paper presents an overview on the dehydration and melting of basic, sedimentary and ultrabasic rocks that occur in the different stages during oceanic subduction processes and their influences on magmatism above subduction zones. During the subduction at the forearc depth of <90–100 km, the basic and ultrabasic rocks from most oceanic slabs can release very small amounts of water, and significant dehydration may occur in the slab superficial sediments. Strong dehydration occurs in both basic and ultrabasic rocks during subduction at the subarc depth of 90–200 km. For example, more than 90% water in basic rocks is released by the successive dehydration of chlorite, glaucophane, talc and lawsonite in the subarc depths. This is diversely in contrast to the previous results from synthetic experiments. Ultrabasic rocks may undergo strong dehydration through antigorite, chlorite and phase 10 Å at the subarc depth of 120–220 km. However, sediments can contribute minor fluids at the subarc depth, one main hydrous mineral in which is phengite (muscovite). It can stabilize to ∼300 km depth and transform into K-hollandite. After phengite breaks down, there will be no significant fluid release from oceanic slab until it is subducted to the mantle transition zone. In a few hot subduction zones, partial melting (especially flux melting) can occur in both sediments and basic rocks, generating hydrous granitic melts or supercritical fluids, and in carbonates-bearing sediments potassic carbonatite melts can be generated. In a few cold subduction zones, phase A occurs in ultrabasic rocks, which can bring water deep into the transition zone. The subducted rocks, especially the sediments, contain large quantities of incompatible minor and trace elements carried through fluids to greatly influence the geochemical compositions of the magma in subduction zones. As the geothermal gradients of subduction zones cannot cross the solidi of carbonated eclogite and peridotite during the subarc subduction stage, the carbonate minerals in them can be carried into the deep mantle. Carbonated eclogite can melt to generate alkali-rich carbonatite melts at >400 km depth, while carbonated peridotite will not melt in the mantle transition zone below a subduction zone. [ABSTRACT FROM AUTHOR]

The subduction channel is defined as a planar to wedge-like area of variable size, internal structure and composition, which forms between the upper and lower plates during slab subduction into the mantle. The materials in the channel may experience complex pressure, temperature, stress and strain evolution, as well as strong fluid and melt activity. A certain amount of these materials may subduct to and later exhume from >100 km depth, forming high to ultra-high pressure rocks on the surface as widely discovered in nature. Rock deformation in the channel is strongly assisted by metamorphic fluids activities, which change composition and mechanical properties of rocks and thus affect their subduction and exhumation histories. In this study, we investigate the detailed structure and dynamics of both oceanic and continental subduction channels, by conducting high-resolution petrological-thermomechanical numerical simulations taking into account fluid and melt activities. The numerical results demonstrate that subduction channels are composed of a tectonic rock melange formed by crustal rocks detached from the subducting slab and the hydrated mantle rocks scratched from the overriding plate. These rocks may either extrude sub-vertically upward through the mantle wedge to the crust of the upper plate, or exhume along the subduction channel to the surface near the suture zone. Based on our numerical results, we first analyze similarities and differences between oceanic and continental subduction channels. We further compare numerical models with and without fluid and melt activity and demonstrate that this activity results in strong weakening and deformation of overriding lithosphere. Finally, we show that fast convergence of orogens subjected to fluid and melt activity leads to strong deformation of the overriding lithosphere and the topography builds up mainly on the overriding plate. In contrast, slow convergence of such orogens leads to very limited deformation of the overriding lithosphere and the mountain building mainly occurs on the subducting plate. [ABSTRACT FROM AUTHOR]