Subduction of continental crust, the origin of post-orogenic granitoids (and anorthosites?) and the evolution of Fennoscandia

Slices of silica-rich continental crust subducted into the mantle during collision may undergo metamorphism and exhumation towards the surface as coherent high-pressure or ultrahigh-pressure (HP or UHP) terranes or, if stalled in the mantle, melting and return towards the surface as magmas, or a combination of these two processes. Some exposed HP or UHP terranes contain anatectic granitoids demonstrating that melting does occur during exhumation. Therefore crust trapped in the mantle should also melt when radioactive heating and/or conductive heating raise temperatures to the appropriate solidus. Terranes with hydrous phases will melt readily through hydrate-breakdown reactions. Terranes lacking hydrous phases may require adiabatic decompression to melt, possibly as heated quartz-rich crust becomes ductile and rises diapirically. The magmas generated will intrude the overlying plate to form late-, post- and possibly anorogenic granitoids, depending on the time required to reach solidus temperatures. Geochemical characteristics will depend on P-T conditions, the chemistry and mineralogy of the subducted terrane (especially the presence of hydrous phases), and the amount of melt interaction with the mantle. The removal of sialic upper crust may strand the denser mafic lower crust, which subsequently could melt to generate anorogenic anorthosites, Fe-K granitoids and related rocks. The evolution of the Fennoscandian Shield documents a change from slab melting in the Mesoproterozoic to combined melting and exhumation in the Neoproterozoic to intact exhumation without significant melting in the Palaeozoic.