Migmatite

"Migmatite is a composite rock found in medium and high-grade metamorphic environments. It consists of two, or more constituents often layered repetitively; one layer was formerly paleosome, a metamorphic rock that was reconstituted subsequently by partial melting; the alternate layer has a pegmatitic, aplitic, granitic or generally plutonic appearance. Commonly, migmatites occur below deformed metamorphic rocks that represent the base of eroded mountain chains, commonly within Precambrian cratonic blocks,

Migmatites form under extreme temperature and pressure conditions during prograde metamorphism, when partial melting occurs in metamorphic paleosome. Components exsolved by partial melting are called neosome (meaning ‘new rock’), which may or may not be heterogeneous at the microscopic to macroscopic scale. Migmatites often appear as tightly, incoherently folded veins (ptygmatic folds). These form segregations of leucosome, light-colored granitic components exsolved within melanosome, a dark colored amphibole and biotite-rich setting. If present, a mesosome, intermediate in color between a leucosome and melanosome, forms a more or less unmodified remnant of the metamorphic parent rock paleosome. The light-colored components often give the appearance of having been molten and mobilized." - (en.wikipedia.org 21.11.2019)