Migmatite

A rock formed under high grade metamorphism by partial melting, called anatexis, that produces a mixture of metamorphic rock and newly formed igneous material within the same body. It develops during prograde metamorphism, as rock is progressively heated and pressurized to the point where it begins to melt, typically in the deeper parts of continental crust and commonly within old Precambrian cratonic blocks. A migmatite is made up of a light colored, quartz and feldspar rich melt portion called the leucosome, a darker residual portion called the melanosome containing minerals such as cordierite, hornblende and biotite, and remnants of the original unmelted rock called the paleosome, sometimes with an intermediate mesosome as well. The rock commonly shows distinctive banding and folded structures, including ptygmatic folds produced as the melt migrated through the rock during ductile deformation, and it is regarded as a transitional stage between metamorphic and igneous rock, central to understanding how continental crust evolves.