ABSTRACT
Partial melting represents the broad boundary between metamorphic and igneous realms. At moderate to high-grade metamorphic conditions (middle amphibolite to granulite facies) many rocks can undergo partial melting. The melt initially coats grain boundaries and, if the melt does not separate from the rock, it will eventually crystallize in situ back onto adjacent unmelted grains. This may leave behind no distinctive textures to indicate that melting ever took place. Commonly, however, the grain boundary melt separates from the rock by segregation into extensional fractures, where the melt prevents fracture closure because it is at or near lithostatic pressure. The residual rock, plus crystallized melt that traveled distances of perhaps centimeters or meters, is the migmatite. Melting can be initiated by rising temperature, decreasing lithostatic pressure, H2O-releasing dehydration reactions, or an influx of fluid from an outside source.
