By exposure to heat and/or pressure through depth of burial, directed pressure, or proximity to a magma source. Existing minerals can recrystallize, non-hydrous minerals can replace hydrous minerals,...
Igneous rock, when exposed, will weather into smaller particles by mechanical and chemical means. Eventually the particles will be eroded and be transported by wind, water, ice, and gravity and...
The direct route would be for the metamorphic rocks to become exposed at the surface and undergo weathering and erosion. The particles broken loose and transported by those processes could eventually...