Metamorphism involves change to a preexisting rock. There are three types:
Where country rock (rock that is in the area where metamorphism occurs) is intruded by magma, there is a zone of heating around the magmatic body, causing local change in a zone called a contact metamorphic aureole.
In the rocks near active metamorphism, water in the pore spaces and fractures in rocks gets heated up. If it wasn't loaded with ions already (if there weren't dissolved chemical elements in the water already), the warming helps water to take on more ions, and chemical reactions happen that otherwise wouldn't, resulting in concentration of new mineral growth in zones where the water moves.
There are several plate tectonic situations where metamorphism happens on large scales. The most extreme is found along the collision of plates in subduction zones and in continent-to-continent collisions. In addition to heat at depth, and the forcing down of cool rock into hot areas, there is great pressure that causes new alignment of existing minerals, and alignment of new minerals as they grow.