What is the metamorphic facies concept?
Metamorphic facies concept – the mineral composition of a metamorphic rock reflects the rock type of the protolith (the original rock that was metamorphosed) AND metamorphic grade. This concept helps geologists determine things like what the P,T of metamorphism were and what the protolith was. That information can help answer questions like – how deeply were rocks buried (i.e. how tall was an ancient mountain belt), and what type(s) of environment(s) or plate boundaries existed prior to or during metamorphism.