How are metamorphic rocks classified?
Metamorphic facies diagram
We classify metamorphic rocks generally on their stable mineral assemblage, see image below for different metamorphic facies in pressure-temperature space:
Each of these different regions in pressure-temperature space have characteristic mineral assemblages. The table below shows what mineral assemblages belong to which metamorphic grade:
So, if my metamorphic rock contains hornblende, plagioclase, sphene (titanite) and ilmenite then I would have an amphibolite. The original rock type (before metamorphism, we call it a protolith) would be a mafic rock.
Sounds simple, yet it is always complicated in real rocks!