How are metamorphic rocks classified?

1 Answer
Dec 7, 2015

Metamorphic facies diagram


We classify metamorphic rocks generally on their stable mineral assemblage, see image below for different metamorphic facies in pressure-temperature space:

Carleton College

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:

Blatt et al., 2006

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!