| Figure 1. Diagram showing metamorphic facies in pressure-temperature space. The domain of the graph corresponds to circumstances within the Earth's crust and upper mantle. |
A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures.[1] The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure (See diagram in Figure 1).[1] Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area.[1] The boundaries between facies (and corresponding areas on the temperature v. pressure graph) are wide because they are gradational and approximate.[1] The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.[1]