Metamorphic facies

A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. 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). Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies (and corresponding areas on the temperature v. pressure graph) are wide because they are gradational and approximate. 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. The name facies was first used for specific sedimentary environments in sedimentary rocks by Swiss geologist Amanz Gressly in 1838. Analogous with these sedimentary facies a number of metamorphic facies were proposed in 1920 by Finnish petrologist Pentti Eskola. Eskola's classification was refined by New-Zealand geologist Francis John Turner throughout his career. A classic work of Turner's was the book he published in 1948 titled Mineralogical and Structural Evolution of Metamorphic Rocks. Turner continued to work in the field, refining the metamorphic facies classifications through the end of his career in the early 1970s. The different metamorphic facies are defined by the mineralogical composition of a rock. When the temperature or pressure in a rock body change, the rock can cross into a different facies and some minerals become stable while others become unstable or metastable. Whether minerals really react depends on the reaction kinetics, the activation energy of the reaction and how much fluid is present in the rock. The minerals in a metamorphic rock and their age relations can be studied by optical microscopy or scanning electron microscopy of thin sections of the rock. Apart from the metamorphic facies of a rock, a whole terrane can be described by the abbreviations LT, MT, HT, LP, MP, HP (from low, medium or high; pressure or temperature). Since the 1980s the term UHP (ultra high pressure) has been used for rocks that experienced extreme pressures. Which minerals grow in a rock is also dependent upon the original composition of the protolith (the original rock before metamorphosis). Carbonate rocks have a different composition than a basalt lava, the minerals that can grow in them are different too. Therefore, a metapsammite and a metapelite will have different mineralogical compositions even though they are in the same metamorphic facies.