REGIONAL METAMORPHISM 407 REEFS See SEDIMENTARY ENVIRONMENTS: Reefs (‘Build-Ups’) REGIONAL METAMORPHISM A Feenstra, GeoForschungsZentrum Potsdam, Potsdam, Germany G Franz, Technische Universitaăt Berlin, Berlin, Germany ò 2005, Elsevier Ltd All Rights Reserved Introduction Regional metamorphic rocks form from other rocks (protoliths) by changes in mineralogy and texture in response to changing physical conditions (temperature, lithostatic pressure, and, in most cases, shear stress) These changes are essentially solid-state reactions, but very often a fluid phase is present, either participating in the reaction or as a reaction medium Many regional metamorphic rocks have a chemical composition that is very similar to that of their sedimentary or igneous precursors, with the exception of removal or addition of volatiles (mainly H2O and CO2) This type of behaviour is termed ‘isochemical metamorphism’ Metamorphism may also take place as a result of a change in chemical environment; this may occur by transport of elements between chemically contrasting rock types (e.g., formation of calcsilicate minerals at a quartzite–marble contact) or by circulation of fluids that dissolve some substances and precipitate others This process of significant chemical change during metamorphism is known as ‘allochemical metamorphism’ or ‘metasomatism’, and rocks formed in this manner are metasomatic rocks Metasomatism is, however, mostly of local significance, and the total volume of metasomatic rocks in regional metamorphic terranes is rather minor The distinction between metasomatism and isochemical metamorphism is also a matter of scale On the scale of individual grains, mass transport takes place during all phase transformations; on the scale of a thin section, it is probably the rule for regional metamorphism; on the scale of a hand (-sized) specimen, it can be observed frequently; and on a larger scale, it is the exception Regional metamorphism is nearly always accompanied by deformation and folding, which happens at converging plate boundaries and leads to orogenic belts Therefore, regional metamorphic terranes follow the contacts between ancient lithospheric plates (suture zones), typically have an elongated shape that is parallel to the mountain belt, and cover areas of several 1000 to 10 000 km2 Two basic types of regional (orogenic) metamorphism may be distinguished The collision type relates to the collision of essentially continental plates and mainly produces metamorphic rocks at medium pressures and medium to high temperatures, often associated with partially molten (migmatites) and magmatic rocks High- and ultrahigh-pressure terranes (P % 1.5 GPa up to >3 GPa) from the deep roots of the collision zone can be preserved within the medium pressure (