402 METAMORPHIC ROCKS/Facies and Zones Stoă ffler D and Grieve RAF (2003) Towards a unified no menclature of metamorphism: Impactites A proposal on behalf of the IUGS Subcommission on the Systemat ics of Metamorphic Rocks http://www.bgs.ac.uk/SCMR/ scmr products.html Wimmenauer W and Bryhni I (2002) Towards a unified nomenclature of metamorphism: Migmatites and related rocks A proposal on behalf of the IUGS Subcommis sion on the Systematics of Metamorphic Rocks http:// www.bgs.ac.uk/SCMR/scmr products.html Winter JD (2001) An Introduction to Igneous and Meta morphic Petrology, 1st edn London: Prentice Hall Yardley BWD (1996) An Introduction to Metamorphic Petrology Addison Wesley Publication Company Facies and Zones K Bucher, University of Freiburg, Freiburg, Germany ß 2005, Elsevier Ltd All Rights Reserved Intensity of Metamorphism The intensity of metamorphism is mainly related to the pressure and temperature conditions prevailing during rock transformation A rock recrystallized at 800 C in the lower crust contains characteristic minerals or mineral assemblages that reflect the high PT conditions under which the rock formed In contrast, a shale in the shallow upper crust equilibrates perhaps at 300 C and is said to be weakly metamorphosed It is of prime interest in geology to characterize the intensity of metamorphism and to use a reliable classification system to compare it within and between metamorphic terrains The metamorphic facies system is such a tool for characterizing the intensity or grade of metamorphism (see Metamorphic Rocks: Classification, Nomenclature and Formation) The distinctive grade of metamorphism of rocks from an outcrop can be evaluated and described by a large variety of techniques and methods Because pressure and temperature are the main variables that control metamorphism, one could simple determine P and T for each collected sample by using methods of geological thermobarometry This is, however, a tedious and costly affair and not necessary, except for special research studies The metamorphic facies system is a much simpler system that can be used in the field and normally does not require laboratory work or even thin section observation The Metamorphic Facies Concept The concept of metamorphic facies, as it is used today, is simple in principle It makes use of the fact, that at a given pressure and temperature the chemical components of a rock are distributed among a unique set of minerals with fixed compositions This is an inevitable consequence of chemical thermodynamics, provided the rocks reach a state of equilibrium The unique sets of minerals that form in a reference material at one P and T can be compared with assembages of minerals that formed in other rock compositions under the same P and T The collective sets of mineral assemblages that form under the same conditions in many kinds of rocks are called a metamorphic facies The reference material that is used in the metamorphic facies scheme is tholeiitic basalt (mid-ocean ridge basalt or MORB) because of its well-defined composition and global abundance Mafic rocks of basaltic composition are extremely widespread and metamorphosed basalts (metabasalt) constitute a major rock type in orogenic belts, the oceanic lithosphere, and continental crust For example, at an outcrop one finds a mafic rock consisting of hornblende (Hbl) and plagioclase (Pl, e.g., andesine), that is termed an amphibolite A metapelitic rock exposed at the same outcrop consists of kyanite, staurolite, biotite, muscovite, and quartz, and an associated band of dolomite marble contains dolomite, calcite, tremolite, and diopside All three kinds of rocks with very diverse compositions have been metamorphosed under the same pressure and temperature conditions They are of the same metamorphic grade At this outcrop, the reference material consists of Hbl ỵ Pl and hence the set of rocks with their characteristic mineral assemblages each belong to the same metamorphic facies The facies name could be the hornblende-plagioclase facies For simplicity and because Hbl ỵ Pl rocks are amphibolites, the name amphibolite facies is used for conditions under which Hbl and Pl are formed in the reference material, namely MOR basalt The kyanite-staurolite micaschist at the same outcrop has also been metamorphosed under amphibolite facies conditions, i.e., where basalt is transformed into amphibolite, and the micaschist is also an amphibolite facies rock However, it contains no hornblende and no andesine Similarly, the