metamorphism, metamorphic facies, and metamorphic rocks

metamorphism, metamorphic facies, and metamorphic rocks ‘Metamorphism’ is a term for the alteration that takes place as a rock, formed originally in an igneous or sedimentary environment, recrystallizes to produce a metamorphic rock as a result of changes in temperature, pressure, or the availability of fluids. The grains that make up metamorphic rocks have typically recrystallized during metamorphism, and commonly also include new mineral species formed by metamorphic reactions. Metamorphism normally takes place in a near-solid state: pore fluids play a vital kinetic role in many instances, but are only a very minor component of the rock. As a result, most metamorphic rocks retain some of the characteristics of the parent material, such as bulk chemical composition or bedding. Extreme metamorphism can lead to the onset of partial melting and the formation of migmatites.

Metamorphic changes are of two types: phase changes (crystallization), which reflect the depth and temperature at which metamorphism has taken place, and textural changes (recrystallization), which are particularly sensitive to strain accompanying metamorphism, and affect all rock-types, irrespective of how chemically reactive they are. For example, pure quartzites or pure calcite limestones do not undergo reactions under most metamorphic conditions because quartz and calcite are stable over a very wide range of crustal conditions. They may, however, experience grain growth or other forms of recrystallization, and may develop mineral orientation fabrics that record much information about the deformational history of the rock during metamorphism. Interbedded clay-rich sediments (pelites) are, however, very reactive during heating, because the original clay minerals are of very limited stability, and so develop a sequence of metamorphic minerals that provide a measure of the temperature and pressure at which metamorphism took place.

Types and settings of metamorphism

Most metamorphism takes place as the result of the progressive burial and accompanying heating of sedimentary sequences in deep basins, accretionary prisms, and subduction zones, or as the result of heating around igneous intrusions. These two types of setting are not mutually exclusive, but are the basis for the distinction between regional metamorphism and contact (or thermal) metamorphism. Regional metamorphism has affected large tracts of rock in orogenic belts, and heating is typically accompanied by deep burial and ductile deformation. As a result, the rocks produced are often strongly foliated slates, schists, or gneisses. Pre-existing crystalline basement rocks formed in much earlier orogenic events can also be caught up in younger regional metamorphism of their cover sequence: for example, the Hercynian massifs of the Alps. Magmas may provide a component of the heating of regional metamorphism, but the classic contact metamorphism caused by the heat of intrusions is unaccompanied by any deformation. Other types of metamorphism are generally rather localized. Hydrothermal metamorphism is caused by the action of circulating hot water, leading to hydration of any original igneous material and growth of cements and veins of minerals stable in the presence of hot, pressurized water (typically at 70–350 °C). Cataclastic metamorphism is the result of intense deformation with little additional heating, and is localized in fault zones. Impact metamorphism takes place at sites of major meteorite impacts; its effects range from melting or even volatilization of the original surface rocks, through the production of dense minerals during the passage of intense shock waves, to the formation of shock-induced deformation structures.

Although it is not possible to see active metamorphism taking place in the same way that we can watch some igneous or sedimentary rocks being formed, hydrothermal metamorphism is occurring near the surface today. For example, in high-temperature geothermal fields, such as those that are exploited for power in Iceland, Italy, New Zealand, and elsewhere, volcanic glass and high-temperature basalt minerals are being actively converted to clays, chlorite, zeolites, epidote, and other minerals that are more stable in the cooler, but wet, environment of the geothermal system at depths of only a few hundred metres. In these settings, drilling into the geothermal system provides both samples and detailed information about the conditions of metamorphism.

Depth and temperature of metamorphism

One of the most interesting things that we can learn from the study of metamorphic rocks is the depth and temperature at which their constituent minerals have grown. This tells us not only about the depths to which they have been buried (and values of about 100 km for the burial of some extreme rock-types accord well with the depth to which continental crust has been reported from modern geophysical measurements), but also about the temperature gradient when they formed. Temperature gradients can be diagnostic of particular tectonic settings, but can also have changed significantly during the early history of the earth. Geologists and chemists at the beginning of the twentieth century had recognized not only that certain minerals were indicative of higher or lower temperatures of metamorphism but also that it was associations or assemblages of minerals, rather than the occurrence of specific individual minerals, that best constrained the conditions under which a rock had formed. V. M. Goldschmidt, and later P. E. Eskola, pioneered the application of the principles of chemical equilibrium to metamorphic mineral assemblages, and this has remained one of the cornerstones of metamorphic petrology. If it can be demonstrated that mineral assemblages formed in equilibrium for the prevailing conditions of pressure (a simple function of depth of burial) and temperature, then laboratory experiments or thermodynamic calculations can in principle be used to quantify those conditions. In practice it has only been in the latter third of the twentieth century that the experimental and latterly the thermodynamic database for metamorphic minerals has been adequate for the purpose in all but the simplest cases, but geothermobarometry—estimating the conditions of formation of a metamorphic assemblage—has now become relatively routine.

Metamorphic grades, zones, and facies

The principal reason for believing that the coexisting metamorphic minerals in a rock (i.e. the metamorphic mineral assemblage) usually form near chemical equilibrium is the observation that, over large areas, rocks of similar chemical composition contain the same assemblage of minerals. In areas such as contact metamorphic aureoles around a pluton, these assemblages may vary systematically towards the heat source in a way that reflects increasing temperature of crystallization. There are a number of ways in which the qualitative variation in the conditions of metamorphism between different rocks can be described. Most generally, metamorphic grade is a useful and rather loosely defined term, used to distinguish rocks that have undergone more extreme conditions of temperature or pressure, or both (high-grade metamorphism), from those that have recrystallized under less extreme conditions (low-grade metamorphism).

Mineral assemblages reflect the initial composition of the rock as well as the conditions of metamorphism, but for any one rock-type it is normally possible to recognize distinct metamorphic zones characterized by a different mineral or assemblage of minerals. A metamorphic zone is a mappable region of rocks in which a particular mineral or association of minerals is present within specific compositions of rock. Metamorphic zones were originally often defined on the basis of the appearance of a specific mineral, known as an index mineral (see regional metamorphism). This index mineral will usually survive to higher metamorphic grades, in the presence of new index minerals indicative of higher metamorphic grades. For example, biotite is the index mineral of the biotite zone, in which it appears at relatively low grades of metamorphism. However, biotite continues to be present in a wide range of metamorphic rocks formed at all but the very highest grades. For this reason, metamorphic zones are generally defined, where possible, by the occurrence of an assemblage of coexisting minerals, rather than on the basis of any one phase; for example, the staurolite zone is characterized by the presence of staurolite in rocks that contain muscovite and quartz but do not contain an aluminium silicate polymorph. Metamorphic zones have been most widely recognized in mica-rich rocks that were originally clay-rich sediments, but zones can also be identified in a number of other rock-types, notably impure limestones and metamorphosed volcanic rocks. The boundaries that can be mapped between zones by recording the distribution of mineral assemblages are known as isograds; they represent surfaces of constant metamorphic grade. Metamorphic zones are generally specific to one particular rock-type; for example, only aluminium-rich pelites normally develop staurolite in the staurolite zone, but some index minerals are more widespread.

In most successions of metamorphic rocks, those with reactive lithologies, such as mature pelites, make up only a small proportion of the rock mass. As a result, while it may be possible to define metamorphic zones precisely in a few places, the zones and their associated isograds cannot be traced through the intervening tracts of ground. The concept of metamorphic facies was introduced by Eskola in order to provide a more general indication of conditions of metamorphism that would be more widely applicable than precisely defined zones. A metamorphic facies is characterized by an assemblage or sub-assemblage of metamorphic minerals, indicative of a specific range of pressure–temperature (P–T) conditions of metamorphism. In general, metamorphic facies have been defined on the basis of the assemblages of metamorphosed basic igneous rocks (metabasites), but equivalent assemblages in other rock compositions are well known. It is interesting to note that at very low grades of metamorphism, where the original igneous mineral assemblages are far removed from their original conditions of formation, a number of discrete zones can be recognized in metamorphosed volcanic rocks, but not in any other rock-types. These zones do not therefore have the status of facies. The approximate ranges of pressure (P) and temperature (T) over which the assemblages of the various facies develop are shown in Fig. 1. The characteristics of the main metamorphic facies can be summarized as follows:

Facies of regional metamorphism (see regional metamorphism.) The most widespread metamorphic facies are those typical of regional metamorphic terrains. The lowest-grade facies, i.e. the zeolite facies and the prehnite– pumpellyite facies, are characterized by the occurrence of hydrous minerals developed from the breakdown of volcanic glass and fragments of igneous minerals, especially in volcanogenic sediments. Massive volcanic rocks are usually too impermeable to undergo significant hydration at these grades, and so may show only local alteration, while siliciclastic sediments undergo normal diagenetic processes at zeolite facies temperatures (below c. 250 °C), and with further heating evolve towards greenschist facies assemblages (white mica and chlorite) without developing any characteristic assemblages at lower grades (e.g. most slates). Increasing regional metamorphic grade is generally accompanied by thorough recrystallization of all but the most massive igneous bodies, with the formation of calcic amphiboles whose chemistry reflects the conditions of formation. Both metabasites and metasediments become progressively dehydrated through the greenschist and amphibolite facies. Mineral abundances generally change progressively, but garnet appears in the upper greenschist facies, while chlorite and muscovite become rarer. Temperatures in the uppermost amphibolite facies are sufficient for melting to take place in dehydrating metasediment, to produce migmatites. Further metamorphism in the granulite facies results in the formation of minerals that are also typical of igneous rocks (e.g. pyroxenes), although some hydrous minerals, notable biotite and hornblende, can persist.

In some regionally metamorphosed terrains, distinct assemblages are present that are indicative of unusually high pressures. In the blueschist facies, the sodic-amphibole glaucophane is diagnostic, most characteristically occurring with lawsonite (a calcium aluminium silicate). In pelitic rocks, biotite is rare or absent, and garnet appears at relatively low grades. Low-grade blueschists may contain aragonite rather than calcite. Eclogite-facies metamorphism is typical of deeply subducted or overthickened crust, and is notable for the absence of plagioclase (see eclogite). In recent years, a number of eclogites have been found to contain evidence for ultra-high-pressure metamorphism, characterized by the presence of the dense silica polymorph coesite, which is preserved as inclusions in garnet; in a few instances, diamonds have also been reported.

Facies of contact metamorphism. The assemblages produced in contact metamorphic aureoles have many similarities to regional assemblages, but dense phases such as garnet are usually absent, while less dense phases such as andalusite and cordierite are widespread in rocks of suitable composition.

Textures of metamorphic rocks

Metamorphic rocks are distinctive for their textures as well as for their mineral assemblages. Regionally metamorphosed rocks commonly recrystallize in response to deformation as well as to changes in pressure and temperature, although the two driving forces can operate together. As a result of the deformation, minerals are commonly aligned in a way that reflects the finite strain over the period during which they recrystallized or grew. Most commonly, platy minerals such as micas are aligned perpendicular to the maximum shortening direction to produce a schistosity. In intensely deformed rocks, many minerals can in fact form aligned tabular grains: quartz, feldspar, and calcite for example. Elongate mineral grains, such as those of amphiboles, may also be aligned in the maximum extension direction, giving rise to a lineation. Many regionally metamorphosed terranes have experienced complex histories of deformation, and fabrics of aligned metamorphic minerals can locally preserve traces of early deformation episodes where they have not been obliterated by later events.

Although contact metamorphism can be accompanied by deformation, many metamorphic aureoles around plutons develop by static recrystallization (i.e. without strain), and so the minerals grow in random orientations, giving rise to a tough rock that does not split in any preferred manner: a hornfels. Hydrothermal metamorphism is likewise commonly without deformation, and often results in the formation of pseudomorphs, in which the patterns of pre-existing minerals or textures are preserved in the shape, distribution, or compositional zoning of newly formed metasomatic minerals.

Many of the distinctive metamorphic minerals that provide indicators of metamorphic grade (e.g. garnet) do not readily align into fabrics, but commonly grow as crystals that are relatively large compared to the rock matrix. Such grains are termed porphyroblasts. It is common for porphyroblasts to contain inclusions of minerals that were present in the matrix when they grew, and in this way they preserve information about the history of metamorphism. Such inclusions may be aligned, preserving evidence of the fabric in the rock when the porphyroblast grew. An important development in the 1950s and 1960s was the recognition of how such textures could be used to deduce the relative timing of burial, heating, and deformation in the development of an orogenic belt.

Metamorphic processes and rates

In the early days of geology, it was assumed that crystalline metamorphic rocks were inevitably older than sedimentary strata, and that their formation required long periods of geological time. It is, however, now known that although the oldest rocks on earth have experienced metamorphism, there are also much younger metamorphic belts exposed at the surface of the planet, some affecting rocks of Cretaceous age or younger. The youngest belts doubtless remain at depth and are continuing to evolve today. Geochronology indicates that while many metamorphic terranes have experienced long histories of crystallization, spanning hundreds of millions of years, individual metamorphic cycles probably have durations of the order of a few tens of millions of years. Progressive burial and metamorphism of sedimentary sequences consumes large amounts of heat, for the reactions are strongly endothermic. It follows from simple heat-flow considerations that heating rates on a regional scale will be of the order of a few degrees to a few tens of degrees per million years. As geochronologists developed techniques for dating metamorphic index minerals themselves, it became possible to address this question directly, and in a pioneering study, K. W. Burton and R. K. O'Nions showed in 1991 that garnet-bearing rocks from the Norwegian Caledonides had indeed been heated up at close to 10 degrees per million years.

Nomenclature of metamorphic rocks

It is impossible to have a rigidly defined system of nomenclature for metamorphic rocks, partly because of the complex history of events that give rise to them, and partly because of the range of purposes for which the name is required. Metamorphic rock names may reflect any or all of the following: the nature of the parent rock; the grade of metamorphism and the minerals present; the texture of the rock. There are also a few special rock names which encompass several of these factors.

The simplest metamorphic rock names are obtained by adding the prefix meta- to the original rock type, as in metagabbro, metabasite, metagranite. For most metasediments, however, there are specific names: marble for metalimestone, or pelite for metaclay. These may be qualified by mineral names to indicate the distinctive members of the mineral assemblage, either based on abundance, to indicate rock composition and aid stratigraphical correlation, or based on the distinctive association that best defines the P–T conditions of metamorphism. In high-grade gneiss terranes, it can be impossible to deduce much about the nature of the parent rock-type in detail, and the term paragneiss is then used to denote a rock with a presumed sedimentary parent; orthogneiss implies an igneous origin. High-grade metamorphic rocks can be very difficult to distinguish from igneous rocks that crystallized at depth.

Textural terms are also an important component of metamorphic rock names, because the texture dominates the appearance of the rock in hand specimen. Foliated rocks may be slate, phyllite, or schist according to grain size. Gneiss is variously used for a uniformly coarse-grained high-grade metamorphic rock lacking any marked foliation and for a banded high-grade rock in which the foliation is defined by alternating bands of distinct composition, rather than by mineral alignment. The term granulite can also be applied to many high-grade rocks; it carries specific connotations of formation under conditions of granulite-facies metamorphism.

Specific rock names that record both the parent rock and its mineralogy are relatively rare, but include eclogite, blueschist, and amphibolite. These are all varieties of metabasite; eclogite is dominated by sodic pyroxene and garnet, and lacks plagioclase; blueschist is named for a slaty blue-grey colour resulting from the presence of glaucophane; and amphibolite denotes a rock dominated by hornblende and plagioclase. There is a complex nomenclature for rocks of the granulite facies that includes a number of traditional names that lack any rational basis and appear to be applied to rocks of both metamorphic and igneous origin. The most commonly encountered is charnockite, a coarse-grained granitic rock containing both orthopyroxene and potassium-feldspar.

Bruce W. D. Yardley

Bibliography

Spry, A. (1969) Metamorphic textures. Pergammon Press, Oxford.
Yardley, B. W. D. (1989) An introduction to metamorphic petrology. Longman, Harlow.
Yardley, B. W. D.,, MacKenzie, W. S.,, and and Guildford, C. (1990) Atlas of metamorphic rocks and their textures. Longmans, Harlow.