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Types of metamorphism

Different kinds of metamorphism may be defined on the basis of different criteria, including (1) the spatial extent over which metamorphism occurred, (2) the geologic setting of metamorphism, (3) the specific cause of metamorphism, (4) whether a rock equilibrated to a single event or if one can discern superposed overprints, (5) whether or not significant change in composition occurred, and (6) whether all or part of the mineral assemblage developed in response to increasing or decreasing temperature. Classifications of metamorphism therefore vary with the proclivity of the author. The following classification is an attempt at compromise between the various viewpoints.

Regional metamorphism occurs over an area of great extent and affects a large volume of rock. It is thus associated with large-scale processes, such as sea-floor spreading or mountain building (orogeny). Local metamorphism is restricted to far more limited rock volumes and can typically be related to a local cause/source such as a magmatic intrusion, and fault zone. When it is possible to relate metamorphism to a particular cause/source (as is generally the case) the following classification takes precedence.

1. Orogenic metamorphism is a type of regional metamorphism related to the development of mountain belts. Dynamic and thermal effects are combined in varying proportions over a wide range of P-T conditions, typically resulting in foliated metamorphic rocks such as slate, schist, and gneiss (listed in order of increasing grade). Multiple deformational and thermal phases may occur, resulting in several metamorphic overprints. Most exposed metamorphic rocks belong to this category.  See also: Gneiss; Schist; Slate

2. Burial metamorphism is a type of regional metamorphism developed in rocks deeply buried under a sedimentary and/or volcanic pile and is typically not associated with orogenic deformation or extensive magmatic intrusion. It commonly involves only low to intermediate metamorphic temperatures and low pressures, resulting in nonfoliated or poorly foliated rocks.

3. Ocean-floor metamorphism is a type of regional metamorphism developed near oceanic spreading centers in response to the high local heat flow and extensive circulation of heated seawater along pervasive fractures.

4. Contact metamorphism is a type of local metamorphism that affects the host rocks adjacent to a magma body. It is typically caused by heat transfer, perhaps accompanied by fluid emanations, from a cooling magmatic intrusion. Metamorphic pressures are generally low, but temperatures vary widely. Textures are predominantly nonfoliated, except in cases of substantial dynamic effects due to the intrusion or when the host rocks are already orogenically metamorphosed and the contact effects are superimposed.

5. Fault-zone metamorphism is a type of local metamorphism associated with fault or shear zones. Dynamic effects predominate, resulting in grain-size reduction and textures governed by the interaction of deformation and recrystallization (typically strain induced).  See also: Earthquake; Fault and fault structures

6. Impact metamorphism is a type of local metamorphism caused by the impact of a meteorite or other projectile. Extreme cases may result in melting and even vaporization.

7. Pyrometamorphism is an extreme type of contact metamorphism characterized by very high temperatures at relatively low pressures, generated by a very hot volcanic or subvolcanic body. It is most typically developed in xenoliths enclosed in such bodies, but may also occur at wall-rock contacts. It is typically accompanied by varying degrees of partial melting.  See also: Xenolith

8. Hydrothermal metamorphism is a type of metamorphism, typically of local extent, caused by hot water-rich fluids. A common situation for this type of metamorphism is in geothermal fields above cooling magmatic intrusions.

9. Polymetamorphism applies to any situation in which one metamorphic event can be demonstrated to partially overprint a previous metamorphism, while retaining relics of the original event.

10. Metasomatism is any metamorphism accompanied by significant chemical alteration. This typically occurs in situations involving fluid emanations from a crystallizing magmatic intrusion, but may encompass a variety of situations involving contrasting rock compositions and/or fluid mobility.  See also: Magma; Metasomatism

11. Prograde metamorphism refers to the changes in a rock that accompany increasing metamorphic grade (the usual case).

12. Retrograde metamorphism refers to changes that accompany decreasing grade as a body of rock cools and recovers from a metamorphic or igneous event. Equilibrium is better maintained during prograde metamorphism when energy is being added, and retrograde effects are generally minor and incomplete. Most metamorphic rocks thus retain the imprint of the maximum metamorphic temperatures attained.

Metamorphic gradients and zonation

Metamorphism is a response to changes in external conditions. There are gradients in temperature, pressure, and fluid composition in nature, so we can expect some sort of zonation in the mineral assemblages constituting the rocks that equilibrate across an expanse of these gradients. We should thus be able to traverse into an eroded metamorphic area and cross from nonmetamorphosed rocks through zones of progressively higher metamorphic grades. Figure 1 shows a cross section through a portion of a contact aureole surrounding an intrusive igneous granite. The granite intrudes into the shallow crust at temperatures in excess of 650°C (1200°F), and a gradient in temperature develops as the granite heats the adjacent sediments. Most granites also release water as they cool and crystallize, which may also set up a gradient in fluid composition across the aureole. Because the different sedimentary layers are of contrasting composition, they respond differently to the thermal and fluid gradients. At the outer limits of rocks affected, shale may develop small crystals of muscovite and chlorite. At higher temperatures inward toward the granite, shale may progressively develop larger grains of biotite + andalusite, cordierite + sillimanite. Very near the contact, the original shale may be thoroughly recrystallized to a hornfels containing coarse quartz, cordierite, and sillimanite. Because limestone and sandstone are composed of virtually a single mineral each [limestone is calcite (CaCO3) and sandstone is quartz (SiO2)], they are less reactive and the principal change developed in these layers is recrystallization of the existing minerals to produce progressively coarser marble and quartzite at higher temperatures. Only very near the granite may some of the released silica-bearing aqueous fluids interact with the limestone to form calcium-silicate minerals (skarn). Figure 2 shows a similar cross section through a mountain range that has been uplifted and eroded to expose rocks that have experienced regional metamorphism. Here gradients in both temperature and pressure over much larger distances (typically hundreds of km) cause similar, but larger-scale transitions in the mineral assemblages and textures of the affected rocks.  See also: Andalusite; Contact aureole; Cordierite; Granite; Limestone; Muscovite; Sandstone; Shale; Sillimanite; Skarn

Fig. 1  Diagrammatic cross section through a contact-metamorphic aureole surrounding an intrusive igneous granite.

Fig. 2  Cross section through an uplifted and eroded mountain belt, exposing the orogenically metamorphosed rocks.

The shales in Fig. 1 exhibit four metamorphic zones of progressively higher-grade metamorphism, and the regionally metamorphosed area in Fig. 2 exhibits four zones of much greater areal extent. The transition from one zone to another is typically caused by changing conditions; hence, a reaction between some or all of the lower-grade minerals to produce the higher-grade assemblage. The transition in the field across which this reaction occurs is called an isograd (line of constant metamorphic grade). George Barrow pioneered the mapping of such zones in orogenically metamorphosed shales in the Scottish Highlands. Barrow named the Scottish zones based on the first appearance of a characteristic index mineral developed by a metamorphic reaction marking the transition into that zone. The lower limit of any zone in the field was thus marked by an isograd named for the index mineral that first appeared at that isograd. Barrow's now-classical isograds and zones (in order of increasing grade) are chlorite, biotite, garnet, staurolite, kyanite, and sillimanite, but it is common practice to name zones on locally recognized mineral sequences.  See also: Garnet; Kyanite; Staurolite

Metamorphic facies

Pentii Eskola developed the concept of metamorphic facies as an extension of the zonal method of characterizing metamorphism and the grades developed. This approach is based on the typical zones developed in metamorphosed basaltic igneous rocks (which span relatively broad ranges of conditions) and attempts to assign pressure and temperature limits to those zones. Figure 3 is a pressure-temperature (P-T) diagram illustrating the most commonly accepted metamorphic facies and the P-T conditions appropriate to each.  See also: Facies (geology)

Fig. 3  Pressure-temperature diagram showing the generally accepted limits of the various facies. Boundaries are approximate and gradational. Included are the typical, or average, continental geotherm and the minimum melting curve for water-saturated granite. Broad gray swaths (1–3) represent metamorphic field gradients from three contrasting terranes. 1, average gradient for orogenic metamorphism in the Scottish Highlands; 2, gradient developed in contact aureoles above large granitic plutons; 3, gradiant from a subduction zone complex (Franciscan formation of California).

In Fig. 3, P-T trajectory 1 is representative of orogenic regional metamorphism and suggests that the typical facies series begins in the low-grade zeolite facies and extends progressively through the prehnite-pumpellyite facies, the greenschist facies, and the amphibolite facies. Large-scale melting is common in the upper amphibolite facies if sufficient water is available, and only dehydrated rocks make it into the granulite facies. Contact metamorphism, developed in aureoles around magmatic plutons, follows a P-T trajectory similar to 2 in Fig. 3, with a series of hornfels facies. Regional metamorphism accompanying the subduction of a cool plate is characterized by an unusually high P/T ratio (similar to trajectory 3 in Fig. 3), resulting in a series culminating in the blueschist facies. Very deep rocks, generally at mantle pressures, are typically metamorphosed in the eclogite facies (along the normal geotherm). It is common practice to combine the facies and zone approaches. Barrow's chlorite zone, for example, is in the greenschist facies and his staurolite zone is in the amphibolite facies.