Metamorphism and its types

Metamorphism

Metamorphism may be defined as the process of transformation of igneous or sedimentary rocks to metamorphic rocks ( changed rocks ) through the Earth’s movements ( producing heat and pressure). The metamorphism can be thermal when the intrusion of the hot mass of igneous rock raises the temperature of surrounding rocks. Similarly, these can be the contact metamorphism when flowing when action by magma affects rocks in the zone near the batholiths or dike. 

Metamorphism produces changes in the minerals. Mostly the change reshapes the rock particles along parallel planes which are different from the bedding. It is called flow cleavage and its best example is slate. Moreover, through the act of metamorphism fine-grained sediments such as shale are turned into Slates and Schists, coarse-grained or crystalline rocks form Quartzite, Gneisses, and Granulites, and most of the rocks show foliation. The processes which operate together in the affected rock to bring about metamorphism are (i) granulation, (ii) plastic deformation, (iii) recrystallization, and (iv) metasomatism.

Types of Metamorphism

On the basis of the agents which bring about the transformation in the rocks, metamorphism can be divided into four types:

• Contact Metamorphism (thermal)
• Regional Metamorphism
• Shock Metamorphism
• Hydrothermal Metamorphism
• Dynamic Metamorphism
• Cataclastic Metamorphism
• Plutonic Metamprhism

(1). Contact Metamorphism:

Contact metamorphism is called “thermal metamorphism”. This metamorphism is caused due to local heating of rocks by the intrusion of hot igneous bodies nearby. The zone of the contact metamorphic rocks which occur surrounding the intrusion is called “aureole”. As temperature decreases away from the intrusive, the outer rocks in the aureole are less intensely metamorphosed than that of the innermost rocks. Thus depending upon the degree of alteration, the rocks in the aureole can be divided into concentric zones, which may differ greatly in mineral assemblages.

In contact-metamorphism heat plays a dominant role and its general effect is to promote recrystallization. In this process, minerals grow haphazardly in all directions and the metamorphic rock acquires a granular fabric which is called the “hornfels textures”. Contact metamorphic rocks do not show schistosity.

During contact metamorphism transfer of magmatic vapours and gases from an igneous body into the country rocks often takes place. These emanations react with the country rocks and form new minerals. Such a process is called “pneumatolytic metamorphism”. A localized burning or baking effect may be produced at the contact of an igneous body and the country rocks. This effect is described as “pyrometamorphism”.

(2). Regional Metamorphism:

When directed pressure and heat act together in the presence of migrating hydrothermal fluids, the rocks are metamorphosed over wider areas. This kind of metamorphism is called the “regional” or “dynamothermal metamorphism”. Regional metamorphism takes place at great depths, such as in root regions of fold mountains, where temperatures and stresses are high.

Heat promotes recrystallization and the stresses cause shearing and flow movements which produce new structures in rocks. The new minerals that grow under directed pressure are usually flat, elongated bladed, or flaky in nature. examples of such minerals are muscovite, biotite, chlorite, tale, and amphibole. These minerals arrange themselves in parallel layers and produce a banded or laminated structure, called “foliation”. The most common foliated metamorphic rocks are slates, phyllites, schists, and gneisses. Foliated rocks split easily into flaky sheets. When shales are subjected to regional metamorphism, the characteristic minerals that develop in succession with the rise in temperature and stress are chlorite, biotite, garnet, staurolite, kyanite, and sillimanite. Thus shale changes to “slate” in the early stages, to “schist” in the middle stage, and finally to “gneiss” at the highest temperatures of regional metamorphism. The schist commonly contains staurolite, garnet, biotite, muscovite, and quartz and the gneiss contains sillimanite, garnet, cordierite, potash felspar, and quartz.

During regional metamorphism sandstones and limestones do not form foliated rocks. They recrystallize into “quartzite” and “marble” respectively. Both of these rocks show granular structures.

Basic igneous rocks, such as basalt, dolerite, or diabase during different grades of regional metamorphism, change into green-schists, amphibolites granulites, or eclogites as follows.

1. During low grades of regional metamorphism, basic igneous rocks change into “green-schists” containing mainly chlorite, albite, epidote.
2. At medium to high grades of metamorphism, green-schists are converted into “amphibole”. Amphibolites are coarse-grained rocks containing garnet, hornblende, or diopside, and plagioclase. Schistosity or bands of dark-colored minerals may be present.
3. At the highest metamorphic grades, “granulites” are formed. Granulites possess a granular texture and contain plagioclase, hypersthene, and diopsite. “Charnochites” are the most common granulite-type rocks which are formed from basic igneous rocks. Hypersthene is the characteristic mineral of charnockites.
4. In the most deep-seated conditions where very high pressures prevail, a red and green rock called “ecoloite” is formed. Eclogite is a coarse-grained granulose rock consisting of pyrope garnet and omphacite (pyroxene). Though eclogites are often classified as metamorphic rocks, their origin is rather obscure. They are also believed to be of igneous origin.

(3). Hydrothermal Metamorphism:

When the rocks come in contact with high-temperature fluid variated composition, they result in hydrothermal metamorphism in them. A set of metamorphic and metasomatic reactions take place due to the difference in the composition of the existing rocks and invading fluid. The hydrothermal fluid may be magmatic (originate in an intruding magma), circulating groundwater, or ocean water. Convective circulation of hydrothermal fluids in the ocean floor basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and other submarine volcanic areas. The fluids eventually escape through vents on the ocean floor known as black smokers. The patterns of this hydrothermal alteration are used as a guide in the search for deposits of valuable metal ores.

(4). Shock Metamorphism:

Shock metamorphism is the result of the collision of external terrestrial bodies like meteorites with the earth’s surface. The shock metamorphism is also known as the Impact metamorphism. Shock metamorphism is, therefore, characterized by ultrahigh-pressure conditions and low temperature. The resulting minerals (such as SiO₂ polymorphs coesite and stishovite) and textures are characteristic of these conditions.

(5). Dynamic Metamorphism:

Dynamic metamorphism is associated with zones of high to moderate strain such as fault zones. Cataclasis, crushing and grinding of rocks into angular fragments, occurs in dynamic metamorphic zones, giving cataclastic texture.

The textures of dynamic metamorphic zones are dependent on the depth at which they were formed, as the temperature and confining pressure determine the deformation mechanisms which predominate. Within depths less than 5  km, dynamic metamorphism is not often produced because the confining pressure is too low to produce frictional heat. Instead, a zone of breccia or cataclastic is formed, with the rock milled and broken into random fragments. This generally forms a mélange. At depth, the angular breccias transit into a ductile shear texture and into mylonite zones.

Within the depth range of 5–10  km, pseudotachylyte is formed because the confining pressure is enough to prevent brecciation and milling and thus energy is focused on discrete fault planes. Frictional heating, in this case, may melt the rock to form pseudotachylyte glass.

Within the depth range of 10–20  km, deformation is governed by ductile deformation conditions and hence frictional heating is dispersed throughout shear zones, resulting in a weaker thermal imprint and distributed deformation. Here, deformation forms mylonite, with dynamo-thermal metamorphism observed rarely as the growth of porphyroblasts in mylonite zones.

Overthrusting may juxtapose hot lower crustal rocks against cooler mid and upper crust blocks, resulting in conductive heat transfer and localized contact metamorphism of the cooler blocks adjacent to the hotter blocks, and often retrograde metamorphism in the hotter blocks. The metamorphic assemblages, in this case, are diagnostic of the depth and temperature and the throw of the fault and can also be dated to give an age of the thrusting.

(6). Cataclastic Metamorphism: 

Cataclastic metamorphism is a very rare type of metamorphism that occurs as a result of mechanical deformation, like when two bodies of rock slide past one another along a fault zone.  Intense heat is generated by the friction of sliding along such a shear zone, and mechanical deformation, crushing, and pulverization of the rocks occur due to shearing. Cataclastic metamorphism is not very common and is confined to a narrow zone along which the shearing occurred. This type of metamorphism operated mainly in the upper part of the earth’s crust, where the temperatures are moderately low. Due to these stresses rocks are crushed, ground, and deformed. New rocks thus formed are called cataclastic rocks. They show mainly mechanical crushing with little new mineral formation. Examples of cataclastic rocks are “mylonites” and “fault breccias”.

(7). Plutonic Metamorphism:

Such a type of metamorphism occurs at a great depth below the earth’s surface, where static pressure and high temperatures operate together. The metamorphism caused by these factors is called “plutonic metamorphism”. High static pressure favours a reduction in volume. Hence during recrystallization mainly denser minerals are formed. The metamorphic rocks produce in this way commonly have an even-grained texture. Such rocks are called “grnulites”.

The Diagenetic and Metamorphic Zones are different zones of heat and temperature below the earth’s crust where rocks are altered from one form to the other. The interior of the earth is very hot, even the rocks present out there are molten. The molten rocks are known as magma. Below the earth’s crust, there is a thick magmatic layer, which is known as the “mantle”. Below the earth’s surface and closer to the magmatic material temperature and pressure both increase. The deeper the zone is, the more there is heat and pressure. Metamorphism is the impact of heat and pressure on the rocks resulting in structural, and textural changes in them. The degree or grade of metamorphism exhibited by a rock, therefore, varies with depth. On the basis of this concept three depth zones of metamorphism have been recognized: (i) epizone, (ii) mesozone, and (iii) Katazone.

Diagenesis and Metamorphic Grades

Diagenetic Zones

But before going through the discussion of metamorphism, and its zones, it is necessary to elaborate on the process of diagenesis. “diagenesis” is commonly considered to include all the changes that affect minerals and sediments from the time of deposition until the stage of metamorphism. In other words, the pre-metamorphic changes are known as diagenesis. While on the other hand, Metamorphism, or “true” metamorphism, is the stage where rocks are completely recrystallized.

1. Shallow Diagenetic Zone: The uppermost diagenetic zone, with a low temperature below 100 degrees Celcius. Zeolite facies come into form in the shallow diagenetic zone.
2. Deep Diagenetic Zone: The zone of deep diagenesis ranges from a temperature of 100 to 200 degrees Celsius. Like a shallow diagenetic zone, Zeolite facies are formed in a deep diagenetic zone.
3. Low Anchizone: Anchizone is the beginning of very low-grade metamorphism at 200-275 degrees Celsius.
4. High Anchizone: A low-grade metamorphism with a temperature ranging from 275 to 300 degrees Celsius. Prehnite Pumpellyite facies are found in both low and high Anchizones.

Metamorphic Zones

When the temperature exceeds 300 degrees Celsius, the zones of “true metamorphism” start. The intensity of metamorphism varies from depth to depth. These different depths where temperature and pressure changes are known as the metamorphic zones.

1. Epizone: This zone of metamorphism occurs near the earth’s surface. In this zone generally, the conditions of cataclastic metamorphism prevail. Zone of low-grade metamorphic rocks characterised by “Illite Kubler Indes/(KI)” mean values less than 0.25 Δ°2θ CuKα indicate the shallow depth of metamorphism.
   Within anchizone the clay mineral smectite is virtually eliminated as illite crystallite thickening is accelerated by tectonic fabric development in metapelites. By the middle of anchizone, a penetrative slaty cleavage develops. Transition to epizone and low-grade metamorphism is marked by the thickening of illite crystals and compositions are those of phengite or muscovite micas. The transition from anchizone (subgreenschist) to epizone (greenschist) occurs at approximately 300 degrees Celsius. 
2. Mesozone: This is the intermediate zone of metamorphism which lies below the epizone. In the mesozone, the conditions of temperatures and pressures are such as to promote regional metamorphism. It is the middle zone in which the temperature factor becomes rather moderate (300°-500°C) and the pressure factor is of both types: shear as well as hydrostatic type. Dynamothermal metamorphism is the typical process of this zone and high-grade schists like biotite-garnet schists are the chief rocks formed.

1. Katazone: The bottom-most zone of metamorphism is called the Katazone. In this zone plutonic metamorphism takes place. It is the high temperature and great depth type metamorphic zone where hydrostatic stresses are quite dominant. Plutonic metamorphism is the representative kind and rocks formed in this zone include great variety of Gneisses.

Grades of Metamorphism

The extent or degree of change in a rock due to metamorphic influence is expressed by the term metamorphic grade. Three terms are used to express the grades: (i) low grade, (ii) medium grade, and  (iii) high grade. Different sets of minerals are present in different grades. These grades are indicated by the presence of a set of minerals that are called index minerals. Each set of minerals is stable only within a specific temperature and pressure range. Therefore, each grade with a specific temperature and pressure has quite different sets of minerals. These sets of minerals are considered to be characteristic of their particular grade.

(a) Low Grade:

It prevails within a temperature range of 200°-400° C and a large pressure range. Important index minerals are – laumonite, prehnite, and lawsonite.

(b) Medium Grade:

This grade prevails up to a temperature range of 650°C and is indicated by the index minerals like staurolite and cordierite. Pressure variations play an important role in determining the stability of various minerals formed in this grade.

(c) High-Grade:

It is believed to begin at temperatures around 580°C under pressure of 3.5 kb and continues up to temperature of 800°C and above. A typical example indicative of high-grade metamorphism is provided by the breakdown of muscovite mica in the presence of quartz and plagioclase. Hypersthene is a typical index mineral of high-grade metamorphism and granulites are the common resulting metamorphic rocks.

Isograd:

It is defined as a line on a geological map of metamorphic rocks joining the points of same grade of metamorphism as indicated by the presence of same type of index minerals. This concept has been found very convenient in tracing the progress of metamorphism in the given region. In practice, an assemblage of index minerals rather an individual mineral is used for drawing isograds.

Another term iso-reaction grade is sometimes used when similar reactions as indicated by mineral assemblages at different places in a metamorphosed area are clearly understood.

Metamorphic Facies 

The set of mineral assemblage at a particular pressure and temperature conditions is known as “metamorphic facies”. In other words, metamorphic facies mean the part of a rock or group of rocks, which is quite distinctive from the rest of the parts of the whole formation. Metamorphic facies demonstrate the various metamorphic events and their mutual relationship.  In the early 20th century, the petrologists introduced the concept of “metamorphic facies”. In 1914, a Finnish petrologist namely Pentti Eelis Eskola defined any rock of a metamorphic formation that has attained chemical equilibrium through metamorphism at constant temperature and pressure conditions, with its mineral composition, being controlled only by the chemical composition. The concept of metamorphic facies is more or less observation-based. The layers of a single outcrop with different chemical compositions will display different mineral assemblages in spite of experiencing the same temperature and pressure conditions during the coarse of metamorphism.

Metamorphism of Shale and other Argillaceous Rocks

Argillaceous rocks which have undergone metamorphism are referred to as Pelites, Low-Grade Spotted Rock, Medium Grade Chiastolite Rock, High-Grade Hornfels. Argillaceous rocks undergo most change as they are composed of chemically complex clay minerals such as kaolinite, illite, smectite, bentonite, and montmorillonite. Contact metamorphism of argillaceous rocks; Contact metamorphism of shale and Contact metamorphism of other rocks, is explained below.

Contact Metamorphism of Shale

Where argillaceous rocks (lutites) such as shales, argillites, siltstones, and mudstones come into contact with an igneous intrusion, a metamorphic aureole is formed. Within this aureole metamorphic zones of increasing intensity can be traced as the contact is approached. These zones are as follows.

1. Outermost Zone of Spotted Slate: The spots in the spotted slates may be composed either of a single mineral or a fine-grained mass of different minerals.
2. Intermediate zone of spotted Hornfels: Nearer the igneous intrusion, the cleavage

   (see physical properties of minerals) 

   of the slate disappears and the rock becomes harder. The spots in this rock are now due to small crystals of andalusite and the groundmass recrystallized to form mica and quartz.

3. Innermost Zone of Hornfels: Close to the contact of intrusion thorough recrystallization takes place and hornfel; a fine-grained hard rock is formed.

In the hornfels, porphyroblasts may be present. “Porphyroblasts” are large, well-shaped crystals, which are set in a fine-grained matrix. These may reach 4 or 5 centimeters in size. Porphyroblasts crystallize late in the solid rock during metamorphism. The minerals which commonly occur as porphyroblasts in the contact metamorphic hornfelses are cordierite, andalusite, and sillimanite. Hornfels may form from any type of parent rock. Metacrysts or porphyroblasts are not limited to contact metamorphic rocks. They also occur in regionally metamorphosed rocks.

Contact Metamorphism of Other argillaceous rocks

The contact metamorphism of sandstones, graywackes, impure limestones, and basic igneous rocks may be summarized as follows.

1. Pure sandstone recrystallize to quartzites composed of quartz with perhaps a little biotite, and magnetite derived from the impurities of clays and iron oxides respectively. Pure limestones yield calcite marbles.
2. when medium to coarse-grained sedimentary rocks such as graywackes and impure sandstones, are subjected to contact metamorphism, aluminium bearing minerals (e.g felspars) are converted into micas and garnets. When carbonate minerals, such as calcite, are present as impurities in the parent rocks, hornblende, epidote, and diopside are formed.
3. Impure limestones containing sand, chert, or clay material during contact metamorphism produce calc-silicate rocks. These rocks contain mostly calcite, lime-garnets, olivine, serpentine, wollastonite, tremolite, and diopside.
4. On contact metamorphisms, basic igneous rocks, such as basalt and gabbro, give rise to hornfels containing generally pyroxene and plagioclase.

Metasomatism

Metasomatism is a process involved in metamorphism. Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock with another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place. Many metamorphic reactions are generally considered to be essentially isochemical. This implies that during recrystallization and other metamorphic reactions, the bulk chemistry of the rocks has remained nearly constant. If other elements are introduced into the rock by circulating fluids derived from igneous magma, the resulting metamorphism is called “metasomatism”. In other words, metasomatism is a type of contact metamorphism in which much material is added to the rock by hydrothermal fluids. During metasomatism, the composition of the parent rock is changed substantially but its volume remains unchanged. As this alteration occurs without any deformation, the textures and structures of the original rock are usually preserved.

Scarns: During contact metamorphism, metasomatism is particularly effective. Scarns are the product of metasomatism formed at the contact of granites with limestones. When granitic magma rich in water and other volatile components comes in contact with limestones, a variety of minerals including magnetite, garnet, diopside, enstatite, and forsterite are formed. metalliferous ore deposits containing sulfides of lead, zinc, copper, and iron are commonly found in association with scarns.

Granitization: Some granites show evidence of former sedimentary bedding. If they are traced towards the margin, they gradually grade into gneiss (migmatite), felspethized schists, mica schists, and finally shale. Such granites appear to have formed by the metasomatism of schists where new mineral matter is added and the old is carried away. This process of transformation of country rocks into granite is called “granitization”. The material which is displaced during granitization, commonly contains Mg, Fe, and Ca. This results in the concentration of ferromagnesium minerals, such as biotite, garnet, pyroxene, or amphibole in the peripheral zone. Thus with the formation of granites, a frontal zone of Fe-Mg enrichment is formed. This zone is called the “basic front”. It must be remembered that all fronts are not basic. Silica fronts of various kinds are developed when quartz-rich rocks are granitized. Granites and granitic rocks are known to have formed from various types of igneous, sedimentary, and metamorphic rocks. During granitization, the transfer of material is caused by the infiltration of gaseous or liquid emanations (granitizing solution) into country rocks or by diffusion of irons in the solid medium.

Lit-par-Lit Injection: Lit-par-Lit is a french word, which means Bed-by-Bed. The characteristic of layered rock, the laminae of which have been penetrated by numerous thin, roughly parallel sheets of igneous material, usually granitic. Near the contact of the intrusive body, emanations from the invading magma are often injected into the metamorphosed country rocks along the foliation and other planes of weakness. When an appreciably large quantity of magmatic fluid is introduced in the country rocks, numerous thin sills are formed between layers of bedding or foliation. This phenomenon is called “lit-par-it injection”. When there is a large-scale transfer of igneous material in the surrounding country rocks, a mixed type of rock having gniessose structure is formed. Such mixed rocks are called the “migmatities”. Most have a somewhat granitic composition.