Metamorphism is the mineralogical and structural adjustment of solid rocks to physical and chemical conditions which have been imposed at depths below the surface zones of weathering and cementation, and which differ from the conditions under which the rocks in question originated. Thus metamorphism is the response of the solid rocks to pronounced changes of temperature, pressure and chemical environment.
Metamorphism stands midway between diagenesis and general melting of rocks. Important features of metamorphic changes:
(i) The bulk chemical composition of the metamorphic rock is the same as that of the rock from which it is formed. Thus metamorphic changes are isochemical changes.
(ii) The structural and textural characteristics of the metamorphic rocks are the outcome of the structure and texture of the pre-existing rocks and temperature-pressure condition of the metamorphic changes.
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(iii) The changes in metamorphism take place in an essentially solid medium.
Metamorphic rocks which are derived from igneous rocks are Known as Orthometamorphic rock, and those which are derived from sedimentary rocks, are known as parametamorphic rocks.
Agents metamorphism:
The agents who are mostly responsible for bringing about metamorphic changes are as follows:
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1. Temperature.
2. Pressure.
(a) hydrostatic or uniform pressure,
(b) directed pressure or stress.
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3- Chemically active fluids.
1. Temperature:
It may be supplied by geothermal gradient, magmatic heat, frictional heat and by radioactive disintegration. The temperature range within which metamorphic changes take place is from 200° to 700°C. However, in certain cases a temperature of 1000° to 1200°C may, be encountered.
Temperature accelerates the processes of reaction, increase the volume of the rocks, remove volatiles and moisture contents of the rocks.
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The following types of metamorphism are said to be the rest of temperatures effect on rocks:
(i) Pyrometamorphism:
At 800° to 1000°C, in the immediate viciniy of the intrusive.
The in duration, backing, burning and fritting effects of lava flows and intrusions on neighboring rocks is known as Caustic metamorphism or optalic metamorphism.
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(ii) Contact metamorphism:
It occurs around larger intrusive at comparatively low-temperature. It includes:
(a) Normal contact metamorphism:
Where rocks are simply crystallized without new mineral formation.
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(b) Pneumatolytic, additive or metasomatic:
The composition of the rocks is vastly modified depending on the addition of material from magmatic emanations.
(c) Injection metamorphism:
Here with the intrusion of magma or its residual liquid may alter the intruded rock substantially.
(iii) Auto-metamorphism:
It is the mineralogic readjustment of an igneous assemblage to the falling temperature as the body of the igneous rock cools. It includes uralitisation, serpentinisation etc.
(iv) Retrograde metamorphism:
Also known as diaphthoresis, where mineralogical rearrangement of high temperature assemblage to a low temperature one takes place.
2. (a) Uniform pressure:
It is the hydrostatic pressure which increases with depth. Uniform pressure and temperature can both dominate together at great depths. There is a reduction in the volume of the rock and a change in the mineralogical composition. It is known as Plutonic Metamorphism, e.g., Granulites, Eclogites.
Load metamorphism:
It is due to the vertically acting stress of superincumbent rock masses aided by high temperature.
(b) Directed pressure:
It is produced mostly by orogenic movements. It dominates at or near the surface. It results in crushing and granulation of minerals, without the formation of any new mineral. It is also known as dynamic metamorphism or cataclastic metamorphism, e.g., Mylonite.
Where both heat and stress dominates, the metamorphism is known as Regional Metamorphism. Here the country rocks are subjected to changes both in mineral composition and texture. It is also known as dynamothermal metamorphism.
3. Chemically active fluids:
These are from the following sources:
(a) Meteoric water.
(b) Juvenile water.
Water carries minerals in some cases in solution and also serves a medium in which chemical changes occur with ease. Chemical activity is more pronounced in the vicinity of the igneous intrusions.
Grades of Metamorphism:
The degrees of metamorphism or grades depend upon the extent to which the agents were in operation during the process. According to the temperature, pressure condition, there are usually three grades of metamorphism and accordingly there are three zones:
1. Epizone:
It is the zone of low-grade metamorphism, where temperature ranges from 100 to 300°C, pressure is low to moderate. It is charcterised by the presence of the hydrous minerals.
Sericite, muscovite, chlorite, biotite, talc, actinolite, epidote, andalusite etc.
Rocks:
Slates, phyllites, chlorite-schists, muscovite-schists, biotite-schists.
2. Mesozone:
This is the zone of medium grade metamorphism, where the temperature ranges from 300 to 500″C, pressure is moderately high. It occurs at an intermediate depth, i e., between 5 to 10 miles.
Minerals:
Biotite, andalusite, cordierite, quartz, hypersthene, almandine, orthoclase, ilmenite etc.
Rocks:
Phyllites and mica-schists.
3. Katazone:
It is the zone of high grade metamorphism, where the temperature ranges from 500 to 650°C, pressure is high. It occurs at a depth of 9 to 13 miles. It is characterised by anhydrous and antistress minerals.
Minerals, Biotite, alkalifeldspar, plagioclase, quartz, garnet, suimanite, kyanite, etc.
Rock:
Gneisses of various types, hornfels etc.
In case of Regional metamorphism, the following grades have been identified:
1. Zone of chlorite
2. Zone of biotite.
3. Zone of garnet.
4. Zone of staurolite.
5. Zone of kyanite.
These zones are according to the progressive grade of regional metamorphism.