Metamorphism & Metamorphic Petrology
M
etamorphism may be defined as the response in solid rocks
to pronounced changes in temperature, pressure and chemical
environment, which takes place in general below the shells
of weathering and cementation. By metamorphism, the
constituent minerals of a rock are changed over to others which
are more stable under the new conditions and these may arrange
themselves with the production of structures which are likewise
better suited to the new environment.
AGENTS OF METAMORPHISM:
Metamorphism is due to the co-operation of the three forces of
temperature, pressure and chemically active fluids. As
metamorphism takes place in depth, it follows that it depends
upon increasing temperatures and pressures and the increased
activity of fluids. The heat may be supplied from the general
increase of temperature downward, or from magma. The pressure is
due ultimately to gravity and may be resolved at any point into
two kinds; hydrostatic or uniform pressure, which leads to change
of volume, and directive or non-uniform pressure, or stress,
which leads to change of shape or distortion.
The environment of chemically active fluids is the most important
factor in metamorphism as the reactions involved can only take
place through partial or complete solution of the minerals. Water
is undoubtedly the chief substance present in this way; but it
may be reinforced locally by CO2, and by substances such as boric
and hydrofluoric acids which emanate from igneous magmas.
TYPES OF METAMORPHISM:
 Thermal Metamorphism(Heat predominant): IT is a general term
including a variety of metamorphic processes, in all of which
the temperature of heat factor dominated, pressure and fluids
playing only secondary roles. A few types of thermal
metamorphism are:
 Contact Metamorphism: It prevails in the immediate vicinity
of magmatic injections, intrusions or flows and involves
changes like recrystallisation of original minerals & the
formation of new ones.
 Pyrometamorphism: This term is used for intense but
essentially localised metamorphic changes brought about by
high temperatures to which a given rock is subjected. It is
illustrated by changes in a block of sedimentary rock that
has incidentally fallen down in a body of flowing lava; the
block may be so heated up that its original minerals are
forced to recrystallise and rearrange themselves.
 Dynamic/Clastic Metamorphism(Directive pressure predominant):
In this kind, the shearing stress is the dominant factor to
bring about changes, which are mostly of mechanical nature. It
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is also called kinetic metamorphism and is restricted to
folded regions. When the pressure is due to the depth factor
(i.e. hydrostatic type) the process of metamorphism is defined
as load metamorphism.
 Dynamothermal Metamorphism(Directive pressure & heat
predominant): It is the most important kind of metamorphism
and includes the changes brought about in rocks under the
combined effects of pressure, temperature and fluids. It is
characteristic of intense folding and may include hundreds of
kilometres of tectonic belts under its influence.
 Plutonic Metamorphism(Uniform pressure & heat predominant):
Where the pressure of uniform nature (i.e. hydrostatic
pressure at depths) and the very high temperatures are
responsible for the bulk of the changes in a rock, the process
is known as plutonic metamorphism. These conditions are
available only at the lower levels in the earth’s crust and
hence the process is characteristic of greater depths only.
 Metasomatism: It can be broadly defined as a type of thermal
metamorphism in which fluids, liquids and gases, which are
invariably at very high temperatures, attack the surrounding
rocks and induce certain changes in them. Metasomatism is
further distinguished into the following two types:
 Hydrothermal Metamorphism: When the rocks are attacked by
chemically active solutions(aqueous or otherwise)
 Pneumatolytic Metamorphism: When the medium of attack is in
the gaseous state or in vapours.
The most important facts regarding metamorphic changes are:
 They take place essentially in the solid state
 The bulk composition of the rock undergoing metamorphism
generally remains unaffected, except in some types, like
metasomatism.
If the temperature and pressure conditions change to such an
extent that rocks become actually “molten” during the process,
then it no longer remains a metamorphic change. The melt thus
produced or “reborn” will crystallise under favourable conditions
in a way similar to a magmatic melt, and hence will give rise to
igneous rocks. This process of regeneration of a melt is termed
anatexis or ultrametamorphism.
METAMORPHIC ZONES & FACIES:
ZONES: The degree or grade of metamorphism generally increases
with depth for the simple reason that temperature and pressure
factors become more powerful at deeper levels. This fact has
given birth to the concept of metamorphic zones which signify the
range of metamorphic effect at different depths below the
surface. The three metamorphic zones are:
 Epizone: It is the near-surface zone and is characterised by a
low temperature, generally less than 300°C, and strong shear
stress. Rocks are therefore deformed chiefly under the
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influence of dynamic metamorphism. The common rocks resulting
in this zone are slates and mica schists.
 Mesozone: It is the middle zone in which temperature becomes
rather moderate (300°C-500°C) and the pressure factor is of
shear as well as hydrostatic character. Dynamothermal
metamorphism is typical of this zone and schists(like the
biotite-garnet schist) are the chief rocks developed.
 Ketazone: It is a zone of high temperature (500°C-800°C) and
hydrostatic stress. Plutonic metamorphism dominates this zone
with the formation of a variety of gneisses.
FACIES: It has been found possible to classify metamorphic rocks
on the basis of the metamorphic environment through which these
rocks have passed. The concept of metamorphic facies simplifies
the classification of these rocks in that it eliminates the
necessity of knowing the nature of the parent rock and original
characters. Eskola has recognised eight different metamorphic
facies:
 Sanidine facies
 Green-schist facies
 Epidote-Amphibolite facies
 Amphibolite facies
 Pyroxene-hornfels facies
 Granulite facies
 Glaucophane schist facies
 Eclogite facies
Metamorphic rocks exhibit
mineralogical composition
 The composition of the
 The type and degree of
a great variation in their
which is defined in most cases by:
parent rock
metamorphism undergone by the rock.
Many rocks consist of reconstituted minerals of the parent rock;
for example, marbles are made up from reconstituted calcite of
limestone, quartzites of reconstituted quartz of sandstones and
so on. It must be made clear here that metamorphic minerals are
produced primarily under the influence of metamorphic agents,
namely temperature and pressure, and hence they must be stable
under new conditions imposed on these rocks. Those minerals which
are produced in metamorphic rocks chiefly under the influence of
the stress factor are known as stress minerals. They are
characterised by flaky, platy or lamellar structures.
Dynamothermal metamorphism chiefly results in stress minerals
which include kyanite, staurolite, amphiboles, muscovite,
chlorite etc. Such minerals are characteristically stable under
conditions of high stress. The other group of minerals include
those that are primarily formed under the influence of the
temperature factor. These anti-stress minerals are generally
equidimensional in character and are the product of plutonic
metamorphism. Examples are sillimanite, olivine, cordierite,
pyroxenes etc. Anti-stress minerals are unstable under high
stress conditions.
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TEXTURES OF METAMORPHIC ROCKS:
The textures of metamorphic rocks can be broadly classified as:
 Crystalloblastic
 Palimpsest or Relict
Crystalloblastic Textures: These include all those types that
have been newly imposed upon the rock during the process of
metamorphism and are, therefore, essentially a product of
metamorphism. Among the crystalloblastic textures, the
porphyroblastic and granoblastic types are the most common. In
the first case, the fine-grained ground mass of the rock shows,
embedded in it, idioblastic crystals (crystals with perfect
outlines) of stronger minerals. In the granoblastic texture, the
rock is composed of equidimensional grains of hard minerals.
Palimpsest/Relict Textures: These comprise the textures which
were present in the original rock and which have resisted the
effects of metamorphism and thus are retained by the rock.
Palimpsest textures are generally described by using the word
“blasto” as prefix to the original texture that has been retained
by the rock. Thus, if an igneous rock with porphyritic texture
undergoes metamorphism whereby its mineral composition is changed
but the same texture is retained in the new rock, the texture of
the rock will now be described as blastoporphyritic.
CARBONATE ROCKS:
Metamorphism of limestone and other carbonate rocks is of
considerable interest and importance.
 When pure limestone recrystallises without any mineral
formation but with definite change in grain size, the
resulting rock is crystalline and is commonly known as marble.
 When the limestone is impure, simple recrystallisation is
substituted by the formation of new minerals. New minerals
that are formed depend upon the nature of the impurities as
well as on the temperature and pressure conditions. When
silica is the sole impurity a mineral wollastonite results,
whereas the presence of alumina and magnesia leads to the
formation of anorthite and amphiboles respectively.
 When the limestone is dolomitic in composition, metamorphic
changes show great variation and many new minerals like
periclase, forsterite, spinel, diopside etc. may be formed,
depending on whether the dolomite is pure of whether
impurities like silica and alumina are also present in it.
IMPORTANT METAMORPHIC ROCKS
SLATE: Slate is a fine-grained metamorphic rock characterised by a
perfect cleavage. The slaty cleavage is due to parallel
arrangement of platy or flaky minerals, under the influence of
metamorphism.
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ORIGIN: Slate is a product of low-grade regional metamorphism on
argillaceous rocks like shale. When slate is subjected to further
action of dynamothermal metamorphism, recrystallisation may lead
to the development, in number and size, of the micas. Such
metamorphic rocks with conspicuous mica layers and slaty
appearance are termed phyllites, which on further metamorphism
change to crystalline schists.
SCHIST: Schists are microscopically crystalline, metamorphic rocks
characterised by a schistose structure. The constituent flaky and
platy minerals are mostly arranged in irregularly parallel layers
of bands.
ORIGIN: Schists are generally the products of dynamothermal
metamorphism of argillaceous rocks like shales.
GNEISS: Gneiss is a megascopically crystalline metamorphic rock,
characterised by segregation of constituent minerals into layers
or bands of contrasting colour, texture and composition.
ORIGIN: Gneisses are generally the produce of advanced stages of
regional metamorphism of a variety of parent rocks, chief among
them which are sandstone, conglomerates and granitic rocks.
QUARTZITE: Quartzite is a fine-grained metamorphic rock composed
of intersutured grains of quartz. The name orthoquartzite is used
for a sedimentary rock of similar composition with siliceous
cement. Metamorphic quartzite is a granular rock characterised by
a tendency of fracturing through the grains under heavy loads.
ORIGIN: Quartzite results from the recrystallisation of sandstone
under the influence of contact and dynamic metamorphism.
PHYLLITE: It is mostly a fine0grained metamorphic rock and
represents an intermediate stage in the metamorphism from slate
to schist. The individual minerals are not recognised by the
unaided eye, bu8t the presence of muscovite is easily indicated
by the shining cleavage surface. The rock shows a foliated
structure.
ORIGIN: Phyllites are formed as a result of dynamothermal
metamorphism of argillaceous rocks like shales.
HORNFELS: It is a term applied to a group of metamorphic roc
developed commonly in contact zones of igneous intrusions
characterised by fine to medium grained textures and a maculose
structure.
ORIGIN: They are formed from fine-grained argillaceous rocks like
shales.
MARBLE: It is essentially a metamorphic rock, composed chiefly of
recrystallised calcite. It is characterised by a granulose
structure, but the grain size shows extreme variations, from
finely saccharoidal to highly coarse. They may often show banded
structure.
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ORIGIN: They result from the recrystallisation of limestones or
dolomites under the influence of contact and regional
metamorphism.
Slate
Texture
Very finegrained,
individual
minerals are
not recognised
by the unaided
eye.
Structure
Foliated
structure;
shows perfect
slaty cleavage.
Composition
Minerals
recognised only
under
microscope and
include chiefly
species of clay
groups.
Phyllite
Fine-grained,
individual
minerals are
not recognised
by the unaided
eye, but mica
flaked are
definitely
better
developed in
number and
size.
Foliated
structure,
better than
slates;
cleavage may be
poor.
Mostly the same
as for slate,
but mica
increased in
number and
size, and give
a shiny
appearance to
cleavage
surface.
Schist
Megascopically
crystalline,
components are
well arranged
and easily
recognisable.
Highly
foliated; rock
shows schistose
structure, in
which
platy/flaky
minerals show
irregularly
parallel
arrangement.
Micaceous
minerals
dominate;
porphyroblasts
of some hard
minerals
present;
chiefly
muscovite,
chlorite,
sericite,
garnet and
tourmaline.
The following rocks are generally classed as of doubtful origin:
 Migmatites: There are essentially mixed roc that consist of
intimately associated members of igneous(granitic) rocks, and
metamorphic(gneisses, schists, etc.) divisions. A streaky or
banded appearance is typical of all, and the bands may be of
alternating igneous and metamorphic minerals.
 Eclogites: These are rocks of uncertain origin containing
garnets, grass-green pyroxene, omphacite and plagioclase
feldspar.
 Charnockites: These are hypersthene-bearing granitic rocks
that are generally garnetiferous.
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TYPE OF
METAMORPHISM
METAMORPHIC
FACIES
Regional
(includes
dynamothermal &
plutonic)
Green
schistose
Epidoteamphibolitic
Amphibolitic
Granulitic
Contact
metamorphic
proper
Contact
metasomatism
Dynamic/
Cataclastic
Eclogite
Hornfelses
METAMORPHIC ROCKS FORMED ACCOERDING
TO PARENT ROCKS
Clay and
Carbonate
Basic and
Feldspar
magmatic
greywackes
and marls
Clay,
Calcareous
Chlorite
schist,
schist
and talc
phyllite
schist,
hornblende,
serpentine
Crystalline Marble
Amphibolite
schist,
quartz
Gneiss
Marble
Amphibolite
Granulite
and
migmatite
Hornfels,
quartzite
Marble
Skarn
Granulite
Eclogite
Amphibolitic and
pyroxene
hornfels
Serpentine
-
Gneisses
-
Tectonic breccia, cataclast, mylonite
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