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GEOLOGY
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GEOLOGY
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Paper – 02
Introduction to Geology – 02
Topic No. & Title
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49
Metamorphic Rocks
Academic Script
INTRODUCTION
It is not uncommon to find a gradual transition from typically sedimentary or igneous
rocks to rocks whose mineralogy and structure can neither be ascribed
to sedimentation or magmatism. Such transition clearly indicate that igneous and
sedimentary rocks are subjected to physic-chemical conditions different from
those under which they were originally formed. Vanhise (1904) introduced the term
metamorphism. The word "Metamorphism" comes from the Greek: meta = change,
morph = form, so metamorphism means to change form. Metamorphic Rocks are
formed by the transformation of pre-existing rocks under the influence of high
temperatures and pressures and chemically active fluids. Note that Diagenesis is
also a change in form that occurs in sedimentary rocks. In geology, however, we
restrict diagenetic processes to those which occur at temperatures below 200 Degree
Celsius and pressures below about 300 MPa (MPa stands for Mega Pascals), this is
equivalent to about 3 kilobars of pressure (1kb = 100 MPa).Metamorphism, therefore
occurs at temperatures and pressures higher than 200oC and 300 MPa. Rocks can be
subjected to these higher temperatures and pressures as they are buried deeper in
the Earth. Such burial usually takes place as a result of tectonic processes such as
continental collisions or subduction. The upper limit of metamorphism occurs at the
pressure and temperature where melting of the rock in question begins. Once
melting begins, the process changes to an igneous process rather than a
metamorphicprocess.
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AGENTS OF METAMORPHISM
Metamorphism is the mineralogical, textural, structural adjustment in the solid state
of the preexisting rock due to change in physical and chemical conditions. This
process leads to transformation of existing rocks due to recrystallisation
of preexisting rocks. This transformation occurs due to marked changes in the
physical nature of the surroundings such as temperature, pressure, etc.
Metamorphism is mainly due to the operation of three agents:
1. The chemical activity of the liquids and gases within the rocks
2. The temperature
3. The pressure
•Liquids, particularly water on and in the earth, are often effective agents of
alteration of rock material, which is dissolved and subsequently crystallizes in new
mineral combinations.
Various gases and vapours, especially those which escape from molten magma into
surrounding rocks, often bring about important chemical changes and rearrangement of mineral matter in rocks.
•Heat is an important metamorphic agency. It renders liquids and gases much more
active chemically. It helps to alter the composition of many minerals and thus to
bring new ones into existence. A sedimentary rock mass may be metamorphosed by
heat along the contact with a magma which is intrusive into it. A rock mass may be
heated not only by a hot igneous body, but also by deep burial within the generally
heated crust of the earth. Some heat may also result from friction between adjacent
parts of the crust moving past one another.
•At any point within the crust of the earth, pressure may be resolved into two kinds:
(a) Lateral pressure &
(b) Downward pressure
Lateral pressure is a very important agency in the metamorphism of igneous as well
as sedimentary rocks. The crust of the earth is in many places subjected to
tremendous stresses and compressive forces causing rocks to be bent, sheared,
fractured and often locally crumpled into mountain ranges. Mineral grains or rocks
fragments may thus be crushed or flattened; rearrangement of minerals may take
place and the rock structure and texture may be greatly changed.
Another important factor in the transformation of rocks is the downward pressure
exerted upon deeply buried rocks, particularly sediments, aided by the heat of the
earth's interior and often by water and other liquids.
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TYPES OF METAMORPHISM
Metamorphism is broadly classified into
1. Contact or thermal &
2. Regional
Contact or Thermal Metamorphism
Contact or thermal metamorphism occurs adjacent to igneous intrusions and results
from high temperatures associated with the igneous intrusion.
Since only a small area surrounding the intrusion is heated by the magma,
metamorphism is restricted to the zone surrounding the intrusion, called a
metamorphic or contact aureole. Outside of the contact aureole, the rocks are not
affected by the intrusive event. The grade of metamorphism increases in all
directions toward the intrusion. Because the temperature contrast between the
surrounding rock and the intruded magma is larger at shallow levels in the crust
where pressure is low, contact metamorphism is often referred to as high
temperature, low pressure metamorphism. The rock produced is often a finegrained rock that shows no foliation, called a hornfels.
Regional Metamorphism
Regional metamorphism occurs over large areas and generally does not show any
relationship to igneous bodies. Most regional metamorphism is accompanied by
deformation under non-hydrostatic or differential stress conditions. Thus, regional
metamorphism usually results in forming metamorphic rocks that are strongly
foliated, such as slates, schists, and gniesses. The differential stress usually results
from tectonic forces that produce compressional stresses in the rocks, such as when
two continental masses collide. Thus, regionally metamorphosed rocks occur in the
cores of fold/thrust mountain belts or in eroded mountain ranges. Compressive
stresses result in folding of rock and thickening of the crust, which tends to push
rocks to deeper levels where they are subjected to higher temperatures and
pressures.
Cataclastic Metamorphism
Cataclastic metamorphism occurs as a result of mechanical deformation, like when
two bodies of rock slide past one another along a fault zone. Heat is generated by
the friction of sliding along such a shear zone, and the rocks tend to be mechanically
deformed, being crushed and pulverized, due to the shearing. Cataclastic
metamorphism is not very common and is restricted to a narrow zone along which
the shearing occurred.
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Hydrothermal Metamorphism
Rocks that are altered at high temperatures and moderate pressures by
hydrothermal fluids are hydrothermally metamorphosed. This is common in basaltic
rocks that generally lack hydrous minerals. The hydrothermal metamorphism results
in alteration to such Mg-Fe rich hydrous minerals as talc, chlorite, serpentine,
actinolite, tremolite, zeolites, and clay minerals. Rich ore deposits are often formed
as a result of hydrothermal metamorphism.
Burial Metamorphism
When sedimentary rocks are buried to depths of several hundred meters,
temperatures greater than 300 degree Celsius may develop in the absence of
differential stress. New minerals grow, but the rock does not appear to be
metamorphosed. The main minerals produced are often the Zeolites. Burial
metamorphism overlaps, to some extent, with diagenesis, and grades into regional
metamorphism as temperature and pressure increase.
Shock Metamorphism(Impact Metamorphism)
When an extraterrestrial body, such as a meteorite or comet impacts with the Earth
or if there is a very large volcanic explosion, ultrahigh pressures can be generated in
the impacted rock. These ultrahigh pressures can produce minerals that are only
stable at very high pressure, such as the SiO2 polymorphs coesite and stishovite. In
addition they can produce textures known as shock lamellae in mineral grains, and
such textures as shatter cones in the impacted rock.
The different kinds of metamorphism are thus due to various combinations of the
four agencies: heat, directed pressure or stress, uniform pressure or hydrostatic
pressure and chemically active fluids. Chemically active fluids are essential to all kinds
of mineral change in metamorphic rocks.
ROLE OF TEMPERATURE & PRESSURE
(a) Heat Predominant- Heat is the dominating factor in the metamorphism which
ensures in the proximity in igneous masses. Nevertheless, the factor of pressure
comes in through the crowding aside of the country rock, its volume expansion due
to heating and the downwardly-acting weight of the superincumbent mass, although
pressure effects are always subordinate to those of heat. In general, the country rock
is soaked with gaseous and liquid emanation form the magma, and these greatly
facilitate the mineral transformations which take place.
(b) Directed Pressure Predominant - Natural pressure within the crust is resolvable
into
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(1) hydrostatic pressure &
(2) stress,
the latter operating in a definite direction. In co-operation with heat, stress is the
principal factor in regional metamorphism. With little or no heat, directed pressure
produces crushing and granulation through the enforced movement of rock masses
upon one another.
(c) Directed Pressure & Heat - The combination of directed pressure and heat,
operating at some depth in the crust, is one of the most powerful in producing
metamorphism. It leads to more or less complete recrystallisation of the rocks
combined with the production of new structures. Under these conditions direction
pressure lowers the melting points of minerals locally and temporarily and thus
facilitates diffusion and recrystallisation of material. The main direction of pressure
and movement is tangential to the earth's surface.
(d) Uniform Pressure and Heat: This combination of factors dominates conditions of
metamorphism at depth in which directed pressure diminished and finally ceases to
operate because of the increasing plasticity of the rocks. Mineral transformations are
carried to completion, but new structures are not greatly in evidence. Those minerals
are formed which help the rock material to best adapt itself to reduced volume.
TEXTURE OF METAMORPHIC ROCKS
The fabric of metamorphic rocks is a result of mechanical deformation and
mineralogical reconstitution of the pre-existing rocks. Texture is a term that
describes the size, shape and orientation of the grains constituting a rock, as well as
the relationship between these grains.
Metamorphic textures can be grouped into three main groups:
A -Relict textures (palimpsest textures): are textures inherited from the original rock
type, and which have survived metamorphism.
B - Typomorphic textures: textures characteristic of metamorphism
C - Superimposed textures: textures characteristic of a post- metamorphic event, e.g.
alteration, weathering, ... etc.
Other smaller groups as “reaction textures”, “polydeformational textures”, etc. may
also be typomorphic or replacement, but are grouped separately because they have
some genetic connotation.
A- Relict Textures
There are several types of relict textures. Relict textures in metamorphic rocks are
indicated by applying the prefix "blasto" to the original textural name. Relict textures
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are best preserved in low-grade rocks. Examples of such textures include: Porphyritic,
ophitic, intergranular, amygdaloidal, spherulitic, variolitic, pisolitic and oolitic.
B- Typomorphic textures
Textures characteristic of thermal metamorphism:
When thermal metamorphism is not associated with any deformation, the mineral
grains are randomly oriented, resulting in either granoblastic or hornfelsic textures.
Please note that the granoblastic texture can also develop in regionally
metamorphosed rocks. The following are some of the types of granoblastic textures:
1- Granoblastic polygonal: where the equidimensional grains may have well
developed crystal faces resulting in straight grain boundaries, and where triple
junctions are common.
2- Granoblastic interlobate: where the grain boundaries are somewhat irregular
3- Granoblastic amoeboid: where all the grains have irregular outlines, and all the
minerals are anhedral.
4- Granoblastic decussate: where the interlocking randomly oriented crystals are
somewhat elongate, prismatic or subidioblastic. Usually applied to rocks with one or
two mineral species only. Triple junctions are common.
d- Snowball: Similar to rotational texture, but where the inclusions define a spiral
shaped trail, which may have developed from the "rolling over" of the poikiloblasts.
e- Helicitic: Where the poikiloblasts overgrow the pre-existing foliation. This texture
therefore indicates post-tectonic crystallization of the poikiloblasts.
C- Replacement textures (superimposed in part!)
Mesh texture: develops in serpentinites, where the needle shaped serpentine
minerals occur in aggregates interwoven like a mesh.
Hour-glass texture: Also in serpentinites, where the serpentine minerals replace the
granular olivine crystals giving rise to hour-glass like appearances.
Bastite texture: A third texture that occurs in serpentinites, where Opx crystals were
completely replaced by aggregates of serpentine minerals retaining the prismatic
shape of the original Opx.
Pseudomorphic replacement textures:
(i) single-crystal
(ii) multicrystal
(iii) multi-phase, multi-crystal
D- Reaction textures
Epitaxial overgrowth: Epitaxial overgrowth is characterized by optical continuity
between the mineral and its overgrowth. Both the mineral and the overgrowth must
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belong to the same structural group, and may possibly be the same mineral. This type
of overgrowth is controlled fully by the the matrix mineral.
CLASSIFICATION OF METAMORPHIC ROCKS
Classification of metamorphic rocks depends on what is visible in the rock and its
degree of metamorphism. Note that classification is generally loose and practical
such that names can be adapted to describe the rock in the most satisfactory way
that conveys the important characteristics. Three kinds of criteria are normally
employed. These are:
1. Mineralogical - The most distinguishing minerals are used as a prefix to a textural
term. Thus, a schist containing biotite, garnet, quartz, and feldspar, would be called a
biotite-garnet schist. A gneiss containing hornblende, pyroxene, quartz, and feldspar
would be called a hornblende-pyroxene gneiss. A schist containing porphyroblasts of
K-feldspar would be called a K-spar porphyroblastic schist.
2. Chemical - If the general chemical composition can be determined from the
mineral assemblage, then a chemical name can be employed. For example a schist
with a lot of quartz and feldspar and some garnet and muscovite would be called a
garnet-muscovite quartzo-feldspathic schist. A schist consisting mostly of talc would
be called a talc-magnesian schist.
3. Protolithic - If a rock has undergone only slight metamorphism such that its
original texture can still be observed then the rock is given a name based on its
original name, with the prefix meta- applied. For example: metabasalt,
metagraywacke, meta-andesite, metagranite.
In addition to these conventions, certain non-foliated rocks with specific chemical
compositions and/or mineral assemblages are given specific names. These are as
follows:
• Amphibolites: These are medium to coarse grained, dark colored rocks whose
principal minerals are hornblende and plagioclase. They result from metamorphism
of basic igneous rocks. Foliation is highly variable, but when present the term schist
can be appended to the name (i.e. amphibolite schist).
• Marbles: These are rocks composed mostly of calcite, and less commonly of
dolomite. They result from metamorphism of limestones and dolostones. Some
foliation may be present if the marble contains micas.
• Eclogites: These are medium to coarse grained consisting mostly of garnet and
green clinopyroxene called omphacite, that result from high grade metamorphism of
basic igneous rocks. Eclogites usually do not show foliation.
• Quartzites: Quartz arenites and chert both are composed mostly of SiO2. Since
quartz is stable over a wide range of pressures and temperatures, metamorphism of
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quartz arenites and cherts will result only in the recrystallization of quartz forming a
hard rock with interlocking crystals of quartz. Such a rock is called a quartzite.
• Serpentinites: Serpentinites are rocks that consist mostly of serpentine. These
form by hydrothermal metamorphism of ultrabasic igneous rocks.
• Soapstones: Soapstones are rocks that contain an abundance of talc, which gives
the rock a greasy feel, similar to that of soap. Talc is an Mg-rich mineral, and thus
soapstones from ultrabasic igneous protoliths, like peridotites, dunites, and
pyroxenites, usually by hydrothermal alteration.
• Skarns: Skarns are rocks that originate from contact metamorphism of limestones
or dolostones, and show evidence of having exchanged constituents with the
intruding magma. Thus, skarns are generally composed of minerals like calcite and
dolomite, from the original carbonate rock, but contain abundant calcium and
magnesium silicate minerals like andradite, grossularite, epidote, vesuvianite,
diopside, and wollastonite that form by reaction of the original carbonate minerals
with silica from the magma. The chemical exchange is that takes place is called
metasomatism.
• Mylonites: Mylonites are cataclastic metamorphic rocks that are produced along
shear zones deep in the crust. They are usually fine-grained, sometimes glassy, that
are streaky or layered, with the layers and streaks having been drawn out by ductile
shear.
CONCLUSION
Metamorphic rocks constitutes a very important part of the study of the different
types of rocks. The study of metamorphic rocks in all its variety enables one to trace
the evolution of the Earth’s crust right from its primitive stage up to the present . The
chemistry of the metamorphosed sedimentary rocks reveals their parentage, the
mineral assemblage of metamorphic rocks present a fossillised record of the P-T
regimes that prevailed in different parts of the crust at different times in the past.
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