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Metamorphic rocks are "derived" rocks that is to say they are
made from pre-existing igneous, sedimentary and metamorphic rocks. Heat and/ or pressure form metamorphic rocks. In being formed metamorphic rocks do not melt but only recrystallise.
Metamorphism is the alteration of pre-existing rocks in the solid state
due to changes in temperature and pressure.
Under increasing temperature and / or pressure existing minerals become
unstable and break down to form new minerals.
Foliation - Foliation forms when pressure squeezes the flat
or elongate minerals within a rock so they become aligned. These rocks
develop a platy or sheet-like structure that reflects the direction
that pressure was applied in. Slate, schist, and gneiss (pronounced
'nice') are all foliated metamorphic rocks.This represents a distinct
plane of weakness in the rock. Foliation is caused by the re-alignment
of minerals when they are subjected to high pressure and temperature.
Individual minerals align themselves perpendicular to the stress field
such that their long axes are in the direction of these planes (which
may look like the cleavage planes of minerals). Usually, a series of
foliation planes can be seen parallel to each other in the rock. Well
developed foliation is characteristic of most metamorphic rocks.
Metamorphic rocks often break easily along foliation planes.
Granular - Non-foliated metamorphic rocks do not have a
platy or sheet-like structure. There are several ways that
non-foliated rocks can be produced. Some rocks, such as limestone are
made of minerals that are not flat or elongate. No matter how much
pressure you apply, the grains will not align! Another type of
metamorphism, contact metamorphism, occurs when hot igneous rock
intrudes into some pre-existing rock. The pre-existing rock is
essentially baked by the heat, changing the mineral structure of the
rock without addition of pressure.This describes a metamorphic rock
consisting of interlocking equant crystals (granules), almost entirely
of one mineral. A granular texture is developed if a rock's
chemical composition is close to that of a particular mineral. This
mineral will crystallise if the rock is subjected to high pressure and
temperature. A if granular texture is characteristic of some
metamorphic rocks. Note: As the grade of metamorphism increases (more
temperature and pressure), both crystal size and the coarseness of
foliation increase. Therefore, gneiss represents more intense
metamorphism (or a higher grade) than does schist.
Some fine-grained metamorphic rocks, e.g. schist, have larger
crystals present. These crystals are called porphyroblasts. Porphyroblasts represent minerals
that crystallise at a faster rate than the matrix minerals. Garnet is
a common porphyroblast mineral.
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Strong foliation, or alignment of grains, particularly micas, in
coarse-grained metamorphic rocks. Phyllitic
Strong foliation in fine-grained metamorphic rocks. Granoblastic
is an equigranular texture in which crystals adopt a polygonal morphology
with grain triple junctions of approximately 120 degrees. The formation of
granoblastic textures occurs to minimise the combined surface energy of
phases within a rock. Porphyroblastic
Metamorphic texture consisting of large grains in a finer grained matrix.
The rocks are subjected to tectonic forces ("pulling apart" =
tension or "pushing together"=compression) to change the local rocks.
Examples include: schist
, slate, and gneiss.
Most metamorphic rocks occur in fold mountain belts or cratonic
areas. Such rocks cover large areas of the Earth's crust and are therefore
termed regional metamorphic rocks. They arise by the combined action of
heat, burial pressure, differential stress, strain and fluids on
pre-existing rocks. The resulting rocks are always deformed (as a result
of the differential stress) and commonly exhibit folds, fractures and
cleavages. Large amounts of granitic intrusions are also associated with
regional metamorphic rocks. Regional metamorphism covers a wide
range of temperature and pressure conditions from 200° C - 750° C and 2
kbar - 10 kbar (or 5 km - 35 km depth). There are three metamorphic facies
within regional metamorphosed rocks, which from lowest to highest grade
Greenschist: can be further divided into chlorite and biotite
zones. The term greenschist gets its name from the rocks themselves as
many rocks of this facies are grey-green in colour and have a
schistose (parallel arrangement of platy minerals) texture.
Amphibolite: can be further divided into the garnet and
staurolite zones. The term amphibolite gets its name from the most
common constituent minerals of this facies, minerals of the amphibole
Granulite: can be further divided into the kyanite and
sillimanite zones. The term granulite reflects the most common texture
of these rocks - granular.
High pressure regional metamorphism
In some parts of the world, geologically young (Cenozoic and Mesozoic)
fold mountain belts contain sequences of metamorphosed fine-grained
sedimentary rocks and basic volcanic rocks that contain unusual blue
amphiboles. These rocks are commonly schistose, may have a characteristic
blue colour, and are termed blueschists. These form at low
temperature but high pressure conditions in the collision zones of
subducting slabs. Amphibolite
is the name of a facies and a dominant rock
When subducting oceanic slabs are dragged down to depths exceeding 50
kilometres, the basalt is metamorphosed at very high pressures to form a
dense rock with the same bulk chemical composition but different
mineralogy (dominantly pyroxene and garnet) and texture. These rocks are
involves metamorphosis through heating by an intruding plutonic body.
Example: hornfels.At shallow
depths within the crust (usually less than 6 km) the heat sources
responsible for contact metamorphism are bodies of hot magma (e.g. igneous
intrusions) which raise the temperature of the surrounding rocks. These
thermal affects are usually restricted to the contact zones of the
intrusions, hence the term contact metamorphism. However, sometimes hot
fluids are released from the intrusions and penetrate the enclosing rocks
along fractures and produce contact metamorphic zones. Determining factors
governing the extent of contact metamorphism are the size of the intrusion
and its temperature.
Basic magmas are much hotter than acid magmas and hence will have a
greater thermal effect. Also, a large intrusion contains much more heat
than a small dyke-like body and its effect on the surrounding country
rocks will be much greater and more widespread.Country rocks surrounding
large, hot bodies of magma are heated, initiating mineral reactions and
forming new minerals. Rocks adjacent to thin dykes and sills are simply
baked and hardened and do not experience any great mineralogical and/or
textural changes. Large plutons give rise to contact aureole zones within
which the country rocks are thermally metamorphosed, with those closest to
the plutons experiencing more heat than those further away (hence they
have a higher metamorphic grade). As large plutons take millions of years
to cool down, the surrounding country rocks also stay hot for tens of
thousands of years allowing chemical reactions to continue to completion.
The metamorphic facies produced by contact metamorphism in order of
increasing grade are as follows:
Albite epidote hornfels
An example of contact metamorphism from Chillagoe Queensland.
Steps in this contact metamorphism from right to left:
you can just make out the shapes of bivalve fossils in limestone
recrystallising and converting to marble as they approach a dolerite /
the green minerals at the contact are "calc-silicates"
you can see calcium from the limestone has entered the dolerite and
contaminated it along the contact
the contact metamorphism in the area has resulted in commercial
deposits of copper, gold and marble
Dynamic metamorphic rocks are restricted to narrow zones adjacent to
faults or thrusts. The high shear stresses associated with faults and
thrusts crush the adjacent rocks. The rise in temperature is produced by
frictional heat generated within the fault zone. The high shear stresses
may be short-lived or long-lived depending on the activity of the fault or
thrust. Dynamic metamorphism involves high shear stress, high pressure,
high strain, high fluid partial pressure and variable temperature. In many
instances, water plays a fundamental role.
Crushed rocks in fault zones are known as fault breccias which consist of
angular fragments of the country rock in a matrix of crushed or powdered
rock, cemented by quartz and/or calcite. Fluids move easily along fault
zones between grain boundaries and through cracks and fissures. These
fluids are able to transport large amounts of silica, carbonates and other
minerals in solution. Pseudotachylite
is a fault-zone rock which is black and glassy. It usually occurs as
narrow dykes and veins and forms by frictional melting of the country
rock. Mylonites are partially
recrystallised rocks with a pronounced foliation that are produced by
intense shearing during large-scale movements along faults and thrusts.
The different rock types produced by dynamic metamorphism vary with depth
from the surface as, with increasing depth, both the surrounding pressure
and temperature increase
Many metamorphic rocks contain evidence of retrograde mineral changes,
that is, alteration of higher grade minerals into lower grade ones. Many
of these changes involve hydration and are the result of a decrease in
temperature and increase in the activity of water. Retrograde
metamorphism is normally produced by repeated regional metamorphism
where a lower grade episode is superimposed on a higher grade one. Most
retrogressive events are probably just a consequence of the metamorphic
system cooling down after peak metamorphism has been reached (i.e. the
system has to cool down with time and as the region undergoes uplift
with time, both pressure and temperature are dramatically reduced). The
secondary minerals produced during retrogressive metamorphism generally
occur as fibrous fringes around, inclusions within, and pseudomorphous
grains after, the higher grade metamorphic minerals. A good example of
retrogressive metamorphism is the occurrence of serpentinites. These form by generally low
temperature hydration of ultramafic rock (containing minerals composed
chiefly of magnesium and iron), commonly at subduction zones.
Approaches used in identification of metamorphic rocks:
Metamorphic Facies - A metamorphic facies is a set of
metamorphic mineral assemblages that were formed under similar
pressures and temperatures.
The assemblage is typical of what is formed in conditions
corresponding to an area on a two dimensional graph of temperature vs.
pressure metamorphic rocks are classified according to the conditions
under which they recrystallised.
These "key minerals" may only be present in small proportions, and in
many cases are difficult to recognise in hand specimen.
Degree of Recrystallisation - useful in the field or for hand
specimens, is based upon the of the original minerals, and so
grain size and the degree of foliation are important.
Deformation - looking for structures that indicate
deformation, such as folding (often shown as crenulations or small
crumpled folds), and small fractures or faults
These observations are useful in determining what type of rock was
present before metamorphism
An important skill if you are looking for a mineral deposit.
Three metamorphic facies shown "key minerals" in red
The geology of eastern Australia is dominated by a number of these fold
mountain belts. The largest are the Lachlan and New England fold belts.
Both of these fold belts contain relatively low-grade regional
metamorphic rocks, along with numerous granitic intrusions. The New
England Fold Belt contains small amounts of blueschists and eclogites
that formed in the collision zones of subducting slabs. These fold belts
have formed over hundreds of millions of years by plate tectonic