Unformatted text preview: Metamorphic Rocks EENS 1110
Tulane University Physical Geology
Prof. Stephen A. Nelson Metamorphism and Metamorphic Rocks
This page last updated on 01-Feb-2011 Definition of Metamorphism
The word "Metamorphism" comes from the Greek: Meta = change, Morph = form, so
metamorphism means to change form. In geology this refers to the changes in mineral
assemblage and texture that result from subjecting a rock to pressures and temperatures
different from those under which the rock originally formed.
The original rock that has undergone metamorphism is called the protolith. Protolith can be
any type of rock and sometimes the changes in texture and mineralogy are so dramatic that is
difficult to distinguish what the protolith was.
Note that diagenesis and weathering are also a changes in form that occur in rocks. In
geology, however, we restrict diagenetic processes to those which occur at temperatures
below 200oC and pressures below about 300 MPa (MPa stands for Mega Pascals), this is
equivalent to about 3,000 atmospheres of pressure.
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
become 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 of wet partial
melting of the rock in question. Once melting begins, the process changes to an igneous
process rather than a metamorphic process.
During metamorphism the protolith undergoes changes in texture of the rock and the mineral
make up of the rock. These changes take place mostly in the solid state and are caused by
changes in physical or chemical conditions, which in turn can be caused by such things as
burial, tectonic stress, heating by magma or interactions with fluids. Factors that Control Metamorphism
Metamorphism occurs because rocks undergo changes in temperature and pressure and may be
subjected to differential stress and hydrothermal fluids. Metamorphism occurs because some
minerals are stable only under certain conditions of pressure and temperature. When pressure
and temperature change, chemical reactions occur to cause the minerals in the rock to change to
an assemblage that is stable at the new pressure and temperature conditions. But, the process is
complicated by such things as how the pressure is applied, the time over which the rock is
subjected to the higher pressure and temperature, and whether or not there is a fluid phase
present during metamorphism. Page 1 of 12 2/1/2011 Metamorphic Rocks Temperature
Temperature increases with depth in the Earth along the Geothermal Gradient.
Thus higher temperature can occur by burial of rock.
Temperature can also increase due to igneous intrusion.
Pressure increases with depth of burial, thus, both pressure and temperature will vary
with depth in the Earth. Pressure is defined as a force acting equally from all directions.
It is a type of stress, called hydrostatic stress, or uniform stress.
If the stress is not equal from all directions, then the stress is called a differential stress. There are two kinds of differential stress. Normal stress causes objects to be compressed
in the direction of maximum principal stress and extended in the direction of minimal
stress. If differential stress is present during metamorphism, it can have a profound
effect on the texture of the rock. Shear stress causes objects to be smeared out in the
direction of applied stress.
Differential stress if acting on a rocks can have a profound affect on the appearance or
texture of the rock. Rounded grains can become flattened in the
direction of maximum stress. Page 2 of 12 2/1/2011 Metamorphic Rocks Minerals that crystallize or grow in the
differential stress field can have a preferred
orientation. This is especially true of the
sheet silicate minerals (the micas: biotite
and muscovite, chlorite, talc, and
These sheet silicates will grow with their sheets orientated perpendicular to the direction
of maximum stress. Preferred orientation of sheet silicates causes rocks to be easily
broken along approximately parallel sheets. Such a structure is called a foliation.
Fluid Phase.- Any existing open space between mineral grains in a rock can potentially
contain a fluid. This fluid is mostly H2O, but contains dissolved ions. The fluid phase is
important because chemical reactions that involve changing a solid mineral into a new
solid mineral can be greatly speeded up by having dissolved ions transported by the
fluid. If chemical alteration of the rock takes place as a result of these fluids, the
process is called metasomatism.
Time - Because metamorphism involves changing the rock while it is solid, metamorphic
change is a slow process. During metamorphism, several processes are at work.
Recrystallization causes changes in minerals size and shape. Chemical reactions occur
between the minerals to form new sets of minerals that are more stable at the pressure
and temperature of the environment, and new minerals form as a result of polymorphic
phase transformations (recall that polymorphs are compounds with the same chemical
formula, but different crystal structures.
Laboratory experiments suggest that the the sizes of the mineral grains produced during
metamorphism increases with time. Thus coarse grained metamorphic rocks involve long
times of metamorphism. Experiments suggest that the time involved is tens of millions
Grade of Metamorphism
Metamorphic grade is a general term for
describing the relative temperature and
pressure conditions under which metamorphic
rocks form. As the temperature and/or
pressure increases on a body of rock we say
that the rock undergoes prograde
metamorphism or that the grade of
metamorphism increases. Low-grade metamorphism takes place at temperatures between about 200 to 320oC, and
relatively low pressure. Low grade metamorphic rocks are characterized by an
abundance of hydrous minerals (minerals that contain water, H2O, in their crystal
Examples of hydrous minerals that occur in low grade metamorphic rocks: Page 3 of 12 2/1/2011 Metamorphic Rocks Clay Minerals
High-grade metamorphism takes place at temperatures greater than 320oC and relatively
high pressure. As grade of metamorphism increases, hydrous minerals become less
hydrous, by losing H2O and non-hydrous minerals become more common.
Examples of less hydrous minerals and non-hydrous minerals that characterize
high grade metamorphic rocks:
Muscovite - hydrous mineral that eventually disappears at the highest
grade of metamorphism
Biotite - a hydrous mineral that is stable to very high grades of
Pyroxene - a non hydrous mineral.
Garnet - a non hydrous mineral.
As temperature and pressure fall due to erosion of overlying rock or due to tectonic uplift, one
might expect metamorphism to a follow a reverse path and eventually return the rocks to their
original unmetamorphosed state. Such a process is referred to as retrograde metamorphism. If
retrograde metamorphism were common, we would not commonly see metamorphic rocks at
the surface of the Earth. Since we do see metamorphic rocks exposed at the Earth's surface
retrograde metamorphism does not appear to be common. The reasons for this include:
chemical reactions take place more slowly as temperature is decreased
during prograde metamorphism, fluids such as H2O and CO2 are driven off, and these
fluids are necessary to form the hydrous minerals that are stable at the Earth's surface.
chemical reactions take place more rapidly in the presence of fluids, but if the fluids are
driven off during prograde metamorphism, they will not be available to speed up
reactions during retrograde metamorphism.
Metamorphic Rock Types
There are two major subdivisions of metamorphic rocks.
1. Foliated – These have a planar foliation caused by the preferred orientation (alignment)
of minerals and formed under differential stress.
They have a significant amount of sheet silicate (platy minerals and are classified by
composition, grain size, and foliation type.
2. Non-foliated – These have no evident planar fabric or foliation, crystallized under
conditions where there was no differential stress, and are comprised of equant minerals
only. These are classified mainly by the minerals present or the chemical composition of
the protolith. Page 4 of 12 2/1/2011 Metamorphic Rocks Foliated Metamorphic Rocks Example - metamorphism of
a shale, made up initially of
clay minerals and quartz all
of clay or silt size. Slate - Slates form at low metamorphic grade by the growth of fine grained chlorite and
clay minerals. The preferred orientation of these sheet silicates causes the rock to easily
break along the planes parallel to the sheet silicates, causing a slatey cleavage. Note that
in the case shown here, the maximum stress is applied at an angle to the original bedding
planes, so that the slatey cleavage has developed at an angle to the original bedding. Because of the nearly perfect breakage along planes, slates are useful for blackboards and
Phyllite - Fine mica-rich rock, formed by low – medium grade metamorphism. In a
phyllite, the clay minerals have recrystallized into tiny micas (biotite and muscovite
which reflect a satiny luster. Phyllite is between slate and schist. Schist - The size of the mineral grains tends to enlarge with increasing grade of
metamorphism. Eventually the rock develops a near planar foliation caused by the
preferred orientation of sheet silicates (mainly biotite and muscovite). Quartz and
Feldspar grains, however show no preferred orientation. The irregular planar foliation at
this stage is called schistosity. Page 5 of 12 2/1/2011 Metamorphic Rocks Schist often has other minerals besides micas. These include minerals like Quartz, Feldspars, Kyanite, Garnet, Staurolite, and Sillimanite.
When these non-mica minerals occur with a grain size greater than the rest of the
rock, they are called pophyroblasts.
Gneiss As metamorphic grade increases, the sheet silicates become unstable and dark
colored minerals like hornblende and pyroxene start to grow. These dark colored
minerals tend to become segregated in distinct bands through the rock, giving the rock a
gneissic banding. Because the dark colored minerals tend to form elongated crystals,
rather than sheet- like crystals, they still have a preferred orientation with their long
directions perpendicular to the maximum differential stress. Granulite - At the highest grades of metamorphism all of the hydrous minerals and sheet
silicates become unstable and thus there are few minerals present that would show a Page 6 of 12 2/1/2011 Metamorphic Rocks preferred orientation. The resulting rock will have a granulitic texture that is similar to a
phaneritic texture in igneous rocks.
Migmatites – If the temperature reaches the solidus temperature (first melting
temperature), the rock may begin to melt and start to co-mingle with the solids. Usually
these melts are felsic with the mafic material remaining metamorphic.
Non-foliated Metamorphic Rocks
Non-foliated rocks lack a planar fabric . Absence of foliation possible for several reasons:
Rock not subjected to differential stress.
Dominance of equant minerals (like quartz, feldspar, and garnet).
Absence of platy minerals (sheet silicates).
Non-foliated rocks are given specific names based on their mineralogy and composition:
Amphibolite - These rocks are dark colored rocks with amphibole (usually hornblende) as their
major mineral. They are usually poorly foliated and form at intermediate to high grades of
metamorphism of basaltic or gabbroic protoliths.
Hornfels - These are very fine grained rocks that usually form as a result of magma intruding
into fined grained igneous rocks or shales. The magma causes a type of metamorphism called
contact metamorphism (to be discussed later).
Quartzite - A rock made up almost entirely of quartz. They are formed by metamorphism of
quartz arenites (sandstones). Since quartz is stable over a large range of temperatures and
pressures, no new minerals are formed during metamorphism, and the only metamorphic effect
that occurs is recrystallization of the quartz resulting in interlocking crystals that make up a
very hard rock.
Marble - A limestone or dolostone made up only of calcite or dolomite will metamorphose to a
marble which is made mostly recrystallized calcite or dolomite. The Recrystallization usually
obliterates all fossils. Marbles have a variety of colors and are often complexly banded. They
are commonly used as a decorative stone.
Although textures and structures of the protolith are usually destroyed by metamorphism, we
can still get an idea about the original rock from the minerals present in the metamorphic rock.
Minerals that form, do so because the chemical elements necessary to form them are present in
General terms used to describe the chemical composition of both the protolith and the resulting
metamorphic rock are:
Pelitic Alumina rich rocks, usually shales or mudstones. These start out originally with clay
minerals and as a result of metamorphism, Alumina rich minerals like micas, chlorite, garnet,
kyanite, sillimanite and andalusite form. Because of the abundance of sheet silicates, pelitic
rocks commonly form slates, phyllites, schists, and gneisses during metamorphism. Page 7 of 12 2/1/2011 Metamorphic Rocks Mafic - These are Mg and Fe rich rocks with low amounts of Si. Minerals like biotite,
hornblende and plagioclase form during metamorphism and commonly produce amphibolites.
Calcareous - These are calcium-rich rocks usually derived from limestones or dolostones, and
thus contain an abundance of Calcite. Marbles are the type of metamorphic rock that results.
Quartzo-Feldspathic - Rocks that contain an abundance of quartz and feldspar fall into this
category. Protoliths are usually granites, rhyolites, or arkose sandstones and metamorphism
results in gneisses containing an abundance of quartz, feldspar, and biotite.
Types of Metamorphism
Metamorphism can take place in several different environments where special conditions exist
in terms of pressure, temperature, stress, conditions, or chemical environments. We here
describe several diff rent types of metamorphism that are recognized.
Contact Metamorphism (also called 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 a zone surrounding the intrusion, called a metamorphic
aureole. Outside of the contact aureole, the rocks are unmetamorphosed. The grade of
metamorphism increases in all directions toward the intrusion. Because temperature
differences between the surrounding rock and the intruded magma are larger at shallow
levels in the crust, contact metamorphism is usually referred to as high temperature, low
pressure metamorphism. The rock produced is often a fine-grained rock that shows no
foliation, called a hornfels. Page 8 of 12 2/1/2011 Metamorphic Rocks Burial Metamorphism - When sedimentary rocks are buried to depths of several hundred
meters, temperatures greater than 300oC 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 the Zeolites. Burial metamorphism overlaps, to some extent,
with diagenesis, and grades into regional metamorphism as temperature and pressure
Dynamic Metamorphism - This type of metamorphism is due to 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 the zone, and the rocks tend to crushed
and pulverized due to the sliding. Dynamic metamorphism is not very common and is
restricted to a narrow zone along which the sliding occurred. The rock that is produced
is called a mylonite. Regional Metamorphism - This type of
metamorphism occurs over large areas
that were subjected to high degrees of
deformation under differential stress.
Thus, it usually results in forming
metamorphic rocks that are strongly
foliated, such as slates, schists, and
gneisses. The differential stress usually
results from tectonic forces that produce a
compression of the rocks, such as when
two continental masses collide with one
another. Thus, regionally metamorphosed
rocks occur in the cores of mountain
ranges or in eroded mountain ranges.
Compressive stresses result in folding of
the rock, as shown here, and results in
thickening of the crust which tends to
push rocks down to deeper levels where
they are subjected to higher temperatures
and pressures (See Figure 8.26 in your
text). A map of a hypothetical regionally metamorphosed area is shown in the figure
below. Most regionally metamorphosed areas can be divided into zones where a
particular mineral, called an index mineral, is characteristic of the zone. The
zones are separated by lines (surfaces in three dimensions) that mark the first
appearance of the index mineral. These lines are called isograds (meaning equal
grade) and represent lines (really surfaces) where the grade of metamorphism is
equal. A maps of a regionally metamorphosed areas are can be seen in figure 8.21
of your text. Page 9 of 12 2/1/2011 Metamorphic Rocks Hydrothermal Metamorphism - Near oceanic ridges where the oceanic crust is broken up by
extensional faults, sea water can descend along the cracks. Since oceanic ridges are areas
where new oceanic crust is created by intrusion and eruption of basaltic magmas, these waterrich fluids are heated by the hot crust or magma and become hydrothermal fluids. The
hydrothermal fluids alter the basaltic oceanic crust by producing hydrous minerals like chlorite
and talc. Because chlorite is a green colored mineral the rocks hydrothermal metamorphic
rocks are also green and often called greenstones.
Subduction Related Metamorphism - At a subduction zone, the oceanic crust is pushed
downward resulting in the basaltic crust and ocean floor sediment being subjected to relatively
high pressure. But, because the oceanic crust by the time it subducts is relatively cool, the
temperatures in the crust are relatively low. Under the conditions of low temperature and high
pressure, metamorphism produces an unusual blue mineral, glaucophane. Compressional
stresses acting in the subduction zone create the differential stress necessary to form schists and
thus the resulting metamorphic rocks are called blueschist
Shock Metamorphism - When a large meteorite collides with the Earth, the kinetic energy is
converted to heat and a high pressure shock wave that propagates into the rock at the impact
site. The heat may be enough to raise the temperature to the melting temperature of the earth
rock. The shock wave produces high enough pressure to cause quartz to change its crystal
structure to more a dense polymorph like coesite or stishovite. Ancient meteorite impact sites
have been discovered on the basis of finding this evidence of shock metamorphism.
In general, metamorphic rocks do not undergo significant changes in chemical composition
during metamorphism. The changes in mineral assemblages are due to changes in the
temperature and pressure conditions of metamorphism. Thus, the mineral assemblages that are
observed must be an indication of the temperature and pressure environment that the rock was Page 10 of 12 2/1/2011 Metamorphic Rocks subjected to. This pressure and temperature environment is referred to as Metamorphic
The sequence of
metamorphic facies observed
in any metamorphic terrain,
depends on the geothermal
gradient that was present
during metamorphism. A
high geothermal gradient
such as the one labeled "A"
in the figure shown here,
might be present around an
igneous intrusion, and would
result in metamorphic rocks
belonging to the hornfels
facies. Under a normal
geothermal gradient, such as
"B" in the figure, rocks
would progress from zeolite
facies to greenschist,
amphibolite, and eclogite
facies as the grade of
metamorphism (or depth of
If a low geothermal gradient was present, such the one labeled "C" in the diagram, then rocks
would progress from zeolite facies to blueschist facies to eclogite facies. Thus, if we know the
facies of metamorphic rocks in the region, we can determine what the geothermal gradient must
have been like at the time the metamorphism occurred. Page 11 of 12 2/1/2011 Metamorphic Rocks Questions on this material that might be asked on an exam
1. Define the following: (a) geothermal gradient, (b) metamorphism, (c) differential stress,
(d) prograde metamorphism, (e) metasomatism (f) protolith, (g) foliation, (i)
metamorphic aureole, (j) isograd, (k) greenstone, (l) blueschist.
2. Starting with a shale, describe the textural changes that would occur to the rock during
prograde metamorphism with differential stress conditions present.
3. Why is retrograde metamorphism uncommon?
4. Describe the following non-foliated metamorphic rocks (a) amphibolite, (b) quartzite, (c)
marble, (d) hornfels.
5. What are the terms used to describe the general chemical composition of metamorphic
rocks?. Describe the type of rocks and minerals four nd in each.
6. What are the various types of metamorphism? Describe the rocks produced by each.
7. What is the progression of metamorphic facies that would occur along a high geothermal
gradient, a normal geothermal gradient, and a low geothermal gradient.
Return to EENS 1110 Page Page 12 of 12 2/1/2011 ...
View Full Document
This note was uploaded on 02/01/2012 for the course EENS 1110 taught by Professor Staff during the Fall '10 term at Tulane.
- Fall '10