Metamorphic rocks

CONTENTS
 Introductions
 Metamorphism and Type of metamorphic agents.
 Metamorphic processes.
 Types of Metamorphism
 Classification of metamorphic rocks
 Textures of metamorphic rocks
 Mineral assemblages and Metamorphic grade and facies of
metamorphic rocks.
 Graphic representation of metamorphic mineral
parageneses.

Metamorphic Rocks
• Recrystallization in the solid state
• Caused by changes in T, P or fluids
• New environment = new minerals
• Growing minerals create a new texture
Meta"= Change (Grk)
"Morph"= form (Grk)
a rock that has been changed from its original form (parent ) by
Heat, pressure , and fluid activity into a new rock (daughter ).

METAMORPHISM
Metamorphism is the process of mineralogical and
structural changes of rocks in their solid state in
response to physical and chemical conditions which
differ from the conditions prevailing during the
formation of the rocks, however, the changes
occurring within the domains of weathering and
diagenesis are commonly excluded.

METAMORPHISM
• Features of Metamorphism
– It principally formed in solid state and before melting,
– Metamorphism can be considered to be isochemical, except
perhaps for removal or addition of volatiles (H2O, CO2),
– The process of extensive chemical changes during
transformation is known as metasomatism.
• Metamorphism characterized by
– Growth of new minerals from pre-existing minerals through
recrystallization
– Deformation of existing minerals
• Change in shape
• Change in orientation

• Growth of new minerals
Mica garnet schist
Garnet crystal

• Deformation of rocks and minerals
Layers are folded and broken

• Recrystallization
– T, P or change in pore fluids initiate
change in the mineral assemblage
– Reaction occur entirely in the solid state
Shale Schist

• Textural changes occur during
metamorphism
– New minerals grow during
metamorphism
– Precursor rock textures are modified

Protolith
The parent rock subjected to Metamorphism
-can be any rock type: igneous,
sedimentary, or metamorphic
Shale Schist

Limits of metamorphism
• The lower temperature limit, at metamorphism takes place, marks the
boundary between metamorphism and diagenesis.
• The temperature lower limit of metamorphism is considered to be between
100 - 200゜C for the more unstable types of protolith, to exclude
diagenetic changes, due to compaction, which result in sedimentary rocks.
There is no agreement as for a pressure lower limit. Some workers argue
that changes in atmospheric pressures are not metamorphic, but some types
of metamorphism can occur at extremely low pressures.
• The upper boundary of metamorphic conditions is related to the onset of
melting processes in the rock. The maximum temperature for
metamorphism is typically between 700 - 900゜ C, depending on the
pressure and on the composition of the rock. Migmatites are rocks formed
at this upper limit, which contain pods and veins of material that has started
to melt but has not fully segregated from the refractory residue. Since the
1980s, it has been recognized that rarely, rocks are dry enough, and of a
refractory enough composition, to record without melting "ultra-high"
metamorphic temperatures of 900 - 1100°C. Once melting begins, the
process changes to an igneous precess rather than a metamorphic process.
• Migmatites (“mixed rocks”) are gradational

• Migmatites: metamorphic or igneous rocks?

Origin of Metamorphic Rocks
• Metamorphism
Temperature exceeds 200OC
Pressure exceeds 300 M Pa (approx.
10,000 ft deep)
• Metamorphism ends when melting
begins
Melting begins at ~700oC

Stability ranges for Al2 SiO5 minerals

TYPES OF METAMORPHIC AGENTS AND LIMITS OF
METAMORPHISM
TYPES OF METAMORPHIC AGENTS :
 Temperature  Pressure  Chemical active fluids
1- Temperatures: Typically the most important factor in
metamorphism (leads to Recrystallization results in new, stable minerals, increase
in grain size, increases the rate of reactions)
-Limits of temperatures
 lower limit (150 50 C)
 higher limit (beginning of melting, 650-1100 C)
- Low limit depend on the original protolith
 lower T (shale, organic matters)
 higher T (Igneous rocks and carbonates)
- Beginning of melting depend on:
 protolith composition
 the presence of aqueous fluids

Example:
- At 5 kbar and presence of aqueous fluid - granites begin to melt at ~ 660 C
- basalts begin to melt at ~800 C
- At 5 kbar and dry conditions - granites begin to melt at ~ 1000 C
- basalts begin to melt at ~1120 C
Source of Temperature for metamorphism:
- heat flowing into the base of the crust from the mantle
- heat brought into the crust by rising magma bodies
- heat generated from radioactive decay
- the effect of rapid uplift and erosion
- heat related to burial effect and geothermal gradient
Geothermal gradient: (rate of increasing temperature with depth, mean = 25
C/km)
- Subduction zone (10 C/km)
- Precambrian Shields (12-20 C/km)
- Collisionl orogens (25-30 C/km)
- Active arc-margin (30-35 C/km)
- Extensional orogens (40-50 C/km)
- Mid-ocean ridges (~ 60 C/km)

Heat as a Metamorphic Agent
Geothermal Gradient

• Source of heat
Proximity to igneous intrusions
•Contact metamorphism
•Different metamorphic grade ring the
intrusion
Depth of burial
•15-30oC increase per km
•Geothermal Gradient

Magma Body
-source of heat
Zone of Contact
Metamorphism

2- Pressures: (leads to reducing grain size and deformation) - --
- Pressure is define as force/unit area
- Unit of pressure (bar, kbar), 1 bar = 0.987 atmosphere = 14.5 pound/inch2
- pressures types  confining pressure
or lithostatic pressure (Plith)
 directive or deviatoric pressure
 fluid pressure (Pfluid)
 effective pressure (Pe)
Pe = Plith – Pfluid

Pressures:
- Limits of pressure
 lower limit (a few of bars, at Earth’s surface)
 Higher limits (30-40 in the collisional orogen or up to 100 kbar
in the ultrahigh pressure metamorphism)
- Source of pressure
 burial influence of an overlying rock column
 Plate tectonic and movement of plate segments
- Geobaric gradient (change of pressure with depth )
 average = 0.285 kbar/km or ~1kbar/3km

Pressure and Stress
Lithostatic Pressure- pressure increases
with depth due to the weight of the
overlying rocks.
-equal in all directions: uniform stress
-a cubic foot of granite weighs 167.9
pounds.

Lithostatic Pressure - equal from all directions

Tectonic Pressure/Stress
-directional/non-uniform
-referred to as deviatoric stress

Folded Metamorphic Rocks
-subjected to non-uniform deviatoric stress

Pressure and fabric changes
Lithostatic pressure = uniform stress (hydrostatic)
Deviatoric stress = unequal pressure in different directions.
Deviatoric stress can be resolved into three mutually
perpendicular stress () components:
i) 1 is the maximum principal stress
ii) 2 is an intermediate principal stress
iii) 3 is the minimum principal stress
In hydrostatic situations all three are equal

Pressure and fabric changes, Cont.
Stress is an applied force acting on a rock (over a particular
cross-sectional area)
Strain is the response of the rock to an applied stress (=
yielding or deformation)
Deviatoric stress affects the textures and structures, but not
the equilibrium mineral assemblage
Deviatoric stresses come in three principal types:
– Tension
– Compression
– Shear

Tension: 3 is negative, and the resulting strain is extension, or
pulling apart. Tension fractures may open normal to the extension
direction and become filled with mineral precipitates.
original shape strain
ellipsoid
1
3

Compression: 1 is dominant; therefore, folding or more
homogenous flattening are caused.
1
3

Shear motion occurs along planes at an angle to  1 and causing
slip along parallel planes and rotation.
1

Foliation is a common result, which allows us to estimate the
orientation of  1
- 1 >  2 =  3  foliation and no lineation
-  1 =  2 >  s3  lineation and no foliation
-  1 >  2 >  3  both foliation and lineation
1

3- Metamorphic fluids (leads to chemical changes)
mostley are H2O and CO2 types
- include  Ascending fluids from Magma chamber
 Descending fluids of the meteoric water
- Proofs of importance of fluids in metamorphism
 most metamorphic minerals are hydrous, so water
should be present
most of metamorphic reactions involves dehydration or
decarbonation
ms + chl  bt + grt + qtz + H2O 
CaCO3 + SiO2  CaSiO3 + CO2 
Fluids could preserved as inclusion
in neoblasts in metamorphic rocks.

• T & P determine degree of
metamorphism
Low-grade metamorphism- 200 to 350 OC
Intermediate-grade metamorphism-350 to
550 OC
High-grade metamorphism - very high
temperatures, above 550OC

Metamorphic Processes
• Recrystallization: The most common process in the formation
of metamorphic rocks is recrystallization. In this process the
grains making up the protolith change shape and size. The
identity of the mineral does not change during this process,
only the texture. Recrystallization occurs due to heating of the
protolith. The temperature at which this occurs can vary
depending on the minerals present. For example, the
sedimentary rock, limestone. Limestone is composed of
microscopic crystals of calcite that once formed the shells of
marine creatures. When metamorphosed, those tiny crystals
are begin to form larger, more visible crystals of calcite found
in the metamorphic rock marble.
• Phase Change: Phase change metamorphism is the creating of
new minerals with the same chemical formula as the protolith.
This involves a rearrangement of the atoms in the crystals.

• Neocrystallization: Neocrystallization involves the
creation of new mineral crystals different from the
protolith. Chemical reactions digest the minerals of
the protolith which yields new minerals. This is a
very slow process as it can also involve the diffusion
of atoms through solid crystals. This is most often
observed with the sedimentary rock shale, which is
composed of fine grained quartz, feldspars, and other
clay minerals. When metamorphosed, these minerals
grains will change to muscovite mica and garnet.
• Pressure Solution: Pressure solution is a
metamorphic process that requires a rock to be under
strong pressure from one direction and in the
presence of hot water. During this process minerals of
the protolith partially dissolve, diffuse through the
water and precipitate elsewhere.

• Plastic Deformation: In plastic deformation pressure
is applied to the protolith, which causes it to shear or
bend, but not break. In order for this to happen
temperatures must be high enough that brittle
fractures do not occur, but not so high that diffusion
of crystals takes place.
• Metasomatism: The process results in the loss or
addition of chemicals. This can best be seen in the
production of anthracite coal, which is almost pure
carbon. The parent rock bituminous coal is produced
from the aggregation of dead plant material, and
anthracite is produced by the loss of the more volatile
materials such as nitrogen, oxygen, and methane.

The Types of Metamorphism
Different approaches to classification
1. Based on principal process or agent
• Dynamic Metamorphism
• Thermal Metamorphism
• Dynamo-thermal Metamorphism

2. Based on setting
• Contact (Thermal) Metamorphism
 Pyrometamorphism
• Regional Metamorphism
 Orogenic Metamorphism
 Burial Metamorphism
 Ocean Floor Metamorphism
• Hydrothermal Metamorphism
• Fault-Zone Metamorphism
• Impact or Shock Metamorphism
We’ll look at the classifivation based on setting

Contact metamorphic rocks form under conditions
of high temperature and low pressure. They are arrayed
in aureoles, or metamorphosed zones, around plutons
and other intrusive igneous bodies.
Contact (thermal) Metamorphism

Contact metamorphism occurs typically around intrusive
igneous rocks as a result of the temperature increase caused by
the intrusion of magma into cooler country rock. The area
surrounding the intrusion where the contact metamorphism
effects are present is called the metamorphic aureole. Contact
metamorphic rocks are usually known as hornfels. Rocks
formed by contact metamorphism may not present signs of
strong deformation and are often fine-grained.
The size and shape of an aureole is controlled by:
1- The nature of the pluton
• Size
• Shape
• Orientation
• Temperature
• Composition
2- The nature of the country rocks
• Composition
• Depth and metamorphic grade prior to intrusion
• Permeability

Dikes generally have small aureoles with minimal
metamorphism whereas large ultramafic intrusions can have
significantly thick and well-developed contact metamorphism.
Contact metamorphism produces non-foliated (rocks without
any cleavage) rocks such as marble, quartzite, and hornfels.
The metamorphic grade of an aureole is measured by the
peak metamorphic mineral which forms in the aureole. This is
usually related to the metamorphic temperatures of pelitic or
alumonisilicate rocks and the minerals they form.The metamorphic
grades of aureoles are andalusite hornfels, sillimanite hornfels,
pyroxene hornfels.
Magmatic fluids coming from the intrusive rock may also
take part in the metamorphic reactions. Extensive addition of
magmatic fluids can significantly modify the chemistry of the
affected rocks. In this case the metamorphism grades into
metasomatism. If the intruded rock is rich in carbonate the result is
a skarn. Fluorine-rich magmatic waters which leave a cooling
granite may often form greisens within and adjacent to the contact
of the granite. Metasomatic altered aureoles can localize the
deposition of metallic ore minerals and thus are of economic interest.

Contact Metamorphism
• Adjacent to igneous intrusions
• Thermal ( metasomatic) effects of hot magma
intruding cooler shallow rocks
• Occurs over a wide range of pressures, including
very low
• Contact aureole

Metamorphic Environments

Regional Metamorphism
Regional Metamorphism : metamorphism that affects a
large body of rock, and thus covers a great lateral
extent
Three principal types:
 Orogenic metamorphism
 Burial metamorphism
 Ocean-floor metamorphism

• Orogenic metamorphism: is the type of
metamorphism associated with convergent plate
margins
Regional or Barrovian metamorphism covers
large areas of continental crust typically associated
with mountain ranges, particularly subduction zones
or the roots of previously eroded mountains.
Conditions producing widespread regionally
metamorphosed rocks occur during an orogenic event.
The collision of two continental plates or island arcs
with continental plates produce the extreme
compressional forces required for the metamorphic
changes typical of regional metamorphism.

These orogenic mountains are later eroded,
exposing the intensely deformed rocks typical of their
cores. The conditions within the subducting slab as it
plunges toward the mantle in a subduction zone also
produce regional metamorphic effects. The techniques
of structural geology are used to unravel the collisional
history and determine the forces involved. Regional
metamorphism can be described and classified into
metamorphic facies or metamorphic zones of
temperature/pressure conditions throughout the
orogenic terrane. This metamorphism usually produces
rocks such as gneiss and schist.

Burial metamorphism : for low-grade metamorphism in
sedimentary basins due to burial. A term coined by Coombs
(1961) for low-grade metamorphism that occurs in
sedimentary basins due to burial by successive layers.
• Mild deformation, no igneous intrusions discovered
• Metamorphic effects attributed to increased temperature and
pressure due to burial
• The main minerals produced are often Zeolites. Burial
metamorphism overlaps, to some extent, with diagenesis, and
grades into regional metamorphism as temperature and
pressure increase.
• The Gulf of Mexico may represent a modern example
• Areas of burial metamorphism may thus become areas of
orogenic metamorphism

Ocean-Floor Metamorphism: affects the oceanic crust at ocean
ridge spreading centers. Variety of metamorphic minerals in
ocean-floor rocks, representing a wide range of temperatures
at relatively low pressure. Alteration concentrated along vein
systems, presumably associated with hydrothermal activity.
Seawater penetrates down ubiquitous fracture systems, where
it becomes heated, and leaches metals and silica from the hot
basalts. Considerable metasomatic alteration, notably loss of
Ca and Si and gain of Mg and Na.
• Highly altered chlorite-quartz rocks- distinctive high-Mg,
low-Ca composition
• Exchange between basalt and hot seawater
• Another example of hydrothermal metamorphism

•Hydrothermal metamorphism
Hydrothermal metamorphism is the result of the
interaction of a rock with a high-temperature fluid of
variable composition. The difference in composition
between existing rock and the invading fluid triggers a
set of metamorphic and metasomatic reactions. The
hydrothermal fluid may be magmatic (originate in an
intruding magma), circulating groundwater, or ocean
water. Convective circulation of hydrothermal fluids in
the ocean floor basalts produces extensive hydrothermal
metamorphism adjacent to spreading centers and other
submarine volcanic areas. The fluids eventually escape
through vents in the ocean floor known as black
smokers. The patterns of this hydrothermal alteration is
used as a guide in the search for deposits of valuable
metal ores.

Hydrothermal Metamorphism

•Shock metamorphism
This kind of metamorphism occurs when
either an extraterrestrial object (a meteorite for
instance) collides with the Earth's surface or during
an extremely violent volcanic eruption. Impact
metamorphism is, therefore, characterized by
ultrahigh pressure conditions and low temperature.
The resulting minerals (such as SiO2 polymorphs
coesite and stishovite) and textures are
characteristic of these conditions.

Impact Metamorphism

•Dynamic metamorphism
Dynamic metamorphism is associated with zones of high to moderate
strain such as fault zones. Cataclasis, crushing and grinding of rocks into
angular fragments, occurs in dynamic metamorphic zones, giving cataclastic
texture.
The textures of dynamic metamorphic zones are dependent on the
depth at which they were formed, as the temperature and confining pressure
determine the deformation mechanisms which predominate. Within depths less
than 5 km, dynamic metamorphism is not often produced because the
confining pressure is too low to produce frictional heat. Instead, a zone of
breccia or cataclasite is formed, with the rock milled and broken into random
fragments. This generally forms a mélange. At depth, the angular breccias
transit into a ductile shear texture and into mylonite zones.

Dynamic metamorphism:
Rock deformation associated with fault zones
Mylonite from shear zone

Within the depth range of 5–10 km pseudotachylite is
formed, as the confining pressure is enough to prevent
brecciation and milling and thus energy is focused into discrete
fault planes. Frictional heating in this case may melt the rock to
form pseudotachylite glass.
Within the depth range of 10–20 km, deformation is
governed by ductile deformation conditions and hence frictional
heating is dispersed throughout shear zones, resulting in a weaker
thermal imprint and distributed deformation. Here, deformation
forms mylonite, with dynamothermal metamorphism observed
rarely as the growth of porphyroblasts in mylonite zones.
Overthrusting may juxtapose hot lower crustal rocks
against cooler mid and upper crust blocks, resulting in conductive
heat transfer and localised contact metamorphism of the cooler
blocks adjacent to the hotter blocks, and often retrograde
metamorphism in the hotter blocks. The metamorphic
assemblages in this case are diagnostic of the depth and
temperature and the throw of the fault and can also be dated to
give an age of the thrusting.

CLASSIFICATION OF METAMORPHIC ROCKS
Classification of metamorphic rocks is based on
mineral assemblage, texture, protolith, and bulk
chemical composition of the rock.
•Texture
In metamorphic rocks individual minerals may or
may not be bounded by crystal faces. Those that are
bounded by their own crystal faces are termed
idioblastic. Those that show none of their own crystal
faces are termed xenoblastic. From examination of
metamorphic rocks, it has been found that metamorphic
minerals can be listed in a generalized sequence in
order of their tendency to be idioblastic known as
the crystalloblastic series.

• Crystalloblastic series
In the series, each mineral tends to develop
idioblastic surfaces against any mineral that occurs
lower in the series. This series is listed below:
– rutile, sphene, magnetite
– tourmaline, kyanite, staurolite, garnet, andalusite
– epidote, zoisite, lawsonite, forsterite
– pyroxenes, amphiboles, wollastonite
– micas, chlorites, talc, stilpnomelane, prehnite
– dolomite, calcite
– scapolite, cordierite, feldspars
– Quartz
This series can, in a rather general way, enable us to
determine the origin of a given rock.

For example a rock that shows euhedral
plagioclase crystals in contact with anhedral amphibole,
likely had an igneous protolith, since a metamorphic rock
with the same minerals would be expected to show
euhedral amphibole in contact with anhedral plagioclase.
Another aspect of the crystalloblastic series is that
minerals high on the list tend to form porphyroblasts (the
metamorphic equivalent of phenocrysts), although K-
feldspar (a mineral that occurs lower in the list) may also
form porphyroblasts. Porphyroblasts are often riddled
with inclusions of other minerals that were enveloped
during growth of the porphyroblast. These are said to
have a poikioblastic texture.

Most metamorphic textures involve foliation.
Foliation is generally caused by a preferred orientation of
sheet silicates. If a rock has a slatey cleavage as its
foliation, it is termed a slate, if it has a phyllitic foliation,
it is termed a phyllite, if it has a schistose foliation, it is
termed a schist. A rock that shows a banded texture
without a distinct foliation is termed a gneiss. All of these
could be porphyroblastic (i.e. could contain
porhyroblasts).
A rock that shows no foliation is called a hornfels
if the grain size is small, and a granulite, if the grain size
is large and individual minerals can be easily
distinguished with a hand lens.

• Protolith
Protolith refers to the original rock, prior to metamorphism.
In low grade metamorphic rocks, original textures are often
preserved allowing one to determine the likely protolith. As
the grade of metamorphism increases, original textures are
replaced with metamorphic textures and other clues, such
as bulk chemical composition of the rock, are used to
determine the protolith.
• Bulk Chemical Composition
The mineral assemblage that develops in a metamorphic
rock is dependent on
– The pressure and temperature reached during metamorphism
– The composition of any fluid phase present during metamorphism,
and
– The bulk chemical composition of the rock.

Just like in igneous rocks, minerals can only form if the
necessary chemical constituents are present in the rock (i.e. the
concept of silica saturation and alumina saturation applies to
metamorphic rocks as well). Based on the mineral assemblage
present in the rock one can often estimate the approximate bulk
chemical composition of the rock. Some terms that describe this
general bulk chemical composition are as follows:
Pelitic. These rocks are derivatives of aluminous sedimentary rocks
like shales and mudrocks. Because of their high concentrations of
alumina they are recognized by an abundance of aluminous
minerals, like clay minerals, micas, kyanite, sillimanite, andalusite,
and garnet.
Quartzo-Feldspathic. Rocks that originally contained mostly
quartz and feldspar like granitic rocks and arkosic sandstones will
also contain an abundance of quartz and feldspar as metamorphic
rocks, since these minerals are stable over a wide range of
temperature and pressure. Those that exhibit mostly quartz and
feldspar with only minor amounts of aluminous minerals are termed
quartzo-feldspathic.

• Calcareous. Calcareous rocks are calcium rich. They are
usually derivatives of carbonate rocks. At low grades of
metamorphism calcareous rocks are recognized by their
abundance of carbonate minerals like calcite and
dolomite. With increasing grade of metamorphism these are
replaced by minerals like brucite, phlogopite (Mg-rich biotite),
chlorite, and tremolite. At even higher grades anhydrous
minerals like diopside, forsterite, wollastonite, grossularite,
and calcic plagioclase.
• Basic. The general term basic refers to low silica
content. Basic metamorphic rocks are generally derivatives of
basic igneous rocks like basalts and gabbros. They have an
abundance of Fe-Mg minerals like biotite, chlorite, and
hornblende, as well as calcic minerals like plagioclase and
epidote.
• Magnesian. Rocks that are rich in Mg with relatively less Fe,
are termed magnesian. Such rocks would contain Mg-rich
minerals like serpentine, brucite, talc, dolomite, and
tremolite. In general, such rocks usually have an ultrabasic
protolith, like peridotite, dunite, or pyroxenite.

• Ferriginous. Rocks that are rich in Fe with little Mg
are termed ferriginous. Such rocks could be
derivatives of Fe-rich cherts or ironstones. They are
characterized by an abundance of Fe-rich minerals
like greenalite (Fe-rich serpentine), minnesotaite (Fe-
rich talc), ferroactinolite, ferrocummingtonite,
hematite, and magnetite at low grades, and ferrosilite,
fayalite, ferrohedenbergite, and almandine garnet at
higher grades.
• Manganiferrous. Rocks that are characterized by the
presence of Mn-rich minerals are termed
manganiferrous. They are characterized by such
minerals as Stilpnomelane and spessartine.

• Classification
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.
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.
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.

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 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.

Metamorphic Rocks
• Classified by texture and
composition
– Texture is divided into foliated &
nonfoliated rocks
– Mineral composition qualifies
textural name

Nonfoliated Rocks
• Marble
– Interlocking, coarse grained calcite
– Recrystallization of limestone or
dolostone
– Sedimentary features
– destroyed

• Quartzite
– Metamorphism of quartz sandstone
– Entire rock is recrystallized

Metamorphic Rock Textures
Metamorphic rocks exhibit a variety of textures. These
can range from textures similar to the original protolith at low
grades of metamorphism, to textures that are purely produced
during metamorphism and leave the rock with little resemblance
to the original protolith.
Foliation
Foliation is defined as a pervasive planar structure that results
from the nearly parallel alignment of sheet silicate minerals
and/or compositional and mineralogical layering in the rock.
Most foliation is caused by the preferred orientation of
phylosilicates, like clay minerals, micas, and chlorite. Preferred
orientation develops as a result of non-hydrostatic or differential
stress acting on the rock (also called deviatoric stress).

Foliation:
-preferred orientation
or location of minerals

Stress and Preferred Orientation
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. Normally geologists talk about stress as
compressional stress. Thus, if a differential stress is
acting on the rock, the direction along which the
maximum principal stress acts is called σ1, the
intermediate principal stress direction is called σ2 and
the minimum principal stress is called σ3. Note that
extensional stress would act along the direction of
minimum principal stress.

• If differential stress is present during metamorphism, it can
have a profound effect on the texture of the rock.
Minerals that crystallize or grow in the differential stress field
may develop a preferred orientation. Sheet silicates and minerals
that have an elongated habit will grow with their sheets or
direction of elongation orientated perpendicular to the direction
of maximum stress.
Rounded grains can become
flattened in the direction of
maximum compressional stress.

• Example - metamorphism of a shale (made up initially of clay
minerals and quartz)
• Shales have fissility that is caused by the
preferred orientation of clay minerals with
their {001} planes orientated parallel to
bedding. Metamorphic petrologists and
structural geologists refer to the original
bedding surface as S0.
• 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 planes parallel to the sheet silicates, causing a
slatey cleavage.
The foliation or surface
produced by this deformation
is referred to S1.

• Phyllite
– Similar to slate
– More Recrystallization
• Crystals are larger
– Common Protolith:
– shales

• 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

• Schist
– Strongly foliated rock
– Medium to coarse grained
-dominated by platy minerals
– Protoliths: multiple??

[Garnet] Schist w/porphyroblasts

• 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 into distinct bands through the rock (this
process is called metamorphic differentiation), 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.

• Gneiss
– Coarse grained granular rock
– Foliation: alternating bands of light
& dark minerals
• Quartz, feldspar, amphibole,
• biotite
– Protoliths: ??

• Granulite - At the highest grades of metamorphism most of
the hydrous minerals and sheet silicates become unstable and
thus there are few minerals present that would show a
preferred orientation. The resulting rock will have a granulitic
texture that is similar to a phaneritic texture in igneous rocks.

Metamorphism and Deformation
The result of compressional stress
acting on rocks that behave in a ductile
manner (ductile behavior is favored by
higher temperature, higher confining stress
[pressure] and low strain rates) is the
folding of rocks. Original bedding is folded
into a series of anticlines and synclines with
fold axes perpendicular to the direction of
maximum compressional stress. Note that
since the axial planes are oriented
perpendicular to the maximum
compressional stress direction, slatey
cleavage or foliation should also develop
along these directions. Thus, slatey
cleavage or foliation is often seen to be
parallel to the axial planes of folds, and is
sometimes referred to axial plane cleavage
or foliation.

Metamorphic Differentiation
• Metamorphic differentiation is alternating somewhat
discontinuous bands or layers of dark colored ferromagnesian
minerals and lighter colored quartzo-feldspathic layers as in
gneisses, and to some extent schists.
Throughout the history of metamorphic petrology, several
mechanisms have been proposed to explain metamorphic
differentiation.
1- Preservation of Original Compositional Layering. In some
rocks the compositional layering may not represent metamorphic
differentiation at all, but instead could simply be the result of
original bedding. For example, during the early stages of
metamorphism and deformation of interbedded sandstones and
shales the compositional layering could be preserved even if the
maximum compressional stress direction were at an angle to the
original bedding.

• In such a case, a foliation might develop in the shale
layers due to the recrystallization of clay minerals or
the crystallization of other sheet silicates with a
preferred orientation controlled by the maximum
stress direction.
Here, it would be easy to determine that the compositional
layers represented original bedding because the foliation
would cut across the compositional layering.

• In highly deformed rocks that have undergone both
folding and shearing, it may be more difficult to
determine that the compositional layering represents
original bedding. As shearing stretches the bedding,
individual folded beds may be stretched out and
broken to that the original folds are not easily seen.

2- Transposition of Original Bedding. Original compositional
layering a rock could also become transposed to a new
orientation during metamorphism. The diagram below shows
how this could occur. In the initial stages a new foliation begins
to develop in the rock as a result of compressional stress at some
angle to the original bedding. As the minerals that form this
foliation grow, they begin to break up the original beds into small
pods. As the pods are compressed and extended, partly by
recrystallization, they could eventually intersect again to form
new compositional bands parallel to the new foliation.

3- Solution and Re-precipitation. In fine grained
metamorphic rocks small scale folds, called kink bands, often
develop in the rock as the result of application of compressional
stress. A new foliation begins to develop along the axial planes of
the folds. Quartz and feldspar may dissolve as a result of
pressure solution and be reprecipitated at the hinges of the folds
where the pressure is lower. As the new foliation begins to align
itself perpendicular to s1, the end result would be alternating
bands of micas or sheet silicates and quartz or feldspar, with
layering parallel to the new foliation.

4- Preferential Nucleation. Fluids present during
metamorphism have the ability to dissolve minerals and
transport ions from one place in the rock to another.
Thus felsic minerals could be dissolved from one part of
the rock and preferentially nucleate and grow in another
part of the rock to produce discontinuous layers of
alternating mafic and felsic compositions.

5- Migmatization. As discussed previously, migmatites
are small pods and lenses that occur in high grade
metamorphic terranes that may represent melts of the
surrounding metamorphic rocks. Injection of the these
melts into pods and layers in the rock could also
produce the discontinuous banding often seen in high
grade metamorphic rocks. The process would be
similar to that described in 4, above, except that it
would involve partially melting the original rock to
produce a felsic melt, which would then migrate and
crystallize in pods and layers in the metamorphic
rock. Further deformation of the rock could then stretch
and fold such layers so that they may no longer by
recognizable as migmatites.

• Amphibolites & Eclogites
– Metamorphism of mafic rocks
– Mineral assemblage
gives green color
Eclogite from Norway
Amphibolite [pillow basalt]

Progressive metamorphism of shale

Fig. 6.5. Stability ranges for Al2O5 minerals

METAMORPHIC GRADE
• One of the primary goals of metamorphic petrology is to interpret
P-T conditions under which a rock (or set of rocks) formed
• Metamorphic grade
– Relative temperature and pressure conditions under which metamorphic
rocks form
• Low-grade metamorphism
– T ~200 to 320ºC, relatively
low pressure
– abundant hydrous minerals
• High-grade metamorphism
– T >320ºC and relatively high
pressure
– Dehydration; less hydrous
minerals common
• Prograde metamorphism
– T and/or P, grade of
metamorphism increases
• Retrograde metamorphism
– T and/or P, grade of metamorphism decreases

The sequence of zones (Barrovian Zones) now recognized, and the
typical metamorphic mineral assemblage in each, are:
• Chlorite zone. Pelitic rocks are slates or phyllites and
typically contain chlorite, muscovite, quartz and albite
• Biotite zone. Slates give way to phyllites and schists, with
biotite, chlorite, muscovite, quartz, and albite
• Garnet zone. Schists with conspicuous red almandine
garnet, usually with biotite, chlorite, muscovite, quartz, and
albite or oligoclase
• Staurolite zone. Schists with staurolite, biotite, muscovite,
quartz, garnet, and plagioclase. Some chlorite may persist
• Kyanite zone. Schists with kyanite, biotite, muscovite,
quartz, plagioclase, and usually garnet and staurolite
• Sillimanite zone. Schists and gneisses with sillimanite,
biotite, muscovite, quartz, plagioclase, garnet, and perhaps
staurolite. Some kyanite may also be present (although
kyanite and sillimanite are both polymorphs of Al2SiO5)

Regional Metamorphic Zones
• Index minerals
– A mineral that forms within a
specific,often narrow range of
conditions
– Identifies a specific grade of
metamorphism
– Allows further subdivision of rock
types

• Protolith controls final mineral
assemblage
– Different mineral assemblages form at a
given P & T for various parent rocks
– Metamorphic facies describe a group of
minerals formed under similar conditions
• May be found in different rocks

MINERAL ASSEMBLAGES AND
METAMORPHIC FACIES
The mineral assemblages that occur in metamorphic rocks
depend on four factors:
1. The bulk chemical composition of the original rock.
2. The pressure reached during metamorphism.
3. The temperature reached during metamorphism.
4. The composition of any fluid phase that was present during
metamorphism.
If a rock is taken to some higher pressure and temperature
then the mineral assemblage that develops should represent stable
chemical equilibrium if the conditions are held for a long enough
period of time that equilibrium can be achieved. Since
metamorphism usually involves long periods of geologic time,
most metamorphic rocks represent an equilibrium mineral
assemblage.

METAMORPHIC FACIES
In general, metamorphic rocks do not drastically
change chemical composition during metamorphism, except
in the special case where metasomatism is involved. 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 subjected to. This
pressure and temperature environment is referred to as
Metamorphic Facies (The metamorphic facies are groups
of mineral compositions in metamorphic rock that are
typical for a certain field in pressure –temperature). (This
is similar to the concept of sedimentary facies). 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" , might be present
around an igneous intrusion,
and would result in
metamorphic rocks belonging
to the hornfels facies. Under a
normal to high geothermal
gradient, such as "B", rocks
would progress from zeolite
facies to greenschist,
amphibolite, and eclogite
facies as the grade of
metamorphism (or depth of
burial) increased.
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.
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.

Metamorphic Facies and
Mineral Assemblages
Contact and Regional Metamorphism

Metamorphic Facies
• = set of rocks
characterized
by equilibrium
mineral
assemblage
that reflect
specific range
of metamorphic
(T-P) conditions
• Mineral assemblage present depends on protolith
composition and P-T conditions
– Ex: marble, metabasalt and schist all in amphibolite facies

Metamorphic Series
• Progression
or sequence
of facies
across a
metamorphic
terrain
• Identified by
field and
experimental
work
1. Contact Series – Hi T, low P = contact metamorphism
2. Dynamic Regional Series – Mod T & P = Barrovian
3. Static Regional Series – Lo T, hi P = Blueschist
1.
2.
3.

Regional
Metamorphic
Gradients

Stable Mineral Assemblages in
Metamorphic Rocks
• Equilibrium Mineral Assemblages
• At equilibrium, the mineralogy (and the
composition of each mineral) is determined by T,
P, and X
• “Mineral paragenesis” refers to such an
equilibrium mineral assemblage
• Relict minerals or later alteration products are
excluded unless specifically stated

THE PHASE RULE FOR METAMORPHISM
Phase rule, as applied to systems at equilibrium:
F = C + 2 – P where
F = the variance of the system or number of degrees of
freedom (the number of independently variable
intensive parameters of state (such as
temperature, pressure, the composition of each
phase, etc.)
C = the number of components in the system (the
minimum number of chemical constituents
required to specify every phase in the system)
P = the number of phases present in the system.

• So, for F=2, C=P, the number of phases present in a
rock for the more common divariant assemblage
will be equal to the number of components. If P is
greater than C, then one of three possibilities exist
for the mineral assemblage:
1. The assemblage represents a non-equilibrium
assemblage (perhaps due to incomplete chemical
reactions or due to the presence of retrograde
minerals that developed during cooling, uplift, or
unroofing of the metamorphic rock).
2. The assemblage represents univariant (F=1) or
invariant (F=0) equilibrium.
3. The number of components have not been chosen
correctly.

• The number of components, as stated in the
phase rule, must be chosen so as to represent
the minimum number necessary to form all
phases possible in the rock. Recall that the
number of components is not strictly the number
of oxide components or the number of elements
as reported in a chemical analysis of the rock. If
we just consider the major phases that make up
metamorphic rocks and consider that some ions
freely substitute for one another in solid
solutions, then the number of components can
often be reduced to 7 or 8. For example:

1. K2O.Al2O3 (based on the ratio of K to Al in the alkali
feldspars)
2. (Ca,Na2)O.Al2O3 (based on the ratio of Ca and Na to
Al in the plagioclase)
3. (Si,Ti)O2 - based on the common substitution of Ti into
tetrahedral sites in most silicates)
4. (Fe,Mn)O - based on the common substitution of Mn
for Fe in minerals.
5. MgO - usually needed because Fe-Mg solid solution
compositions are both temperature and pressure
dependent. (although sometimes these two are
combined, which would reduce the total number of
components by 1).
6. (Al,Fe+3) 2O3 - based on the commonly observed
substitution of Fe+3 for Al+3Al in minerals.
7. H2O - usually present in a fluid phase, but also an
important component of hydrous minerals.
8. CO2 - also usually present in a fluid phase, but also an
important component in carbonate minerals.

• If H2O and CO2 are assumed to be always
present and available to form hydrous and
carbonate minerals, then the number of
components can be reduced to 5 or 6. Thus for
a divariant assemblage (F=2) we would expect
to find 5 or 6 different mineral phases present
in a metamorphic rock, or up to 8 phases if the
assemblage is invariant.
• This is the basis for the construction of the
AKF and ACF diagrams, where the number of
components have been reduced to 4

TRIANGULAR PLOTS IN
METAMORPHIC PETROLOGY
• Recall, that the number of components in any given
system is the minimum number required to define the
composition of all phases in the system. Thus, since
constituents like Na2O and K2O are not usually found
as separate mineral phases, we can combine these
with other constituents, like Al2O3 and SiO2 in the
feldspars, and thus reduce the number of components
required to define our system.
• the best way to graphically display chemical rock and
mineral data is to attempt to reduce the number of
components to 3, so that we can plot the compositions
of rocks and minerals on a triangular composition
diagram.

General Three Component
Compositional Diagrams
• Specific minerals present within a facies (P-T conditions)
depends largely on protolith composition
• Equilibrium assemblages can be shown on ternary phase
diagrams (with some assumptions and simplifications)
• Hypothetical A-B-C
– 7 possible minerals can occur at
this temp & pressure
– Mineral phases that coexist with
each other at this temperature
and pressure are connected by
lines, called tie lines.
– Smaller triangles show regions
where sets of 3 minerals are
stable together

• Changing the P-T conditions changes
– Location of the tie lines
– Regions of coexisting stable mineral assemblages
• Due to chemical reactions between minerals
Change in conditions
AB + A2C => 2A + ABC

• Many minerals that occur in nature are
solid solutions, and thus they can have a
variable composition. Solid solution
minerals, because of their possible range
in chemical composition, do not plot at
single point on the composition diagrams,
but instead plot along a line or within a
field that represents the possible range in
chemical compositions.

• When solid solutions are present,
the tie lines become spread out
over a range of compositions as is
seen in the diagram shown here for
the hypothetical system X, Y, Z.
• In this diagram the mineral X(Y,Z)
shows limited solid solution with
variable amounts of Z substituting
for Y. This is shown by a solid
line extending from pure XY into
the ternary diagram.
Similarly, mineral X2(Z,Y) shows limited solid solution of Y
substituting for Z.
The minerals XYZss and Zss show a range of possible
compositions that are represented by a shaded field on the
diagram.

Common Triangular Plots Used in
Metamorphic Rocks
• Most common natural rocks contain the major
elements: SiO2, Al2O3, K2O, CaO, Na2O,
FeO, MgO, MnO , H2O and CO2 such that
C = 10
• Three components is the maximum number
that we can easily deal with in two
dimensions
• What is the “right” choice of components?
• Some simplifying methods:

1) Simply “ignore” components
• Trace elements
• Elements that enter only a single phase
(we can drop both the component and the
phase without violating the phase rule)
• Perfectly mobile components

2) Combine components
Components that substitute for one
another in a solid solution: (Fe + Mg)
3) Limit the types of rocks to be shown
Only deal with a sub-set of rock types for
which a simplified system works
4) Use projections
That is assume that a constituent will always be
present and project compositions from that
constituent in a four or five component system
to the 3 component system.

The ACF Diagram
• Illustrate metamorphic mineral assemblages in mafic rocks
on a simplified 3-C triangular diagram
• Concentrate only on the minerals that appeared or
disappeared during metamorphism, thus acting as
indicators of metamorphic grade
• Useful for plotting most common metamorphic rocks

• However, the A value we want is the
value of excess Al2O3 left after
allotting Na2O and K2O to form alkali
feldspar. The CaO value we want is
the excess CaO after allotting P2O5 to
form apatite, assuming that any P2O5
in the rock will suck up CaO to form
apatite. We will assume then that all
mineral assemblages plotted may also
contain alkali feldspar and quartz
(and apatite).
To plot a rock on the ACF diagram, the chemical analysis of
the rock is first recalculated to molecular proportions
A = Al2O3
C = CaO
F = FeO + MgO
a = [Al2O3 + Fe2O3] - [Na2O + K2O]
c = [CaO] - 3.33[P2O5]
f = [FeO + MgO + MnO]
Since we are only plotting these 3
components, they have to be
normalized so that they add up to 1 (or
100 if we are plotting %).

The ACF Diagram
Example 1:
Anorthite CaAl2Si2O8
A = 1 + 0 - 0 - 0 = 1, C = 1 - 0 = 1, and F = 0
A+C+F= 2, so we can normalize to 100
A = 100* 1/2 = 50
C = 100* 1/2 = 50
F = 0
Plotting minerals on the ACF diagram is somewhat easier if you know
the chemical formula of the mineral, since mineral formulae are
already in the form of molecular proportions.
Where does Ab plot? Plagioclase?

Figure 24.4. After Ehlers and Blatt (1982).
Petrology. Freeman. And Miyashiro (1994)
Metamorphic Petrology. Oxford.

Example 2:
hypersthene, (Mg,Fe)SiO3 , we have 1 molecule of (FeO +
MgO) for every 1 molecule of SiO2. Thus:
a = 0
c = 0
f = 1
t =1
• so, the plotting parameters become
• A = 100 * 0/1 = 0
• C = 100 * 0/1 = 0
• F = 100 * 1/1 = 100%
• and we see that hypersthene would plot at the F corner
of the ACF diagram.

The AKF Diagram
Because pelitic sediments are high in Al2O3 and K2O, and low
in CaO, Eskola proposed a different diagram that included K2O to
depict the mineral assemblages that develop in them.
In AKF diagrams we assume that both alkali feldspar and
plagioclase feldspar can be present, thus the amount of Al2O3 that we
use is the excess Al2O3 left after allotting it to all of the feldspars. To
obtain the plotting parameters for AKF diagrams, calculate the
following:
a = [Al2O3 + Fe2O3] - [Na2O + K2O + CaO]
k = [K2O]
f = [FeO + MgO + MnO]
Let t = a + k + f, then the plotting parameters in % are:
A = 100 * a/t
K = 100 * k/t
F = 100 * f/t

Figure 24.6. After Ehlers and Blatt
(1982). Petrology. Freeman.
Minerals are plotted in the same way as was done for the ACF
diagrams, and an example AKF diagram showing the potting
positions of common metamorphic minerals is shown below.

• Example 1: chemical formula of K-feldspar KAlSi3O8
• Rewrite it in oxide form as 1/2K2O 1/2Al2O3 3SiO2.
Then:
a = ½ - ½ = 0
k = ½
f = 0
t = ½
• So,
A = 100 * 0/½ = 0%
K = 100 * ½ /½ = 100%
F = 100 * 0/½ = 0%
• The K-feldspar plots in the lower right hand corner

• Example 2: Muscovite KAl3Si3O10 (OH)2 or
• 1/2K2O 3/2Al2O3 3SiO2 H2O. For muscovite:
a = 3/2 - 1/2 = 1
k = 1/2
f = 0
t = 1½ = 1.5
• So,
A = 100 * 1/1.5 = 66.7%
K = 100 * 0.5/1.5 = 33.33%
F = 100 * 0 = 0%
• Note that AKF diagrams are used for CaO-poor,
K2O-rich rocks, whereas ACF diagrams should be
used for Al2O3 and CaO - rich rocks.

Three of the most common minerals in metapelites:
andalusite, muscovite, and microcline, all plot as
distinct points in the AKF diagram
• And & Ms plot as the
same point in the ACF
diagram, and Micr
doesn’t plot at all, so
the ACF diagram is
much less useful for
pelitic rocks (rich in K
and Al)

AKFM Projection onto AFM
• One of the problems associated with ACF and AKF diagrams is
that Fe and Mg are assumed to substitute for one another and act
as a single component.
• J.B. Thompson developed a projected diagram that takes into
account possible variation in the Mg/(Mg+Fe) ratios in
ferromagnesium minerals, and has proven very useful in
understanding metamorphosed pelitic sediments.
• Thompson starts with the 5 component system SiO2 - Al2O3 - K2O
- FeO - MgO and ignores minor components in pelitic rocks like
CaO and Na2O. Because quartz is a ubiquitous phase in
metamorphosed pelitic rocks, the five component system is
projected into the four component system Al2O3 - K2O - FeO -
MgO as shown below.

• Next, because muscovite is also a common mineral in these
rocks, all compositions are projected from muscovite onto the
front face of the diagram. (Al2O3 - FeO - MgO). The front face
of the diagram becomes the AFM diagram.
• Minerals that contain no K2O like
andalusite, kyanite and sillimanite plot at
the A corner of the diagram, and minerals
like staurolite, chloritoid (Ctd), chlorite,
and garnet plot on the front face of the
diagram.
Biotite, however, does contain K2Oand has
varying amounts of Al2O3 and thus is a
solid solution that lies in the four
component system. Because muscovite is
relatively K - poor, this results in biotite
being projected to negative values of Al2O3.

• A = [Al2O3 - 3 K2O]
• F = [FeO]
• M = [MgO]
• Using these parameters, one
can grid off the AFM diagram
with the vertical scale
represented by the normalized
values for the A parameter -
• [Al2O3 - 3 K2O]/[Al2O3 - 3
K2O + FeO + MgO]
• and the horizontal position
based on the ratio of
MgO/(FeO + MgO).
• Of course these values are
obtained after converting the
chemical analysis of the rock to
molecular proportions.
• To calculate the plotting parameters for the AFM diagram the
following formulae are used:

• The projection from muscovite works well for metamorphic
rocks that contain muscovite. But, at higher grades of
metamorphism, in the upper amphibolite facies and the
granulite facies, muscovite becomes unstable and is replaced
by K-feldspar + quartz + an Al2SiO5 mineral. In order to show
rocks and mineral assemblages at these higher grades of
metamorphism, a new projection is made from K-spar, as
shown below.
• For this diagram the plotting parameters
are much more straight forward, with -
A = [Al2O3]
F= [FeO]
M =[ MgO]
• All on a molecular basis and then
renormalized to sum to 100%.
• Note the absence of all hydrous phases
(staurolite, chloritoid, muscovite) except
biotite in this projection.

Resolving Problems
• a common medium grade assemblage in a pelitic rock is
staurolite, garnet, muscovite, biotite, quartz, and
plagioclase. Plotting these minerals on ACF, AKF, and AMF
diagrams, as shown below creates a problem. For divariant
equilibrium we expect the number of components to equal the
number of phases (c=3, so p=3) at least for the ternary part of
our system.
• Thus, in the ACF diagram, a
rock like composition x would
have plagioclase, garnet, staurolite
(+quartz + muscovite), but biotite
cannot be resolved from garnet
because they plot near the same
point(s). Still, in the ACF
diagram, x plots within a 3 phase
triangle.

• In the AFM diagram the
same rock of composition
x is seen to have garnet,
staurolite, and biotite
(+quartz + muscovite).
Plagioclase is ignored by
the diagram (because CaO
is not plotted), but we can
resolve biotite and garnet
because they clearly have
different compositions in
the AFM plot.

• The problem in the AKF diagram implies one of the following:
1. The assemblage represents one where equilibrium has not
been achieved in the rock.
2. The assemblage is really a univariant assemblage, rather
than a divariant assemblage.
3. The AKF diagram is not truly behaving as a three
component system because of information lost to create the
projection.
• In the AKF plot there is an
ambiguity. Composition x plots
in the correct 4 phase field of
muscovite, garnet, staurolite,
and Biotite, but divariant
equilibrium requires that it plot
in a 3 phase triangle.

149
Example of Progressive Metamorphism
• Minerals present reflect equilibrium P-T conditions for
different protoliths
• As P-T conditions increase (prograde metamorphism)
minerals react with each other and with fluids
– Some minerals no longer stable; new minerals appear

What happened in jumping from the pressure/
temperature conditions of the first diagram to those
of the second diagram.
Phases Disappeared New Phases
andalusite sillimanite
epidote/zoisite plagioclase
chlorite grossularite
talc cordierite
actinolite biotite
albite anthophyllite
hornblende
The chemical reactions that must
have occurred between the two
sets of pressure/temperature
conditions that would explain the
new mineral assemblages are as
the following:
Al2SiO5 => Al2SiO5
Andalusite Sillimanite
4Ca2Al3Si3O12(OH) + SiO2 => 5CaAl2Si2O8 + Ca3Al2Si3O12 + 2H2O
Epidote/Zoisite Qtz Anorthite (plag) grossularite fluid
(Mg,Fe)5Al2Si3O10(OH)2 + KAl3Si3O10(OH)2 + 2SiO2 =>
Chlorite Muscovite Qtz
K(Fe,Mg)3AlSi3O10(OH)2 + (Mg,Fe)2Al4Si5O18 + H2O
Biotite Cordierite fluid
the reactions result in the evolution of H2O in a fluid phase are called
dehydration reactions.

Retrograde Metamorphism
If retrograde metamorphism were a common
process then upon uplift and unroofing metamorphic
rocks would progressively return to mineral
assemblages stable at lower pressures and temperatures
Three factors inhibit retrograde metamorphism,
two of which involve the fluid phase.
1. Chemical reactions run faster at higher temperatures.
Thus, during prograde metamorphism reaction rates are
faster. As temperature is lowered on a rock, the
reaction rates are much slower.

2. During prograde metamorphism a fluid phase is
driven off as a result of the devolatilization
reactions. As pressure increases, porosity of rocks
also decreases, and thus this fluid phase will likely be
driven out of the rock body. In the absence of the
fluid phase it is impossible to form hydrous minerals
and carbonates, since H2O and CO2, two of the key
components needed in such reactions, may not be
present.
3. The fluid phase also helps to catalyze chemical
reactions. If the fluid phase is driven off during
prograde metamorphism, then it will not be available
to catalyze the reactions to produce the retrograde
mineral assemblage as pressure and temperature are
lowered.

TYPES OF METAMORPHIC
REACTIONS
A metamorphic reaction is an expression of
how the minerals got to their final state, but a reaction
does not necessarily tell us the path that was actually
taken to arrive at this state. Sometimes it is possible to
deduce the path by means of a reaction mechanism.
Types of metamorphic reactions:
1. Univariant Reactions
2. Divariant Reactions

Univariant Reactions
• A univariant reaction is one that plots as a line or
curve on a pressure-temperature diagram. If all
phases in the reaction are present in the rock, then we
know that the rock must have been metamorphosed at
some pressure and temperature along the reaction
boundary. Consider for example the simple Al2SiO5
system with excess SiO2 and H2O. In low grade
metamorphic in this system, the reaction:
Al2Si4O10(OH) 2 <=> Al2SiO5 + 3SiO2 + H2O
Pyrophyllite Ky or Andal Qtz fluid

Metamorphic Reactions and
Critical Minerals
• plot a reaction boundary on
a P-T diagram. This
boundary can be
determined experimentally
or can be calculated using
thermodynamic properties
of the phases involved.
• For example, if the mineral is
andalusite, then we know the
rock was metamorphosed at a
pressure less than about 2.5
kilobars. If the mineral is kyanite, then we know that the pressure
was greater than about 2.5 kilobars.

• we can use the diagram to illustrate another point. Imagine
that a group of rocks are buried along the geothermal gradient
shown in the diagram. From this diagram we can deduce that:
1. Rocks buried to a pressure less than about 4 kb and a
temperature less than about 420oC should have pyrophyllite
2. Rocks buried to pressures between about 4 and 5 kb and
temperatures between 420 and about 600oC should have
kyanite + quartz
3. Rocks buried to pressures along
the geothermal gradient greater
than about 5 kb and temperatures
greater than about 600oC should
have Sillimanite + Quartz.

Metamorphic Isograds
• Reactions and appearance/disappearance of critical
minerals can be used to determine P-T conditions
• Isograd = “line” to indicate same grade of metamorphism
– First appearance of mineral of increasing metamorphic grade

Divariant Reactions
• The univariant reactions can be considered
discontinuous reactions because they result in a
sudden change in mineral assemblage which
occur along specific pressure temperature
curves.
• Divariant reactions result in a gradual change in
composition of the solid solutions minerals,
which occur over a wide range of pressure and
temperature conditions.

Consider the hypothetical case of rocks that contain minerals
like chlorite and garnet, which are both Mg-Fe solid solutions.
The reaction that occurs with increasing temperature (at
constant pressure) is:
Chlorite + Qtz => Garnet + Mg-richer Chlorite + H2O
We say that this reaction is a
continuous reaction because there
is no change in mineral
assemblage between T1 and T3,
but there is a reaction occurring
and its effect is to change the
compositions of the solid solution
minerals.

Assemblages, Facies, and Series
• For a given parent rock composition, mineral assemblage
reflects P-T conditions
• A variety of
different rocks
metamorphosed
under the same
conditions
indicate facies
• Regional
changes
between facies
indicate series
(and tectonic
setting)
Metamorphic Reaction Mechanisms

• Contact aureole = surrounding rocks metamorphosed by
heat of intrusion
• Size/shape of aureole depends on:
– Size, temp, cooling history of the igneous intrusion
– Properties of the country rock (conductivity, presence of water)
• Result of high geothermal
gradient produced locally
around intruding magma
• Restricted to relatively shallow
depths (low pressure)
• Rocks generally not foliated
• Produce fine grained rocks
called hornfels

Contact Metamorphism: Hornfels
• Mineral
assemblage
characterized by
hornfels facies
series
– Albite-epidote
hfls
– Hornblende hfls
– Pyroxene hfls
– Sanidinite
1.
Pyroxene hornfels

163
Contact Metamorphism: Skarn
• Contact metamorphism of carbonate or siliceous carbonate
• Usually involves significant metasomatism (chemical
exchange between magmatic fluids and rock)
• Can be hosts of Au, Cu, Fe, Mo, Sn, W, Zn-Pb
Wollastonite-garnet
skarn

Regional Metamorphism
• Affects large
areas of earth’s
crust
• Produced during
orogenic
(mountain-
building) event
1.
• Foliated rocks developed under med-high T and P:
– 1. Buchan 2. Barrovian
• Rocks developed under low-med T and high P:
– 3. Sanbagawa 4. Franciscan
3.
2.
4.

Regional Metamorphism: Barrovian
• “Normal” metamorphic series recognized in mountain belts
world-wide
• Medium to high T; low to
high P
• Moderate to high
geothermal gradient
• Critical mineral sequence:
– Kaolinite => pyrophyllite =>
kyanite => sillimanite
• Textural sequence (in
pelitic rocks):
– Slate => phyllite => schist => gneiss
• Facies sequence:
– Zeolite => prehnite-pumpellyite => greenschist => amphibolite =>
granulite

Regional Metamorphism: Franciscan
• Low T; high P
• Low geothermal gradient
• Little directed stress
(little deformation)
• Distinguished by
presence of blue, sodic
amphiboles
• Facies sequence:
– Zeolite => prehnite-
pumpellyite => blueschist
=> eclogite

Regional Metamorphism and
Tectonics
• Barrovian (mod-high T and P):
– Develop in orogenic belts at convergent margins
– Regional heat flow increased by upward movement of magma and
migration of fluids
– Thrust faulting thickens crust, increasing pressure and directed
stress
– Geothermal gradient of ~20-40ºC/km
• “Blueschist” (Franciscan) (low T and high P):
– Occur world-wide in orogenic belts of Phanerozoic age
– Associated with subduction zones
• rapid subduction of oceanic crust depresses isotherms
– Geothermal gradient of ~10-20ºC/km

Regional Metamorphism and
Tectonics
• Paired metamorphic belts – specific to subduction zones
• Barrovian
– Formed in rocks
beneath and
around the arc
– Heat due to
magmatic
activity
• Blueschist
– formed in the
mélange of the
trench

Identification of Metamorphic
Rocks
Foliation Grain Size Hardness Usual Color Other Rock Type
foliated fine very soft light greasy feel Soapstone
foliated fine soft dark "tink" when struck Slate
foliated fine soft dark shiny; crinkly foliation Phyllite
foliated coarse hard mixed dark and light crushed and stretched fabric; deformed large crystals Mylonite
foliated coarse hard mixed dark and light wrinkled foliation; often has large crystals Schist
foliated coarse hard mixed banded Gneiss
foliated coarse hard mixed distorted "melted" layers Migmatite
foliated coarse hard dark mostly hornblende Amphibolite
nonfoliated fine soft greenish shiny, mottled surface Serpentinite
nonfoliated fine or coarse hard dark dull and opaque colors, found near intrusions Hornfels
nonfoliated coarse hard red and green dense; garnet and pyroxene Eclogite
nonfoliated coarse soft light calcite or dolomite by the acid test Marble
nonfoliated coarse hard light quartz (no fizzing with acid) Quartzite

References
• Vernon, Ronald Holden, 2008, Principles of Metamorphic Petrology,
Cambridge University Press, p.1
• Bucher, Kurt, 2002, Petrogenesis of Metamorphic Rock, Springer, p. 4
• Marshank, Stephen, 2009, Essentials of Geology, Third Edition, Norton, p.
187.
• Marshank, Stephen, 2009, Essentials of Geology, Third Edition, Norton, p.
190.
• Gillen, Cornerlius, 1982, Metamorphic geology : an introduction to tectonic
and metamorphic processes, London; Boston: G. Allen & Unwin, p. 31.
• Winkler, H.G.F., 1979, Petrogenesis of Metamorphic Rock, Springer-
Verlag New York Inc.
• Ron H. Vernon, Geoffrey Clarke: Principles of Metamorphic Petrology.
• Kurt Bacher, Martin Frey: Petrogenesis of Metamorphic Rocks. 8th Edition,
Springer.
• Norman Frey: The Field Description of Metamorphic rocks.

Metamorphic Rocks & Tectonics
• Most metamorphic rocks develop due
to plate collisions
– Processes are not observable in the field
– Conditions at various plate boundaries
define possible P & T range
– Parent rocks present control mineral
assemblages

Metamorphic rocks

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