Metamorphic petrology general courses.ppt

Metamorphic Petrology
Melesse Alemayehu (PhD)
E-mail: melesse555@yahoo.com
Melesse A.

Course Outline
Chapter I: Introduction
► Composition of the Earth ► Rock types
► Plate tectonic theory ► Why we study metamorphic rocks?
Chapter II: Definition, limits and types of metamorphism
► Factors and limits of metamorphism
► Types and environment of metamorphism
► Types of metamorphism
•Regional and local metamorphism
► Metamorphism and plate tectonics
Chapter III: Metamorphic Reactions and Protoliths of
Metamorphic Rocks
► Development of Metamorphic minerals and rocks
► Metamorphic reactions and P-T path ► Types of metamorphic reactions
► Protoliths of metamorphic rocks
Melesse A.

Course Outline
Chapter IV: Metamorphic Textures, Grade, and Facies
► Metamorphic fabric and textures
► Foliation
► Lineation
► metamorphic grade
► Metamorphic facies
► Metamorphic series
► Graphical representation of metamorphic mineral assemblages
Chapter V: Nomenclature of Metamorphic rocks
► Metamorphic Rocks Nomenclature
► Classification of high strain rocks
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Cont. Course Outline
Chapter VI: Metamorphism of Pelitic rocks
► Pre-metamorphism
► Low grade metamorphic changes
► Barrovian Zonal scheme of metapelites
► Buchan zonal scheme of metapelites
►Higher temperature metapelites
Chapter VII: Metamorphism of calcareous Rocks
► Calcite marble
► Dolomite marble
► Calc-silicate rocks
Chapter VIII: Metamorphism of basic Igneous and
ultramafic rocks
► Metamorphism of basalts and others
► Metamorphism of peridotites and pyroxenites
Melesse A.

Required Textbooks
-An Introduction to metamorphic petrology
Yardley (1989)
-Petrogenesis of metamorphic rocks
Bucher and Frey (1994)
-Introduction to metamorphic textures and microstructures
Barker (1998)
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Chapter ONE
Introduction
Melesse A.

Introduction
The composition of the Earth:
-Lithosphere (5-70 km, solid and
rocky, 5 km thick under the oceans
and up to 70 km thick under the
continents). It composes of:
- sedimentary cover (10 km)
-Sial (granitic in composition)
-Sima (basaltic in composition)
-Mantle Asthenosphere (250 km
thick, molten rocks, 780 °C)
-Mantle Mesosphere (2550 km thick,
Si, O, Fe, Mg)
-Outer core (2200 km thick, Thick
liquid, Fe, Ni)
-Inner Core (1228 km thick, Solid,
Fe and Ni) Melesse A.

Rocks
Rocks are defined as a component of the Earth’s crust, composed of one
or more minerals with geologic extension
Rocks are classified into:
- Primary - Igneous rocks
- Secondary - Sedimentary rocks
- Metamorphic rocks
-The metamorphic rocks are secondary rocks formed from pre-
existing igneous, sedimentary, and/or prior metamorphic rocks, which are
subjected to physicochemical conditions (P, T, and chemical active fluids)
higher than that at the earth’s surface. The yielded metamorphic rocks differ
than the original ones in mineralogy, structure (textures), and/or chemical
composition. Note: Metamorphism should be occur in solid state.
-Due to higher P-T conditions, metamorphic rocks undergo partial melting
and a hybrid rock between igneous and metamorphic, know as
migmatites, could form.
Melesse A.

Types of the Plate motion
Plate boundaries includes:
i- Divergent plate boundaries ():
- Formation of the Red Sea and Atlantic Ocean
Ii- Convergent plate boundaries ()
-Oceanic-continental convergence (Oceanic Nazka – S
American plate)
- Oceanic-oceanic convergence (Pacific plate – Philippine
plate)
-Continental-continental convergence (Indian plate-
Eurasian plate)
iii- Transform or shear plate boundaries:
- The San Andreas fault zone, and Gulf of Aqaba fault
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i- Divergent plate boundaries
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i- Divergent plate boundaries, Cont.
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ii- Convergent plate boundaries
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ii- Convergent plate boundaries, Cont.
a) oceanic-oceanic subduction
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b) Oceanic-continental subduction
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c) Continental - continental collision
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iii- Transform or shear plate boundaries
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iii- Transform or shear plate boundaries, Cont.
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Why we study metamorphic rocks?
Goals of study metamorphic petrology includes:
- Academic goals: to deduce the following
- Protolith (original rock) composition
- Grade and conditions of metamorphism
- Tectonic setting under which the metamorphism have done
- Applied goals: Metamorphic rocks like other rock types hosted
mineral resources e.g:
- Graphite, Talc, Magnesite, Asbestos, Corundum, vermiculites,
garnets, etc.
- They used also as ornamental stones as Slates, Marbles,
gneisses, metaconglomerates, greenstones and others
Melesse A.

Chapter TWO
Definition, Limits and Agents of
Metamorphism
Melesse A.

Metamorphism
Metamorphism (Meta=change, Morph=form or character). So,
metamorphism means to change form or character).
It is define as a subsolidus process leading to change in
mineralogy, structures (textures) and/or chemical composition of
an igneous, sedimentary, or prior metamorphic rocks. These
changes were made due to subjection of these rocks to
physicochemical conditions (P, T, active chemical fluids) higher
than those occurring in the zone of weathering, cementation and
diagenesis
 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.
Melesse A.

Factor and limits of metamorphism
Factor of metamorphism include three variables:
 Temperature  Pressure  Chemical active fluids
1- Temperatures: (leads to increase in grain size)
-Limits of temperatures
- Limits of Temperature
 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
Melesse A.

Example:
- At 5 kbar and presence of aqueous fluid - granites begin to melt at ~ 660 °C
- basalts begin to melt at ~800
- 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)
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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
Melesse A.

Pressures:
- Limits of pressure
 lower limit (a few of bars, at Earth’s surface)
 Higher limits (30-40 kbar 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
Melesse A.

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
Melesse A.

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
► Strain energy may overcome kinetic barriers to reactions
Deviatoric stresses come in three principal types:
– Tension
– Compression
– Shear Melesse A.

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
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Compression: 1 is dominant; therefore, folding or more
homogenous flattening are caused.
1
3
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Shear motion occurs along planes at an angle to  1 and causing
slip along parallel planes and rotation.
1
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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 of
decarbonation
ms + chl  bt + grt + qtz + H2O 
CaCO3 + SiO2  CaSiO3 + CO2 
Fluids could preserved as inclusion in neoblasts in
metamorphic rocks.
Melesse A.

Types of metamorphism
On the basis of (i) Geological setting, and (ii) agents of
metamorphism, the type of metamorphism includes:
A. Regional metamorphism (over a wide area)
- Orogenic metamorphism (T, P, active fluids)
- Ocean floor metamorphism (T)
- Subduction zone metamorphism (HP/LT)
- Burial metamorphism (LT/LP)
B. Local metamorphism (cover local area)
- Contact or thermal metamorphism (T)
- Cataclastic or shear zone metamorphism (P)
- Hydrothermal metamorphism (active fluids)
- Impact or shock metamorphism (extreme P-T)
Melesse A.

A1: Orogenic metamorphism
(Regional or dynamothermal metamorphism)
Features of orogenic
metamorphism :
- Where?: Restricted to orogenic belts and
extent over distance of hundreds to
thousands Kms, e.g. East-African orogen
- The agents of metamorphism: include T, P
& active chemical solution
-Time duration is long (million or tens of
millions years)
- The yielded rocks suffered deformation and
recrystallization, and exhibit penetrative
fabric with preferred orientation of mineral
grains. They could suffered phases of
crystallization and deformation
- At higher P-T conditions, partial to
complete melting may accompanied and
both migmatites and granites may
associates, or granulite could be develop.
Melesse A.

A2: Ocean-floor metamorphism
Features of ocean-floor
metamorhism :
- where?: Restricted to transformation
of the oceanic crust at the vicinity of
mid-ocean ridge
- Occur in the upper part of the oceanic
crust, typically in sheeted dykes
- The agents of metamorphism include
T & sea water percolation
- The yielded rocks are mostly basic
(sheated dykes) in composition, with no
penetrative fabric (non-foliated texture)
Melesse A.

A3: Subduction zone metamorphism
Features of subduction zone
metamorphism :
- where?: At convergence plate
margins, where subduction of cold
oceanic lithosphere and overlying
sediments against an adjacent
continental or oceanic plate.
- The agents of metamorphism include
higher pressure, low temperature
conditions
- The yielded rocks contain high
pressure mineral assemblage such
glucophane, and kyanite should
formed
-To preserve such environment , the
rock requires rapid uplift Melesse A.

A4: Burial metamorphism
Features of burial
metamorphism :
- Where?: in subsidence basins, where
sediments and interlayered volcanics
suffered low temperature regional
metamorphism
- Agent of metamorphism include low
temperature-low pressure conditions
due to burial affect without any
influence of orogenesis or magmatic
intrusions.
- The yielded rocks lack schistosity
and the original fabrics are largely
preserved. So, the yielded rocks are
distinguished only in thin section
- In Extensional regime, Diatathermal
metamorphism is used Melesse A.

B1: Contact or thermal metamorphism
Features of Contact or thermal
metamorphism :
- Where ?: At vicinity of contacts with
intrusive or extrusive igneous rock bodies
-Agent of metamorphism is the higher
temperature resulted from heat emanating
from the magma, and sometimes by
deformation connecting with the
emplacement of the igneous bodies.
- The zone of the contact metamorphism is
known as contact aureole, various from
meter to few kms.
-The width of the zone depend up on:
1- volume of the magmatic bodies
2- nature of the magmatic bodies (basaltic
or granitic composition)
3- The intrusion depth of magmatic bodies.
Melesse A.

B1: Contact or thermal metamorphism, cont.
4- Type of country rocks (Shale,
limestones or igneous rocks)
5- Structures of the country rocks (cracks
and fissures)
- Duration of metamorphism is short time
(up to hundred years)
- The yielded rocks are generally fine
grained and lack schistosity (hornfels)
- In case of higher temperature influence,
Pyrometamorphism, is used.
- Migmatites could produced in such
conditions.
Melesse A.

B2- Cataclastic or shear zone metamorphism
Features of cataclastic or shear
zone metamorphism :
- where?: Restricted to the vicinity of faults
of overthrusts in the upper crust level
(brittle deformation)
-Agents of metamorphism is pressure in
form of mechanical forces.
-The yielded rocks suffered crushing,
granulation and pulverization (reducing in
grain size).
- The yielded rocks are non-foliated and
braccia-like, cataclasite, mylonite,
ultramylonite to pseudotachylite.
Melesse A.

B2- Cataclastic or shear zone metamorphism
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B3- Hydrothermal metamorphism
Features of hydrothermal
metamorphism :
- where?: Localized at interaction of hot, largely
aqueous fluids (from igneous source) with
country rocks.
- Similar to regional ocean-floor metamorphism
- the aqueous hydrothermal fluids usually
transported via fractures and shear zones at
some distance either near or far from their source
- The yielded rocks are mineralogically and
chemically changed than the protolith and ore
deposits are occasionally originated
-If the gases instead the
aqueous fluids,
Pneumatolytic
metamorphism, is used
Melesse A.

B4- Impact or shock metamorphism
Features of impact metamorphism :
- Where?: Impact of fall meteorites with different size on the Earth’s crust.
- This impact yielded shock waves with extreme higher P-T conditions, up to
1000 kbar and 5000 °C
- Duration time is very short, microsecond.
- The impacted rocks were vaporized, but in less condition, they melted to
produce vesicular glass containing coesite and stishovite, as well as minute
diamond
Melesse A.

Metamorphism and plate tectonic
1- Divergent plate margin:
-Ocean floor metamorphism (HT/LP & seawater fluids)
- Diatathermal metamorphism (HT/LP)
- Contact metamorphism (HT/LP)
- Hydrothermal metamorphism (Hydrothermal fluids)
Melesse A.

Metamorphism and plate tectonic
2- Convergent plate metamorphism
-Orogenic condition (various P-T)
- Cataclastic and Subduction zone metamorphism (LT/HP)
3- Transform plate boundaries
- Cataclastic or Subduction zone metamorphism (LT/HP)
Melesse A.

Development of Metamorphic Rocks
The yielded metamorphic rocks, with specific
mineral composition and textures is a function of:
 Protolith nature i.e: whole rock chemistry
(pelitic (Argillaceous), semipelitic, calcareous
(limestone and dolomite), mafic-ultramafic, basic
igneous, granitic, Mn-rich sediments, ironstone,
laterites... etc.)
 P-T-X conditions (the intensity of temperature and
the intensity and type of pressure (simple
compressed or twisted and broken) influence and the
presence or absence of fluids and their chemistry
during metamorphism)
 Time (how long the rock subjected to HT and
HP?. By which the grain size was evolved, and the
reactions were proceed)
Melesse A.

Metamorphic Rocks components and development
A metamorphic rock consists of individual grains of several solid
minerals and a network of grain boundaries, which at the time of
metamorphism may have held an aqueous fluid, providing pathways for
transport through the rock.
Omphacite
garnet
garnet
Melesse A.

CHAPTER THREE
Metamorphic Reactions and
Protoliths of Metamorphic Rocks
Melesse A.

Development of Metamorphic minerals
 Nucleation: nuclei (embryo crystals) of
the new mineral appear
 Interface reactions - dissolution:
reactant minerals break down, their chemical
constituents going into solution
 Interface reactions - growth: material
is added onto the nuclei to build larger
crystals
 Mass transfer: material is transported
through the rock from sites of breakdown to
sites of growth
For a new mineral to appear by a chemical reaction, a number of
processes have to operate in concern:
Melesse A.

Nucleation, Mineral growth and Grain size
-Completed reaction produces an amount
of product (mineral phases). The
microstructure, will depend on the
relative rates of nucleation and growth of
minerals
- Grain size in a metamorphic rocks is a
function in:
- Intensity of P-T conditions,
- nuclation rate, and
-Time interval of metamorphism
-Coarse-grained rocks are the product of
long sustained metamorphic conditions
(possibly over millions of years) at HT
and HP (e.g. in high grade regional
metamorphic rocks)
-Fine-grained rocks are products of LP,
LT, in some cases, short reaction time
(e.g. in contact metamorphiic rocks)
Melesse A.

Metamorphic Reactions and P-T path
- The P-T path include three
segments:
 Prograde segment: With
increasing the P-T conditions
(such as burial effect)
 Peak segment: at maximum P-
T conditions (at the summit
metamorphic conditions)
 Retrograde segment: With
decreasing P-T conditions (such
as uplift)
- With increasing P-T conditions, metamorphic reaction take place (e.g.
burial effect) until the maximum pressure and temperature (peak
condition), then with decreasing the P-T conditions (e.g. uplift) until
cooling of the rock. This is known as Metamorphic P-T path
- The metamorphic P-T can be simple (clockwise or anticlockwise) or
complex due to multiphase metamorphism
Melesse A.

Types of Metamorphic reactions
- With either progressing or retrogressing metamorphism,
various types of metamorphic reactions are proceeds e.g.:
1- Univarient reactions: reactions that plot as line or
curve on the P-T diagram and depend on temperature and
pressure only e.g:
Al2Si4O10(OH)2  Al2SiO5 + 3SiO2 + H2O
Pyrophyllite Al-silicate + Qtz + fluid
Melesse A.

Types of Metamorphic reactions cont.
2- Divarient reactions: reactions
occur over wide range of P-T. This
because most minerals involved
in the reaction exhibit solid
solution (e.g garnet, mica,
plagioclase); therefore, the
reaction boundaries can changed
depend on the composition of
solid solution.
KAl2Si3AlO10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O
Ms Qtz Kfs Sill W
Melesse A.

3- Solid-solid reaction: only
involve the solid-phases for both
reactant and products (with no
fluid phases). So reactions
involves phase transformation
e.g.
Calcite  aragonite,
andalusite  sillimanite,
graphite  diamond
Albite  jadite + quartz
Melesse A.

4- Dehydration reactions: reactions
that liberate H2O. This always occur in
the prograde reaction, i.e. with
increasing temperature e.g
chlorite + muscovite 
orthoclase + andalusite + H2O
5- Decarbonation reaction : reactions
that liberate CO2 e.g
Calcite + quartz  Wallstonite + CO2 
Melesse A.

6- Oxidation-reduction reaction: reactions that involve change the
valence state of Fe-Ti oxide phases (Fe+2 and Fe+3) e.g: the
breakdown of biotite to K-feldspars and magnetite at high P-T
conditions
biotite + O2  K-feldspars + Magnetite + H2O
7- Cation exchange reaction: reaction involves ionic substitution
of two or more phases in the system e.g:
Fe-garnet + Mg-biotite  Mg-biotite + Fe-garnet
8- Ionic reactions: reaction that balanced by inferring involvement
of ionic species derived from the fluid phase
Melesse A.

Protoliths of metamorphic rocks
As we discussed, The yielded metamorphic rocks is
function of:
-Protolith (original rock) nature  bulk-rock chemistry
- P (pressure)-T (temperature)-X (active fluids) conditions
- Time
At specific P-T-X conditions, reactions in solid state take place in
the rock and new equilibrated mineral assemblage and
corresponding textures are arise, which equivalent to the
influence metamorphic conditions.
Melesse A.

The protoliths of the metamorphic rocks could be:
-Sedimentary rocks
-Shales (Pelitic rocks)
-Sandstones (Arenaceous rocks and semipelitic rocks)
-Carbonate (Calcareous rocks)
-marl rocks
- Igneous rocks
- Basic igneous rocks (metabasalts)
- Ultramafic rocks
- Granitoid rocks
- Prior metamorphic rocks
Protoliths of metamorphic rocks
Melesse A.

Metapelites
Shales (Pelites): very fine-grained
sedimentary rocks, composed of silicate clay
minerals rich in the elements (SiO2, Al2O3,
FeO, K2O, Na2O, H2O), beside other minor
elements.
- Common metamorphic minerals include
-Quartz
- Feldspars (plagioclase, K-feldspars)
- Mica (sericite, muscovite, biotite, chlorite)
- garnet,
- staurolite,
- cordierite,
- Al-silicate (andalusite, kyanite, silliminite)
- Pyroxene Melesse A.

1. Sillimanite ( C )
Melesse A.

Metacarbonate rocks
Limestone and dolomite (Calcareous rocks), composed
essentially of calcite (CaCO3), doilomite (CaMg(CO3)2, with
minor quartz and clay minerals. If the clay minerals are
excess, the rock known as marl
- the yielded rocks is known as Marble (mainly calcite) if the
calcareous rocks are pure. In case of non-pure calcareous
rocks, marble contain silicate minerals such as: wollastonite,
grossular-andradite garnet, diopside and tremolite
Melesse A.

(a) Limestone (fiossiliferous)
Melesse A.

Metamorphosed sandstone
Sandstones (Arenaceous rocks): of variable composition,
arenite (wholly quartz), arkose (quartz + feldspars) and
graywackes (quartz, lithic fragment and clayey matrix)
-Metamorphism of sandstone yield quarzites (quartz +
plagioclase + K-feldspars + biotite + granet + Fe-Ti oxides)
- Metamorphism of graywackes formed a metamorphic rocks
intermediate between metapelites and gneisses)
- Arenites yield quartizites
Melesse A.

Metamorphosed basic igneous rocks
Basic rocks including basalts and gabbros, are low silica
igneous rocks that contain plagioclase and pyroxene.
- Metamorphism of basic igneous rocks yielded
metabasites (quartz, plagioclase, amphiboles, garnet,
epidotes, chlorite)
- If the rock is composed of plagioclase and amphibole,
amphibolites term are used
- At extreme P-T conditions, Eclogites are formed (garnet
+ omphacite) Melesse A.

CPX: Diopside - Augite
Melesse A.

Tremolite-actinolite
Melesse A.

Metamorphosed Ultramafic rocks
Ultramafic rocks, Mg-Fe rich silicate minerals (Diopside,
Olivine)
-When metamorphosed yielded produce serpentinites and
talc schists with mineral assemblage include serpentine
minerals, talc, enstatite, diopside, anthophyllite, tremolite,
hornblende, garnets and chlorite
Melesse A.

Chapter Four
Metamorphic Textures, Grade,
and Facies
Melesse A.

Metamorphic fabric and textures
 Again, Identification of a given metamorphic rock depend on:
1- Mineral composition 2- Texture
Metamorphic rocks undergo deformation during their
crystallization as a result of pressure influence.
 Orogeny is described to long-term mountain-building e.g:
Pan African Orogeny. The orogeny may:
comprise several Tectonic Events
have several Deformational Phases
have an accompanying Metamorphic Cycles with one or
more Reaction Events
 Tectonite is a deformed rock with a texture that records the
deformation Melesse A.

Melesse A.

 Texture (grain-grain relationships) refer to:
1) shape and size of the individual grains
2) orientation of the individual grains
3) arrangements of the mineral grains in metamorphic rock
structure used for large features
Fabric refer to the complete spatial and geometric
configuration of textural and structural elements
Importance of textures in metamorphic rocks to:
1) decipher the order of recrystallization of minerals,
2) sequence of events involved in forming the metamorphic
rocks,
3) Intensity of P-T condition during metamorphism, and
4) used to nominate the metamorphic rocks
Melesse A.

A- Grain size
 Remember that, the grain size of a given metamorphic rocks
is a function of:
 Intensity of P-T conditions
- Very low conditions  very fine grain size texture
- Very high conditions  very coarse-grained texture
 rate of nucleation (high rate donate finer grain sizes)
 Subsequent time internal (shorter time donate more finer
grain size)
Melesse A.

A- Grain size Categories
 Metamorphic rocks have different
sizes:
- Fine-grained (<0.75 mm)
- Medium grained (0.75-1.0 mm)
- Coarse grained (1-2 mm)
- Very coarse grained (>2 mm)
Melesse A.

B- Textures donating planar or linear elements:
 These textures described in metamorphic rocks that
composed of unequal mineral assemblage with preferred
orientation. They include:
Foliation- planar textural elements
Lineation- linear textural elements
- Rocks without preferred orientation  massive or isotropic
Foliation Lineation
Massive/isotropic
Melesse A.

Foliation: defined by any layering
in a metamorphic rock as a result of
parallel arrangement or distribution of
planar elements that include:
I- Compositional layering: defined by
alternating layers composed of
different mineral composition and/or
different grain sizes. Easily
recognized by differences in color of
layers.
1- Foliation Types
Melesse A.

II-Gneissosity: defined by
compositional layering of equent
crystals (e.g. quartz, feldspars)
alternate with platy or elongate
mineral layers (e.g. micas). It is
usually coarse-grained size.
1- Foliation (Cont.)
Melesse A.

III- Schistosity: defined by
alignment of platy (mica, chlorite)
or inequent (amphiboles, quarz)
minerals
- Minerals defining schistosity are
said to posses preferred orientation
and usually are medium-grained.
Melesse A.

IV- Cleavage: Schistosity
surface along which the rock
may break (cleave). It include:
a- Slaty cleavage in very fine-
grained mica and/or chlorite in
slate and phyllite,
b- Crenulation cleavage:
alignments with cm- to mm-
scale periodic folding
Melesse A.

V- Mylonite layering: defined by layers of highly strained rock
with elongated grains due to grain size reduction and dynamic
recrystalization during shearing
Melesse A.

Lineation: parallelism or alignment
of linear elements in the rock
Types of lineations:
a. Preferred orientation of
elongated mineral aggregates
(e.g. quartz pebbles in
metaconglomerates)
b. Preferred orientation of elongate
minerals (feldspars & Hb)
c. Lineation defined by platy
minerals
d. Fold axes (especially of
crenulations)
e. Intersecting planar elements.
2- Lineation
Melesse A.

Foliation and Lineation
Melesse A.

C- Textures donating lack of preferred orientation or
equigranular grains:
- Hornfelsic textures: random orientation of fine-grained rocks,
due to lack of stresses, granofelsic texture for the medium to
coarse grained rock
Melesse A.

C- Textures donating lack of preferred orientation or
equigranular grains (Cont.)
- Granoblastic texture: A mosic of fine to coarse grained
anhedral grains, such as marble and granulites
Melesse A.

D- Textures donating Large grains within the rock:
-Porphyroblastic texture: A relatively
large crystal (e.g. garnet, staurolite) in
smaller fine grained matrix. It could be
-Idioblast (Euhedral),
-subidioblast (subhedral) or,
- xenoblast (anedral).
Melesse A.

D- Textures donating Large grains within the rock:
-Porphroclastic texture: A large
strained or broken grain in fine
grained matrix
-Blastoporphyritic texture: A relict
of porphyritic volcanic texture in
metamorphic rocks
- Augen texture: Porphyroblast of
feldspars with eye-shape cross
section in fine grained gneissic
matrix
Melesse A.

E- Textures donating inclusion within or rim on a
porphyroblasts:
- Poikiloblastic or sieve texture:
porphyroblast containing
numerous inclusions of one or
more fine grains.
Melesse A.

E- Textures donating inclusion within or rim on a
porphyroblast:
Corona or reaction rim: A zone
consisting of grains of a new
minerals that have formed at rim
around mineral.
Melesse A.

F- Textures donating fragmental nature of whole rock:
- Cataclastic texture: sheared or crushed rock fabric. The
nature of original rock still recognized
Melesse A.

F- Textures donating fragmental nature of whole rock:
Mylonite texture: Extremely sheared, stretched and
recrystallized grains, typically foliated and containg ovoid relict
crystal.
- Slightly sheared: Protomylonitic texture
- exteremely sheared: ultra-mylonitic texture
Melesse A.

Metamorphic Grade and Facies
Melesse A.

Metamorphic grade and Facies
Systematic spatial distribution in mineral assemblages that formed
during metamorphism in metamorphic terrains allow to delineate mineral
zonation, using index minerals (e.g. chlorite, biotite, garnet, staurolite,
cordierite, sillimanite, andalusite, wollastonite, diopside… etc.)
- Mineral zone: Zones in the field,
which mark the first appearance of
an index mineral, such as chlorite
zone, garnet zone.
-Mineral isograd: boundary marked
between two mineral zones, which
include:
-Mineral-in: first appearance
-Mineral-out: the last appearance of
Melesse A.

Distribution of minerals within metamorphic zones
Melesse A.

Metamorphic grades
Metamorphic grades is a general term for describing the relative P-T
conditions under which the metamorphic rocks form. The grades
could subdivided into:
- Very low grade
- Low grade
- Medium grade
- High grade
- Very high grade
The boundaries between the grades are chosen to correspond to
important discontinuous reactions (could recognized as major
isograde), and they correlate with the scheme of metamorphic facies.
Melesse A.

Metamorphic facies
Metamorphic facies is defined as a set of metamorphic mineral
assemblage, repeatedly associated in space and time.
Melesse A.

Metamorphic grades and facies
Index mineral assemblages
Metamorphic facies
Metamorphic grade
Metabasalt
Pelitic rocks
Laumonite, analcite,
heulandite, wairakite
Mixed layer clays,
sericite
Zeolite
Very low grade
- prehnite-pumpellyite,
Chl. Ab, Ep, Act, Law.
Illite-muscovite, Chl,
Ab, Pyr,
Prehnite-pumpellyite
Glu, Law
Chl, Tlc, Ms, Grt, Chld
Blue schist
Act, Ep, Ab, Chl, Act,
Grt, Hbl
Chl, Ms, Ab, Bt, Grt
Greenschist
Low grade
Pl, Ep, Act, Chl
Ms, Bt, Chl
Albite-epidote
hornfels
Hbl, Pl, Ep, Grt
Grt, St, And, Ky, Sill,
Ms, Crd
Amphibolite
Medium grade
Hbl, Pl, Cumm
Crd, Chl, bt, Ms, And
Hornblende hornfels
Cpx, Grt, Opx, Pl, Ol,
Hbl
Crd, Grt, Kfs, Sill, Ky,
Hy, Sap.
Granulite
High grade
Cpx, Opx, Pl, Ol, Hbl
Crd, And, Kfs
Pyroxene honfels
Not well defined
Cor, Mgt, An, glass
Sanidinite
Omph (Cpx), grt
Tlc, ky, Grt, Ms
Eclogite
Very high grade
Melesse A.

Metamorphic grades and facies
Melesse A.

Graphical representation of metamorphic mineral
assemblage
1- ACF and AKF diagrams: Suitable for mafic rocks and calc-
silicates.
2- AFM diagram: Useful for pelitic rocks.
3- CAS (CaO - Al2O3 - SiO2) diagram: Useful for marly rocks
(calcareous mudstones).
4- MCS (MgO - CaO - SiO2) diagram: Useful for ultramafic rocks.
Melesse A.

Plotting of mineral assemblage and rock categories in
ACF and AKF
Melesse A.

Chapter FIVE
Nomenclature of Metamorphic
rocks
Melesse A.

Metamorphic Rocks Nomenclature
 The igneous rocks are classified according to IUGS system.
 The sedimentary rocks are classified according to the
genesis of the rocks.
 But, the classification of metamorphic rocks are differ and
depends on what is visible in the rock and its degree of
metamorphism. Four kinds of criteria are normally employed:
1- The nature of the parent material (protolith composition)
2- The rock's texture (grain size and fabric development)
3- The metamorphic mineralogy (mineral content)
4- Appropriate special name
Melesse A.

1- The nature of the parent material (protolith composition)
As mentioned above, the metamorphic rocks are derived from pre-
existing rocks, which could be sedimentary, igneous or prior
metamorphic nature.
The metamorphic rock categories are principally nominated according
to the nature of the protoliths such as:
Metamorphic equivalent
Rock type
Parent Material
metapelites
pelites
Clay-rich sediments
metapsammites
psammites
Sandstones
metapsammopelites
Semi-pelite
Clay-sand mixtures
metaquartizites
quartzite
Quartz-sand (Qtz arenites)
Calc-silicates
calcareous
Marl (lime mud)
Metacarbonate / marble
Carbonate / calcareous
Limestone or dolomite
metabasalt (metamafic)
Basalt
Metagranitoids
Granitoids
metaultramafics
Ultramafic
As well if the rock subjected to low grade metamorphism, name of
original rock is used with prefix (meta-) Such as: metamudstone,
metagraywacke, metagabbros, and metabasalt
Melesse A.

2- Rock textures (grain sizes and fabrics)
A- when the rock is mica rich (i.e. metapelites and/or
metapsammo-pelites and exhibit preferred orientation:
1- If the rock is very fine grained (not visible with 10X
magnification, not luster (dull) and freshly cleaved Slate name
is used
2- if the rock is fine grained schistose (not visible with naked
eyes but easily recognized with 10X and sheen to foliation in
strong sunlight Phyllite name is used
3- if the mica is easily visible with the naked eye (the rock is
medium grained) and possess schistosic foliation
Schist name is used
4- if the rock is possessing gneissic foliation and medium to
coarse grained gneiss name is used
Melesse A.

2- Rock textures (grain sizes and fabrics)
Note that:
Schist: In common usage, schists are restricted to those
metamorphic rocks in which the foliated minerals are coarse
enough to see easily in hand specimen.
 If the gneisses contain aguen texture, Augen gneiss name is
used
 The prefix ortho- and para- is used to an igneous and
sedimentary parentage, respectively. For example, many
gneisses could easily be derived from either an impure arkose
or a granitoid rock. If some mineralogical, chemical, or field-
derived clue permits the distinction, terms such as orthogneiss,
paragneiss, may be useful.
Melesse A.

2- Rock textures (grain sizes and fabrics) cont.
B- When the rock have no preferred orientation (i.e. random
orientation of individual minerals or isotropic):
1- if the minerals are unequent (prismatic) and fine grained size.
It occurs in contact aureoles and is tough, and tend to splinter
when broken Hornfels name is used
2- if the minerals are unequent (prismatic) and medium grained
size Granofels name is used
3- if the minerals are equent and fine to coarse grained size
granulite name is used
Marble name is used (>50% carbonate minerals)
Melesse A.

Granulite
(Eclogite)
Melesse A.

2- Rock textures (grain sizes and fabrics) cont.
C- When the rock is subjected to dynamic metamorphism and
granulation:
1- if the rock is coarse to very coarse grained and similar to
that of braccia Fault braccia name is used
2- if the rock is medium grained and still the original mineral
easily recognized Cataclasite name is used
3- if the rock is granulated to fine or very fine-grained with
pronounced foliation mylonite / ultramylonitename is used
4- If the rock is highly strained and the matrix become glassy
Pseudotachylite is used
Melesse A.

Cataclasite
Mylonite
Melesse A.

Classification of high strain rocks
Melesse A.

3- Metamorphic mineralogy (mineral content)
Most distinguished (index) minerals are used as prefix to the
textural name. The mineral is arranged in order of their percent
content such as:
-Garnet biotite schist
-Sillimanite K-feldspar gneiss
- hornblende biotite gneiss
- spotted andalusite hornfels
-Garnet-andalusite-sillimanite-K-feldspars granulite.
Melesse A.

4- Appropriate (Special) names
Mafic schist: A term used to describe foliated or non-foliated
metamorphic rocks that containing >50 mafic minerals (chlorite,
epidote, amphiboles {actinolite-tremolite-hornblende-glucophane-
cummingtonite), pyroxene (ortho- and para-types).
-Their color differs from green, black to blue colour and include:
1- greenstone / greenschist: low-grade, fine grained rock
composed of (Chl + Act + Ep + Ab). Most of the mineral except
the latter are green colors. The greenstone is non-foliated, while
the grrenschist is foliated. The protolith is either a mafic igneous
rock or graywacke
2- amphibolites: foliated or non-foliated fine to medium grained
rock composed dominantly of hornblende and plagioclase
3- Blue schists: fine to medium foliated rock (Gluc + Pl + Ep)
Melesse A.

4- Eclogite: a green and red coarse-grained metamorphic
rock that contains clinopyroxene and garnet (omphacite +
pyrope). The protolith is typically basaltic.
Melesse A.

Marble: a foliated or non-foliated metamorphic rock composed
predominantly of calcite or dolomite (> 50 vol.%). The protolith
is typically limestone or dolostone. Prefix with dominate
minerals is used such as: wollastonite marble, tremolite-marble,
calcite marble, dolomite marble, diopside-grossular marble.
Calc-schist a foliated rock with >50 vol.% Ca-silicate minerals
(e.g. tremolite, diopside, hornblende, wollastonite, grossular).
Calc-silicate a non-foliated equivalent of calc-schist.
Skarn: a calc-silicate rock formed as a result of metasomatism
of carbonate rocks as a result of fluid action from a magmatic
rocks.
Melesse A.

Quartizite: a foliated or non foliated metamorphic rock
composed predominantly of quartz (> 90 vol. % quartz). The
protolith is typically sandstone.
Soapstone: a non-foliated rock with abundant talc and greasy
feel.
Talc-schist: a foliated equivalent of soapstone.
Serpentinite: a foliated or non-foliated rock with >50 serpentine
minerals.
Migmatite: a composite silicate rock that is heterogeneous on
the 1-10 cm scale, commonly having a dark gneissic matrix
(melanosome) and lighter felsic portions (leucosome).
Migmatites may appear layered, or the leucosomes may occur
as pods or form a network of cross-cutting veins
Melesse A.

Chapter SIX Metamorphism of
pelitic rocks (Metapelites – Part I)
Melesse A.

What is the Metapelites?
Metapelites, are metamorphic rocks, which derived from
contact or regional metamorphism of shale or mudstones (clay
rich sediments).
 Metapelites are the most distinguished family in
metamorphic rocks because the clays are very sensitive to
variations in temperature and pressure, undergoing extensive
changes in mineralogy during progressive metamorphism.
Pelitic sediments are mineralogy dominated by fine Al-K-rich
phyllosilicates (50 vol%), such as clays (montmorillonite,
kaolinite, or smectite), fine white micas (sericite, paragonite, or
phengite) and chlorite, all of which may occur as detrital or
authigenic grains (10-30%).
Chemically, the pelitic rocks are rich in Al2O3 and SiO2, Na2O,
K2O, and poor in CaO, therefore, the yielded metamorphic
minerals during progressive metamorphism will be rich in
Al2O3. Melesse A.

Mineralogy of metapelites
Metapelites contain the
following mineral
assemblage:
 Mica (Muscovite, biotite),
pyrophyllite, chlorite,
chloritoid,
 Feldspars (plagioclase
and K-feldspars)
 Garnet, staurolite,
cordierite
 Al-silicate (andalusite,
Kyanite, and sillimanite)
 Quartz, orthopyroxene,
spinel
+Qtz
+Pl
+ Ms
Melesse A.

The metapelites will discuss their metamorphism in the
following conditions:
Pre-metamorphic – low-grade metamorphic conditions
 In Middle Pressure metamorphism (Barrovian zonal
scheme)
 In the low pressure metamorphism (Buchan zonal scheme)
 In the high temperature conditions
Melesse A.

-1-
Pre-metamorphic – low grade metamorphic
changes
Melesse A.

1- Pre-metamorphic – low grade metamorphic changes
During compaction and diagenesis, changes in mudstones and
shale include:
– Reducing of porosity (> 50 Vol.%) during burial &
compaction
– Original clay, smectite, are replaced by mixture of chlorite
and illite (sericitic muscovite)
– With progress increase of P-T condition the following
assemblage could be formed
chlorite + illite + kaolinite
chlorite + sericite + pyrophyllite + illite + koalinite
– Illite crystallinity, as defined from XRD, used to measure
the degree of diagenetic and very-low metamorphism.
There is no sharp contact between diagenesis and low-
Temperature metamorphism
Melesse A.

-2-
Medium Pressures metamorphism
(Barrovian Zonal scheme)
Melesse A.

2- Barrovian Zonal Scheme (MP metamorphism)
The classical zones of metamorphism in the Scottish Highlands
and many other parts of the world include six distinct mineral
assemblages that occur in the metapelites.
Melesse A.

I- Chlorite zone
-Metapelites of the chlorite zone are very fine-grained, so it makes
difficult to investigate under the microscope,
-They tyically contain mineral assemblage: chlorite + Mg-Fe-bearing
muscovite (phengitic) + quartz + Na-plagioclase (albite) ± K-
feldspars ± stilpnomelane ± calcite.
Melesse A.

II- Biotite zone
- Metapelites of the biotite zone are defined by first appearance of
biotite through one of two mineral reactions (depending upon the
presence or absence of K –feldspar):
K-feldspar + chlorite  biotite + muscovite + quartz + H2O
Phengitic Ms + chlorite  biotite + phengitic-poor Ms + quartz + H2O
-They are typically Phyllite and contain mineral assemblage: chlorite
+ muscovite + biotite + quartz + Na-plagioclase (albite) ± calcite.
Melesse A.

III- Garnet zone
- Metapelites of the garnet zone are defined by first appearance of
garnet porphyroblasts (Fe-rich almandine) through the following
mineral reaction:
Chlorite + muscovite  garnet + biotite + quartz + H2O
They are typically medium to coarse grained schists and contain
mineral assemblage: garnet + biotite + chlorite + quartz + Na-
plagioclase (albite) ± epidote.
Melesse A.

IV- Staurolite zone
- Staurolite is only form in Al-rich, Ca-poor pelites. This will
depend on the stability of plagioclase, which allow
available Ca to combined Al. Therefore, Al is reduced and
other Al-silicate minerals does not form.
- staurolite forming through the following mineral reaction:
Chld + Qtz  St + Grt + H2O
Grt + Ms + Chl  St + Bt + Qtz + H2O (Grt consuming reaction)
Ms + Chl  St + Bt + Qtz + H2O
They are typically medium to coarse grained schists and
contain mineral assemblage: staurolite + garnet +
biotite + muscovite + quartz + plagioclase ± chlorite
(retrograde).
Melesse A.

V- Kyanite zone
- Kyanite zone is typified by the range of the assemblages:
Ky + St + Bt + Ms + Qtz, Ky + Grt + Bt + Ms + Qtz,
Ky + Grt + St + Bt + Ms + Qtz, Ky + Bt + Ms + Qtz
- Kyanite formed through the reaction:
Ms + St + Chl  Ky + Bt + Qtz + H2O
Ms + St + Qtz  Ky + Bt + H2O
They are typically coarse grained schists and contain above
mentioned diagnostic mineral assemblage.
Melesse A.

V- Sillimanite zone
- this zone is the highest zone in the Barrovian series
-It characterize by presence of Sillimanite in the form of fibrolite,
and/or coarse prismatic crystals. It could form as Psedudomorph of
andalusite via solid-solid reaction AndSill
-Sillimanite coud also formed as a result of the following reaction:
St + Ms + Qtz  Grt + Bt + Sill+ H2O
Ms + St + Chl  Bt + Sill+ H2O
They are typically coarse grained schists/gneisses and contain
mineral assemblage of Sill ± St + Grt + Bt + Ms + Qtz + Pl ± Ky.
Melesse A.

Stability of metamorphic minerals
Melesse A.

Metamorphism of pelitic rocks
(Metapelites – Part II)
Melesse A.

-3-
Low Pressures metamorphism
(Buchan Zonal scheme)
Melesse A.

2- Buchan Zonal Scheme (LP metamorphism)
- At lower pressure, such as contact metamorphism or
shallow level regional metamorphism, where pressure <3.5
kbar, metamorphism of metapelites exhibit the Buchan
Zonal Scheme
The principle characteristic features of Buchan zonal
scheme are:
1- Cordierite is common and forms at relatively LT,
2- Kyanite does not occur, but andalusite may be present,
3- Garnet is less abundant or absence, and staurolite may
be lacking
4- Migmatities are not developed until well above the
sillimaninte zone
Sequence of metapelites metamorphic zones in the Buchan type
metamorphism include:
Melesse A.

I- Biotite zone
- Biotite zone is the lowest grade of Buchan series
-Metapelites are typically fine-grained Slates and contain mineral
assemblage: biotite + chlorite + muscovite + quartz + Na-
plagioclase (albite).
Melesse A.

2- Cordierite zone
- Cordierite appears as the first distinctive index mineral via the
reaction:
Chl + Ms  Crd + Bt + Qtz+ H2O
Cordierite occur as spots; therefore, the common rock is Spotted
slates/schists and contain mineral assemblage: Cordierite + biotite
+ chlorite + muscovite + quartz + Na-plagioclase (albite) ±
garnet.
Melesse A.

III- Andalusite zone
Andalusite can form in most pelites at low pressures as a result of the
discontinuous reactions:
chlorite + muscovite + quartz  cordierite + andalusite + biotite + H2O
Cordierite + muscovite + quartz  biotite + andalusite + H2O
-Metapelites are typically medium grained schists and contain mineral
assemblage: Andalusite + biotite + muscovite + quartz + Na-
plagioclase (albite) ± garnet ± staurolite
Melesse A.

IV- Sillimanite zone zone
Sillimanite in this zone can occur due to the occurrence of the
polymorphic solid-solid reaction:
andalusite  sillimanite
but as with regional metamorphism the occurrence of muscovite +
cordierite + quartz in this zone suggests that a separate reaction may
occur:
chlorite + muscovite + quartz  cordierite + sillimanite + biotite + H2O
- Mineral assemblage include: Sill + Qtz + Bt + Pl ± Grt ± St ± Crd
Melesse A.

V- Upper sillimanite zone
The highest grade of contact metamorphism of pelites is characterized
by the assemblage sillimanite + cordierite + biotite + K-feldspar +
quartz + muscovite.
This assemblage resulting from the reaction:
Muscovite + quartz  sillimanite + K-feldspar + H2O
Melesse A.

-4-
High Temperature metamorphism
Melesse A.

4- High temperature metapelites
At high-temperatures, above or
coeval to sillimanite zone,
metapelites undergo partial
melting, and the yielded rock is
known as Migmatites.
The Migmatites are mixed rocks
predominantly schists but with
pads, veins or layers of
leucocratic material of granitic
composition. The leucocratic
(granitic) materials are well
known as leucosomes, while the
metamorphic parts are known
as mesosome (resistite) and
melansomes.
Stromatic
Agmatitic Vein-type
Nebulitic Philibitic
Melesse A.

Migmatization processes
migmatization processes could form as a result of :
A- closed system (no gains or loses during migmatization)
1- Aanatexis (partial melting) at higher temperature
2- Metamorphic differentiations at higher temperature
B- Open system
3- K-Na rich external fluid metasomatism
4- Injection of granitic materials to the schistose rocks
In the closed system migmatites three mineral zones develop:
Melesse A.

2- upper sillimanite zone
Melesse A.

2- Cordierite-garnet-K-feldspar zone
 At higher grade, pelitic rocks develop
assemblages with: Cordierite + garnet +
K-feldspar + sillimanite + muscovite +
Qtz
 This mineral assemblage is typical for
the high grade pelitic migmatites, and is
often taken to mark the beginning of the
granulite facies
 The assemblages result from
continuous reaction such as:
biotite + sillimanite + quartz  K-
feldspar + cordierite garnet + melt
Melesse A.

3- Ultra–high grade zone
 Higher grade granulite facies, metapelites with mineral assemblage
orthopyroxene + sillimanite can be formed as a related to breakdown
of common corderite-garnet assemblages through the equilibrium:
Crd + Grt  Opx + Sill
 At even higher temperatures, sillimanite + orthopyroxene assemblage
becomes not stable, and assemblage of sapphirine + quartz has been
formed through the following reaction:
Sill + Opx  sapphirine + quartz (at 850-1000 °C)
Melesse A.

Chapter SEVEN
Metamorphism of carbonate rocks
(Metacarbonates)
Melesse A.

What is the Metacarbonates?
Metacarbonates, are metamorphosed calcareous (limestone
and dolomite) rocks in which the carbonate component is
predominant, with granoblastic polygonal texture
 Metacarbonates include:
i) Marbles are nearly pure carbonate (carbonate >50%)
ii) Calc-silicate rocks: carbonate is subordinate (carbonate <50%)
and may be composed of Ca-Mg-Fe-Al silicate minerals, such
as diopside, grossular, Ca-amphiboles, vesuvianite, epidote,
wollastonite, plagioclase, talc, anthophyllite, etc.
iii) Skarn: calc-silicate rock formed by metasomatism between
carbonates and silicate-rich rocks or fluids
Carbonate rocks are predominantly carbonate minerals, usually
limestone or dolostone. They may be pure carbonate, or they
may contain variable amounts of other precipitates (such as
chert or hematite) or detrital material (sand, clays, etc.)
Chemically, the carbonate rocks are rich in CaO, CO2, MgO,
and mad may SiO2, Al2O3, FeO, and other subordinate oxides if
the carbonate are impure. Melesse A.

Mineralogy of Metacarbonates
Metacarbonate contain the
following mineral
assemblage:
 Carbonate minerals (Calcite
and dolomite),
 Amphibole (anthophyllite,
Tremolite)
 Pyroxene (Diopside, Enstatite)
 Olivine
 talc,
 wollastonite
 quartz
Melesse A.

The metacrbonates will discuss for metamorphism in
the following conditions:
Pure limestone and dolomite
 Impure limestone and dolomite
Melesse A.

-1-
Pure Metacarbonates
Calcite and dolomite marbles
Melesse A.

1- Pure Carbonates (Limestone and dolomite)
 Metamorphism of pure carbonate rocks yielded calcite and/or
dolomite marbles. Many marbles are composed only of calcite and/or
dolomite with minor quartz and phyllosilicates, originally of detrital
origin.
 The grade of metamorphism is function in grain size, where grain
size increases with grade increase.
 At very HP, the polymorph aragonite becomes stable and aragonite
marble is known from high pressure terrains.
 At HT/LP (>600°C) calcite and quartz react to produce wollasonite
and CO2. The reaction occurs only at high temperature thermal
aureole, and is inhibited by high fluid pressures of CO2.
CaCO3 +SiO2  CaSiO3 + CO2
A- Calcite marble
Melesse A.

 At HT/LP, dolomite marble loses CO2 to form periclase (MgO) in
condition <900 °C, and consequently reacts with water to form brucite
(MgO(OH)2). Therefore, the common result of decarbonation of
dolomite or dolomitic marble is a mixture of brucite and calcite.
 Quartz bearing dolomitic marbles (calcite + dolomite + quartz)
develop a characteristic sequence of Ca- and/or Mg-silicate as
follows:
(i) talc
dolomite + qurtz + H2O = talc + calcite + CO2
(ii) tremolite in the greenschist facies,
talc + calcite + quartz = tremolite + H2O + CO2 (quartz rich)
talc+calcite = tremolite + dolomite + CO2 + H2O (quartz poor)
A- Dolomite marble
Melesse A.

(iii) diopside and/or forsterite in the amphibolite facies
tremolite+calcite+quartz = diopside+H2O +CO2
tremolite + dolomite = forsterite + calcite + H2O + CO2
And,
(iv) diopside + forsterite at higher grade.
tremolite + calcite = diopside + forsterite + H2O+CO2
 Sheet-silicate impurity in calcite and dolomite marble adds variety by
the following Al-bearing minerals to feature in the assemblage:
typically they include zoisite, epidote and Ca-rich garnet in the
greenschist facies and anorthite in the amphibolite facies.
A- Dolomite marble, cont.
Melesse A.

Metamorphic zones developed in regionally metamorphosed
dolomitic rocks of the Lepontine Alps
Melesse A.

-2-
Metamorphism of impure carbonates and
marls (Calc-silicates)
Melesse A.

2- Calc-silicates
 Calc-silicates are rocks rich in Ca-Mg-silicate minerals but
poor in carbonate,
 They form via the metamorphism of very impure calcite
or dolomite limestones, or from limy mudstones (marls).
 Since calc–silicates contain significant amounts of other
chemical components, such as Al, K and Fe, minerals such
as zoisite (epidote group), garnet, Ca-plagioclase, K-
feldspar, hornblende and diopside could formed. A
generalized zonal sequence can be summarized as
follows:
Melesse A.

I- Ankerite zone
-The lowest grade rocks
- It characterized by the assemblage ankerite
Ca(Mg,Fe)(CO3)2) + quartz + albite + muscovite ± chlorite
II- Biotite zone
This zone is characterized by the coexistence of biotite and
chlorite without amphibole, via a reaction such as:
Ms +Qtz + ankerite + H2O  Cal + Chl + Bt + CO2
The upper part of this zone also characterize by the
replacement of albite by a more Ca-rich plagioclase and a
reduction in the amount of muscovite present:
Chl + Cal + Ms + Qtz + Ab  Bt + Pl + H2O + CO2
Melesse A.

III- Amphibole zone
The appearance of Ca-amphibole is accompanied by a
further increase in the Ca content of the plagioclase:
Chl + Cal + Qtz + Pl  Ca-amph + Ca-Pl + H2O + CO2
IV- Zoisite zone
Zoisite (Ca2(Al,Fe)3[SiO4](OH)) often first appears rimming
plagioclase at contacts with calcite grains, suggesting
growth is due to the reaction:
Ca-plagioclase + calcite + H2O  zoisite + CO2
V- Diopside zone
At the highest grades diopside appears due to the
breakdown of amphibole:
Ca-amphibole + calcite + quartz  diopside + H2O + CO2
Melesse A.

Chapter EIGHT
Metamorphism of Ultramafic rocks
(Metaultramafics)
Melesse A.

What is the Metaultramafics?
Metaultramafics, are metamorphosed ultramafic igneous
rocks.
Ultramafic rocks are generated in the upper mantle and
composed of <90% mafic minerals, which include:
- Olivine (Ol) (Mg,Fe)2SiO4,
- Orthopyroxene (Opx) (Mg,Fe)SiO3 and
- Clinopyroxene (Cpx) Ca(Mg,Fe)Si2O6
and comprises rocks such as:
- Dunite (Ol), Pyroxenite (Cpx+Opx), Peridotite (Cpx+Opx+Ol)
include harzburgite (Ol+Opx) and lherzolite (Ol+Cpx).
Chemically, the Utramafic chemical system of the ultramafic is
simple (SiO2, MgO, FeO, CaO)Melesse A.

Mineralogy of Metaultramafics
Metaultramafics contain the following mineral assemblage:
 Talc
 Serpentine
Olivine + H2O = serpentine
Mg2SiO4 + H2O = Mg3Si2O5(OH)4
 Anthophyllite
 Olivine
 Pyroxene
Melesse A.

1- Rock types of the metamorphosed ultramafic rocks
 The rock types of the metamorphosed ultramafic rocks
are usually mono-mineralic and include:
- Serpentintes: a massive to schistose rock with abundant
serpentine minerals.
- Talc fels / talc schist: Talc rich foliated/non-foliated rocks
- Ophicalcites: serpentine + carbonate calcite
- Anthophyllite schist.
- Peridotites: Olivine bearing ultrmafics
Melesse A.

THANKS FOR YOUR ATTENTIONS!
END!!!

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