Adamson University

College of Engineering

Mining, Geology and Ceramic Engineering

Jerali C. Rodrigo

Metamorphic Rocks

TOPICS

Definitions

Factors Controlling the Characteristics of

Metamorphic rocks

Grade of Metamorphism

Types of Metamorphism

Metamorphic Structures

Characteristics of Depth Zone Metamorphism

Metamorphic Minerals

Metamorphic Textures

Metamorphic Fabrics

Metamorphic Facies

Classification of Metamorphic rocks

Metamorphism and Plate Tectonics

Definitions Metamorphic Rocks

- are rocks changed from one form to another by

intense heat, intense pressure, or the action of

hot fluids. These rocks underwent the process of

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.

Factors controlling the characteristics of

metamorphic rocks

1. Composition of the parent rock

2. Temperature

3. Pressure and stress

4. Fluids

5. Time

Pressure and Stress

confining pressure (or geostatic

pressure) – pressure applied on all

direction.

Differential stress

Compressive stress – due to compression

Shearing stress – due to sliding of one

body past another.

Foliation - is a planar texture that develops

as a result of differential stress. Minerals

that are subjected or formed under

differential stress tend to follow the

direction of shearing or align themselves

perpendicular to the compressive stress.

10

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

3. Contact metamorphism – the

transformation of rocks caused by heat escaping

from an igneous intrusion. The host rock is

baked, recrystallized, or changed through

reactions driven by the infusion of heat and by

the fluids mobilized within the magma and the

intruding rock itself.

4. Burial metamorphism – results in

response to the pressure exerted by the weight

of the overlying rock; occurs deep into thick

sedimentary basins; includes hardening of the

matrix, cementation, precipitation in pore spaces

and the dissolving and recrystallization of

mineral grains at points of contact.

5. Shock Metamorphism (Impact

Metamorphism)

6. Pyrometamorphism

7. Autometamorphism

8. Pneumatolytic metamorphism

9. Injection metamorphism

10. Geothermal metamorphism

11. Plutonic metamorphism

12. Ultrametamorphism

Metamorphic Structures

1. Fold – it is a curved or bent layer of rock. Folded rocks

2. Crenulations – parallel sets of very tiny folds up to 1 cm long.

3. Lineations – lines on rocks at the edges of foliations, shear planes, slaty cleavages, or folds.

4. Stretched or sheared grains – deformed

pebbles, fossils, or mineral crystals that have

been stretched out or shortened by shearing.

5. Kink bands – in rock are small, abrupt folds

that occur in pairs and results in narrow zones

having a different orientation of foliation that that

which characterizes the rock as a whole.

6. Fault – is a fracture, break, or line of

failure along which movement parallel to

the fracture has occurred.

7. Boudin and Boudinage - A boudin is a

rod-like structure, shaped like a sausage

or football in cross-section. Boudinage is

the term applied to the structure

composed of a series of boudins.

8. Mullion – rod-like or columnar masses of

metamorphic rock representing folded layers.

9. Rock cleavage – cleavage in rocks is the

tendency of a rock to split along parallel to sub

parallel planes. The term cleavage is also used

to name the planes along which splitting

occurs.

10. Banding - As a consequence of metamorphic

crystallization or as a result of the presence of

original layering in a rock, metamorphic rocks

may exhibit bands.

11. Hydrothermal veins – fractures that

have been “healed” (filled) by minerals

that precipitated from hydrothermal

fluids.

Characteristic Aspects of Depth

Zone of Metamorphism

1. Epizone

– Less than 300°C

– Strong Shearing stress, low hydrostatic pressures

– Minerals; Sericite, Chlorite, Chloritoid, Talc,

Stilpnomelane, Antigorite, Brucite, Actinolite,

Epidote, Zoisite, Albite, Glaucophane, Magnesium

Garnet, Calcite, Dolomite and Magnetite.

– Slate, Chlorite and Mica Schist

– Kinetic Metamorphism

2. Mesozone

– 300-500°C

– Moderate shearing stress and hydrostatic

pressure; intermediate depth

– Minerals; Biotite, Muscovite, Staurolite,

Kyanite, Anthophyllite, Epidote, Zoisite,

Sodic Plagioclase, Common Hornblende,

Alkalic Hornblende, Cummingtonite,

Grunerite, Actinolite, Almandine, Calcite,

Brucite, Associations of epizone minerals

with some katazone minerals

– Regional Metamorphism

3. Katazone – 500-700°C

– Strong hydrostatic pressure, deep-seated crustal conditions

– Minerals; Biotite, Potassium Feldspars, Sillimanite, Andalusite, Enstatite Hypersthene, Olivine, Diopside-Hedenbergite, Omphacite, Common Hornblende, Alkalic Hornblende, Aegirine Augite, Jadeite, Cordierite, Garnets ( Almandine, Grossularite, Andradite), Calcic Plagioclase, Idocrase, Scapolite, Humite, Monticellite, Calcite and Brucite.

– Gneisses, Granulites, Eclogites, Amphibolites, High Grade Schists

– Deep-seated Metamorphism associated with Igneous Intrusions.

Characteristic Aspects of Depth Zones of

Metamorphism Zone Tempera-

ture

Dominant

pressure

condition

Typical minerals

formed

Rock type

formed

Types of

metamor-

phism

Epiz

one

Less

than

300oC

Strong

shearing

stress, low

hydrostatic

pressure

Sericite,

chlorite, talc,

epidote,

albite,calcite,

dolomite, etc

Slate,

chlorite,

and mica

schists

Kinetic

Meso

zone

300 –

500oC

Moderate

shearing

stress &

hydrostatic

pressure:

intermediate

depth

Biotite,

muscovite,

kyanite, epidote,

plagioclase,

hornblende,

calcite,

Brucite, etc.

Schists,biot

ite,&

hornblende

schist,garn

et schist

Regio-

nal

Kata

zone

500 –

700oC

Strong

hydrostatic

pressure,

deep-seated

crustal

condition

Biotite,silimanite

Andalusite,

olivine,enstatite-

hypersthene,

common

hornblende, etc

Gneisses,

granulites,

eclogites,

amphibolites

High grade

schist

Deep-seated

metamorphis

m

associated

with igneous

intrusions

Metamorphic Minerals

Mineral Formula

Quartz SiO2

Andalusite, Sillimanite,

Kyanite

Al2SiO

5

Cordierite Mg2Al

4Si

5O18

Pyrope Mg3Al

2(SiO

4)2

Chlorite Mg5Al(AlSi

3O10)(OH)

8

Enstatite MgSiO3

Anthophyllite Mg7(Si

4O11)2(OH)

2

Talc Mg3Si

4O10(OH)

2

Serpentine Mg3Si

2O5(OH)

4

Forsterite Mg2SiO

4

Staurolite Fe2Al

9Si

4O23(OH)

Chloritoid Fe2Al

2(Al

2Si

2O10)(OH)

4

Almandite Fe3Al

2(SiO

4)3

Cummingtonite Fe7(Si4O11)2(OH)

2

Wollastonite CaSiO3

Grossularite Ca3Al

2(SiO4)

3

Zoisite Ca2Al

3(SiO

4)3(OH)

Anorthite CaAl2Si

2O3

Diopside CaMgSi2O6

Tremolite Ca2Mg

5(Si

4O11)2(OH)

2

Jadeite NaAlSi2O6

Glaucophane Na2Mg

3Al

2(Si

4O11)2(OH)

2

Albite NaAlSi3O8

Potash Feldspar KALSi3O8

Muscovite KAl2(AlSi

3O10)(OH)

2

Phlogopite KMg3(AlSi

3O10)(OH)

2

Minerals common in metamorphic rocks than in igneous rocks

1. Diopside

2. Chondrodite

3. Graphite

4. Garnet

5. Tourmaline

6. Axinite

Minor Accessory minerals in igneous rocks reappear in metamorphic rocks

1. Apatite

2. Zircon

3. Sphene

4. Corundum

5. Magnetite

6. Ilmenite

Most common feldspars

1. Orthoclase

2. Microcline

3. Perthite

• All members of the plagioclase series may be

found in metamorphic rocks.

• The Higher the grade of metamorphism, the

higher the anorthite content in the plagioclase.

Metamorphic Textures

Granoblastic (granular): containing

equidimensional generally xenoblastic

crystals of approximately equal size.

Porphyroblastic: containing porphyroblasts.

Maculose or knotted rocks contain scattered

porphyroblasts, generally all of the same

species and of similar size, in an otherwise fine-

grained hornfels matrix.

Poikiloblastic: texture denotes a

porphyroblast containing inclusions of

other minerals; corresponds to poikilitic

texture of igneous rocks. Also called sieve

texture.

Porphyroclast: Large grain surviving in an

otherwise granulated or mylonitized rock.

Feldspar is the most common

porphyroclasts.

Crystalloblastic: textures result from growth of crystals in a solid medium.

Xenoblastic: Crystal faces are poorly developed

Idioblastic: Proper crystal form

Crystalloblastic Series or Idioblastic Order

Usually

Idioblastic Magnetite, Sphene, Rutile,Specularite,Ilmenite

Tourmaline, Garnet, Kyanite, Staurolite,Andalusite

Epidote, Zoisite

Pyroxene, Amphiboles, Wollastonite

Micas, Chlorite

Calcite

Scapolite, Cordierite

Quartz, plagioclase

Rarely Orthoclase, Microcline

Idioblastic

Foliation: a texture characterized by

parallelism of platy and acicular minerals

in the rock. Cleavage, schistosity, and

gneissic foliation or banding are types of

foliation.

1. Slaty cleavage – a planar foliation of fine-

grained platy minerals (mainly clay

minerals, chlorite, or muscovite)

developed parallel and subparallel to

shear planes (microscopic faults) in tightly

folded clayey and mica-rich rocks.

2. Schistosity – a parallel to subparallel foliation

medium- to coarse-grained platy minerals

(mainly micas and chlorite) or an alignment of

long, prismatic crystals (tourmaline, amphibole).

3. Gneissic texture – a parallel to subparallel

foliation of medium- to coarse-grained platy

minerals, in alternating layers of different

composition (dominated by different minerals).

4. Phyllite texture – a wavy foliation of fine-

grained platy minerals (mainly muscovite or

chlorite) that exhibit a somewhat metallic (silvery

) luster and wrinkled form.

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Fabrics Cataclastic Fabric

Those of the broken and fragmented rocks developed by mechanical deformation of Cataclastic or dynamic metamorphism upon hard, brittle rocks.

Flinty Rock and Pseudotachylite

Glassy texture caused by frictional heat generated during thrust faults may cause fusion of minerals particles.

Hornfelsic Fabric

Developed from pelitic sediments undergoing thermal metamorphism and the resulting rock produced is a hornfel.

Decussate Fabric

Randomly arranged crystals of biotite.

Granulose Fabric

Developed largely in rocks with granoblastic

minerals.

Porphyroblastic Fabric

A fabric in which relatively metamorphic

minerals are porphyroblasts of one or

more species occur in a matrix of smaller

grains.

Metamorphic facies

Facies – Assemblage of mineral, rock (or fossil) features reflecting environment in which rock was formed; such features are used to differentiate one rock facies from other neighboring units. Rocks having the same mineral assemblage that formed within a well-defined set of pressure-temperature conditions are regarded as belonging to the same metamorphic facies.

Such mineral that characterizes a given intensity of metamorphism are called index minerals.

42

Metamorphic Facies

• Mineral assemblage present depends on protolith composition and P-T conditions

– Ex: marble, metabasalt and schist all in amphibolite facies

1. Zeolite Facies – represents the lowest grade of metamorphism, the mineral assemblage includes zeolite, chlorite, muscovite and quartz.

2. Greenschist Facies – low grade metamorphic facies of many regionally metamorphosed terranes. The mineral assemblage may include chlorite, epidote, muscovite, albite and quartz.

3. Amphibolite Facies – occurs in medium- to high-grade metamorphic terranes. The mineral constituents include hornblende, plagioclase, and almandine. This facies occurs where Staurolite and sillimanite-grade metamorphic conditions have prevailed.

4. Glaucophane-lawsonite schist (or blueschist) Facies - represented by relatively low temperatures but elevated pressures of metamorphism in young orogenic zones. Characteristic constituents are lawsonite, jadeite, albite, Glaucophane, muscovite, and garnet.

5. Granulite Facies – reflects the maximum temperature conditions of regional metamorphism. Characteristic mineral constituents are plagioclase, orthopyroxene, garnet, and diopside.

6. Eclogite Facies – represents the most deep-seated conditions of metamorphism. Characteristic mineral constituents are Pyrope-rich garnet and Omphacite.

Classification of Metamorphic

Rocks • 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 biotite-garnet schist. A gneiss containing hornblende, pyroxene, quartz, and feldspar would be called hornblende-pyroxene gneiss. A schist containing porphyroblasts of K-feldspar would be called K-spar porphyroblastic schist.

• Chemical - If the general chemical composition can be determined from the mineral assemblage, then a chemical name can be employed. For example schist with a lot of quartz and feldspar and some garnet and muscovite would be called garnet-muscovite quartzo-feldspathic schist. Schist consisting mostly of talc would be called talc-magnesian schist.

• Protolithic - If a rock has undergone only slight metamorphism such that its original texture can still be observed then the rock is given a name based on its original name, with the prefix meta- applied. For example: metabasalt, metagraywacke, meta-andesite, metagranite.

Protolith Protolith refers to the original rock, prior to metamorphism.

• 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, although they contain other minerals that result from reaction of the carbonates with associated siliceous detrital minerals that were present in the rock. 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 - Just like in igneous rocks, 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

Spessartite.

Slate – fine-grained rocks that have

remarkable property known as slaty

cleavage which permits them to be split

into thin, broad sheets. Their color is

commonly gray to black but may be green,

yellow brown, and red.

Phyllite – Fine grained and very schistose

rock, the platy mineral of which consists

mainly of phengite. Phengite sericite gives

an overall silky sheen to the schistosity

planes. The grain size is coarser than in

slates but finer than in mica schists.

Schists –Distinguished by the presence of

well-developed foliation or schistosity,

along which rock may be easily broken.

Low grade Schists – presence of low

temperature minerals; Albite, Muscovite,

Chlorite, Actinolite, and Talc.

High Grade Schists – foliated

porphyroblastic and are characterized by

the presence of high-temperature minerals

as garnet, biotite, sillimanite, staurolite,

cordierite, andalusite and kyanite.

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Gneiss – medium- to coarse-grained rock having

a gneissic fabric. Light and dark minerals are

found separate, parallel layers or lenses. Dark

layers commonly include biotite and hornblende;

light-colored layers consist of quartz and

feldspars. Layers may be folded or contorted.

Amphibolite - These are medium to coarse

grained, dark colored rocks whose

principal minerals are hornblende and

plagioclase. They result from

metamorphism of basic igneous rocks.

Blueschist – a blue amphibole-bearing

metamorphosed mafic igneous rock or mafic

greywacke. This term is so commonly applied to

such rocks that it is even applied to non-

schistose rocks.

Greenschist - a low-grade metamorphic rock that

typically contains chlorite, actinolite, epidote, and

albite. Note that the first three minerals are

green, which imparts the color to the rock. Such

a rock is called greenschist if foliated, and

greenstone if not. The protolith is either a mafic

igneous rock or greywacke.

Argillites – a low-grade, nonfoliated

metamorphic rock made from a mudstone

or other clay-rich sedimentary rock.

Argillite breaks with an irregular or

conchoidal fracture. Its lack of foliation

may be attributable partly to a lower grade

of deformation and partly to abundance in

the parent mudstone of quartz silt or other

minerals that are neither platy nor

elongate.

Hornfels – a high-temperature contact metamorphic

rock of uniform grain size that has undergone little or no

deformation. Its platy or elongate crystals are oriented

randomly, and foliated texture is absent. Hornfels have a

granular texture overall, even though they commonly

contain pyroxene, which makes elongate crystals, and

some micas.

Greenstone – metamorphosed mafic volcanic rocks.

Many of these low-grade rocks form when mafic lava

and ash deposits react with percolating seawater or

other solutions. Large areas of the seafloor are covered

with basalts slightly or extensively altered in this way at

mid-ocean ridges. On the continents, buried volcanic and

plutonic mafic igneous rocks react with groundwater at

temperatures of 150 to 300°C and form similar

greenstones. An abundance of chlorite gives these rocks

their greenish cast.

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.

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 plate

tectonics

Seafloor metamorphism

Fractures that develop within the MOR act

as passageways for seawater circulating

within the crust. The seawater heated by

magma rises and reacts with the basaltic

crust, converting it to hydrous rocks such

as serpentinite. Metals extracted from the

crust are redeposited and concentrated

high within the crust and on the surface.