Chapter7 metamorphicrocks-110708162052-phpapp02

Essentials of GeologyEssentials of Geology
33rdrd
EditionEdition
Chapter 7
Norton Media LibraryNorton Media Library

Metamorphism: A Process of
Change
Prepared by:Prepared by:
Ronald ParkerRonald Parker,, Senior GeologistSenior Geologist
Fronterra GeosciencesFronterra Geosciences
Houston, Oklahoma City, Denver, Anchorage, Dallas, Midland, Aberdeen, Vienna, Buenos Aires, NeuquénHouston, Oklahoma City, Denver, Anchorage, Dallas, Midland, Aberdeen, Vienna, Buenos Aires, Neuquén
www.fronterrageo.comwww.fronterrageo.com

Essentials of Geology, 3rd
edition, by Stephen Marshak Chapter 7: Metamorphism: A Process of Change
IntroductionIntroduction
 Metamorphic – Changed from an original “parent.”Metamorphic – Changed from an original “parent.”
 Meta = Change.Meta = Change.
 Morph = Form or shape.Morph = Form or shape.
 Parent rocks are calledParent rocks are called
““protoliths.”protoliths.”
 Metamorphism canMetamorphism can
occur to any protolith.occur to any protolith.

IntroductionIntroduction
 Protoliths undergo pronounced changes in…Protoliths undergo pronounced changes in…
 Texture.Texture.
 Mineralogy.Mineralogy.
 Due to changes in…Due to changes in…
 Temperature.Temperature.
 Pressure.Pressure.
 Tectonic stress.Tectonic stress.
 Reaction with heated water.Reaction with heated water.

Metamorphic CharacterMetamorphic Character
 Metamorphic rocks have distinctive properties.Metamorphic rocks have distinctive properties.
 Unique texture – Intergrown and interlocking grains.Unique texture – Intergrown and interlocking grains.
 Unique minerals – Some that areUnique minerals – Some that are onlyonly metamorphic.metamorphic.
 Unique foliation – A planar fabric from aligned minerals.Unique foliation – A planar fabric from aligned minerals.
 These transformations can change the rock completely.These transformations can change the rock completely.

Metamorphic ProcessesMetamorphic Processes
 Metamorphic change occurs slowly in the solid state.Metamorphic change occurs slowly in the solid state.
 Several processes are at work.Several processes are at work.
 Recrystallization – Minerals change size and shape.Recrystallization – Minerals change size and shape.
 Phase change – New minerals form with…Phase change – New minerals form with…
Same chemical formula.Same chemical formula.
Different crystal structure.Different crystal structure.
Example: Andalusite to kyanite.Example: Andalusite to kyanite.
Kyanite

 Neocrystallization – New minerals with changes inNeocrystallization – New minerals with changes in
temperature and pressure.temperature and pressure.
 Initial minerals become unstable and change to newInitial minerals become unstable and change to new
minerals.minerals.
Original protolith minerals are digested in reactions.Original protolith minerals are digested in reactions.
Elements restructure to form new minerals.Elements restructure to form new minerals.
 In this way, a shale can transform into a garnet mica schist.In this way, a shale can transform into a garnet mica schist.

 Pressure solution – Mineral grains partially dissolve.Pressure solution – Mineral grains partially dissolve.
 Dissolution requires small amounts of water.Dissolution requires small amounts of water.
 Minerals dissolve where their surfaces press together.Minerals dissolve where their surfaces press together.
 Ions from the dissolution migrate in the water film.Ions from the dissolution migrate in the water film.

 Plastic deformation – Mineral grains soften and deform.Plastic deformation – Mineral grains soften and deform.
 Requires elevated temperatures.Requires elevated temperatures.
 Rock is squeezed or sheared.Rock is squeezed or sheared.
 Minerals act like plastic, changing shape without breaking.Minerals act like plastic, changing shape without breaking.

Causes of MetamorphismCauses of Metamorphism
 The agents of metamorphism are…The agents of metamorphism are…
 Heat (T).Heat (T).
 Pressure (P).Pressure (P).
 Compression and/or shear.Compression and/or shear.
 Hot water.Hot water.
 Not all agents are required; they often do co-occur.Not all agents are required; they often do co-occur.
 Rocks may be overprinted by multiple events.Rocks may be overprinted by multiple events.

Heat (Temperature, T)Heat (Temperature, T)
 Metamorphism occurs as the result of heat.Metamorphism occurs as the result of heat.
 Temperature (T) ranges between 200Temperature (T) ranges between 200oo
C and 850C and 850oo
C.C.
 The upper T limit is…melting. It varies based upon rockThe upper T limit is…melting. It varies based upon rock
mineral composition and water content.mineral composition and water content.
 Heat energy breaks and reforms atomic bonds.Heat energy breaks and reforms atomic bonds.
 Sources of heat.Sources of heat.
 The geothermal gradient.The geothermal gradient.
 Magmatic intrusions.Magmatic intrusions.
 Compression.Compression.

Pressure (P)Pressure (P)
 P increases with depth in the crust.P increases with depth in the crust.
 270 to 300 bars per km (1 bar is almost 1 atm = 14.7 psi).270 to 300 bars per km (1 bar is almost 1 atm = 14.7 psi).
 Metamorphism occurs mostly in 2 to 12 kbar range.Metamorphism occurs mostly in 2 to 12 kbar range.
 T and PT and P bothboth change with depth.change with depth.
 Mineral stability is highly dependent upon T and P.Mineral stability is highly dependent upon T and P.
 This stability can be graphed on a “phase diagram.”This stability can be graphed on a “phase diagram.”
 Changes in T and P leadChanges in T and P lead
to changes in minerals.to changes in minerals.

 Pressure that is greater in one orientation.Pressure that is greater in one orientation.
 A commonplace result of tectonic forces.A commonplace result of tectonic forces.
 Two kinds of differential stress: Normal and shear.Two kinds of differential stress: Normal and shear.
 Normal Stress – Operates perpendicular to a surface.Normal Stress – Operates perpendicular to a surface.
Tension – Pull-apart normal stress.Tension – Pull-apart normal stress.
Compression – Push-together normal stress.Compression – Push-together normal stress.
Differential StressDifferential Stress

 Two kinds of differential stress: Normal and shear.Two kinds of differential stress: Normal and shear.
 Shear Stress – Operates sideways across a surface.Shear Stress – Operates sideways across a surface.
Causes material to be “smeared out.”Causes material to be “smeared out.”

 At higher T and P, differential stress deforms rock.At higher T and P, differential stress deforms rock.
 Rocks change shape slowly without breaking.Rocks change shape slowly without breaking.

 Deformation acts on minerals with specific shapes.Deformation acts on minerals with specific shapes.
 Equant – Roughly equal in all dimensions.Equant – Roughly equal in all dimensions.
 Inequant – Dimensions not the same.Inequant – Dimensions not the same.
Platy (pancake-like) – 1 dimension shorter.Platy (pancake-like) – 1 dimension shorter.
Elongate (cigar-shaped) – 1 dimension longer.Elongate (cigar-shaped) – 1 dimension longer.
 Differential stress causes these minerals to align.Differential stress causes these minerals to align.
 Alignment fabric records stress trajectory.Alignment fabric records stress trajectory.

 Preferred platy mineral alignment is called foliation.Preferred platy mineral alignment is called foliation.
 Foliation imparts a layered or banded appearance.Foliation imparts a layered or banded appearance.
 Rocks commonly break parallel to foliation planes.Rocks commonly break parallel to foliation planes.
 Foliation develops perpendicular to compression.Foliation develops perpendicular to compression.
 Minerals flatten, recrystallize and rotate.Minerals flatten, recrystallize and rotate.
 Inequant grains align by rotation and new growth.Inequant grains align by rotation and new growth.

Hydrothermal FluidsHydrothermal Fluids
 Hot water with dissolved ions and volatiles.Hot water with dissolved ions and volatiles.
 Hydrothermal fluids facilitate metamorphism.Hydrothermal fluids facilitate metamorphism.
 Accelerate chemical reactions.Accelerate chemical reactions.
 Alter rocks by adding or subtracting elements.Alter rocks by adding or subtracting elements.
 Hydrothermal alteration is called metasomatism.Hydrothermal alteration is called metasomatism.

Metamorphic Rock TypesMetamorphic Rock Types
 Two major subdivisions of metamorphic rocks.Two major subdivisions of metamorphic rocks.
 Foliated – Has a through-going planar fabric.Foliated – Has a through-going planar fabric.
Subjected to differential stress.Subjected to differential stress.
Has a significant component of platy minerals.Has a significant component of platy minerals.
Classified by composition, grain size, and foliation type.Classified by composition, grain size, and foliation type.

Metamorphic Rock TypesMetamorphic Rock Types
 Two major subdivisions of metamorphic rocks.Two major subdivisions of metamorphic rocks.
 Nonfoliated – No planar fabric evident.Nonfoliated – No planar fabric evident.
Crystallized without differential stress.Crystallized without differential stress.
Comprised of equant minerals only.Comprised of equant minerals only.
Classified by mineral composition.Classified by mineral composition.

Metamorphic RocksMetamorphic Rocks
 Slate – Fine clay, low-grade metamorphic shale.Slate – Fine clay, low-grade metamorphic shale.
 Has a distinct foliation called slaty cleavage.Has a distinct foliation called slaty cleavage.
Develops by parallel alignment of platy clay minerals.Develops by parallel alignment of platy clay minerals.
Slaty cleavage oriented perpendicular to compression.Slaty cleavage oriented perpendicular to compression.
Slate breaks along this foliation creating flat sheets.Slate breaks along this foliation creating flat sheets.

 Phyllite - Fine mica-rich rock.Phyllite - Fine mica-rich rock.
 Formed by low- to medium-grade alteration of slate.Formed by low- to medium-grade alteration of slate.
 Clay minerals neocrystallize into tiny micas.Clay minerals neocrystallize into tiny micas.
 Micas reflect a satiny luster.Micas reflect a satiny luster.
 Phyllite is between slate and schist.Phyllite is between slate and schist.

 Schist – Fine or coarse rock with larger micas.Schist – Fine or coarse rock with larger micas.
 Medium- to high-grade metamorphism.Medium- to high-grade metamorphism.
 Has a distinct foliation called schistosity.Has a distinct foliation called schistosity.
Parallel alignment of large mica crystals.Parallel alignment of large mica crystals.
Micas are visible because they have grown at higher T.Micas are visible because they have grown at higher T.
 Schist often has other minerals due to neocrystallization.Schist often has other minerals due to neocrystallization.
Quartz.Quartz.
Feldspars.Feldspars.
Kyanite.Kyanite.
Garnet.Garnet.
Staurolite.Staurolite.
Sillimanite.Sillimanite.
 Large non-mica minerals are called porphyroblasts.Large non-mica minerals are called porphyroblasts.

 Gneiss – Has a distinct banded foliation.Gneiss – Has a distinct banded foliation.
 Light bands of felsic minerals (quartz and feldspars).Light bands of felsic minerals (quartz and feldspars).
 Dark bands of mafic minerals (biotite or amphibole).Dark bands of mafic minerals (biotite or amphibole).

 Compositional banding develops in several ways.Compositional banding develops in several ways.
 Original layering in the protolith.Original layering in the protolith.
 Extensive high-temperature shearing.Extensive high-temperature shearing.

 Compositional banding - Solid state differentiation.Compositional banding - Solid state differentiation.

MigmatiteMigmatite
 Migmatite is a partially melted gneiss.Migmatite is a partially melted gneiss.
 It has features of igneousIt has features of igneous andand metamorphic rocks.metamorphic rocks.
 Mineralogy controls behavior.Mineralogy controls behavior.
 Light-colored (felsic) minerals melt at lower T.Light-colored (felsic) minerals melt at lower T.
 Dark-colored (mafic) minerals melt a higher T.Dark-colored (mafic) minerals melt a higher T.
 Felsics melt first; mafics remain metamorphic.Felsics melt first; mafics remain metamorphic.

 Nonfoliated rocks lack a planar fabric.Nonfoliated rocks lack a planar fabric.
 Absence of foliation possible for several reasons:Absence of foliation possible for several reasons:
Rock not subjected to differential stress.Rock not subjected to differential stress.
Dominance of equant minerals.Dominance of equant minerals.
Absence of platy minerals like clays or micas.Absence of platy minerals like clays or micas.

 Amphibolite – Dominated by amphibole minerals.Amphibolite – Dominated by amphibole minerals.
 Basalt or gabbro protolith.Basalt or gabbro protolith.
 Usually not well foliated.Usually not well foliated.
 Hornfels – Alteration by heating.Hornfels – Alteration by heating.
 Associated with plutonic intrusions.Associated with plutonic intrusions.
 Finely crystalline.Finely crystalline.

 Quartzite – Almost pure quartz in composition.Quartzite – Almost pure quartz in composition.
 Forms by alteration of quartz sandstone.Forms by alteration of quartz sandstone.
 Sand grains in the protolith recrystallize and fuse.Sand grains in the protolith recrystallize and fuse.
 Like quartz, it is hard, glassy, and resistant.Like quartz, it is hard, glassy, and resistant.
Metamorphic Alteration

 Marble - Coarsely crystalline calcite or dolomite.Marble - Coarsely crystalline calcite or dolomite.
 Forms from a limestone or dolostone protolith.Forms from a limestone or dolostone protolith.
 Extensive recrystallization completely changes the rock.Extensive recrystallization completely changes the rock.
 Original textures and fossils in the parent are obliterated.Original textures and fossils in the parent are obliterated.
 Used as a decorative and monument stone.Used as a decorative and monument stone.
 Exhibits a variety of colors.Exhibits a variety of colors.
Metamorphic Alteration

 Type depends on protolith.Type depends on protolith.
 Minerals contribute elements.Minerals contribute elements.
 Some protoliths yield specific rocks.Some protoliths yield specific rocks.
 Broad compositional classes:Broad compositional classes:
 Pelitic.Pelitic.
 Mafic.Mafic.
 Carbonate.Carbonate.
 Quartzofeldspathic.Quartzofeldspathic.

Metamorphic IntensityMetamorphic Intensity
 Different minerals are stable as T and P changes.Different minerals are stable as T and P changes.
 Grade is a measure of metamorphic intensity.Grade is a measure of metamorphic intensity.
 Low-grade – Slight.Low-grade – Slight.
 High-grade – Intense.High-grade – Intense.

Metamorphic GradeMetamorphic Grade
 Prograde – Metamorphism via increasing T and P.Prograde – Metamorphism via increasing T and P.
 Common in rocks that are buried in orogenic belts.Common in rocks that are buried in orogenic belts.
 Progressive changes.Progressive changes.
Recrystallization causes mineral growth.Recrystallization causes mineral growth.
Neocrystallization results in new mineral assemblages.Neocrystallization results in new mineral assemblages.
Mineral changes release water.Mineral changes release water.

 Example: Prograde metamorphism of a pelitic rock.Example: Prograde metamorphism of a pelitic rock.
 Low grade – Shale protolith.Low grade – Shale protolith.
Clays recrystallize into larger, aligned clays to yield a slate.Clays recrystallize into larger, aligned clays to yield a slate.
Clays neocrystallize into tiny, aligned micas in a phyllite.Clays neocrystallize into tiny, aligned micas in a phyllite.
 Intermediate grade –Intermediate grade –
Micas recrystallize and grow large to form a schist.Micas recrystallize and grow large to form a schist.
New minerals grow in the schist.New minerals grow in the schist.
 High grade -High grade -
Micas decompose; elements recombine into new minerals.Micas decompose; elements recombine into new minerals.
Neocrystallization yields quartz and feldspars in a gneiss.Neocrystallization yields quartz and feldspars in a gneiss.

 Retrograde – Metamorphism via decreasing T and P.Retrograde – Metamorphism via decreasing T and P.
 Common in rocks that are brought from depth by erosion.Common in rocks that are brought from depth by erosion.
 Accompanied by addition of HAccompanied by addition of H22O by hydrothermal fluids.O by hydrothermal fluids.
 Many prograde rocks aren’t “retrograded.”Many prograde rocks aren’t “retrograded.”
 Rocks at the surface can preserve prograde conditions.Rocks at the surface can preserve prograde conditions.

Index MineralsIndex Minerals
 Certain minerals have a limited P-T range.Certain minerals have a limited P-T range.
 These “index minerals” record metamorphic grade.These “index minerals” record metamorphic grade.
 Index mineral maps.Index mineral maps.
 Define metamorphic zones.Define metamorphic zones.
 Grade boundaries calledGrade boundaries called
isograds.isograds.

Metamorphic FaciesMetamorphic Facies
 Metamorphic facies – Mineral assemblage from aMetamorphic facies – Mineral assemblage from a
specific protolith at specific P-T conditions.specific protolith at specific P-T conditions.
 The same minerals result from the same…The same minerals result from the same…
 Protoliths.Protoliths.
 T and P conditions.T and P conditions.
 Named for dominantNamed for dominant
mineral.mineral.

Metamorphic EnvironmentsMetamorphic Environments
 Metamorphism occurs in different settings.Metamorphism occurs in different settings.
 Different settings yield different effects via…Different settings yield different effects via…
 Geothermal gradient.Geothermal gradient.
 Differential stresses.Differential stresses.
 Hydrothermal fluids.Hydrothermal fluids.
 These characteristics are governed by tectonics.These characteristics are governed by tectonics.

Metamorphic EnvironmentsMetamorphic Environments
 The types (and settings) of metamorphism are...The types (and settings) of metamorphism are...
 Thermal – Heating by a plutonic intrusion.Thermal – Heating by a plutonic intrusion.
 Burial – Increases in P and T by deep burial in a basin.Burial – Increases in P and T by deep burial in a basin.
 Dynamic – Shearing in a fault zone.Dynamic – Shearing in a fault zone.
 Regional – P and T alteration due to orogenesis.Regional – P and T alteration due to orogenesis.
 Hydrothermal – Alteration by hot-water leaching.Hydrothermal – Alteration by hot-water leaching.
 Subduction – High P to low T alteration.Subduction – High P to low T alteration.
 Shock – Extremely high P attending a bolide impact.Shock – Extremely high P attending a bolide impact.

Contact MetamorphismContact Metamorphism
 Due to heat from magma invading host rock.Due to heat from magma invading host rock.
 Creates zoned bands of alteration in host rock.Creates zoned bands of alteration in host rock.
 Called a contact (or metamorphic) aureole.Called a contact (or metamorphic) aureole.
 The aureole surrounds the plutonic intrusion.The aureole surrounds the plutonic intrusion.
Zoned from high-grade (near pluton) to low-grade (far fromZoned from high-grade (near pluton) to low-grade (far from
pluton).pluton).

Contact MetamorphismContact Metamorphism
 Grades of alteration form bands around the pluton.Grades of alteration form bands around the pluton.
 Bands range from highly altered to slightly altered.Bands range from highly altered to slightly altered.
 Analogous to changes in pottery with increased heating.Analogous to changes in pottery with increased heating.
 The width of each aureole zone is due to…The width of each aureole zone is due to…
 The size of the plutonic intrusion.The size of the plutonic intrusion.
 The degree of metasomatism.The degree of metasomatism.
 The dominant rock is hornfels.The dominant rock is hornfels.

Burial MetamorphismBurial Metamorphism
 As sediments are buried in a sedimentary basin…As sediments are buried in a sedimentary basin…
 P increases because of the weight of the overburden.P increases because of the weight of the overburden.
 T increases because of the geothermal gradient.T increases because of the geothermal gradient.
 Requires burial below diagenetic effects.Requires burial below diagenetic effects.
 This is ~ 8–15 km depending on the geothermal gradient.This is ~ 8–15 km depending on the geothermal gradient.

Dynamic MetamorphismDynamic Metamorphism
 Breakage of rock by shearing at a fault zone.Breakage of rock by shearing at a fault zone.
 Fault location determines type of alteration.Fault location determines type of alteration.
 Shallow crust – Upper 10–15 km.Shallow crust – Upper 10–15 km.
Rocks behave in a brittle fashion.Rocks behave in a brittle fashion.
Mineral grains crush-forming fault breccia.Mineral grains crush-forming fault breccia.
 Deeper crust – Below 10–15 km.Deeper crust – Below 10–15 km.
Rocks behave in a ductile manner.Rocks behave in a ductile manner.
Minerals smear like taffyMinerals smear like taffy
to form mylonite.to form mylonite.

Regional MetamorphismRegional Metamorphism
 Tectonic collisions deform huge “mobile belts.”Tectonic collisions deform huge “mobile belts.”
 Directed compression thickens mountains.Directed compression thickens mountains.
 Rocks caught up in mountain building are…Rocks caught up in mountain building are…
Heated via the geothermal gradient and plutonic intrusions.Heated via the geothermal gradient and plutonic intrusions.
Squeezed and heated by deep burial.Squeezed and heated by deep burial.
Smashed and sheared by differential stresses.Smashed and sheared by differential stresses.

Regional MetamorphismRegional Metamorphism
 Regional metamorphism creates foliated rocks.Regional metamorphism creates foliated rocks.
 This type of metamorphism is, by far, the most importantThis type of metamorphism is, by far, the most important
in terms of the amount of rock altered.in terms of the amount of rock altered.
 Collisional belts are often…Collisional belts are often…
Thousands of km long.Thousands of km long.
Hundreds of km wide.Hundreds of km wide.

Hydrothermal MetamorphismHydrothermal Metamorphism
 Alteration by hot, chemically aggressive water.Alteration by hot, chemically aggressive water.
 A dominant process near mid-ocean ridge magma.A dominant process near mid-ocean ridge magma.
 Cold ocean water seeps into fractured crust.Cold ocean water seeps into fractured crust.
 Heated by magma, this water then reacts with mafic rock.Heated by magma, this water then reacts with mafic rock.
 The hot water rises and is ejected via black smokers.The hot water rises and is ejected via black smokers.

Subduction MetamorphismSubduction Metamorphism
 Subduction creates the unique blueschist facies.Subduction creates the unique blueschist facies.
 Trenches and accretionary prisms have…Trenches and accretionary prisms have…
 A low geothermal gradient – low temperature.A low geothermal gradient – low temperature.
 High pressures.High pressures.
 High P – Low T favorHigh P – Low T favor
glaucophane, a blueglaucophane, a blue
amphibole mineral.amphibole mineral.

Shock MetamorphismShock Metamorphism
 Rarely, Earth is struck by a comet or an asteroid.Rarely, Earth is struck by a comet or an asteroid.
 Impacts generate a compressional shock wave.Impacts generate a compressional shock wave.
 Extremely high pressure.Extremely high pressure.
 Heat that vaporizes or melts large masses of rock.Heat that vaporizes or melts large masses of rock.
 These conditions generate high-pressure minerals.These conditions generate high-pressure minerals.

ExhumationExhumation
 How do metamorphic rocks return to the surface?How do metamorphic rocks return to the surface?
 Exhumation is due to uplift, collapse, and erosion.Exhumation is due to uplift, collapse, and erosion.

Finding MetamorphicsFinding Metamorphics
 Large regions of ancient high-grade rocks, calledLarge regions of ancient high-grade rocks, called
shields, are exposed in continental interiors.shields, are exposed in continental interiors.
 Shields are eroded remnants of orogenic belts.Shields are eroded remnants of orogenic belts.
 Shield rocks form the basement under sedimentary cover.Shield rocks form the basement under sedimentary cover.

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