Chapter 8,Metamorphism and
Metamorphic Rocks:New Rocks
from Old
What is Metamorphism? (1)
? Metamorphism is the change in form that
happens in Earth’s crustal rocks in response to
changes in temperature and pressure.
What is Metamorphism? (2)
? There are six major factors in metamorphism:
? Chemical composition.
? The change in temperature.
? The change in pressure.
? The presence or absence of fluids.
? How long a rock is subjected to high pressure
or high temperature.
? Whether the rock is simply compressed or is
twisted and broken during metamorphism.
Chemical Composition of Original Rock
? The greatest factor in determining the
mineral assemblage of a metamorphic
rock.
? The chemical composition of the original
rock controls the mineralogy of the
metamorphosed rock.
Temperature And Pressure (1)
? The heat source is Earth’s internal heat.
? Rock can be heated by burial or by nearby
igneous intrusion.
? Burial is inevitably accompanied by an increase
in pressure due to the weight of the overlying
rocks.
? An intrusion may be shallow,resulting in low
pressure,or deep,resulting in high pressure.
A grain of garnet in a garnet-biotite gneiss,the small inclusions are
relicts from early assemblage formed under P-T conditions different
from those that formed the gneiss
P-T path for a body of rock undergoing metamorphism along a subduction zone
Temperature And Pressure (2)
? Low-grade metamorphism is the result of
metamorphic processes that occur at
temperatures from about 100oC to 500oC,
and at relatively low pressures.
? High-grade metamorphism is the result
of metamorphic processes at high
temperatures (above 500oC),and at high
pressure.
The metamorphic grade
Stress
? Stress is applied pressure that results in
deformation in a solid,and the development of
new textures,
? Uniform stress occurs if pressure is equal in all
directions.
? Differential stress occurs if pressure is different
in different directions.
? Texture is controlled by differential versus
uniform stress.
Uniform stress and differential stress
Deformation of a conglomerate during metamorphism
Role of Time in Metamorphism
? Coarse-grained rocks are the products of
long sustained metamorphic conditions
(possibly over millions of years) at high
temperatures and pressures.
? Fine-grained rocks are products of lower
temperatures,lower pressures or,in
some cases,short reaction times.
The Upper And Lower Limits Of
Metamorphism
? At the lower end,metamorphism occurs in
sedimentary and igneous rocks that are
subjected to temperatures greater than about
100oC,usually under pressures of hundreds of
atmospheres,caused by the weight of a few
thousand meters of overlying rock.
? At the upper end,metamorphism ceases to
occur at temperatures that melt rock.
Role of Water in Determining the
Limits of Metamorphism
? The water present controls the temperature at
which wet partial melting commences and the
amount of magma that can form from a
metamorphic rock.
? When a tiny amount of water is present,only a
small amount of melting occurs.
? Migmatites 混合岩 are composite rocks that contain an
igneous component formed by a small amount of
melting plus a metamorphic portion,
How Rocks Respond To Temperature and Pressure
Change In Metamorphism
? Lower-grade Metamorphism,Slaty Cleavage.
? the newly forming sheet-structure minerals create
foliation that tends to be parallel to the bedding
planes of the sedimentary rock being metamorphosed.
? Higher-grade Metamorphism,Schistosity.
? At intermediate and high grades of metamorphism,
grain size increases.
? Foliation in coarse-grained metamorphic rocks is
called schistosity (the parallel arrangement of coarse
grains of the sheet-structure minerals),
Microscopic thin section of a metamorphic rock showing pronounced
foliation due to the roughly arrangement of mica grains
Slaty cleavage
Mineral Assemblage Change
? As temperature and pressure rise,one
mineral assemblage,morphs” into
another,
? Each assemblage is characteristic of a
given rock composition.
From shale to schist
and gneiss，
（ kyanite 篮晶石
phyllite 矽线石 ）
Metamorphism of Shale and Mudstone
? Slate (low grade):
? The low grade metamorphic product of shale.
? Phyllite (intermediate grade):
? Pronounced foliation,larger mica grains.
? Schist and gneiss (high grade):
? Schist is a coarse-grained rock with pronounced
schistosity.
? Gneiss is a high grade,coarse grained rock with
layers of micaceous minerals segregated from layers
of minerals such as quartz and feldspar.
Metamorphism of Basalt
? Greenschist has pronounced foliation like
phyllite,but also a very distinctive green color
because of its chlorite content.
? Amphibolite and granulite.
? When greenschist is subjected to intermediate-grade
metamorphism,amphibole replaces the chlorite,
Foliation is present in amphibolites,but is not
pronounced because micas and chlorites are usually
absent,
? At the highest grade of metamorphism,amphibole is
replaced by pyroxene and an indistinctly foliated rock
called a granulite develops.
Progressive metamorphism of shale and basalt,Mineral assemblage and
foliation change as a result of increasing T and P and different stress
epidote 绿帘石
Amphibolite resulting from metamorphism of a pillow basalt.
Metamorphism of Limestone
? Marble is the metamorphic derivative of
limestone.
? Coarsely crystalline.
? Pure marble is snow white.
? Pure grains of calcite.
? Many marbles contain impurities that
result in various colors.
Metamorphism of Sandstone
? Quartzite is the metamorphic derivative
of quartz.
? It is derived from quartz sandstone by
filling of the spaces between the original
grains with silica and by recrystallization
of the entire mass.
Types of Metamorphism (1)
There are four types of metamorphism:
? Cataclastic metamorphism
? Dominated by mechanical deformation.
? Contact metamorphism
? Dominated by recrystallization due to
contact with magma.
Undeformed granite consisting of quartz,feldspar and biotite,and a
xenolith of amphibolite
The granite has been cataclasticlly transformed to a gneiss
with a distinct foliation,and amphibolite xenolite have been
flattened and elongated
Types of Metamorphism (2)
? Burial metamorphism
? Dominated by recrystallization aided by
water,
? Regional metamorphism
? Both mechanical deformation and chemical
recrystallization.
Cataclastic Metamorphism
? Mechanical deformation of a rock can
occur with only minor chemical
recrystallization.
? Usually localized and seen in igneous
rocks when a coarse-grained granite
undergoes intense differential stress.
? Grain and rock fragments become
elongated and a foliation develops.
Contact Metamorphism (1)
? Occurs when bodies of hot magma
intrude into cool rocks of the crust.
? Vapors given off by the intruding magma
play a role.
? Mechanical deformation is minor or
absent.
Contact metamorphism around a granite porphyry
Contact Metamorphism (2)
? Rock adjacent to the intrusion becomes
heated,developing a metamorphic
aureole.
? Hornfels.
? Aureoles reach more than 100 m in thickness.
? Metamorphism that involves a lot of fluid
and a large change in rock’s composition
is called metasomatism 交代作用,
Burial Metamorphism
? When buried deeply in a sedimentary basin,
sediments may attain temperatures of a few
hundred degrees Celsius,causing burial
metamorphism.
? Zeolites 沸石 are group of minerals with fully
polymerized silicate structures containing the
same chemical elements as feldspars,plus water.
? As temperatures and pressures increase,burial
metamorphism grades into regional
metamorphism.
Regional Metamorphism
—A Consequence of Plate Tectonics
? Regional metamorphism results from
tectonic forces that build mountains.
? It results from pronounced differential
stresses and extensive mechanical
deformation in addition to chemical
recrystallization.
? Regional metamorphism produces
greenschists and amphibolites.
Regional metamorphism
and metamorphic zones
Metamorphic Facies
? Mineral assemblages caused by specific sets of
temperature/pressure conditions:
? Granulite facies,- hornfels facies
? Amphibolite facies,- zeolite facies
? Epidote-amphibolite facies,
? Greenschist facies,
? Blueschist facies,
? Eclogite facies
Metamorphic Facies plotted with respect to temperature and depth
Metasomatism
? Metasomatism is the process in which
rock compositions are distinctively
altered through exchange with ions in
solution.
? Metasomatic fluids may carry valuable
metals and form mineral deposits.
Metasomatism,elements needed to form the garnet,pyroxene
and fluorite were added to the marble by metasomatic fluids
Plate Tectonics And Metamorphism (1)
? There are five geologic settings where plate
tectonics encourages metamorphism:
? Burial metamorphism.
? Subduction (blueschist and eclogite
metamorphism).
? Regional metamorphism.
? Zone where wet fractional melting starts.
? Contact metamorphism.
Diagram of a convergent plate boundary showing the different
regions of metamorphism
? Burial metamorphism occurs today in the
sediment accumulated in ocean-floor trenches.
? When oceanic crust with a covering of
sedimentary rocks is dragged down by a
rapidly subducting plate,pressure increases
faster than temperature,subjecting the rock to
high pressure but relatively low temperature,
? This is observed today along the subduction
margin of the Pacific Plate where it plunges
under the coast of Alaska and the Aleutian
Islands.
Plate Tectonics And Metamorphism (2)
? Regional metamorphism,where continental
crust is thickened by plate convergence and
heated by rising magma,greenschist and
amphibolite facies metamorphic condition
occur.
? Examples include the Appalachians,Alps,
Himalayas,and Andes.
Plate Tectonics And Metamorphism (3)
continue
? If the crust is sufficiently thick,when 10
percent or more of the crust has melted
the magma so formed will rise forming
stock or batholith.
? As the granitic magma formed by wet
partial melting rises,it heats and
metamorphoses the rocks with which it
comes in contact.
Subduction and rich
mineral deposits
Metamorphic Rocks:New Rocks
from Old
What is Metamorphism? (1)
? Metamorphism is the change in form that
happens in Earth’s crustal rocks in response to
changes in temperature and pressure.
What is Metamorphism? (2)
? There are six major factors in metamorphism:
? Chemical composition.
? The change in temperature.
? The change in pressure.
? The presence or absence of fluids.
? How long a rock is subjected to high pressure
or high temperature.
? Whether the rock is simply compressed or is
twisted and broken during metamorphism.
Chemical Composition of Original Rock
? The greatest factor in determining the
mineral assemblage of a metamorphic
rock.
? The chemical composition of the original
rock controls the mineralogy of the
metamorphosed rock.
Temperature And Pressure (1)
? The heat source is Earth’s internal heat.
? Rock can be heated by burial or by nearby
igneous intrusion.
? Burial is inevitably accompanied by an increase
in pressure due to the weight of the overlying
rocks.
? An intrusion may be shallow,resulting in low
pressure,or deep,resulting in high pressure.
A grain of garnet in a garnet-biotite gneiss,the small inclusions are
relicts from early assemblage formed under P-T conditions different
from those that formed the gneiss
P-T path for a body of rock undergoing metamorphism along a subduction zone
Temperature And Pressure (2)
? Low-grade metamorphism is the result of
metamorphic processes that occur at
temperatures from about 100oC to 500oC,
and at relatively low pressures.
? High-grade metamorphism is the result
of metamorphic processes at high
temperatures (above 500oC),and at high
pressure.
The metamorphic grade
Stress
? Stress is applied pressure that results in
deformation in a solid,and the development of
new textures,
? Uniform stress occurs if pressure is equal in all
directions.
? Differential stress occurs if pressure is different
in different directions.
? Texture is controlled by differential versus
uniform stress.
Uniform stress and differential stress
Deformation of a conglomerate during metamorphism
Role of Time in Metamorphism
? Coarse-grained rocks are the products of
long sustained metamorphic conditions
(possibly over millions of years) at high
temperatures and pressures.
? Fine-grained rocks are products of lower
temperatures,lower pressures or,in
some cases,short reaction times.
The Upper And Lower Limits Of
Metamorphism
? At the lower end,metamorphism occurs in
sedimentary and igneous rocks that are
subjected to temperatures greater than about
100oC,usually under pressures of hundreds of
atmospheres,caused by the weight of a few
thousand meters of overlying rock.
? At the upper end,metamorphism ceases to
occur at temperatures that melt rock.
Role of Water in Determining the
Limits of Metamorphism
? The water present controls the temperature at
which wet partial melting commences and the
amount of magma that can form from a
metamorphic rock.
? When a tiny amount of water is present,only a
small amount of melting occurs.
? Migmatites 混合岩 are composite rocks that contain an
igneous component formed by a small amount of
melting plus a metamorphic portion,
How Rocks Respond To Temperature and Pressure
Change In Metamorphism
? Lower-grade Metamorphism,Slaty Cleavage.
? the newly forming sheet-structure minerals create
foliation that tends to be parallel to the bedding
planes of the sedimentary rock being metamorphosed.
? Higher-grade Metamorphism,Schistosity.
? At intermediate and high grades of metamorphism,
grain size increases.
? Foliation in coarse-grained metamorphic rocks is
called schistosity (the parallel arrangement of coarse
grains of the sheet-structure minerals),
Microscopic thin section of a metamorphic rock showing pronounced
foliation due to the roughly arrangement of mica grains
Slaty cleavage
Mineral Assemblage Change
? As temperature and pressure rise,one
mineral assemblage,morphs” into
another,
? Each assemblage is characteristic of a
given rock composition.
From shale to schist
and gneiss，
（ kyanite 篮晶石
phyllite 矽线石 ）
Metamorphism of Shale and Mudstone
? Slate (low grade):
? The low grade metamorphic product of shale.
? Phyllite (intermediate grade):
? Pronounced foliation,larger mica grains.
? Schist and gneiss (high grade):
? Schist is a coarse-grained rock with pronounced
schistosity.
? Gneiss is a high grade,coarse grained rock with
layers of micaceous minerals segregated from layers
of minerals such as quartz and feldspar.
Metamorphism of Basalt
? Greenschist has pronounced foliation like
phyllite,but also a very distinctive green color
because of its chlorite content.
? Amphibolite and granulite.
? When greenschist is subjected to intermediate-grade
metamorphism,amphibole replaces the chlorite,
Foliation is present in amphibolites,but is not
pronounced because micas and chlorites are usually
absent,
? At the highest grade of metamorphism,amphibole is
replaced by pyroxene and an indistinctly foliated rock
called a granulite develops.
Progressive metamorphism of shale and basalt,Mineral assemblage and
foliation change as a result of increasing T and P and different stress
epidote 绿帘石
Amphibolite resulting from metamorphism of a pillow basalt.
Metamorphism of Limestone
? Marble is the metamorphic derivative of
limestone.
? Coarsely crystalline.
? Pure marble is snow white.
? Pure grains of calcite.
? Many marbles contain impurities that
result in various colors.
Metamorphism of Sandstone
? Quartzite is the metamorphic derivative
of quartz.
? It is derived from quartz sandstone by
filling of the spaces between the original
grains with silica and by recrystallization
of the entire mass.
Types of Metamorphism (1)
There are four types of metamorphism:
? Cataclastic metamorphism
? Dominated by mechanical deformation.
? Contact metamorphism
? Dominated by recrystallization due to
contact with magma.
Undeformed granite consisting of quartz,feldspar and biotite,and a
xenolith of amphibolite
The granite has been cataclasticlly transformed to a gneiss
with a distinct foliation,and amphibolite xenolite have been
flattened and elongated
Types of Metamorphism (2)
? Burial metamorphism
? Dominated by recrystallization aided by
water,
? Regional metamorphism
? Both mechanical deformation and chemical
recrystallization.
Cataclastic Metamorphism
? Mechanical deformation of a rock can
occur with only minor chemical
recrystallization.
? Usually localized and seen in igneous
rocks when a coarse-grained granite
undergoes intense differential stress.
? Grain and rock fragments become
elongated and a foliation develops.
Contact Metamorphism (1)
? Occurs when bodies of hot magma
intrude into cool rocks of the crust.
? Vapors given off by the intruding magma
play a role.
? Mechanical deformation is minor or
absent.
Contact metamorphism around a granite porphyry
Contact Metamorphism (2)
? Rock adjacent to the intrusion becomes
heated,developing a metamorphic
aureole.
? Hornfels.
? Aureoles reach more than 100 m in thickness.
? Metamorphism that involves a lot of fluid
and a large change in rock’s composition
is called metasomatism 交代作用,
Burial Metamorphism
? When buried deeply in a sedimentary basin,
sediments may attain temperatures of a few
hundred degrees Celsius,causing burial
metamorphism.
? Zeolites 沸石 are group of minerals with fully
polymerized silicate structures containing the
same chemical elements as feldspars,plus water.
? As temperatures and pressures increase,burial
metamorphism grades into regional
metamorphism.
Regional Metamorphism
—A Consequence of Plate Tectonics
? Regional metamorphism results from
tectonic forces that build mountains.
? It results from pronounced differential
stresses and extensive mechanical
deformation in addition to chemical
recrystallization.
? Regional metamorphism produces
greenschists and amphibolites.
Regional metamorphism
and metamorphic zones
Metamorphic Facies
? Mineral assemblages caused by specific sets of
temperature/pressure conditions:
? Granulite facies,- hornfels facies
? Amphibolite facies,- zeolite facies
? Epidote-amphibolite facies,
? Greenschist facies,
? Blueschist facies,
? Eclogite facies
Metamorphic Facies plotted with respect to temperature and depth
Metasomatism
? Metasomatism is the process in which
rock compositions are distinctively
altered through exchange with ions in
solution.
? Metasomatic fluids may carry valuable
metals and form mineral deposits.
Metasomatism,elements needed to form the garnet,pyroxene
and fluorite were added to the marble by metasomatic fluids
Plate Tectonics And Metamorphism (1)
? There are five geologic settings where plate
tectonics encourages metamorphism:
? Burial metamorphism.
? Subduction (blueschist and eclogite
metamorphism).
? Regional metamorphism.
? Zone where wet fractional melting starts.
? Contact metamorphism.
Diagram of a convergent plate boundary showing the different
regions of metamorphism
? Burial metamorphism occurs today in the
sediment accumulated in ocean-floor trenches.
? When oceanic crust with a covering of
sedimentary rocks is dragged down by a
rapidly subducting plate,pressure increases
faster than temperature,subjecting the rock to
high pressure but relatively low temperature,
? This is observed today along the subduction
margin of the Pacific Plate where it plunges
under the coast of Alaska and the Aleutian
Islands.
Plate Tectonics And Metamorphism (2)
? Regional metamorphism,where continental
crust is thickened by plate convergence and
heated by rising magma,greenschist and
amphibolite facies metamorphic condition
occur.
? Examples include the Appalachians,Alps,
Himalayas,and Andes.
Plate Tectonics And Metamorphism (3)
continue
? If the crust is sufficiently thick,when 10
percent or more of the crust has melted
the magma so formed will rise forming
stock or batholith.
? As the granitic magma formed by wet
partial melting rises,it heats and
metamorphoses the rocks with which it
comes in contact.
Subduction and rich
mineral deposits
