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Chapter 7
Metamorphism and
Metamorphic Rocks
PowerPoint Presentation
Stan Hatfield . SW Illinois College
PowerPoint Presentation
Ken
Pinzke. .Southwestern
SW Illinois
College
Stan Hatfield
Illinois
College
Ken
Pinzke . Southwestern
Illinoisof
College
Charles
Henderson
. University
Calgary
Charles Henderson . University of Calgary
Tark
Hamilton . Camosun College
Copyright (c) 2005 Pearson Education Canada, Inc.
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Metamorphism
The transition of one rock into another by
temperatures and/or pressures unlike
those in which it formed
Metamorphic rocks are produced from
• Igneous rocks
• Sedimentary rocks
• Other metamorphic rocks
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Metamorphism
During metamorphism the rock must
remain essentially solid
The type and degree of change in the
transformation of a parent rock to a
metamorphic rock depends on the type
and intensity of metamorphic processes or
agents
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Agents of Metamorphism
Temperature
• The most important agent
• Chemical reactions and recrystallization
result in new minerals that are stable for the
new conditions
• Two sources of heat
– Intrusive igneous body baking surrounding
rocks
– Earth’s internal heat when brought to great
depth down the geothermal gradient
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Agents of Metamorphism
Pressure (stress)
• Increases with depth
• Confining pressure (uniform stress)
applies forces equally in all directions;
harder and denser metamorphic rocks
• Rocks may also be subjected to directed
pressure (differential stress), which is
unequal in different directions (near
faults)
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Agents of Metamorphism
Pressure (stress) continued
• Three basic types include compressional,
tensional, and shear stress
• Rocks are brittle at the surface and fracture
when subjected to differential stress
• At depth, where temperatures are higher,
rocks are ductile and minerals can flatten
and elongate and spectacular folds form
when subjected to differential stress
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Agents of Metamorphism
Differential stress forms a typical
metamorphic texture called foliation (texture
refers to the size, shape, and arrangement of
grains within a rock)
Foliation – any planar arrangement of mineral
grains or structural features within a rock
• Three factors influence foliation development
1. Rotation of platy and/or elongate mineral grains
into a new orientation
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Agents of Metamorphism
Foliation
• Three factors influence foliation development
2. Changing the shape of equidimensional grains
into elongate shapes aligned in preferred
orientation
3. Recrystallization of minerals to form new grains
growing in direction of preferred orientation
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Agents of Metamorphism
The effects of pressure on the texture of metamorphic rocks.
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Agents of Metamorphism
Confining versus directed pressure.
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Development of preferred orientations.
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Agents of Metamorphism
Chemically active fluids
• Mainly water with other volatile
components; become more reactive with
higher temperature
• Enhance migration of ions; acts as catalyst
to promote textural changes
• Aid in recrystallization of existing minerals
• Play role in changing overall composition of
rock
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Agents of Metamorphism
Chemically active fluids
• Sources of fluids
– Pore spaces of sedimentary rocks
– Fractures in igneous rocks
– Hydrated minerals such as clays and micas
become dehydrated
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Metamorphic Grade and Index Minerals
The importance of parent rock
• Most metamorphic rocks have the same
overall chemical composition as the parent
rock from which they formed
• Mineral makeup determines, to a large
extent, the degree to which each
metamorphic agent will cause change
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Metamorphic Grade and Index Minerals
Systematic variations in the mineralogy and
often the textures of metamorphic rocks are
related to the variations in the degree of
metamorphism (grade)
Index minerals and metamorphic grade
• Changes in mineralogy occur from regions of
low-grade metamorphism to regions of highgrade metamorphism; index minerals are
stable under certain pressure-temperature
conditions (P-T)
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Metamorphic Grade and Index Minerals
Index minerals and metamorphic grade
• Certain minerals, called index minerals,
are good indicators of the metamorphic
conditions in which they form
• Low grade = chlorite (around 200ºC)
• High grade = sillimanite (around 600ºC)
• Minerals will be foliated or nonfoliated
depending on pressure conditions
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Metamorphic Grade and Index Minerals
Index minerals provide information about the P-T conditions of metamorphism.
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Metamorphic Grade and Index Minerals
Metamorphic grade reflects the intensity of metamorphism experienced by a rock.
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Metamorphic Environments and Rocks
There are several types including burial,
contact, regional, and hydrothermal.
Burial Metamorphism
• Associated with very thick sedimentary strata
• Rocks become denser, but not distorted
• Required depth varies from one location to
another depending on the prevailing geothermal
gradient; on average, low-grade metamorphism
begins at about 8 km
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Metamorphic Environments and Rocks
Basic types of metamorphism associated with changes in temperature and pressure.
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Metamorphic Environments and Rocks
Contact Metamorphism
• Occurs due to a rise in temperature when
magma invades a host rock
• A zone of alteration called an aureole
forms in the rock surrounding the magma
• Most easily recognized when it occurs at
the surface, or in a near-surface
environment
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Metamorphic Environments and Rocks
Contact Metamorphism
• Clay + heat = hornfels (nonfoliated rock
with mica and amphibole)
• Sometimes large metamorphic minerals
impart a spotted appearance =
porphyroblastic
• Quartzite and Marble often (not always)
form by contact metamorphism from
sandstone and limestone respectively
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Metamorphic Environments and Rocks
Contact metamorphism produces an alteration zone or aureole around an intrusive body.
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Metamorphic Environments and Rocks
Hornfels with porphyroblasts of cordierite.
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Metamorphic grade decreased away
from intrusive body.
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Metamorphic Environments and Rocks
A. Quartzite is metamorphosed quartz sandstone, and B. Marble is metamorphosed
limestone; it is coarsely crystalline and often used as a decorative stone.
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Metamorphic Environments and Rocks
Regional metamorphism
• Produces the greatest quantity of
metamorphic rock
• Associated with mountain building at
convergent plate boundaries
• Intensity is greatest when continents
collide giving a metamorphic core to a
mountain range
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Metamorphic Environments and Rocks
Regional metamorphism occurs where rocks are squeezed between converging plates.
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Metamorphic Environments and Rocks
Regional Metamorphism of Shale
• Slate
– Very fine-grained
– Excellent rock cleavage
– Most often generated from low-grade
metamorphism of shale, mudstone, or siltstone
– Colour depends on specific minerals (black
contains organic material; red contains iron
oxide; green contains chlorite)
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Metamorphic Environments and Rocks
Regional Metamorphism of Shale
• Phyllite
– Gradation in the degree of metamorphism
between slate and schist
– Platy minerals not large enough to be
identified with the unaided eye
– Glossy sheen and wavy surfaces
– Exhibits rock cleavage
– Composed mainly of fine crystals of muscovite
and/or chlorite
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Metamorphic Environments and Rocks
Regional Metamorphism of Shale
• Schist
– Medium- to coarse-grained
– Platy minerals predominate
– Commonly include the micas
– The term schist describes the texture (strongly
foliated texture or schistosity)
– To indicate composition, mineral names are
used (such as mica schist)
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Metamorphic Environments and Rocks
Regional Metamorphism of Shale
• Gneiss
– Medium- to coarse-grained
– Banded appearance
– High-grade metamorphism
– Often composed of white or light-coloured
feldspar-rich layers with bands of dark
ferromagnesian minerals (called gneissic
texture or gneissosity)
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Metamorphic Environments and Rocks
Rocks produced from the regional metamorphism of shale.
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Metamorphic Environments and Rocks
Development of one type of rock cleavage.
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Metamorphic Environments and Rocks
Regional Metamorphism of Basalt
• Foliation is less pronounced compared
with mudrock-derived metamorphics
• Greenschist
– Low grade
– Ferromagnesian minerals (olivine and
pyroxene) are hydrated to form chlorite
– Phyllite-like foliation
– Green colour
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Metamorphic Environments and Rocks
Regional Metamorphism of Basalt
• Amphibolite
– Higher grade than greenschist
– Chlorite and other minerals lose water and are
converted to amphibole
– Preferred orientation of needle-like amphibole
crystals produces a foliation, although not as
distinct as in greenschist
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Metamorphic Environments and Rocks
Regional Metamorphism of Basalt
• Granulite
– High grade
– Amphiboles are further dehydrated to
produce pyroxene and garnets
– Little preferred orientation to minerals due to
their shape
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Metamorphic Environments and Rocks
Rocks produced from regional metamorphism of basalt. Grade increases to right.
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Metamorphic Environments and Rocks
The Upper Limit of Regional Metamorphism
• Migmatite
– Rocks begin to partially melt at transition zone
between metamorphic and igneous conditions
– Light-coloured silicates like quartz and K-feldspar
melt at lower temperature
– Igneous bands are folded intensely
– Dark layers maintain metamorphic origin
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Metamorphic Environments and Rocks
Regional Metamorphism and Nonfoliated Rocks
• Depending on composition regional metamorphic
rocks may be nonfoliated
– Includes quartzite and marble which lack platy and
elongate minerals
– May be difficult to discern from similar rocks
produced by contact metamorphism
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Metamorphic Environments and Rocks
Subduction Zone Metamorphism
• Technically a form of regional metamorphism
• Special conditions of high pressure, but low
temperature occur due to rapid subduction so
compression & shear exceed heating
• Na-rich blue-coloured amphibole called
glaucophane or riebeckite forms
• Exhibits foliation and is called blueschist
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Metamorphic Environments and Rocks
Hydrothermal metamorphism
• Chemical alteration caused when hot, ionrich fluids, called hydrothermal solutions,
circulate through fissures and cracks that
develop in rock
• Most widespread along the axis of the midocean ridge system but occurs above most
intrusions in any setting.
• Involves interaction of hot rocks and
circulated seafloor (also called seafloor
metamorphism)
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Metamorphic Facies and Plate Tectonics
A Metamorphic Facies is defined by distinctive
assemblage of minerals
• Names of facies refer to metamorphic rocks derived
from basaltic parent rock at certain P-T (pressuretemperature) conditions
• Hornfels (high T, low P) and zeolite (low T, low P)
are low grade
• Greenschist-Amphibolite-Granulite are increasing
regional grade facies
• Blueschist (low T, high P) at subduction zones
• Eclogite (high T, high P) at subduction zones
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Metamorphic Facies and Plate Tectonics
Metamorphic facies and corresponding temperature and pressure conditions.
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Metamorphic Facies and Plate Tectonics
The association of metamorphic facies with plate tectonic environments.
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Other Types of Metamorphism
Other metamorphic environments
• Cataclastic Metamorphism
– Caused by extreme directed pressures (shear),
usually along faults and fault systems,
– Rocks and minerals are either flattened or
destroyed entirely
– At depth where rocks deform by ductile flow
minerals in fault zones become very lineated
and are called mylonites
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Other Types of Metamorphism
Other metamorphic environments
• Impact (Shock) Metamorphism
– Occurs when high speed projectiles called
meteorites strike Earth’s surface
– Also in nuclear bomb craters
– Distinctive structures include shock lamellae
and shatter cones
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Ancient Metamorphic Environments
Metamorphics occur as linear belts along the
axes of mountain belts
Paired metamorphic belts denote subduction
Also occur as a large expanse in the stable
continental interior and are called shields
• Shields represent the amalgamated remnants
of earlier mountain building periods
• Canadian Shield ranges in age from 1.8 to 4.03
billion years
• Resources include metasedimentary BIF’s,
corundum, garnet, talc, graphite, diamonds
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Occurrence of Metamorphic Rocks
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End of
Chapter 7
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