chapter 8: metamorphism & metamorphic rocks
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Chapter 8: Metamorphism & Metamorphic Rocks
Chapter 8: Metamorphism & Metamorphic Rocks
IntroductionIntroduction “Meta” = change & “morph” = form
Metamorphic = to change form Transition of one rock into another by
application of pressure and/or temperature unlike those from which it formed.
Metamorphic rocks are produced from Sedimentary rocks Igneous rocks Other metamorphic rocks
Parent rock = protolith Rock from which the metamorphic rock was
formed
“Meta” = change & “morph” = form Metamorphic = to change form Transition of one rock into another by
application of pressure and/or temperature unlike those from which it formed.
Metamorphic rocks are produced from Sedimentary rocks Igneous rocks Other metamorphic rocks
Parent rock = protolith Rock from which the metamorphic rock was
formed
Types of MetamorphismTypes of Metamorphism
Progresses incrementally Low-grade metamorphism (< 200oC) High-grade metamorphism (> 600oC)
** Rock must remain solid!! If melting occurs, then igneous rocks are
formed Agents of Metamorphism
Heat Pressure & differential stress Chemically active fluids
Progresses incrementally Low-grade metamorphism (< 200oC) High-grade metamorphism (> 600oC)
** Rock must remain solid!! If melting occurs, then igneous rocks are
formed Agents of Metamorphism
Heat Pressure & differential stress Chemically active fluids
Settings of MetamorphismSettings of Metamorphism Three settings:
Thermal metamorphism AKA contact metamorphism Intrusion of magma body Change driven by rise in temperature
Hydrothermal metamorphism Chemical alterations that occur as hot ion-rich
water circulates through rock Regional metamorphism
AKA large-scale deformation Large quantities of rock are subject to P and
high T
Three settings: Thermal metamorphism
AKA contact metamorphism Intrusion of magma body Change driven by rise in temperature
Hydrothermal metamorphism Chemical alterations that occur as hot ion-rich
water circulates through rock Regional metamorphism
AKA large-scale deformation Large quantities of rock are subject to P and
high T
Heat as Metamorphic AgentHeat as Metamorphic Agent Most important agent
b/c provides energy for chemical reactions & recrystallization
Sources: Geothermal gradient Contact metamorphism
Most important agent b/c provides energy for chemical reactions &
recrystallization
Sources: Geothermal gradient Contact metamorphism
Pressure & Differential Stress
Pressure & Differential Stress Confining
pressure compaction Equal
pressure in all directions
Differential stress = directed pressure Causes folds
& faults
Confining pressure compaction Equal
pressure in all directions
Differential stress = directed pressure Causes folds
& faults
Brittle vs. Ductile BehaviorBrittle vs. Ductile Behavior At surface, rocks are brittle
Tend to fracture & break into smaller pieces
At high-T, rocks are ductile Grains tend to flatten and elongate
At surface, rocks are brittle Tend to fracture & break into smaller
pieces At high-T, rocks are ductile
Grains tend to flatten and elongate
Chemically Active FluidsChemically Active Fluids Mostly water (H2O) and carbon
dioxide (CO2) Sources of fluids:
Hydrated minerals e.g. Clays & amphiboles
Movement: Pore spaces of sedimentary rocks Fractures in igneous rocks
Mostly water (H2O) and carbon dioxide (CO2)
Sources of fluids: Hydrated minerals
e.g. Clays & amphiboles
Movement: Pore spaces of sedimentary rocks Fractures in igneous rocks
Importance of ProtolithImportance of Protolith
Remember: protolith = parent rock Most metamorphic rock have same
chemical composition of their parent rock Except for gain or loss of volatiles e.g., H2O & CO2
Remember: protolith = parent rock Most metamorphic rock have same
chemical composition of their parent rock Except for gain or loss of volatiles e.g., H2O & CO2
Metamorphic TexturesMetamorphic Textures Texture = size, shape & orientation
(arrangement) of grains in rock Foliation = any planar arrangement of
mineral grains or structural features within a rock e.g., parallel alignment of platy and/or
elongated minerals e.g., parallel alignment of flattened mineral
grains or elongated pebbles e.g., compositional banding e.g., slaty cleavage where rocks can be easily
split into thin, tabular sheets
Texture = size, shape & orientation (arrangement) of grains in rock
Foliation = any planar arrangement of mineral grains or structural features within a rock e.g., parallel alignment of platy and/or
elongated minerals e.g., parallel alignment of flattened mineral
grains or elongated pebbles e.g., compositional banding e.g., slaty cleavage where rocks can be easily
split into thin, tabular sheets
Foliation: Alignment of Platy Minerals
Foliation: Alignment of Platy Minerals
Foliation: Alignment of Elongated
Pebbles
Foliation: Alignment of Elongated
Pebbles
Foliation: Compositional Banding
Foliation: Compositional Banding
Granite protolith resultant Gneiss with compositional banding
Foliation: Slaty Cleavage
Foliation: Slaty Cleavage
Foliated TexturesFoliated Textures Rock cleavage (AKA slaty cleavage)
Closely spaced planar surfaces along which rocks split
e.g., slate (originally shale) Schistosity
Platy minerals (e.g., micas) are discernible with unaided eye
Exhibit planar or layered structure e.g., schist (originally slate)
Gneissic banding During higher grades of metamorphism, ion migration
results in separation of light and dark minerals Exhibit distinctive light & dark compositional banding
Rock cleavage (AKA slaty cleavage) Closely spaced planar surfaces along which rocks
split e.g., slate (originally shale)
Schistosity Platy minerals (e.g., micas) are discernible with
unaided eye Exhibit planar or layered structure e.g., schist (originally slate)
Gneissic banding During higher grades of metamorphism, ion migration
results in separation of light and dark minerals Exhibit distinctive light & dark compositional banding
Formation of SlateFormation of Slate
Garnet – Mica Schist
Garnet – Mica Schist
Gneissic BandingGneissic Banding
Other Metamorphic Textures
Other Metamorphic Textures
Nonfoliated Form in environments where deformation
is minimal e.g., fine-grained limestone + heat = marble
Porphyroblastic texture Large grains surrounded by smaller grains Porphyroblast = large grains Matrix = fine-grains around porphyroblast
Nonfoliated Form in environments where deformation
is minimal e.g., fine-grained limestone + heat = marble
Porphyroblastic texture Large grains surrounded by smaller grains Porphyroblast = large grains Matrix = fine-grains around porphyroblast
Common Metamorphic Rocks
Common Metamorphic Rocks
Foliated Rocks: Slate Phyllite Schist Gneiss
Foliated Rocks: Slate Phyllite Schist Gneiss
Nonfoliated Rocks: Marble Quartzite
Nonfoliated Rocks: Marble Quartzite
SlateSlate Very fine-grained Excellent rock cleavage
Slaty cleavage Most often generated from low-grade
metamorphism of shale, mudstone or siltstone
Different colors: Black = carbonaceous Red = Fe-oxide Green = chlorite
Very fine-grained Excellent rock cleavage
Slaty cleavage Most often generated from low-grade
metamorphism of shale, mudstone or siltstone
Different colors: Black = carbonaceous Red = Fe-oxide Green = chlorite
PhyllitePhyllite Degree of
metamorphism between slate & schist
Platy minerals not large enough to be identified with unaided eye
Glossy sheen & wavy surfaces
Has rock cleavage Composed of platy
minerals such as micas & chlorite
Degree of metamorphism between slate & schist
Platy minerals not large enough to be identified with unaided eye
Glossy sheen & wavy surfaces
Has rock cleavage Composed of platy
minerals such as micas & chlorite
Slate Phyllite
SchistSchist Medium- to coarse-
grained Medium-grade
metamorpism Platy minerals dominate
Mainly micas Schistositic texture To indicate composition,
mineral names are used e.g., mica schist
Medium- to coarse-grained
Medium-grade metamorpism
Platy minerals dominate Mainly micas
Schistositic texture To indicate composition,
mineral names are used e.g., mica schist
GneissGneiss Medium- to coarse-
grained Banded layered
appearance High-grade
metamorphism Often composed of
white or light-colored feldspar-rich layers with bands of dark ferromagnesian-rich layers
Medium- to coarse-grained
Banded layered appearance
High-grade metamorphism
Often composed of white or light-colored feldspar-rich layers with bands of dark ferromagnesian-rich layers
MarbleMarble Coarse-grained Crystalline Protolith = limestone Composed of
essentially calcite [CaCO3] and/or dolomite [CaMg (CO3)2] crystals
Exhibits a variety of colors Used as decorative &
monument stone
Coarse-grained Crystalline Protolith = limestone Composed of
essentially calcite [CaCO3] and/or dolomite [CaMg (CO3)2] crystals
Exhibits a variety of colors Used as decorative &
monument stone
QuartziteQuartzite Medium- to coarse-
grained Crystalline Protolith = sandstone Composed dominately
of quartz [SiO2]
Medium- to coarse-grained
Crystalline Protolith = sandstone Composed dominately
of quartz [SiO2]
Classification of Metamorphic Rocks
Classification of Metamorphic Rocks
Metamorphic EnvironmentsMetamorphic Environments Contact Metamorphism
AKA Thermal Metamorphism Hydrothermal Metamorphism Regional Metamorphism Other Metamorphic Environments
Burial Metamorphism Metamorphism along fault zones Impact metamorphism
AKA Shock Metamorphism
Contact Metamorphism AKA Thermal Metamorphism
Hydrothermal Metamorphism Regional Metamorphism Other Metamorphic Environments
Burial Metamorphism Metamorphism along fault zones Impact metamorphism
AKA Shock Metamorphism
Contact MetamorphismContact Metamorphism
Hydrothermal MetamorphismHydrothermal Metamorphism
Regional MetamorphismRegional Metamorphism
Burial MetamorphismBurial Metamorphism
Fault Zone MetamorphismFault Zone Metamorphism
Impact MetamorphismImpact Metamorphism
Metamorphic ZonesMetamorphic Zones Metamorphic Grade:
Systematic variation in mineralogy & textures are related to the variations in the degree of metamorphism
Changes in mineralogy occur from regions of low-grade metamorphism to regions of high-grade metamorphism
Index Minerals: Certain minerals which are good
indicators of the degree of the metamorphic conditions under which they form
e.g., chlorite ~200oC
Metamorphic Grade: Systematic variation in mineralogy &
textures are related to the variations in the degree of metamorphism
Changes in mineralogy occur from regions of low-grade metamorphism to regions of high-grade metamorphism
Index Minerals: Certain minerals which are good
indicators of the degree of the metamorphic conditions under which they form
e.g., chlorite ~200oC
Progressive Regional Metamorphism
Progressive Regional Metamorphism
With increasing P & T, higher degree of metamorphism
With increasing P & T, higher degree of metamorphism
Index MineralsIndex Minerals Typical transition in mineralogy due
to progressive metamorphism of shale
Typical transition in mineralogy due to progressive metamorphism of shale
Metamorphic Zones in New England, USA
Metamorphic Zones in New England, USA
Grades concentric with high-grade in center of mountain range
Grades concentric with high-grade in center of mountain range
MigmatitesMigmatites Highest grade metamorphic rock
Transitional to Igneous rock Partial melting of felsic minerals
Highest grade metamorphic rock Transitional to Igneous rock Partial melting of felsic minerals
Contain light bands of igneous components along with dark metamorphic rock
Fault Zone MetamorphismFault Zone Metamorphism Fault
breccia at and near surface
Mylonite (AKA migmatite) forms at depth
Fault breccia at and near surface
Mylonite (AKA migmatite) forms at depth
Metamorphism & Plate Tectonics
Metamorphism & Plate Tectonics Most metamorphism occurs along
convergent boundaries Compressional stresses deform edges of plates Formation of Earth’s major mountain belts
e.g. Alps, Himalayas, Appalachians Mountainous terrains along subduction zones
contain linear belts of metamorphic rocks High P, low T rocks nearest the trench
e.g. blueschist facies (glaucophane = index min) High T, low P zones further inland
In region of igneous activity Large-scale metamorphism also occurs along
subduction zone at convergent boundaries Several metamorphic environments exist here Important site for magma generation
Most metamorphism occurs along convergent boundaries Compressional stresses deform edges of plates Formation of Earth’s major mountain belts
e.g. Alps, Himalayas, Appalachians Mountainous terrains along subduction zones
contain linear belts of metamorphic rocks High P, low T rocks nearest the trench
e.g. blueschist facies (glaucophane = index min) High T, low P zones further inland
In region of igneous activity Large-scale metamorphism also occurs along
subduction zone at convergent boundaries Several metamorphic environments exist here Important site for magma generation
Metamorphism & Plate Tectonics
Metamorphism & Plate Tectonics
Decompression melting
Blueschist facies
Migmatites
High P deformation
Mountain Building
Ancient MetamorphismAncient MetamorphismCanadian shield
AKA craton
Homework Assignment #1Homework Assignment #1 Investigate rocks in courtyard
Determine if igneous, sedimentary, metamorphic
Give five observations to support choice
Investigate rocks in courtyard
Determine if igneous, sedimentary, metamorphic
Give five observations to support choice
F-A
F-B
F-C
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