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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