GEOL3025, Section 096 Lecture #7 30 August 2007

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Transcript GEOL3025, Section 096 Lecture #7 30 August 2007

Chapter 8: Metamorphism &
Metamorphic Rocks
Introduction
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“Meta” = change & “morph” = form
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Metamorphic rocks are produced from
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Metamorphic = to change form
Transition of one rock into another by application of
pressure and/or temperature unlike those from which it
formed.
Sedimentary rocks
Igneous rocks
Other metamorphic rocks
Parent rock = protolith
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Rock from which the metamorphic rock was formed
Types of Metamorphism
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Progresses incrementally
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** Rock must remain solid!!
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Low-grade metamorphism (< 200oC)
High-grade metamorphism (> 600oC)
If melting occurs, then igneous rocks are formed
Agents of Metamorphism
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Heat
Pressure & differential stress
Chemically active fluids
Settings of Metamorphism
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Three settings:
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Thermal metamorphism
AKA contact metamorphism
 Intrusion of magma body
 Change driven by rise in temperature
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Hydrothermal metamorphism
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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
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Heat as Metamorphic Agent
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Most important agent
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b/c provides energy for chemical reactions &
recrystallization
Sources:
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Geothermal gradient
Contact metamorphism
Pressure & Differential Stress
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Confining
pressure 
compaction
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Equal pressure
in all
directions
Differential
stress =
directed
pressure
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Causes folds &
faults
Brittle vs. Ductile Behavior
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At surface, rocks are brittle
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Tend to fracture & break into smaller pieces
At high-T, rocks are ductile
Grains tend to
flatten and
elongate
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Chemically Active Fluids
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Mostly water (H2O) and carbon dioxide
(CO2)
Sources of fluids:
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Hydrated minerals
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e.g. Clays & amphiboles
Movement:
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Pore spaces of sedimentary rocks
Fractures in igneous rocks
Importance of Protolith
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Remember: protolith = parent rock
Most metamorphic rock have same chemical
composition of their parent rock
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Except for gain or loss of volatiles
e.g., H2O & CO2
Metamorphic Textures
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Texture = size, shape & orientation (arrangement)
of grains in rock
Foliation = any planar arrangement of mineral
grains or structural features within a rock
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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 Elongated Pebbles
Foliation:
Compositional Banding
Granite protolith
resultant Gneiss with compositional banding
Foliation:
Slaty Cleavage
Foliated Textures
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Rock cleavage (AKA slaty cleavage)
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Schistosity
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Closely spaced planar surfaces along which rocks split
e.g., slate (originally shale)
Platy minerals (e.g., micas) are discernible with unaided
eye
Exhibit planar or layered structure
e.g., schist (originally slate)
Gneissic banding
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During higher grades of metamorphism, ion migration
results in separation of light and dark minerals
Exhibit distinctive light & dark compositional banding
Formation of Slate
Garnet – Mica
Schist
Gneissic Banding
Other Metamorphic Textures
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Nonfoliated
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Form in environments where deformation is
minimal
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e.g., fine-grained limestone + heat = marble
Porphyroblastic texture
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Large grains surrounded by smaller grains
Porphyroblast = large grains
Matrix = fine-grains around porphyroblast
Common Metamorphic Rocks
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Foliated Rocks:
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Slate
Phyllite
Schist
Gneiss
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Nonfoliated Rocks:
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Marble
Quartzite
Slate
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Very fine-grained
Excellent rock cleavage
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Slaty cleavage
Most often generated from low-grade
metamorphism of shale, mudstone or
siltstone
Different colors:
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Black = carbonaceous
Red = Fe-oxide
Green = chlorite
Phyllite
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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
Slate
minerals such as micas &
chlorite
Phyllite
Schist
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Medium- to coarse-grained
Medium-grade
metamorpism
Platy minerals dominate
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Mainly micas
Schistositic texture
To indicate composition,
mineral names are used
e.g., mica schist 
Gneiss
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Medium- to coarsegrained
Banded layered
appearance
High-grade
metamorphism
Often composed of white
or light-colored feldsparrich layers with bands of
dark ferromagnesian-rich
layers
Marble
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Coarse-grained
Crystalline
Protolith = limestone
Composed of essentially
calcite [CaCO3] and/or
dolomite [CaMg (CO3)2]
crystals
Exhibits a variety of
colors
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Used as decorative &
monument stone
Quartzite
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Medium- to coarse-grained
Crystalline
Protolith = sandstone
Composed dominately of
quartz [SiO2]
Classification of Metamorphic Rocks
Metamorphic Environments
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Contact Metamorphism
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AKA Thermal Metamorphism
Hydrothermal Metamorphism
Regional Metamorphism
Other Metamorphic Environments
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Burial Metamorphism
Metamorphism along fault zones
Impact metamorphism
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AKA Shock Metamorphism
Contact Metamorphism
Hydrothermal Metamorphism
Regional Metamorphism
Burial Metamorphism
Fault Zone Metamorphism
Impact Metamorphism
Metamorphic Zones
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Metamorphic Grade:
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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 highgrade metamorphism
Index Minerals:
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Certain minerals which are good indicators of
the degree of the metamorphic conditions under
which they form
e.g., chlorite ~200oC
Progressive Regional Metamorphism
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With increasing P & T, higher degree of
metamorphism
Index Minerals
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Typical transition in mineralogy due to
progressive metamorphism of shale
Metamorphic Zones in
New England, USA
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Grades
concentric
with highgrade in center
of mountain
range
Migmatites
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Highest grade metamorphic rock
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Transitional to Igneous rock
Partial melting of felsic minerals
Contain light bands of
igneous components
along with dark
metamorphic rock
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Fault Zone Metamorphism
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Fault breccia
at and near
surface
Mylonite
(AKA
migmatite)
forms at
depth
Metamorphism & Plate Tectonics
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Most metamorphism occurs along convergent
boundaries
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Compressional stresses deform edges of plates
Formation of Earth’s major mountain belts
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Mountainous terrains along subduction zones contain
linear belts of metamorphic rocks
High P, low T rocks nearest the trench
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e.g. blueschist facies (glaucophane = index min)
High T, low P zones further inland
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e.g. Alps, Himalayas, Appalachians
In region of igneous activity
Large-scale metamorphism also occurs along subduction
zone at convergent boundaries
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Several metamorphic environments exist here
Important site for magma generation
Metamorphism & Plate Tectonics
High P
deformation
Mountain Building
Migmatites
Decompression
melting
Blueschist facies
Ancient Metamorphism
Canadian shield
AKA craton
Homework Assignment #1
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Investigate rocks in courtyard
F-A
2 3,4 5
1
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F-C
F-B
6
7
Determine if igneous, sedimentary,
metamorphic
Give five observations to support choice