Transcript Document

VII. Metamorphic Rocks
A.
B.
C.
D.
E.
Evidence of metamorphism
The ingredients of metamorphism
Prograde metamorphism of shale
Classification of Metamorphic Rocks
Metamorphism and Plate Tectonics
The
Rock Cycle

Geological Materials
Transformation Processes

Rock
Metamorphism
(Increased T & P)

Metamorphic
rock
Partial Melting

Magma
Metamorphism
Metamorphic Rocks
Definition:
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Re-crystallization of
minerals
While still solid
Into minerals that are
stable at different
temperatures and
Different pressures
(Usually progressively
higher temperatures and
pressures)
A. Evidence of Metamorphism
Shale
E.g., Sedimentary Rocks
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Made of minerals derived
from weathering of a parent
rock
Stable at atmospheric
temperatures and pressures
(low T & P)
Originally horizontal,
continuous and uniform
layers
1. Bent (deformed) layers
Gneiss
Evidence of Metamorphism

2. Flattened Pebbles
Conglomerate
Differential
pressure
“squashes” rock
and included
features
Metaconglomerate
Evidence of Metamorphism
Quartz Sandstone
Quartzite

3. Crystalline Texture
Minerals tightly interlocking
due to recrystallization under
pressure
Evidence of Metamorphism
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4. New mineral
assemblages
E.g., Shale: Clay minerals
(some quartz) 
Metamorphism
(Mid-grade)

Forms Schist: Mica,
Feldspar and other silicate
minerals 
B. Recipe of Metamorphism
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1. Parent Rock
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Even though
minerals will change
Most elements are
provided by parent
rock
Except water and
some dissolved ions
Shale
Schist
B. Ingredients of Metamorphism
A
C
B
Temp. (ºC)
0 600 1200
A B C
See Kehew,
Fig. 5-1, 5-2
 2. Increased Temperature (geothermal gradients)
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Minerals stable at lower temperatures converted to
minerals stable at higher temperatures
Solid state chemical reactions are accelerated
Ingredients of Metamorphism
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3. Increased Pressure (and stresses)
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Increased Confining Pressure as rocks are buried
Compression at convergent plate boundary or
Sheared as plates slide past each other
Results of Stress
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Cause Foliation
Compressive Stress
Shear Stress
See Kehew,
Fig. 5-8
Ingredients of Metamorphism
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4. Addition or removal of fluids (and elements)
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Water (and other fluids) within rocks and minerals
Moving during metamorphism
Accelerates solid-state chemical reactions and
May change rock composition
5. Time  Millions of years!
C. Prograde Metamorphism of
Shale (and the classification of
metamorphic rocks)
Shale: Fine grained Clay
(and quartz)
 Minerals stable under low
T&P (atmospheric)
 Compaction due to
accumulation of sediment
 Fissility along laminations
 Water bound in crystalline
structure of clay
Prograde Metamorphism of
Shale
1: Low Grade
MetamorphismSlate
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Fine grained Mica, (chlorite
and Quartz)
Low grade metamorphic T&P
(Water is expelled from crystalline
structure of clay)
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Minerals stable under low grade metamorphic conditions
Slaty Cleavage due to realignment of platy minerals
Prograde Metamorphism of
Shale
2: Medium-Grade
Metamorphism  Phillite
 Courser grained Mica and
quartz
 Medium-grade
metamorphic T&P Causes
minerals to grow

Slaty cleavage becomes rippled and
rock has a sheen
Prograde Metamorphism of
Shale
3: Medium to High-Grade
Metamorphism  Schist
 Course grained Mica and
Quartz
 Medium-grade
metamorphic T&P Causes
minerals to grow
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Minerals stable under medium grade metamorphic
conditions appear: Garnet, amphibole and biotite
Schistosity due to alignment of platy and needle
likeminerals
Prograde Metamorphism of
Shale
4: High-Grade
Metamorphism  Gneiss
 Course grained Feldspar,
Quartz, Amphibole,
Biotite
 High-grade metamorphic
Fig 7.12
T&P Causes minerals to
separate into bands
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Minerals stable under high grade metamorphic
conditions appear: feldspar
Gneissic banding bands of dark and light minerals
Prograde Metamorphism of
Shale
5: Very High-Grade
Metamorphism and
partial melting
Migmatite
 Silica rich minerals melt
first (quartz and
feldspar)
 Forming silicic magma
 Injected into fractures
resulting in silicic veins
if intrusive igneous rock
Fig 7.12
Metamorphic Rocks of other
Parent Rocks
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Limestone
 Marble
Nonfoliated
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Bioclastic calcite

Crystalline calcite
Metamorphic Rocks of other
Parent Rocks
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Quartz Sandstone  Quartzite
Nonfoliated
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Granular quartz  Crystalline quartz
Metamorphic Rocks of other
Parent Rocks
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Basalt
Granite
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Uniform texture
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Amphibole Schist
Gneiss
Foliation (schistosity and
gneissic banding)
Prograde Metamorphism in
Mountain Belts
Regional
Metamorphism
Northeastern
North America
Kehew,
Fig. 5-5
Metamorphic
Grade
High
Northwestern
Africa
Low
Reading the Grand Canyon
The Grand Canyon
Fig. 8.16
The Grand Staircase
5. Coconino Sandstone
4. Redwall Limestone
Sedimentary Rocks
Intrusive Igneous
Metamorphic
3. Bright Angel Shale
2. Grand Canyon
Pink Granite
1. Vishnu Schist (and gneiss)
The Grand Staircase
Fig. 8.16
Eastern Zion National Park
Checkerboard Butte
Zion National Park
Vishnu Schist and Pink Granite