VII. Metamorphic Rocks
A.
Evidence of metamorphism
B.
The ingredients of metamorphism
C.
Prograde metamorphism of shale
D.
Classification of Metamorphic Rocks
E.
Metamorphism and Plate Tectonics

 Geological Materials
Transformation Processes
 Rock
Metamorphism
(Increased T & P)
 Metamorphic rock
Partial Melting
 Magma


Definition :
 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.
Shale
E.g., Sedimentary Rocks
 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

 2.
Flattened Pebbles
Conglomerate
Differential pressure
“squashes” rock and included features
Metaconglomerate

Quartz Sandstone
Quartzite
 3.
Crystalline Texture
Minerals tightly interlocking due to recrystallization under pressure

 4.
New mineral assemblages
E.g., Shale : Clay minerals
(some quartz) 
Metamorphism
(Mid-grade)
 Forms Schist : Mica,
Feldspar and other silicate minerals 

B.
 1.
Parent Rock
 Even though minerals will change
 Most elements are provided by parent rock
 Except water and some dissolved ions
Shale
Schist

B.
Temp. (ºC)
0 600 1200
A
C
B
A B C
See Kehew,
Fig. 5-1, 5-2
 2.
Increased Temperature (geothermal gradients)
 Minerals stable at lower temperatures converted to
 minerals stable at higher temperatures
 Solid state chemical reactions are accelerated

 3.
Increased Pressure (and stresses)
 Increased Confining Pressure as rocks are buried
 Compression at convergent plate boundary or
 Sheared as plates slide past each other

 Cause Foliation
 Compressive Stress
 Shear Stress
See Kehew,
Fig. 5-8

 4.
Addition or removal of fluids (and elements)
 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


Fine grained Mica, (chlorite and Quartz)
Low grade metamorphic T&P
(Water is expelled from crystalline structure of clay)


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


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
T&P Causes minerals to separate into bands
Fig 7.12


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
 Limestone  Marble
Nonfoliated
 Bioclastic calcite  Crystalline calcite

Metamorphic Rocks of other
Parent Rocks
 Quartz Sandstone  Quartzite
 Granular quartz  Crystalline quartz
Nonfoliated

Metamorphic Rocks of other
Parent Rocks


Basalt 
Granite 
Amphibole Schist
Gneiss
 Uniform texture  Foliation (schistosity and gneissic banding)

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