VII. Metamorphic Rocks A.Evidence of metamorphism B.The ingredients of metamorphism C.Prograde...
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Transcript of VII. Metamorphic Rocks A.Evidence of metamorphism B.The ingredients of metamorphism C.Prograde...
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
The Rock Cycle
Rock
Metamorphism (Increased T & P)
Meta- morphic rock
Partial Melting
Magma
Geological Materials
Transformation Processes
MetamorphismMetamorphic Rocks
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. Evidence of Metamorphism
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
Shale
Gneiss
Evidence of Metamorphism
Differential pressure “squashes” rock and included features
2. Flattened Pebbles
Conglomerate
Metaconglomerate
3. Crystalline Texture Minerals tightly interlocking
due to recrystallization under pressure
Evidence of Metamorphism
Quartz Sandstone
Quartzite
Evidence of Metamorphism
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
1. Parent Rock Even though
minerals will change Most elements are
provided by parent rock
Except water and some dissolved ions
Shale
Schist
B. Ingredients of Metamorphism
2. Increased Temperature (geothermal gradients) Minerals stable at lower temperatures converted to minerals stable at higher temperatures Solid state chemical reactions are accelerated
0 600 1200Temp. (ºC)
A B C
See Kehew,
Fig. 5-1, 5-2
A C B
Ingredients of Metamorphism
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
Results of Stress
Compressive Stress
Shear Stress
Cause Foliation
See Kehew,
Fig. 5-8
Ingredients of Metamorphism
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 MetamorphismSlate
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
Minerals stable under high grade metamorphic conditions appear: feldspar
Gneissic banding bands of dark and light minerals
Fig 7.12
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
Bioclastic calcite Crystalline calcite
Nonfoliated
Metamorphic Rocks of other Parent Rocks
Quartz Sandstone Quartzite
Granular quartz Crystalline quartz
Nonfoliated
Metamorphic Rocks of other Parent Rocks
Basalt Amphibole Schist Granite Gneiss
Uniform texture Foliation (schistosity and
gneissic banding)
Prograde Metamorphism in Mountain Belts
Regional Metamorphism
MetamorphicGrade
High
Low
NortheasternNorth America
NorthwesternAfrica
Kehew, Fig. 5-5
Reading the Grand CanyonReading the Grand Canyon
2. Grand Canyon Pink Granite
The Grand Canyon
Intrusive IgneousSedimentary Rocks
Metamorphic
1. Vishnu Schist (and gneiss)
3. Bright Angel Shale
4. Redwall Limestone
5. Coconino SandstoneThe Grand Staircase
Fig. 8.16
The Grand Staircase
Fig. 8.16
Eastern Zion National Park
Checkerboard Butte Zion National Park
Vishnu Schist and Pink Granite