Metamorphic Rocks

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Metamorphic Rocks
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Metamorphism

• Literally translates to change of form
• In geology it refers to solid-state changes in
  mineral assemblages of a rock, and/or the texture
  of these minerals
• Due to changes in temperature and/or pressure

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Sources of Heat for Metamorphism

• Heat from Earths interior
• Geothermal gradient is the increase in
  temperature with depth
• Typical continental geothermal gradient is
  25-30C/km
• Volcanically active areas have geothermal
  gradients of 30-50C/km
• Oceanic trenches have geothermal gradients as low
  as5-10C/km

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Sources of Heat for Metamorphism

• Heat from magma
• Emplacement of magma chambers will add heat to
  the immediately surrounding rock
• Gabbroic magma 1300C
• Granitic magma 700C

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Pressure Associated with Metamorphism

• Lithostatic pressure the confining pressure
  created by the material that sits above a
  particular location. Lithostatic pressure is
  equal in all directions and compresses the volume
  of rock.
• Basalt 3 g/cm3 (3000 kg/m3)
• Granite 2.7 g/cm3 (2700 kg/m3)
• The lithostatic pressure at a 10 kmdepth is 3
  kbar 0.3 GPa

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Pressure Associated with Metamorphism

• Directed pressure pressure is imposed in a
  particular direction due to a regional stress
  field.
• Directed pressure affectsthe shape and
  arrangementof the  minerals
• Directed pressure varies with tectonic
  environment
• Compressional environments Horz gt Vert Pressure
• Extensional environments Vert gt Horz Pressure

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Types of Metamorphism

• Contact Metamorphism
• Thermal variation controls processes
• Regional Metamorphism
• Orogenic Metamorphism
• Combination of temperature and directed pressure
• Burial Metamorphism
• Combination of temperature and lithostatic
  pressure
• Fault-Zone Metamorphism
• Directed pressure controls processes (GEOL 41.1)

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Contact Metamorphism

• Occurs adjacent to igneous intrusions
• Temperature contrast between magma chamber and
  host rock
• Most evident in low-pressure (near-surface)
  environments

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Regional Metamorphism

• Increase in temperature is accompanied by an
  increase in pressure
• Usually there is directed pressure, so rock
  deformation increases with metamorphic grade

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Development of Foliations
Recrystallization
Pressure Solution
Rotation
Remobilization
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Metamorphic Zonesand Facies
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Mineral Zones of G.W. Barrow (1893)

• Barrow noted that pelitic rocks of the Scottish
  Highlands had distinct mineral zones (Gt, Ky,
  Sil)
• He concluded that this was the result of
  increasing metamorphic grade (T)
• Tilley (1925) added the low-grade Biotite and
  Chlorite zones
• Bt, Gt, St, Ky, and Sil are Index Minerals in
  metapelitic rocks

Excerpted From Gillen (1982) Metamorphic Geology.
An Introduction to Tectonic and Metamorphic
Processes.
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Index Mineral Isograds

• The line that defines the first appearance of an
  index mineral corresponds to a line of equal
  metamorphic grade
• Introduction of the concept of an isograd

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Eskola and Metamorphic Facies

• Eskola (1914, 1915) noted that metapelitic rocks
  in southern Finland (Orijärvi) contained the
  assemblage Bt-Ms whereas near Oslo, rocks
  contained the compositionally equivalent mineral
  assemblage Kf-Cd
• If rocks are the same composition, then the
  mineralogical difference must be due to a
  difference in physical conditions

2 KMg3AlSi3O10(OH)2 6 KAl2AlSi3O10(OH)2 15
SiO2 ? 3 Mg2Al4Si5O18 8 KAlSi3O8 8 H2O
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Eskola and Metamorphic Facies

• Eskola (1915) introduced the concept of
  metamorphic facies
• In any rock or metamorphic formation which has
  arrived at a chemical equilibrium through
  metamorphism at constant temperature and pressure
  conditions, the mineral composition is controlled
  only by the chemical composition.
• A metamorphic facies is a set of repeatedly
  associated metamorphic mineral assemblages
• If you find a specified mineral assemblage, then
  you can assign a metamorphic facies to the area,
  and thereby assign a range of pressure and
  temperature conditions.

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Eskola and Metamorphic Facies

• In 1920, Eskola introduced five metamorphic
  facies that were defined by mineral assemblages
  in metabasites
• Greenschist
• Amphibolite
• Hornfels
• Sanidinite
• Eclogite
• In 1939, Eskola added an additional 3 metamorphic
  facies
• Granulite
• Epidote-amphibolite
• Glaucophane-schist (now called Blueschist)
• In 1959 and 1960, Coombs added two additional
  metamorphic facies
• Zeolite
• Prehnite-Pumpellyite (now called Subgreenschist)

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Progressive Metamorphism of Shales
Increasing Temperature
Gneiss
Schist
Slate
Phyllite
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Non-Foliated Metamorphic Rocks