Classroom presentations to accompany Understanding Earth, 3rd edition

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Classroom presentations to accompany
Understanding Earth, 3rd edition

• prepared by
• Peter Copeland and William Dupré
• University of Houston

Chapter 8 Metamorphic Rocks
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Metamorphic Rocks
Gregory G. Dimijian/Photo Researchers
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Metamorphic rocks

• Metamorphism to change form
• Metamorphic rock any rock (sedimentary,
  volcanic, plutonic, metamorphic) that has
  undergone changes in texture or mineralogical
  composition in the solid state.

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No single mode of origin

• Metamorphic rocks result from the partial or
  complete recrystallization of minerals in rocks
  over long periods of time.
• Rocks remain essentially solid during
  metamorphism.

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Metamorphic processes

• HEAT stability region of mineral sensitive to
  T. With increased T, pore fluid decreases.
• PRESSURE greater pressure tends to decrease
  space available metamorphic mineral tend to be
  dense. Increased P can come from any directed
  stress (burial). Stress will bring about a
  preferred orientation of minerals.
• FLUIDS (H2O) acts as a catalyst during
  metamorphism aids the exchange of ions between
  growing crystals.

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Metamorphic grade

• Refers to the intensity of
• metamorphism.
• High grade high T, P
• Low grade low T, P

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Pressure and Temperature vs. Depth
Fig. 8.1
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Metamorphic Grade
Fig. 8.2
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Types of metamorphism

• Regional Widespread changes in temperature and
  pressure bring about changes in rocks due to
  tectonic forces.
• Contact Intrusion of magma against colder rocks
  (affected area is proportional to the size and
  temperature of the intrusion but always only a
  local phenomenon).
• Deformational Changes in rocks associated with
  faulting and folding (regional or local).

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Types of metamorphism (cont.)

• Burial Changes in a rock due to the gradual
  changes in T and P due to successive burial
  (regional).
• Impact Changes due to rapid increase in
  pressure (only localized).

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Augen Mylonitic Gneiss
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Plate Tectonics and Metamorphism
Fig. 8.3
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Metamorphic reactions

• Mineralogical changes (e.g., clay to mica) Many
  complicated reactions depend on pressure,
  temperature, composition.
• Common metamorphic minerals include amphiboles,
  garnet, mica, staurolite, and kyanite.
• Textural changes recrystallization (grain
• boundaries more compact) and foliation
  (preferred orientation of minerals).

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How much can a rock change?

• The amount of change during
• metamorphism depends on
• ? grade of metamorphism
• ? duration of metamorphism
• ? composition of the rock

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Changes in texture

• Grain size
• Recrystallization
• Mineral size can either decrease or increase.
• Orientation of minerals
• Recrystallization
• Directed stress will orient minerals
• Lineation
• Foliation

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Slate with Foliation and Relict Bedding
Fig. 8.4
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Metamorphic Foliation
Fig. 8.5a
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Metamorphic Foliation
S. Dobos
Fig. 8.5b
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Direction of Compressive Forces
S. Dobos
Fig. 8.5b
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Slaty Cleavage
Fig. 8.6
Martin Miller
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Classification of Metamorphic Rocks Based on
Texture Table 8.1
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Classification of Foliated Rocks
Fig. 8.7
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Slate
Fig. 8.8a
Andrew J. Martinez/Photo Resrachers
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Schist
Fig. 8.8b
Biophoto Associates/Photo Researchers
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Schist in Thin Section
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Gneiss
Breck P. Kent
Fig. 8.8c
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Quartzite
Fig. 8.9a
Breck P. Kent
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Quartzite in Thin Section
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Marble
Breck P. Kent
Fig. 8.9b
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Marble in Thin Section
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Garnet Porphyroblasts
Schist Matrix
Fig. 8.10
Chip Clark
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Stability of minerals

• Most minerals are stable over a relatively narrow
  range of P and T (e.g., ice unstable above 0C).
• The stability range of different minerals overlap
  and provide constraints on the metamorphic
  history of rocks.

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Metamorphic facies

• A given set of metamorphic conditions
• Each facies is characteristic of particular
  tectonic environments and will have certain
  minerals that are diagnostic of those conditions.
• Therefore, the minerals in a rock can be clues to
  the (P,T) history of the rock.

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Metamorphic reactions

• Prograde Mineral changes that take place
  during an increase in temperature.
• Retrograde Mineral changes that take place
  during an decrease in temperature.

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Mineralogic Changes in Metamorphosed Shales
Fig. 8.11a
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Mineralogic Changes in Metamorphosed Shales
Fig. 8.11b
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Mineralogic Changes in Metamorphosed Mafic Rocks
Fig. 8.12
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Metamorphic Facies
Fig. 8.13
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Classification of metamorphic rocks

• Based on the texture and composition of
• the rock
• Low grade Slate
• Phyllite
• Schist
• Gneiss
• High grade Migmatite

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Major Minerals of Metamorphic Facies from Parent
Rocks of Different Composition Table 8.2
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Contact Metamorphism of Limestones
Fig. 8.14
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Contact Metamorphism of Sandstones and Shales
Fig. 8.15
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Fig. 8.16