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					                           Metamorphic Rocks

Gregory G. Dimijian/Photo Researchers
          Metamorphic rocks

Metamorphic rock: any rock (sedimentary,
volcanic, plutonic, metamorphic) that has
undergone changes in texture or mineralogical
composition in the solid state due to increase in
heat and pressure
          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 etamorphism.
           Metamorphic Processes
HEAT: stability region of mineral sensitive to T.
 With increased T, minerals contain water or
 CO2 will break down and form new minerals .
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 and CO2): acts as a catalyst
  during metamorphism; aids the exchange of
  ions between growing crystals.
         Metamorphic Grade

Refers to the intensity of

     High grade: high T, P
     Low grade: low T, P
Normal Geothermal       Grade
Gradient (~30°/km)

                           Fig. 8.2
           Types of Metamorphism
Regional: Widespread changes in temperature and
  pressure bring about changes in rocks due to tectonic
Contact: Associated with the ―baking‖ of rocks in the
 vicinity of an intrusion (only a local phenomenon).
Deformational (or Dynamic): Changes in rocks
 associated with faulting and folding (regional or
       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 and intense increase in
  pressure (>106 atmospheres)) associated with
  meteorite impact.
Hydrothermal metamorphism – water heated by
 igneous intrusions can leach and change the
 mineralogy of ―country rocks.‖
 Changes in Texture during Metamorphism
Grain size
• Recrystallization
• Mineral size can either decrease or increase.
Orientation of minerals
• Recrystallization
• Directed stress will orient minerals:
      • Foliation
      • Lineation
   Metamorphic Reactions
Mineralogical changes (e.g., clay to mica): Many
 complicated reactions — depend on pressure,
 temperature, composition.
 Common metamorphic minerals include amphiboles,
 garnet, micas, staurolite, and kyanite.

Textural changes: recrystallization (grain boundaries
  more compact) and foliation (preferred orientation of
Slate with Foliation and Relict Bedding

                                          Fig. 8.4
Augen Mylonitic Gneiss
Metamorphic Foliation

                        Fig. 8.5a
  Metamorphic Foliation

S. Dobos

                          Fig. 8.5b
            Direction of

S. Dobos
                Fig. 8.5b
                Slaty Cleavage

Martin Miller                    Fig. 8.6
Classification of Metamorphic Rocks Based on
               Texture: Table 8.1
Classification of Foliated Rocks

                                   Fig. 8.7

Andrew J. Martinez/Photo Resrachers   Fig. 8.8a

Biophoto Associates/Photo Researchers            Fig. 8.8b
Schist in Thin Section

Breck P. Kent
                Fig. 8.8c

Breck P. Kent
                            Fig. 8.9a
Quartzite in Thin Section

Breck P. Kent
                         Fig. 8.9b
Marble in Thin Section

                  Schist Matrix
Chip Clark
                                  Fig. 8.10
              Stability of Minerals

• Most minerals are stable over a relatively narrow
  range of P and T (e.g., ice unstable above 0°C).

• The stability range of different minerals overlap
  and provide constraints on the metamorphic
  history of rocks.
                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.
            Metamorphic reactions

Prograde: Mineral changes that take   place during
  an increase in temperature.

Retrograde: Mineral changes that take place during an
  decrease in temperature.
Mineralogic Changes in Metamorphosed Shales

                                  Fig. 8.11a
Mineralogic Changes in Metamorphosed Shales

                                  Fig. 8.11b
Mineralogic Changes in Metamorphosed
             Mafic Rocks

                                  Fig. 8.12
Metamorphic Facies

                     Fig. 8.13
Classification of metamorphic rocks
Based on the texture and composition of
the rock:
  Low grade            Slate
  High grade           Migmatite
Major Minerals of Metamorphic Facies from Parent Rocks of Different
                      Composition: Table 8.2
Metamorphism of

                  Fig. 8.14
  Metamorphism of
Sandstones and Shales

                        Fig. 8.15
Fig. 8.16
Plate Tectonics and Metamorphism

                               Fig. 8.3

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