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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
metamorphism.

     High grade: high T, P
     Low grade: low T, P
                     Metamorphic
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
  forces.
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
 local).
       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
  minerals).
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
           Compressive
              Forces




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
                    Slate




Andrew J. Martinez/Photo Resrachers   Fig. 8.8a
                                        Schist




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




Breck P. Kent
                Fig. 8.8c
                Quartzite




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




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



                  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
                       Phyllite
                       Schist
                       Gneiss
  High grade           Migmatite
Major Minerals of Metamorphic Facies from Parent Rocks of Different
                      Composition: Table 8.2
    Contact
Metamorphism of
  Limestones




                  Fig. 8.14
      Contact
  Metamorphism of
Sandstones and Shales




                        Fig. 8.15
Fig. 8.16
Plate Tectonics and Metamorphism




                               Fig. 8.3

				
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