Metamorphic Rocks of North America

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Metamorphic Rocks of North America Powered By Docstoc
					  Metamorphism and metamorphic
         rocks (CH 7)

   Rocks created by heat, pressure and/or
   chemically reactive fluids
   Metamorphic rocks are produced from
      • Igneous rocks
      • Sedimentary rocks
      • Other metamorphic rocks

                                                                           The Rock Cycle

Metamorphic                                                 low-grade to high-grade
Rocks                                                       remains essentially solid
                                                            characterized by
of North                                                        1) Growth of new minerals
America                                                         2) Deformation of existing minerals
                                                                    – Change in shape
                                                                    – Change in orientation

                     Figure 7-3

         Growth of new minerals                         Deformation of rocks and minerals

                                  Mica garnet schist

                                                       Layers are folded
                                                       and broken

          Garnet crystal

          Agents of metamorphism                                                 Origin of pressure in metamorphism
     • The most important agent
     • Two sources of heat                                                         Confining pressure
         – Contact with magma                                                      (equally in all
         – Regional geothermal gradient                                            directions)
 Pressure (stress)                                                                 Differential stress
                                                                                   (unequal in different
     • Increases with depth
     • transports elements (ions) including Au and Ag
     • Mainly water (H2O) with other volatile

          Metamorphic textures                                                              Examples of foliation
Texture refers to the size, shape, and arrangement of grains within a rock

                                                                                       Parallel alignment of flattened mineral grains and
Foliation – planar arrangement of mineral grains or                                    pebbles
structural features within a rock                                                      Compositional banding
Foliation can form through:
        – Rotation of platy and/or elongated minerals
        – Recrystallization of minerals in the direction
          of preferred orientation
        – Changing the shape of equidimensional
          grains into elongated shapes that are aligned

                                                                                  Directed pressure (stress) effects on
    Development of foliation due                                                  minerals
     to directed pressure (stress)                                           Deformation
                                                                                                       Dissolution &
                                                                                                       of crystal

                                                                                                                       Deformation within crystal
                                                                                                                       Dissolution and precipitation
                                                            Gneiss                                                     of crystal

                                                                       Foliated textures
                                                             Slaty cleavage
                                                                 • Closely spaced planar surfaces along which rocks
                                                                   split, fine grained (shale is parent)
Foliation                                                    Schistosity
                                                                 • Platy minerals visible with unaided eye and
                                                                   exhibit a planar or layered structure
                                                                 • Rocks having this texture are referred to as
                                                             Gneissic texture
                                                                 • distinctive banded appearance

            Non-foliated textures                                Importance of parent rock

  Non layered appearance                                    Most metamorphic rocks have the same overall
  Typically composed of minerals that exhibit               chemical composition as the parent rock from which
  equidimensional crystals such as quartz and calcite       they formed

             Metamorphic rocks                                      Metamorphism settings
   Main groups based on whether or not rocks are foliated   Contact metamorphism
   Foliated rocks                                               • Occurs at contacts with magma
       • In this group, changing degree of metamorphism     Regional metamorphism
         leads to characteristic rock series
                                                                • Occurs during mountain building
       • (Shale) slate  phyllite  schist  gneiss
                                                                • Produces the greatest volume of metamorphic
       • (Basalt) greenschist  amphibolite                       rock
       • (Basalt) blueschist  eclogite
   Non-foliated rocks
       • Quartzite (sandstone)
       • Marble(limestone)
       • Hornfels (claystone)
       • Coal (peat)

                                                                  Very fine-grained
                                                                  Excellent rock cleavage
                                                                  low-grade metamorphism of shale, mudstone, or siltstone
                                                                  Gray to black color

Metamorphic rock series

                             Phyllite                                                      Schist
                                                                  Medium- to coarse-grained can id minerals present
  Glossy sheen and exhibits rock cleavage
                                                                  Platy minerals predominate
  metamorphism between slate and schist
  Platy minerals not large enough to be identified with the       To indicate composition, mineral names are used (such
  unaided eye                                                     as mica schist)
  fine crystals of muscovite
  and/or chlorite
                                                              • Varieties:
                                                              •Mica schist (biotite, muscovite)
                                                              •Greenschist (green chlorite)
                                                              •Blueschist (blue amphibole)

                         Gneiss                                                           Marble
    Medium- to coarse-grained                                       Metamorphosed limestone or dolostone
    Banded appearance                                               Non-foliated
    High-grade metamorphism                                         Composed essentially of calcite or dolomite crystals
    Often light-colored feldspar-rich layers with dark              Coarse, crystalline, variety of colors
    bands of ferromagnesian minerals                                Used as a decorative and monument stone, table top

                                                                 Microscopic picture

   Formed from sandstone                                            Metamorphic
   Quartz grains are fused together                                 rock

Microscopic picture

                                                                                 Contact metamorphism
         Metamorphic environments                                        Occurs when magma invades a host rock
                                                                         low P, high T
      Contact metamorphism (low P, high T)                               Gradient??
      Regional metamorphism
          • Type A: Pressure and Temperature both
            increase comparable
          • Type B: Pressure increases relatively faster than
            temperature (high P, low T metamorphism)

                                                                   Metamorphic Index Minerals
                      Regional Metamorphism
 Regional metamorphism
       • Produces the greatest quantity of
         metamorphic rock
       • Associated with mountain building and
         the subducting plate (high P, low T

                                                                                                                 Figure 7-19

  Metamorphic grade and index
          minerals                                   Metamorphic Facies

E.g., chlorite indicates low Grade (low P, low T)       Figure 7-21

garnet indicates high Grade (high P and/or high T)

               Metamorphic Rocks & Plate Tectonics
                                                         Metamorphic environments
                                                        associated with plate tectonics

 Figure 7-20

Metamorphism and plate tectonics
  Low-temperature, High-pressure
  zones nearest the trench
  High-temperature, High-pressure
  zones further inland in the region of
  igneous activity