Metamorphic Rocks

Pamela J. W. Gore
Georgia Perimeter College

Objectives

  1. List the agents of metamorphism and briefly describe the effects of each agent.
  2. Briefly describe the textural differences exhibited by metamorphic rocks (foliated, non-foliated).
  3. Describe the use of index minerals in the interpretation of metamorphic zones.
  4. Discuss the types of metamorphism.
  5. Discuss the relationship between metamorphism and plate tectonics.


Metamorphism means "changed form".

Agents of Metamorphism

Changes occur because of:

Rocks adjust to become more stable under new, higher temperatures and pressures.

  1. Heat
  2. There are several sources of heat for metamorphism.

    1. Geothermal gradient
      Temperature increases with depth at a rate of 20 - 30 degrees C per km in the crust.
      Ultimate source of the heat? Radioactive decay.

      Increase of temperature and pressure with depth causes Regional Metamorphism
      Heat may come from large bodies of molten rock rising under a wide geographic area.

    2. Intrusions of hot magma can bake rocks as it intrudes them. Lava flows can also bake rocks on the ground surface.

      Lava or magma in contact with other rock causes Contact Metamorphism.

      Hornfels is a common contact metamorphic rock.


      Contact metamorphism along a narrow (approx. 1 meter wide) diabase dike in the Deep River Basin of North Carolina. Diabase weathers tan. Contact metamorphic aureole rocks (hornfels) are gray. Host rocks are red siltstones.


  3. Pressure
    1. Burial Pressure. Pressure increases with depth due to the weight of the overlying rocks. A cubic foot of granite weighs 167.9 pounds. Increase of pressure and temperature with depth causes Regional Metamorphism.

      Regional metamorphism occurs at depths of 5 - 40 km.

    2. Tectonic pressures associated with convergent plate boundaries and continental collision also cause Regional Metamorphism.

    3. Pressure along fault zones causes Dynamic Metamorphism, the crushing and ductile flow of rock.

      Rocks formed along fault zones are called mylonites.

  4. Chemical Fluids
  5. In some metamorphic settings, new materials are introduced by the action of hydrothermal solutions (hot water with dissolved ions). Many metallic ore deposits form in this way.

    1. Hydrothermal solutions associated with magma bodies

    2. Black smokers - Sea water percolates through newly formed oceanic crust, dissolving out metallic sulfide minerals. The hot sea water rises along fractures and pours from vents in the seafloor as black clouds of dark mineral-rich water. Sulfide minerals (such as pyrite, sphalerite, and galena) and copper precipitate when the hot water comes in contact with cold sea water.


    How do rocks change?

    Metamorphism causes changes in:
    1. Texture
    2. Mineralogy

    Texture

    The processes of compaction and recrystallization change the texture of rocks during metamorphism.

    1. Compaction
      • The grains move closer together.
      • The rock becomes more dense.
      • Porosity is reduced.
      • Example: clay to shale to slate

    2. Recrystallization
      Growth of new crystals. No changes in overall chemistry. New crystals grow from the minerals already present.

      A preferred orientation of minerals commonly develops under applied pressure. Platy or sheet-like minerals such as muscovite and biotite become oriented perpendicular to the direction of force. This preferred orientation is called foliation.


    Metamorphic Textures


    The foliated metamorphic rocks

    As shale is subjected to increasing grade of metamorphism (increasing temperatures and pressures), it undergoes successive changes in texture associated with an increase in the size of the mica grains.

    1. Slate - very fine grained rock. Resembles shale. Has slaty cleavage which may be at an angle to the original bedding. Relict bedding may be seen on cleavage planes. Often dark gray in color. "Rings" when you strike it. (Unlike shale, which makes a dull sound. Temperature about 200 degrees C; Depth of burial about 10 km.


    2. Phyllite - fine-grained metamorphic rock. Has a frosted sheen, resembling frosted eye shadow. This is no coincidence. Cosmetics commonly contain ground up muscovite (ground to a size similar to that occurring naturally in phyllite.)


    3. Schist - metamorphic rock containing abundant obvious micas, several millimeters across. Several types of schist may be recognized, based on minerals which may be present:
      • mica schist
      • garnet schist
      • chlorite schist
      • kyanite schist
      • talc schist


    4. Gneiss - (pronounced "nice") - a banded or striped rock with alternating layers of dark and light minerals. The dark layers commonly contain biotite, and the light layers commonly contain quartz and feldspar.


    5. Migmatite - a very high grade metamorphic rock that has been subjected to such high temperatures that it has partially melted. It is intermediate between the metamorphic and the igneous rocks. Look for swirled banding. The light colored minerals have undergone melting and flow. The dark colored minerals have been contorted by flow. Example - the Lithonia Gneiss in the area southeast of Atlanta.


    The non-foliated (and weakly foliated) metamorphic rocks

    1. Marble - fizzes in acid because its dominant minerals is calcite (or dolomite). The parent rock is limestone (or dolostone).


    2. Quartzite - interlocking grains of quartz. Scratches glass. The rock fractures through the grains (rather than between the grains as it does in sandstone). The parent rock is quartz sandstone.


    3. Metagraywacke - metamorphosed graywacke or "dirty sandstone".

    4. Metaconglomerate and stretched pebble metaconglomerate - the parent rock is conglomerate. The clasts are fairly easily recognized. May be more difficult to recognize if the clasts have been stretched.

    5. Hornfels - A fine-grained, tough, dense, hard, massive rock. Usually (but not always) dark in color. Finer grained than basalt, which it may superficially resemble. This rock forms through contact metamorphism. The parent rock is commonly siltstone or basalt, but may be other types of rock.

    6. Serpentinite - A dark green, dense, tough, massive, hard rack. May contain veins of asbestos. The parent rock is peridotite, an ultramafic rock.


    7. Soapstone (sometimes called steatite) - a soft, easily carved rock with a slippery feel because it contains talc and chlorite. The parent rock is peridotite (ultramafic), probably with more water associated with it than in the formation of serpentinite. Example - Soapstone Ridge southeast of Atlanta.

    8. Metabasalt (sometimes called greenstone if massive and green, or greenschist if foliated and green) - the green color comes from chlorite (soft and bluish green) and epidote (pea green). The parent rock is basalt. The grade of metamorphism is LOW.

    9. Amphibolite - Abundant amphibole is present; may be lineated. Usually black. The parent rock is basalt. The grade of metamorphism is HIGH. Has been subjected to higher temperatures and pressures than metabasalt, greenstone, or greenschist.

    10. Mylonite - A dynamic metamorphic rock which forms along fault zones.


      Mylonite along the Linville Falls Fault, Linville Falls, NC. Relatively undeformed conglomeratic quartzite lies above the layered mylonite zone.


    Mineral changes in metamorphic rocks

    1. Recrystallization - rearrangement of crystal structure of existing minerals. Commonly many small crystals merge to form larger crystals, such as the clay in shale becoming micas in slate, phyllite, and schist.

      Note the chemical compositions of clay and muscovite.

      Also, fine-grained calcite in limestone recrystallizes to the coarse-grained calcite mosaic in marble.

    2. Formation of new minerals - there are a number of metamorphic minerals which form during metamorphism and are found exclusively (or almost exclusively) in metamorphic rocks:
      • Garnet - dark red dodecahedrons (12 sides)
      • Staurolite - brown lozenge-shaped minerals, commonly twinned to form "fairy crosses". State mineral of Georgia.

      • Kyanite - sky blue bladed minerals with differential hardness. Scratch lengthwise with a knife or nail, but not sideways.

      • Chlorite - dark bluish green, soft. Fe, Mg
      • Talc - white or pale green and soft.
      • Graphite - metamorphosed carbon
      • Tourmaline - commonly black. Forms elongated crystals with a rounded triangular cross-section. Can see at Stone Mountain.
      • Asbestos - fibrous mineral. Commonly light greenish. Occurs in veins (seems to fill a crack) with the fibers oriented perpendicular to the edged of the vein. Associated with lung diseases. Mesothelioma and asbestosis. Found in serpentinite. "Serpent rock" name due to snake-like veins of asbestos.
      • Micas - muscovite (silvery), biotite (dark brown), phlogopite (light brown)


    Metamorphic index minerals

    In regional metamorphic terranes, the temperature and pressure regime is indicated by the distribution of metamorphic minerals across a large area.

    Low metamorphic grade (low temperatures and pressures) - about 200 degrees C
    Slate and phyllite

    Intermediate metamorphic grade
    Schist

    High metamorphic grade - 800 degrees C (verging on melting)
    Gneiss and migmatite

    Know which minerals are the metamorphic index minerals. Know which ones are characteristic of low, intermediate, and high metamorphic grades.


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    Animated gif of regional metamorphism used with permission of Bruce E. Herbert, Texas A & M University, Big Bend Virtual Field Trip

    This page created by Pamela J. W. Gore
    Georgia Perimeter College, Clarkston, GA
    Pamela.Gore@gpc.edu

    October 23, 1995
    Modified May 6, 1997
    Modified December 12, 1998
    Last modified July 17, 1999
    Updated to facstaff, email updated September 7, 2008