METAMORPHISM

Metamorphism - Solid state changes in sedimentary or igneous rocks. Takes place within the crust and in response to the agents of metamorphism.

Agents of Metamorphism

1) Heat

a) frictional sliding of plates

b) radioactivity

c) gravitational compression

2) Pressure

a) burial (lithostatic)

b) directed pressure due to tectonism

3) Chemically Active Fluids

a) Water - circulates in response to heat generated by cooling magmas. Exchanges ions between the solution and the rock through which it is traveling.

Types of Metamorphism

Dynamic metamorphism - Metamorphism along faults zones in response to pressure. Involves a brittle deformation of the rock during which it is ground into fine particles. Heat and chemical fluids are less important. Most important rock is mylonite a very distinctive lineated rock.

Contact metamorphism - Alteration of rocks at or near the contact of a cooling pluton. Most important agents of metamorphism are heat and circulating fluids. Pressures usually less important, often in the range from 1-3 kilobars. Temperatures 300-800 degrees C. Produces a series of zones characterized by the presence of one or more diagnostic minerals.

Regional metamorphism - Occurs over a very large area in response to increased temperature and pressure. Circulating fluids are unimportant due to the great depth of regional metamorphism. Pressure seals pore space in the rocks and fluids can't circulate. A variation on regional metamorphism is burial metamorphism, the latter occurs solely in response to burial. Generally, regional metamorphism occurs in tectonically active areas (i.e. plate margins).

Classification of Metamorphic Rocks

Process oriented classification just as is that for the igneous rocks. The two dominant processes are regional and contact metamorphism.

Foliated - Contain linear or planar features. Form in response to active pressure during regional metamorphism. Foliation is not to be confused with the original sedimentary layering.

Slate - Fine grained, with nice rock cleavage. Cleavage due to the parallel orientation of the mica grains.

Phyllite - Well developed foliation. Grains slightly larger than those of a slate. Again composed of mica. Poorer rock cleavage.

Schist - Contains grains that can be seen by the eye. Still has noticeable foliation. May be most common of all metamorphic rocks.

Gneiss - Consists of alternating light (feldspar-quartz) and dark (amphibole-biotite) bands. Requires a higher degree of metamorphism.

Non-foliated - Show no evidence of foliation and are apt to form in a contact metamorphic environment where pressure is unimportant.

Marble - Recrystallized limestone (calcite).

Quartzite - Metamorphosed quartz sandstone.

Hornfels - "Spotted rock" due to the presence of large crystals in a fine-grained matrix.

Contact Metamorphism

Metamorphic aureole - Zone characterized by a certain mineral or assemblage of minerals which differ from those originally present in the protolith (starting material). Index Mineral - The mineral that characterizes each contact metamorphic zone. Isograd - line on a map that marks the first appearance of that mineral.

Figure: the Onawa Pluton in Maine. Note the various zones and index minerals. Several factors control this zoning.


Look at phase diagram for the Al2SiO5 polymorphs (Figure) and notice how the zoning reflects the stability of the various mineral phases. Why is there no sillimanite zone in the Onawa pluton? Answer - it didn't get hot enough adjacent to the pluton.


Look at the Marysville Pluton in Utah (Figure). What has happened here? Why is the zoning on the northwest side of the pluton different? What did we fail to take into consideration?


This creates a problem if a large area has undergone metamorphism as is the case in regional metamorphism. The chances that only a single rock type will be present over a wide area is small. Sometimes it works, such as in southern Vermont (Figure). Here we are looking at regional metamorphism of a single rock type, shale.


Regional Metamorphism

To attack complex regional metamorphism we obviously needed a different approach since index minerals often will not work. There would be one for each different starting rock type and the result would be so complex it would be difficult to interpret.

Characteristics of regional metamorphism:

1. Occurs over large areas (1000's of sq. miles)

2. Closely related to episodes of mountain building

3. Both temperature and pressure important

Metamorphic facies - An assemblage of minerals that reached equilibrium under a specific set of temperature and pressure conditions. Each facies named for a readily recognizable characteristic mineral or other feature. Remember that any one mineral does not have to be present, the facies is characterized by several different minerals (Figure).


  1. Zeolite - Transitional from sedimentary conditions. P 2-4 kb and T 200-300 degrees C.

  2. Greenschist - Low temperature and pressure facies of regional metamorphism. P 3-8 kb and T 300-500 degrees C. Characterized by the green minerals chlorite, epidote and actinolite.

  3. Amphibolite - Moderate to high temperature and low pressure regional metamorphic facies. P 3-8 kb and T 500-700 degrees C. Characterized by the presence of amphibole.

  4. Granulite - High temperature and low to moderate pressure regional metamorphic facies. P 3-12 kb and T >650 degrees C. Characterized by quartz, feldspar, same minerals in a granite, hence the name.

  5. Blueschist - Low temperature and high pressure metamorphic facies. Occurs only in areas of abnormally low geothermal gradients. P >4 kb and T 200-450 degrees C. Name from the blue mineral glaucophane. Common rock type on Catalina Island. Actually very rare in much of the world.

  6. Eclogite - Mantle rock, probably not a valid metamorphic facies. Requires P >10 kb and T from 350-750 degrees C.

Myoshira and the "Paired Metamorphic Belts" of Japan

Figure shows Myoshira's geologic map of Japan. Published in the early 1960's. Shows a series of paired belts with a low temperature, high pressure belt (oceanward) juxtaposed against a high temperature, low pressure belt (landward). This presented problems to geologists. What was this map showing>


(Figure) Myoshira was actually mapping the location of ancient subduction zones. High P low T belt marked the trench where plates were colliding, hence high pressure. Low temperature due to the cooling effect of seawater. Landward the rising plutons from the zone of partial melting caused the local high temperatures at relatively shallow depth. Oceanward facies is the blueschist and landward a combination amphibolite and granulite.