metamorphic rocks

Metamorphism and Metamorphic Rocks

 
Contents of Entire Course

Definition of Metamorphism   
Grade of Metamorphism
Retrograde Metamorphism
Factors that Control Metamorphism
Responses of Rock to Increasing Metamorphic Grade
Metamorphism of Basalts and Gabbros
Metamorphism of Limestone and Sandstone
Types of Metamorphism  
Metamorphic Facies
Metamorphism and Plate Tectonics

adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane University


Definition of Metamorphism

The word "Metamorphism" comes from the Greek:  Meta = change, Morph = form, so metamorphism means to change form.  In geology this refers to the changes in mineral assemblage and texture that result from subjecting a rock to pressures and temperatures different from those under which the rock originally formed.

  • Note that Diagenesis is also a change in form that occurs in sedimentary rocks.  In geology, however, we restrict diagenetic processes to those which occur at temperatures below 200oC and pressures below about 300 MPa (MPa stands for Mega Pascals), this is equivalent to about 3000 atmospheres of pressure.

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  • Metamorphism, therefore occurs at temperatures and pressures higher than 200oC and 300 MPa.  Rocks can be subjected to these higher temperatures and pressures as rocks become buried deeper in the Earth.  Such burial usually takes place as a result of tectonic processes such as continental collisions or subduction.

  • The upper limit of metamorphism occurs at the pressure and temperature of wet partial melting of the rock in question.  Once melting begins, the process changes to an igneous process rather than a metamorphic process.


    Grade of Metamorphism

    metagrade.gif

    As the temperature and/or  pressure increases on a body of rock we say the rock undergoes prograde metamorphism or that the grade of metamorphism increases.   Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form.
     
  • Low-grade metamorphism takes place at temperatures between about 200 to 320oC, and relatively low pressure.  Low grade metamorphic rocks are characterized by an abundance of hydrous minerals, minerals that contain water, H2O, in their crystal structure.

  • Examples of hydrous minerals that occur in low grade metamorphic rocks:

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  • Clay Minerals

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  • Serpentine

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  • Chlorite

  • High-grade metamorphism takes place at temperatures greater than 320oC and relatively high pressure.  As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H2O and non-hydrous minerals become more common.
  • Examples of less hydrous minerals and non-hydrous minerals that characterize high grade metamorphic rocks:


  • Retrograde Metamorphism

    As temperature and pressure fall due to erosion of overlying rock or due to tectonic uplift, one might expect metamorphism to a follow a reverse path and eventually return the rocks to their original unmetamorphosed state.  Such a process is referred to as retrograde metamorphism.  If retrograde metamorphism were common, we would not commonly see metamorphic rocks at the surface of the Earth.  Since we do see metamorphic rocks exposed at the Earth's surface retrograde metamorphism does not appear to be common.  The reasons for this include:

  • chemical reactions take place more slowly as temperature is decreased

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  • during prograde metamorphism, fluids such as H2O and CO2 are driven off, and these fluids are necessary to form the hydrous minerals that are stable at the Earth's surface.
  • Chemical reactions take place more rapidly in the presence of fluids, but if the fluids are driven off during prograde metamorphism, they will not be available to speed up reactions during retrograde metamorphism.


    Factors that Control Metamorphism

    Metamorphism occurs because some minerals are stable only under certain conditions of pressure and temperature.  When pressure and temperature change, chemical reactions occur to cause the minerals in the rock to change to an assemblage that is stable at the new pressure and temperature conditions.  But, the process is complicated by such things as how the pressure is applied, the time over which the rock is subjected to the higher pressure and temperature, and whether or not there is a fluid phase present during metamorphism.

  • Temperature

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  • Temperature increases with depth in the Earth along the Geothermal Gradient.  Thus higher temperature can occur by burial of rock.

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  • Temperature can also increase due to igneous intrusion.

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  • Pressure increases with depth of burial, thus, both pressure and temperature will vary with depth in the Earth.  Pressure is defined as a force acting equally from all directions.  It is a type of stress, called hydrostatic stress, or uniform stress.  If the stress is not equal from all directions, then the stress is called a differential stress

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    stress.gif




    These sheet silicates will grow with their sheets orientated perpendicular to the direction of maximum stress.  Preferred orientation of sheet silicates causes rocks to beeasily broken along approximately parallel sheets.  Such a structure is called a foliation.
    Responses of Rock to Increasing Metamorphic Grade
    Metamorphism of Basalts and Gabbros
    Metamorphism of Limestone and Sandstone
    Types of Metamorphism Contact Metamorphism
    - Occurs adjacent to igneous intrusions and results from high temperatures associated with the igneous intrusion.  Since only a small area surrounding the intrusion is heated by the magma, metamorphism is restricted to zone surrounding the intrusion, called a metamorphicaureole.  Outside of the contact aureole, the rocks are unmetamorphosed.  The grade of metamorphism increases in all directions toward the intrusion.  Because temperature differences between the surrounding rock and the intruded magma are larger at shallow levels in the crust, contact metamorphism is usually referred to as high temperature, low pressure metamorphism.  The rock produced is often a fine-grained rock that shows no foliation, called a hornfels Regional Metamorphism
    - This type of metamorphism occurs over large areas that were subjected to high degrees of deformation under differential stress.
    Thus, it usually results in forming metamorphic rocks that are strongly foliated, such as slates, schists, and gniesses.
    The differential stress usually results from tectonic forces that produce a compression of the rocks, such as when two continental masses collide with one another.
    Thus, regionally metamorphosed rocks occur in the cores of mountain ranges or in eroded mountain ranges.
    Compressive stresses result in folding of the rock, as shown below, and result in thickening of the crust   which tends to push rocks down to deeper levels where they are subjected to higher temperatures and pressures
    A map of a hypothetical regionally metamorphosed area is shown in the figure below.  Most regionally metamorphosed areas can be divided into zones where a particular mineral, called and index mineral,  is characteristic of the zone .  The zones are separated by lines (surfaces in three dimensions) that mark the first appearance of  the index mineral.  These lines are called isograds (meaning equal grade) and represent lines (really surfaces) where the grade of metamorphism is equal.
    Metamorphic Facies In general, metamorphic rocks do not change chemical composition much during metamorphism.  The changes in mineral assemblages are due to changes in the temperature and pressure conditions of metamorphism.  Thus the mineral assemblages that are observed must be an indication of the temperature and pressure environment that the rock was subjected to.  This pressure and temperature environment is referred to as metamorphic Facies.  (This is similar to the concept of  sedimentary facies, in that a sedimentary facies is also a set of environmental conditions present during deposition).
     
     
    The sequence of metamorphic facies observed in any metamorphic terrain, depends on the geothermal gradient that was present during metamorphism.&nbrp; A high geothermal gradient such as the one labeled "A"  in the figure shown here, might be present around an igneous intrusion, and would result in metamorphic rocks belonging to the hornfels facies.  Under a normal geothermal gradient, such as "B" in the figure, rocks would progress from zeolite facies to greenschist, amphibolite, and eclogite facies as the grade of metamorphism (or depth of burial) increased.
    If a low geothermal gradient was present, such the one labeled "C" in the diagram, then rocks would progress from zeolite facies to blueschist facies to eclogite facies.  Thus, if we know the facies of metamorphic rocks in the region, we can determine what the geothermal gradient must have been like at the time the metamorphism occurred.
    Metamorphism and Plate Tectonics

    At present, the geothermal gradients observed are strongly affected by plate tectonics. Along a subduction zone, relatively cool oceanic lithosphere is pushed down to great depths.  This results in producing a low geothermal gradient (temperature increases slowly with depth).  This low geothermal gradient ("C") in the diagram above, results in metamorphism into the blueschist and eclogite facies.