Retrograde Metamorphism

Prograde and Retrograde Metamorphism

  • In general, the changes in mineral assemblage and mineral composition that occur during burial and heating are referred to as prograde metamorphism. Whereas, those that occur during uplift and cooling of a rock represent retrograde metamorphism.
  • Prograde metamorphism is a change of mineral composition with increased heat and pressure.
  • Retrograde metamorphism is a change in mineral composition during uplift (releasing of pressure) and cooling (decreasing temperature) to reconstitute a rock, which is a rare process.
  • Retrograde metamorphism is covered in details later in this book.

Retrograde Metamorphism

The theory of Retrograde Metamorphism was proposed by Becke in 1909, and later elaborated by Harker.

  • As temperature and pressure decreases due to erosion of overlying rock or due to tectonic uplift, a reverse metamorphism path may be initiated. It eventually returns the rocks to the original unmetamorphosed state. This process is known as retrograde metamorphism.
  • Retrograde metamorphism can be considered as a reverse of prograde metamorphism. Retrograde reactions are usually very slow and may impact only some parts of the rock.

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
  • 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.

There are two factors that mitigate against complete retrogression

  • Efficient removal of water and carbon dioxide released.
  • Metamorphic reactions do not typically operate in reverse during cooling and reaction rates are increased by rising temperatures.

Given enough time, all metamorphic rocks will eventually undergo a change in the mineral composition (Prograde metamorphism or retrograde), under the atmospheric conditions present near the earth. This process, however, is called weathering, and occurs near the earth's surface.

Factors inhibiting retrograde metamorphism:

Only 3 factors prevent retrograde metamorphism, two of which involve the fluid phase.

  1. Fast chemical reactions at high temperatures
  • At higher temperature diffusion rates are higher and molecular vibration required to break chemical bonds is higher. 
  • Thus, during prograde metamorphism reaction rates are faster.  As temperature is lowered on a rock, the reaction boundaries are over-stepped to low temperature, and as a result the reaction rates are much slower.
  • Of course, this depends on the rate at which temperature is lowered.  It is certainly possible for temperatures to decrease at such a slow rate during uplift and unroofing, that retrograde mineral assemblages would have time to form. 
  • Also, a second episode of prograde metamorphism may occur during which temperatures are increased again.
  • Minerals stable at the new temperature may start to grow and overprint the minerals that were formed at higher temperature during the first episode of metamorphism.
  1. During the process of prograde metamorphism, a fluid phase vanishes as an outcome of the devolatilization reactions.
  • As pressure increases, porosity of rocks also decreases, and thus this fluid phase will likely be driven out of the rock body.
  • In the absence of the fluid phase it is impossible to form hydrous minerals and carbonates, since H2O and CO2, two of the key components needed in such reactions, may not be present.
  1. The fluid phase also helps to catalyze chemical reactions.
  • Although the net reactions may appear to be solid-solid reactions, in reality there may be more involved. 
  • For example the fluid phase could dissolve a mineral in one part of the rock and precipitate a new mineral in another part of the rock, just as happens during diagenesis of sedimentary rocks.
  • If the fluid phase is driven off during prograde metamorphism, then it will not be available to catalyze the reactions to produce the retrograde mineral assemblage as pressure and temperature are lowered.