What Is Retrograde Metamorphism

The process of altering the mineral composition of relatively high-grade metamorphic rocks at temperatures lower than those of their initial metamorphism is known as retrograde metamorphism (also known as diaphthoresis or retrogressive metamorphism).

Retrograde metamorphism has an origin.

  • Fluid Phase. Any open area between the mineral grains of a rock may be home to a fluid.

While primarily water, this liquid also has dissolved ions in it. The fluid phase is crucial because the movement of dissolved ions through the fluid can significantly speed up chemical reactions that entail converting one solid mineral into another. The process is known as metasomatism if the rock is chemically altered as a result of these fluids.

  • Time – Metamorphic transformation is a long process since it entails altering the rock while it is still solid. There are various processes active during metamorphism. The size and shape of minerals change as a result of recrystallization. In addition, new minerals are created as a result of polymorphic phase transformations, which are reactions between the minerals that result in new mineral sets that are more stable under the pressure and temperature of the environment. Polymorphs are compounds with the same chemical formula but different crystal structures.

According to laboratory research, the size of the mineral grains formed during metamorphism grows over time.

Thus, the metamorphism of rocks with coarse grains took place over a lengthy period of time.

According to experiments, it took tens of millions of years.

The temperature range for low-grade metamorphism is between 200 to 320oC.

and low pressure, respectively. Low grade metamorphic rocks exhibit an increased

a profusion of hydrous minerals (water-containing minerals, H2O,

in their crystalline composition).

  • Some illustrations of hydrous minerals found in low-grade metamorphic rocks include:
  • The process of high-grade metamorphism occurs at temperatures above 320C and

quite a bit of pressure Hydrous minerals appear when metamorphism grade increases.

become less hydrous as a result of losing water, and non-hydrous minerals are more prevalent.

Examples of non-hydrous and less hydrated minerals that make up high grade

metamorphic rocks

At the greatest grade of muscovite, a hydrous mineral that gradually dissolves

metamorphism

  • A hydrous mineral with exceptionally high metamorphism stability is biotite.
  • A non-hydrous mineral is pyroxene.
  • A non-hydrous mineral is garnet.

As temperatures and pressure drop as a result of tectonic or overlaying rock erosion

uplift, one may anticipate metamorphism to take the opposite course and finally bring back the

restoring rocks to their untainted state.

Retrograde refers to such a procedure.

metamorphism.

Retrograde metamorphism would not occur frequently if it were common.

Observe metamorphic rocks on the Earth’s surface.

Since metamorphic rocks are present,

Retrograde metamorphism does not seem to be widespread near the Earth’s surface.

These are some of the causes:

  • The slower the temperature drops, the slower the chemical processes are.
  • Prograde metamorphism is a process in which fluids like CO2 and H2O are

the fluids are required to create the hydrous minerals that are stable at

the outside of Earth.

  • In the presence of fluids, chemical reactions proceed more quickly, however if the fluids

are ejected during prograde metamorphosis, they won’t be able to accelerate

retrograde metamorphism responses

  • Foliated These were created under differential stress and feature a planar foliation due to the preferred orientation (alignment) of the minerals.

They are categorized by composition, grain size, and foliation type and contain a considerable quantity of sheet silicate (platy minerals).

  • Non-foliated These are made up of of equal minerals and lack any visible planar fabric or foliation. They crystallized under non-differential stress conditions. These are mostly categorized according to the minerals present or the protolith’s chemical makeup.
  • Slate – The development of fine-grained rocks causes slates to form at low metamorphic grades.

clay minerals and chlorite. These sheet silicates’ preferred orientation

causes the rock to break easily along the planes perpendicular to the sheet silicates, resulting

a slate-colored cleavage

You should be aware that in this instance, the maximum

the original bedding planes are stressed at an angle, causing the slatey cleavage

has grown away from the initial bedding at an angle.

Slates are useful for blackboards and shingles due to the almost flawless fracture along planes.

  • Fine, mica-rich rock called phyllite is the result of low- to medium-grade metamorphism. The clay minerals have re-crystallized into microscopic micas (biotite and muscovite) that have a satiny sheen in phyllites.

Between slate and schist is phyllite.

– As the grade increases, the size of the mineral grains tends to increase.

of transformation.

The rock eventually forms a near-planar foliation as a result of the

Silicate sheet orientation that is desired (mainly biotite and muscovite).

Agate and

However, feldspar granules do not exhibit a preferential orientation.

The planar irregularity

Schistosity refers to the foliation at this level.

Micas are not the only minerals found in schist.

Among them are minerals like –

Quartz,

Feldspars,

Kyanite,

Garnet,

both Staurolite and

Sillimanite.

Pophyroblasts are these non-mica minerals that have a grain size larger than the remainder of the rock.

Gneiss The stability of the sheet silicates decreases with metamorphic grade.

and the growth of minerals with dark colors, such as hornblende and pyroxene, begins. These gloomy

Throughout the rock, colorful minerals frequently segregate into discrete bands, providing the

gneissic banding in rock.

Because the minerals with dark colors tend to

Despite the fact that they create elongated crystals as opposed to sheet-like crystals,

ideal positioning with their long axes parallel to the largest differential

stress.

Granulite – During the most advanced metamorphic stages, all of the hydrous

There are few minerals exist that remain stable as minerals and sheet silicates do.

would demonstrate an inclination.

The final rock will have a granulitic composition.

akin to the phaneritic texture found in igneous rocks.

  • MigmatitesThe rock may start to melt and start to mix with the solids if the temperature hits the solidus temperature, which is the initial melting point.

These melts typically contain felsic material, leaving the mafic material metamorphic.

Prograde and retrograde metamorphism: what are they?

Increased heat and pressure cause a change in the mineral composition known as progressive metamorphism. While retrograde metamorphism, a rare event, is a shift in the mineral assemblage and its composition that takes place during uplift (the release of pressure) and cooling (the reduction of temperature) to rebuild a rock.

A retrograde mineral is what?

  • the original rock’s overall chemical makeup.
  • the level of pressure that metamorphism attained.
  • the degree of heat that metamorphism reaches.
  • any fluid phase’s make-up that existed during metamorphism.

When a rock is subjected to greater pressure and temperature, the mineral assemblage changes.

assuming the circumstances are maintained for, should indicate stable chemical equilibrium.

a sufficient amount of time for equilibrium to be reached.

after metamorphism

Most metamorphic rocks indicate an era of geologic time that typically spans considerable distances.

mineral assemblage that is in balance.

Consider this: In metamorphic rocks, when pressure and temperature

can both change during metamorphism, the presence of a divariant (F=2) would be the most plausible scenario.

a combination of phases.

It would be less probable for a monovariant assemblage (F=1) to occur.

and a stable equilibrium would be represented by an invariant assemblage (F=0).

It would be even less likely to occur given the difference in temperature and pressure.

Therefore, for F=2, C=P, the more phases there are in a rock, the more

The number of components will be equal to the common divariant assemblage.

If

P is

larger than C, one of three mineral assemblage scenarios is possible.

The assemblage is a non-equilibrium assemblage (perhaps as a result of

due to incomplete chemical processes or the development of retrograde minerals

while the metamorphic rock is cooling, being lifted, or being unroofed).

The assemblage depicts equilibrium that is either univariant or invariant, as

mentioned earlier.

If scenario (1) explains why there isn’t a correspondence with the

phase rule, it is typically possible to identify by closely examining the rock.

Reaction

Rock textures could be a sign of an incomplete reaction.

Popular retrograde

minerals that can withstand pressures and temperatures that are lower than the majority of the

The rock’s minerals may be recognized.

Then, these retrograde periods could

phase rule could be added to the number of phases being thought about, then

applicable exclusively to the known equilibrium phases.

(For instance, the existence

Occurrence chlorite in rocks with the amphibolite and granulite facies would suggest that the

Chlorite is a retrograde mineral that forms as a result of weathering; as such, it would not

be taken into account when implementing the phase rule.)

The second scenario is always conceivable, particularly if there are

This may be the case if the minerals are properly picked and retrograde minerals are not taken into account.

The phase rule specifies the number of components that must be picked.

so as to depict the very minimum required to create every potential rock phase.

Remember that the number of components is not just the number of components that are oxide.

or the number of elements listed in the rock’s chemical analysis.

Simply said,

Take a look at the primary metamorphic rock phases and remember that some ions

if the components can readily swap places in stable solutions, the number of components

frequently be lowered to 7 or 8.

For instance:

Based on the frequent substitution of Ti into (Si,Ti)O2,

most silicates include tetrahedral sites)

MgO is typically required since the compositions of Fe-Mg solid solutions are both

dependent on temperature and pressure (although these two factors can be mixed,

It would shave off one from the overall number of components.

Based on the generally used formula (Al,Fe+3)2O3,

Minerals have been found to substitute Fe+3 for Al+3Al.

H2O, which is often found in a fluid phase but is also a vital

element found in hydrous minerals.

Typically present in a fluid phase, CO2 is also an

significant carbonate mineral component.

If it is assumed that H2O and CO2 are always present and can be combined to generate hydrous

the number of components can be lowered to 5 or 6 and carbonate minerals.

So for a

two-variant assemblage There should be 5 or 6 separate mineral phases present, as expected.

if the assemblage is invariant, there could be up to 8 phases in a metamorphic rock.

This serves as the foundation for creating the AKF and ACF diagrams.

where the number of components has been decreased to four as previously described, by

Alkali feldspar and quartz are always possible assumptions.

However, you are

advised that the methodology above is not always universally applicable and that each rock

be taken into account on a case-by-case basis.

either prograde or progressive metamorphism

happens on the rock as the temperature and pressure rise.

pressure increases

A rock with a specific chemical composition and rising temperature is anticipated to go through

between its constituent minerals and any fluid, in a constant cycle of chemical reactions

phase to create several novel mineral assemblages that are stable at the higher temperatures

temperatures and pressures.

Here, we provide an example of how the mineral assemblages may

a hypothetical group of rocks undergo alteration, beginning with a low grade mineral assemblage as

represented in the following ACF diagram.

Because of the emergence of sillimanite and the absence of andalusite,

Simple reaction:

Muscovite and quartz could have interacted with chlorite to create biotite, cordierite, and

also fluid

Anthophyllite, quartz, and fluid would result from the breakdown of talc:

and hornblende would result from the reaction of quartz, actinolite, epidote (zoisite), and chlorite.

also fluid

Next, let’s raise the temperature and pressure to allow for the formation of a fresh batch of minerals.

for each rock, grows.

At these novel pressure and temperature levels,

Plagioclase, sillimanite, cordierite, and quartz would be found in pelitic rocks.

k-spar, too.

Plagioclase would be present in quartzo-feldspathic rocks,

cordierite, quartz, k-spar, and almandine/pyrope garnet.

In reference to the disappearance of Moscowite, we can say:

We can record the following for the appearance of wollastonite and the elimination of calcite:

Hyperstne and quartz would result from the breakdown of anthophyllite.

The following reaction must occur in order for the breakdown of biotite to produce hypersthene and k-spar:

have taken place:

Lastly, in regards to the synthesis of almandine/pyrope from cordierite and anthophyllite,

the following response might have taken place:

Retrograde metamorphism would be a frequent occurrence upon elevation and

Eventually, unroofing metamorphic rocks would recover to stable mineral compositions.

lower temperatures and pressures.

However, high-grade metamorphic rocks are widespread at

the Earth’s surface and typically only display modest retrograde minerals.

Three

Two of the factors that prevent retrograde metamorphosis involve the fluid phase.

Higher temperatures hasten chemical processes.

During

retrograde transformation The response thresholds are often breached to higher levels

temperature.

Diffusion rates increase and become more molecular at higher temperatures.

It takes more vibration to break chemical bonds.

So, when in prograde

Reaction times during metamorphosis are quicker.

On a rock, when the temperature drops, the

reaction boundaries are crossed at low temperatures, which causes the reaction to occur.

The pace is substantially slower.

Of course, how quickly the temperature is dropped will determine this.

It is undoubtedly

temperatures might possibly drop so slowly during uplift and unroofing,

that time would allow for the formation of retrograde mineral assemblages.

Moreover, another episode

prograde metamorphism, during which temperatures may rise once more.

Minerals that are stable at the increased temperature could begin to expand and cover the existing minerals.

that during the initial metamorphic phase formed at a higher temperature.

As we’ve just seen, prograde metamorphism involves a fluid phase.

pushed away by the devolatilization reactions.

As the pressure builds,

Rocks’ porosity likewise declines, therefore this fluid phase will probably be forced out of

the body of rock.

The formation of hydrous compounds is not possible without the fluid phase.

minerals and carbonates, as water and carbon dioxide are two essential

There could not be all the necessary ingredients for such reactions.

Chemical processes can also be catalyzed by the fluid phase.

Despite the fact that the net reactions can seem to be solid-solid reactions, there

possibly more complex.

For instance, the fluid phase might completely dissolve a mineral

a new mineral to precipitate in a different area of the rock, just as

occurs when sedimentary rocks are forming.

When the fluid phase is removed

Consequently, it won’t be available to catalyze the processes to during prograde metamorphism.

As pressure and temperature are reduced, the retrograde mineral assemblage is produced.

All metamorphic rocks will eventually undergo a transformation to an

a collection of minerals that can withstand the conditions found close to the Earth’s surface.

However, this procedure, known as weathering, takes place close to

the surface of the earth.

The following time, we’ll delve deeper into some of the elements that influence

chemical processes that take place during metamorphosis.

What sorts of metamorphosis are there?

occurs when magma interacts with a piece of rock that already exists. When this occurs, the magma fluid infiltrates the existing rocks and raises their temperature. Magma contact typically only affects a small area of 1 to 10 kilometers. Non-foliated (rocks with no cleavage) rocks like marble, quartzite, and hornfels are the result of contact metamorphism.

Isograd metamorphism: what is it?

A line depicting the first instance of a metamorphic mineral in a rock sample near the Earth’s surface is called a metamorphic isograd. The line on the map is generated by the junction of the erosional plane and what is actually an isograd surface.

Barrovian metamorphism: what is it?

The form of metamorphism affects the type of metamorphic zones in a terrane as well. This depends on the geodynamic (magmatic and tectonic) environment in which metamorphism occurred. A metamorphic facies series is a collection of metamorphic zones, with the Barrovian being the most prevalent (called after George Barrow who first mentioned it in 1912). Along the metamorphic gradient, the pressure and temperature in this group of zones gradually rise. Barrovian metamorphism occurs as a result of crustal thickening in an orogenic belt’s roots during regional metamorphism (under mountain chains). Pelitic rocks make it particularly simple to identify Barrovian zones. The Barrovian zone prograde sequence is as follows:

What distinguishes a prograde from a retrograde?

the direction in which an object spins with reference to its solar orbit. An object that spins in the same direction as its orbit is said to be prograde. An object that spins counterclockwise to the direction of its orbit is said to be retrograde.

The asteroid Bennu rotates in the opposite direction from Earth because it has a retrograde rotation and Earth has a prograde rotation.

What does prograde metamorphism mean?

Prograde and retrograde metamorphism are additional divisions of metamorphism. Prograde metamorphism is characterized by the transformation of mineral assemblages (paragenesis) under conditions of rising temperature and (often) pressure. These reactions include the loss of volatiles like water or carbon dioxide and are solid state dehydration processes. Rock that has undergone prograde metamorphism is characterized by the highest temperatures and pressures ever. When metamorphic rocks are brought back to the surface, they often do not alter any further.

The process of retrograde metamorphism includes reconstituting a rock through volatilization at lower temperatures (and typically lower pressures), which enables the mineral assemblages created during prograde metamorphism to return to those that are more stable under less extreme circumstances. Because the volatiles created during prograde metamorphism typically migrate out of the rock and are not accessible to recombine with the rock after cooling, this process is exceedingly uncommon. When cracks in the rock allow groundwater to enter the cooling rock through a channel, localized retrograde metamorphism may occur.

What types of rocks undergo prograde metamorphism?

ultramafic rock Depending on the concentration of enstatite and forsterite in the protolith, the starting assemblage for prograde metamorphism of diopside-free peridotite is serpentine in the low temperature form (chrysotile), brucite, and/or talc.