- The lowest grade of rock in prograde metamorphism is low-grade metamorphic rocks.
- Low-grade metamorphic rocks form at relatively low temperatures of 150-450°C.
- They contain abundant hydrous minerals like clay, serpentine, and chlorite.
- As temperature and pressure increase, rocks undergo higher grades of metamorphism.
- The progression from low to high metamorphic grade is called prograde metamorphism.
What is metamorphism and how does it work??
Metamorphism is the process by which rocks are transformed when subjected to high temperatures and pressures. Pre-existing rocks like sedimentary and igneous rocks undergo mineralogical, chemical and structural changes when metamorphosed, resulting in new metamorphic rocks.
Metamorphism typically occurs deep below the Earth’s surface where temperatures and pressures are elevated. As rocks get buried ever deeper, they encounter higher temperatures and pressures which drives metamorphic reactions.
There are two main types of metamorphism:
- Contact metamorphism – Caused by heat from nearby igneous intrusions. Occurs in a narrow zone.
- Regional metamorphism – Caused by burial and heating of rocks over a wide area. Progressive with depth.
Regional metamorphism leads to a complete gradation in metamorphic grade depending on the maximum temperature and pressure conditions. This progression from low grade to high grade metamorphism is known as prograde metamorphism.
What are the grades of metamorphic rocks??
Metamorphic rocks are classified into grades based on the temperatures and pressures they form at:
- Low grade – 150-450°C, relatively low pressure.
- Medium grade – 450-650°C, moderate pressure.
- High grade – 650-800°C, high pressure.
- Very high grade – Above 800°C, very high pressure.
This spectrum from low grade to high grade represents the prograde progression of metamorphic grade. Each grade is characterized by a typical mineral assemblage reflective of the P-T conditions.
What minerals characterize low-grade metamorphic rocks??
Low-grade metamorphic rocks form at relatively low temperatures of 150-450°C and pressures equivalent to burial depths of about 8-25 km.
They are characterized by an abundance of hydrous minerals which contain water in their mineral structure such as:
- Clay minerals – Formed by metamorphism of mudstones and shales. Examples are chlorite, kaolinite.
- Serpentine – Formed by metamorphism of magnesium-rich rocks like dunites.
- Chlorite – Formed by metamorphism of mafic igneous rocks and greywackes.
In addition to the hydrous minerals, low-grade metamorphic rocks can also contain minerals like quartz, calcite, hematite and epidote.
The presence of hydrous minerals indicates that low-grade rocks contain appreciable water. This water is released by reactions to produce higher grade mineral assemblages as metamorphism progresses.
How do low-grade metamorphic rocks form??
Low-grade metamorphic rocks form through the process of diagenesis whereby compaction, mild heating and chemical reactions produce slates and phyllites from shale precursors.
Slate – Formed by compaction and realignment of clay minerals in shale. Has slaty cleavage.
Phyllite – Formed by further heating of slate. Fine grained with silky luster.
With increased temperatures, metasomatism leads to growth of hydrous minerals like chlorite, serpentine and clay minerals in the original rock fabric.
For example, at 200-300°C, montmorillonite clay transforms to illite clay. At 300-400°C, chlorite begins forming from precursor clay minerals, ferro-magnesian minerals and quartz.
Low-grade regional metamorphism affects wide areas, transforming shale to slate/phyllite, greywacke to greenshist, basalt to greenstone and granite to meta-granite.
How do low-grade rocks progress in prograde metamorphism?
As burial depth increases in regional metamorphism, so do the temperature and pressure conditions. This pushes the metamorphic reactions into medium and high-grade mineral assemblages.
Some key reactions include:
- 350-500°C – Clay minerals transform to biotite micas, releasing water.
- 400-650°C – Chlorite breaks down releasing water, enabling growth of garnet + biotite.
- 500-700°C – Feldspar crystals grow in size. Muscovite mica forms.
- Above 650°C – Melting begins generating migmatites.
So low-grade rocks like phyllite and greenstone progressively metamorphose into schists and gneisses of amphibolite or granulite grade as temperature and pressure increases.
The prograde sequence represents increasing metamorphic grade through the low–>medium–>high grade spectrum.
What are some examples of low-grade metamorphic rocks?
Some common examples of low-grade metamorphic rocks include:
- Slate – Metamorphosed shale with well developed slaty cleavage.
- Phyllite – Fine grained metamorphic rock with silky sheen. Formed by low-grade metamorphism of mudstones.
- Schist – Medium grade metamorphic rock. Formed by metamorphism of basalt, greywacke or shale protoliths.
- Greenstone – Metamorphosed basalt containing green minerals like chlorite and epidote. Derived from oceanic crust.
- Marble – Metamorphosed limestone composed of calcite or dolomite.
- Quartzite – Metamorphosed sandstone dominated by quartz grains.
- Soapstone – Massive rock composed predominantly of talc. Formed by metamorphism of dolomitic rocks.
What are the main factors controlling metamorphic grade?
The two primary factors controlling metamorphic grade are:
- Temperature – Increases with deeper burial and proximity to heat sources like igneous intrusions or hot mantle rocks. Directly influences mineral stability.
- Pressure – Also increases with burial depth. At higher pressures, certain mineral reactions are facilitated or inhibited.
Other secondary factors like rock composition, strain/deformation and fluid composition also affect metamorphic reactions and mineral assemblages.
Higher temperatures and pressures are required to transform low-grade meta-sedimentary rocks into high-grade gneisses and schists. So metamorphic grade correlates strongly with the depth of burial.
How does metamorphic grade vary on a regional scale?
On a regional scale in areas of high heat flow, metamorphic grade progressively increases towards the center of the geothermal gradient. Thus isograds are concentric zones of equal metamorphic grade.
The lowest grade metamorphic rocks like slate and phyllite occur in the outermost isograds. Grade progressively increases inwards to schists, gneisses and migmatites at the core.
This reflects the prograde sequence of metamorphism as temperature and pressure increases towards the zone of peak regional metamorphism.
Uplift and erosion can expose this vertical section through the metamorphic pile as horizontally oriented isograds across the landscape.
What other geological processes are associated with regional metamorphism?
Regional metamorphism is associated with:
- Mountain building – Rocks buried to great depths in mountain belts undergo high-grade metamorphism.
- Magmatism – Heat from magmatic underplating metamorphoses the overlying rocks.
- Continental rifting – Upwelling mantle heat metamorphoses the crust.
So in convergent and divergent plate boundaries, metamorphism often accompanies crustal thickening, magmatism and mantle convection. The metamorphic belts record the P-T paths of burial, heating and exhumation.
What is the role of fluids in metamorphic reactions?
Fluids play several key roles:
- Transport heat – Fluids like water efficiently transfer heat to enable metamorphic reactions.
- Catalyze reactions – Fluids facilitate mineral transformations by aiding dissolution and precipitation.
- Change rock composition – Hydration and chemical exchange reactions with fluids alter bulk composition.
- Cause retrogression – Influx of external fluids during exhumation leads to retrograde metamorphism.
So the availability of fluids controls the physical and chemical environment for metamorphic changes to occur.
How do we determine metamorphic conditions from mineral assemblages?
Mineral assemblages in metamorphic rocks provide clues to determine the P-T conditions of formation:
- Indicator minerals – Minerals like garnet, staurolite, kyanite which only form at specific P-T.
- Mineral composition – Chemical zoning and elemental substitutions reflect changing P-T.
- Reaction textures – Textural equilibrium between minerals indicates metamorphic stage.
- Phase diagrams – To quantify mineral equilibria and derive P-T estimates.
So detailed petrological studies can fingerprint the progressive metamorphic history recorded in the rocks.
What other geological information helps determine metamorphic grade?
- Field relationships – Relative age and cross-cutting relations between rock units.
- Fabric analysis – Orientations of minerals and structures to determine deformational history.
- Radioisotopic dating – To determine the absolute timing of metamorphic events.
- Geophysical surveys – Imaging rock properties and structures at depth.
When integrated together, these observations enable geologists to establish the spatial distribution of metamorphic grade in a terrane as well as the temporal evolution of grade through time.
What are some remaining questions about metamorphic processes?
Some unresolved questions include:
- How is heat transferred through rocks and localized in specific areas?
- How do fluids migrate through compacted metamorphic rocks?
- What is the precise role of organic matter and microbiological activity?
- How will ultrahigh pressure metamorphic rocks be preserved back to surface?
- How can peak metamorphic conditions be reliably identified?
Further research on reaction mechanics, advective fluid transport and geochronology will provide fresh perspectives on these issues. Advanced imaging and modeling techniques will also aid visualization in 4D.
In summary, low-grade metamorphic rocks constituting the lowest metamorphic grade in regional prograde metamorphism are characterized by hydrous minerals like clays, chlorite and serpentine. Formed at relatively low temperatures and pressures, they represent the incipient stages of metamorphism before the higher grade transformations happen. A spectrum of processes involving heat, pressure, fluids and deformation collectively drives the prograde progression to progressively higher metamorphic grades