Metamorphic rocks

Metamorphism:

• Metamorphism is the change that takes place within a body of rock as a result of it being subjected to conditions that are different from those in which it formed.
• The rock is deeply buried beneath other rocks, where it is subjected to higher temperatures and pressures than those under which it formed.
• Metamorphic rocks typically have:
• different mineral assemblages
• different textures
from their parent rocks

• Most metamorphism results from the burial of igneous, sedimentary, or pre-existing
metamorphic to the point where they experience different pressures and
temperatures than those at which they formed

The main factors that control metamorphic processes are:

•The mineral composition of the parent rock
•The temperature at which metamorphism takes place
•The amount and type of pressure during metamorphism
•The types of fluids (mostly water) that are present during metamorphism
•The amount of time available for metamorphism

Parent Rock:

• The parent rock is the rock that exists before metamorphism starts.
• In most cases, this is sedimentary or igneous rock, but metamorphic rock that reaches the surface and is then reburied can also be considered a parent rock.
• The critical feature of the parent rock is its mineral composition because it is the stability of minerals that counts when metamorphism takes place

Temperature:

• The temperature that the rock is subjected to is a key variable in controlling the type of metamorphism that takes place
• All minerals are stable over a specific range of temperatures.
• EX. most clay minerals are only stable up to about 150° or 200°C; above that, they transform into micas.
• Some minerals will crystallize into different polymorphs (same composition, but different crystalline structure) depending on the temperature and pressure.

Pressure:

• Pressure is important in metamorphic processes for two main reasons.
• First, it has implications for mineral stability.
• Second, it has implications for the texture of metamorphic rocks
• Rocks that are subjected to very high confining pressures are typically denser than others because the mineral grains are squeezed together.

Fluids:

• Water is the main fluid present within rocks of the crust

The presence of water is important for two main reasons:
1- water facilitates the transfer of ions between minerals and within
minerals, and therefore increases the rates at which metamorphic
reactions take place.
2- water, especially hot water, can have elevated concentrations of
dissolved substances, and therefore it is an important medium for moving certain elements around within the crust

Time:

• Most metamorphic reactions take place at very slow rates.
• EX. the growth of new minerals within a rock during metamorphism has been estimated to be about 1 mm per million years.

Two main types of metamorphic rocks:

• Foliated:
  formed in an environment with either directed pressure
  or shear stress.

• Non Foliated:
  formed in an environment without directed pressure or
  relatively near the surface with very little pressure at all.

• When a rock is squeezed under directed pressure during metamorphism this
  can result in a textural change such that the minerals are elongated in the
  direction perpendicular to the main stress. This contributes to the formation
  of foliation.

• When a rock is both heated and squeezed during metamorphismthere is a
  likelihood that the new minerals will be forced to grow with their long axes
  perpendicular to the direction of squeezing.

The various types of foliated metamorphic rocks, listed in order of:

 the grade or intensity of metamorphism and
 the type of foliation
are slate, phyllite, schist, and gneiss

slate:

• is formed from the low-grade metamorphism of shale.
• has microscopic clay and mica crystals that have grown perpendicular to the
stress.
• Slate tends to break into flat sheets

Phyllite:

• is similar to slate, but has typically been heated to a higher temperature.
• the micas have grown larger and are visible as a sheen on the surface.
• Where slate is typically planar, phyllite can form in wavy layers.

Schist:

• the temperature has been hot enough so that individual mica crystals are visible, and other mineral crystals, such as quartz, feldspar, or garnet may also be visible.
• schist, and especially gneiss, can form from a variety of parent rocks, including mudrock, sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks.

Gneiss:

• In gneiss, the minerals may have separated into bands of different colours.
• Most gneiss has little or no mica because it forms at temperatures higher than those under which micas are stable.
• Schist and gneiss can be named on the basis of important minerals that are present.
• Ex. a schist derived from basalt is typically rich in the mineral chlorite, so we call it chlorite schist
• a gneiss that originated as basalt and is dominated by amphibole, is an amphibole gneiss or, more accurately, an amphibolite.

migmatite:

• If a rock is buried to a great depth and encounters temperatures that are close to its melting point, it will partially melt.
• The resulting rock includes both metamorphosed and igneous material.

Marble:

• Marble is metamorphosed limestone.
• When it forms, the calcite crystals tend to grow larger, and any sedimentary
textures and fossils that might have been present are destroyed.
• If the original limestone was pure calcite, then the marble will likely be white,
but if it had various impurities, such as clay, silica, or magnesium, the marble
could be “marbled” in appearance

Quartzite:

• Quartzite is metamorphosed sandstone .
• It is dominated by quartz, and in many cases, the original quartz grains of the
sandstone are welded together with additional silica.
• Most quartzite has some impurities with the quartz.

Hornfels:

• Hornfels forms during contact metamorphism of fine-grained rocks like
mudstone or volcanic rock .
• In some cases, hornfels has visible crystals of minerals like biotite or andalusite.
• If the hornfels formed in a situation without directed pressure, then these
minerals would be randomly orientated, not foliated as they would be if
formed with directed pressure

• All of the important processes of metamorphism can be directly related
to geological processes caused by plate tectonics: Regional metamorphism

• Most regional metamorphism takes place within continental crust.
• The potential for metamorphism is greatest in the roots of mountain ranges where there is a strong likelihood for burial of relatively young sedimentary rock to great depths

• All of the important processes of metamorphism can be directly related
to geological processes caused by plate tectonics: Regional metamorphism

• Most regional metamorphism takes place within continental crust.
• The potential for metamorphism is greatest in the roots of mountain ranges where there is a strong likelihood for burial of relatively young sedimentary rock to great depths

• regional metamorphism occurs when rocks are buried deep in the crust
at significant depths
• The greatest likelihood of attaining those depths, and then having the once-buried
rocks eventually exposed at the surface, is where mountain ranges existed and have
since been largely eroded away.
• As this happens typically at convergent plate boundaries, directed pressures can
be strong, and regionally altered rocks are almost always foliated.

Many different patterns of regional metamorphism exist, depending on
 Plate Tectonics and Metamorphism
 Regional Metamorphism
 parent rocks,
 the geothermal gradient,
 the depth of burial,
 the pressure regime,
 the amount of time available
• geologists tend to look at specific minerals within the rocks that are
indicative of different grades of metamorphism

• All of the important processes of metamorphism can be directly related
to geological processes caused by plate tectonics: Retrograde metamorphism

• At an oceanic spreading ridge, recently formed oceanic crust of gabbro and basalt
is slowly moving away from the plate boundary.
• Water creates a convective system where cold seawater is drawn into the crust
and then out again onto the sea floor near the ridge.
• The passage of this water through the oceanic crust at 200° to 300°C promotes
metamorphic reactions that change the original pyroxene in the rock to chlorite
and serpentine.
• Because this metamorphism takes place at temperatures well below the temperature at which the rock originally formed (~1200°C),

• rock that forms in this way is known as greenstone if it isn’t
foliated, or greenschist if it is

• At a subduction zone, oceanic crust is forced down into the hot mantle. But because the oceanic crust is now relatively cool, especially along its sea-floor upper surface, it does not heat up quickly, and the subducting rock remains several hundreds of degrees cooler than the surrounding mantle
• A special type of metamorphism takes place under these very high-pressure but relatively low- temperature conditions, producing an amphibole mineral known as glaucophane (which is blue in colour, and is a major component of a rock known as blueschist (rare to find).

Contact Metamorphism and Hydrothermal Processes:

• Contact metamorphism takes place where a body of magma intrudes into the upper part of the crust
• Any type of magma body can lead to contact metamorphism, from a thin dyke to a large stock
• The type and intensity of the metamorphism, and width of the metamorphic aureole will depend on a number of factors, including:
 Plate Tectonics and Metamorphism
 Contact Metamorphism and Hydrothermal Processes
 the type of country rock,
 the temperature of the intruding body
 the size of the body

Contact Metamorphism and Hydrothermal Processes 2:

Contact metamorphic aureoles are typically quite small, from just a few centimetres around small dykes and sills, to as much as 100 m around a large stock.
• Contact metamorphism can take place over a wide range of temperatures — from around 300° to over 800°C — and of course the type of metamorphism, and new minerals formed, will vary accordingly.
• The nature of the country rock is also important. EX. Mudrock or volcanic rock will be
converted to hornfels. Limestone will be metamorphosed to marble, and sandstone to
quartzite.

Contact Metamorphism and Hydrothermal Processes 3:

• In many cases, water is released from the magma body as crystallization takes place, and this water is dispersed along fractures in the country rock.
• The water released from a magma chamber is typically rich in dissolved minerals. As this water cools, minerals are deposited, forming veins within the fractures in the country rock.
• EX. Quartz veins are common in this situation, and they might also include pyrite, hematite, calcite, and even silver and gold.

Contact Metamorphism and Hydrothermal Processes 4:

• Heat from the magma body will cause surrounding groundwater to expand and then rise toward the surface.
• Hot water circulating through the rocks can lead to significant changes in the mineralogy of the rock, including alteration of feldspars to clays, and deposition of quartz, calcite, and other minerals in fractures and other open spaces
• Metamorphism in which much of the change is derived from fluids passing through the rock is known as metasomatism
• When hot water contributes to changes in rocks, including mineral alteration and formation of veins, it is known as hydrothermal alteration.

Skarn:

• A special type of metasomatism takes place where a hot pluton intrudes into carbonate rock such as limestone. When magmatic fluids rich in silica, calcium, magnesium, iron resulting in the deposition of minerals as garnet, epidote (another silicate), magnetite, pyroxene, and a variety of copper and other minerals.