Metamorphic rocks

When does metamorphism happen?

When a rock is subjected to an increase in temperature and pressure following progressive burial…

Another cause of metamorphism?

Contact with a magmatic intrusion

What appears during metamorphism?

Schistosity and foliation” + “new mineral associations at the expense of pre-existing minerals

Define a metamorphic rock.

Formed by the recrystallization… under the action of temperature and/or pressure.

Does metamorphism melt the rock

No, metamorphism occurs entirely in the solid state. without melting the rock

Two main metamorphism factors?

“Pressure” and “Temperature.”

What is hydrostatic pressure?

Exerted in an identical manner in all directions.

What are oriented pressures linked to

Rock cracking” and “circulation of fluids

Average geothermal gradient?

30°C/Km

What causes pressure to increase at depth?

“Generally the weight of the overlying rocks causes an increase in pressure at depth.

How can temperature affect minerals?

Temperature alters the crystal lattice of minerals.

Main modifications due to metamorphism?

Textural changes, chemical modifications, mineralogical changes.

Schistosity (definition).

“Preferential orientation of minerals whose flattening and elongation develop on the same plane.

Schistosity: how is the texture characterized?

A family of subparallel and regularly spaced sheets, resulting from recrystallization and stretching (flattening)

Examples of rocks with schistosity (given).

slate, schist.

Schistosity is specific to which grain sizes?

Specific to rocks with more or less fine or clayey grain sizes

Define foliation

Mineral segregations… newly formed mineral species concentrate along certain preferred planes

Foliation: what is the texture characterized by?

Alternation of beds of different mineralogy visible to the naked eye, most often micas.

Define granoblastic texture.

Mosaic of crystals of approximately equal size… without particular orientation.”

Examples for granoblastic texture

Cipolin, Quartzite, Corneal

Define lepidoblastic texture

Juxtaposition of crystals in sheets typically oriented micas

Examples for lepidoblastic texture

Schist, Micaschist

Define granolepidoblastic texture

Alternation of beds with a granoblastic texture and beds with a lepidoblastic texture

Example for granolepidoblastic texture

Gneiss

Chemical modifications are often due to what?

Additions or departures from fluids

What are polymorphic transformations?

Modifications of the structure of a mineral without changing its mineralogical composition

Give the Al₂SiO₅ polymorphs listed.

Andalusite, kyanite, sillimanite (general formula Al2SiO5)

What is paragenesis

New associations of minerals, called paragenesis.

Example reaction: muscovite + quartz → ?

Potassium feldspar + andalusite + H2O.

Example sequence: kaolinite + quartz →

Pyrophyllite → andalusite, kyanite

Which types are responsible for the majority of metamorphic rocks

Contact metamorphism and regional metamorphism and dynamo-metamorphism

Define contact metamorphism

Occurs in the surrounding rock in contact with intrusives

What happens when hot magma meets cold rocks?

Heat transfer and cooking of the surrounding rock at the edges

Structure of contact metamorphism rocks

Have a granoblastic structure

Flamanville halo: what do corneals contain?

Quartz, feldspar, mica, andalusite, cordierite

What does regional metamorphism form?

Large metamorphic regions, characteristic of many mountain range roots

Regional metamorphism is due to what

An increase in temperature and pressure… burial producing high temperatures controlled by the depth reached.

Texture of regional metamorphism rocks

schistose and foliation texture due to the influence of oriented pressures

With increasing temperature and pressure, what sequence is obtained?

Slate, schist, mica schist and gneiss.

Dynamo-metamorphism is due to what?

Due to the effect of pressure (intensely deformed rocks with relatively little new formation of minerals

What are these rocks called

Cataclasite rocks

Where does dynamo-metamorphism take place?

At the border between two rock blocks… in a fault zone or base of a thrust sheet

When does mylonite form?

In case the rock is stuck between the two blocks and ductile, it transforms into mylonite.

What is special about deformation into mylonite?

The rock never breaks, but its minerals rearrange themselves and allow it to deform

Mylonite: one property compared to gneiss

A hard rock that has foliation, like gneiss.

Slates: definition line (grain + origin)

Fine-grained, homogeneous clay shales… ancient clays having undergone very low intensity metamorphism.

Slates: possible coloring.

Gray, black, bluish or wine-colored.

Slates: role of schistosity

The schistosity allows them to be cleaved into thin plates

Slates: common recrystallizations

Small cubes of pyrite (iron sulphide)

Marbles: origin.

Resulting from the transformation of limestone or dolomites.

what happens to calcite elements?

Calcite elements are entirely recrystallized

Marble: effect of recrystallization

Gives greater hardness to the rock and promotes its polishing

Marble: why can color vary

Impurities, oxides, etc.

Schists: key property.

Capable of being cut into sheets (schistosity).

Many schists come from what?

Metamorphism of ancient clays.

Mica schists: what makes them shiny?

Rich in micas which give them a shiny appearance on their cleavage surface.

Schists: mineralogical composition

Mica, a little feldspar, sometimes quartz… staurolite, tourmaline and andalusite.

Quartzite: definition.

completely recrystallized sandstone… inseparable quartz grains are intertwined with each other

Quartzites: under what effect do they recrystallize?

Under the effect of pressure and temperature.

Gneiss: relation to granite.

Having the same mineralogical composition as the granites.

Gneiss: what distinguishes it?

Succession of parallel beds… alternation of light beds… and darker beds of micas

Most gneisses come from what?

Metamorphism of ancient granite

Gneisses can also come from which sedimentary rocks?

Ancient sedimentary rocks such as sandstones rich in feldspars.

What is the rock cycle?

A continuous process where igneous, sedimentary, and metamorphic rocks transform into each other through melting & crystallization, weathering/erosion & deposition + lithification, and heat/pressure (metamorphism)—driven by Earth’s internal heat and surface processes.