Metamorphic Rock Textures

what are metamorphic rocks

Rocks that change structure or composition without melting due to pressure or temperature changes

 

Pressure/temperature change can cause the form of the rock to change in two ways

• Form = texture/structure and composition

• Protolith = original rock

 

Note: if rock melts it then becomes an igneous rock NOT metamorphic

 

What causes a rock to undergo metamorphism

Subduction of plates	Increase in pressure and temperature in a subduction zone Buries rock at great depth Rock subjected to high temperature and pressure These conditions will change structure and composition Rock metamorphose
Collision zones	Collision between continental and continental lithosphere Rocks are being buried due to different forces Will change chemical composition from ∆ pressure and ∆ temp Also change structure due to tectonic forces
Meteorites	Impact shock of meteorites landing Exert a large force on earth = increases temperature and pressure of rocks in landing area Causes rocks to metamorphose

how does metamorphism change rock texture

• Pressure/temperature change can cause the texture of the rock to change

• Chemical composition remains the same

• Texture = grain shale and orientation

  • Layers inside rock allow us to tell direction of forces

  • Direction or force is perpendicular to plane of rock layer

 

example of metamorphosed rock changing texture

E.g. Marble

• Comes from fossiliferous limestone

• Both made out of calcite and some dolomite

• Main differences

  • Limestone contains fossils

  • Fossils cannot be preserved when exposed to high pressure and temperature

  • Marble has layering

    • Marble flows

    • Due to high temperature

  • Limestone has fractures

  • Grains in marble are more uniform

  • Grains in limestone are more varied in size and shape

example of how metamorphism changes rock chemical compositoin

 shale -> schist

• Shale

  • Very fine grains

  • Clay minerals

  • Small fragments of rock

• Schist

  • Contains elongated course minerals (chloritoid)

  • Chloritoid not originally in shale

  • Metamorphism increased grain size and made new minerals

Economic significance of metamorphic rocks

• Create rare earth minerals

• Create ore deposits (e.g. vein gold deposits)

• Metamorphism during subduction = deep crustal rocks release fluid = fluid moves through overlying rocks

• Hotter temperature in lower crust carries gold to upper crustal levels

• Gold occurs in quartz, carbonate veins, adjacent rocks or faults

How to identify a metamorphic rock

Metamorphic texture

• Grains have grown in place and interlock

  
  

Metamorphic minerals

• Presence of new minerals that can only form under metamorphic conditions

• These minerals are not stable where igneous or sedimentary rocks form

Metamorphic foliation

• repetition of planar surfaces within a rock

• Parallel alignment of platy minerals AND/OR alternation of light-dark minerals

process of metamorphic evolution of shale

1. Slate: When subjected to slight increase in temperature shale becomes slate

   1. Low grade, temperature metamorphism

   2. Mudstone protolith

   3. Larger grain size

      1. Grain size increases with temperature

      2. Also true for igneous rocks

   4. Distinct foliation -> slaty cleavages

2. Phyllite

   1. Low-medium grade metamorphism

   2. Clay minerals neocrystalise into mica

   3. Fine grained, mica-rich rock

   4. Satiny lustre

   5. Larger grain size

3. Schist

   1. Medium-high grade metamorphism

   2. Fine to coarse grained

   3. Larger micas

   4. Distinct foliation occurs

   5. Other minerals neocrystalise

4. Gneiss

   1. Highest metamorphic grade

      1. Subduction zones

      2. Contact metamorphism

      3. Converging plates

   2. Banded foliation -> felsic and mafic layering

      1. Light bands = feldspars, quartz

      2. Dark bands = amphibole, biotite

why are some rocks not foliated? what is their name?

Non-foliated metamorphic rock = hornfel

• Either occurs in absence of differential stress

• New crystals are equant

  • From recrystallisation or neocrystallisation

• Similar composition but different structure to protolith

e.g.

• Hornfel

  • Metamorphism because of heating

• Quartzites

  • Recrystallisation of quartz protolith

  • Metamorph of quartz sandstone or chert

  • Forms new, larger grains

• Marble

  • Metamorphism of limestone

  • Recrystallisation of calcite protolith

  • Original fossils are destroyed or hard to recognise

what are Metamorphic grades

range of metamorphic change a rock undergoes

• Low = little metamorphic change

• High = significant metamorphic change

• Increasing metamorphic grade

  • Shale: slate -> phyllite -> schist -> gneiss (all foliated)

what are Metamorphic facies

set of mineral assemblies that can be indicative of the main temperature and pressure of metamorphic conditions

• Different minerals have specific ranges of temperature and pressures that it is stable/will form

• How to read graph

  • Horizontal axis = temp

  • Vertical axes = pressure and depth

    • Increase in depth/pressure at 600°C = amphibolite -> eclogite

    • Increase in temperature at 4kbar = greenschist -> amphibolite

  • Each field/coloured section is a metamorphic facies

  • Each dotted line is a different geotherm at a specific tectonic location

• For a given set of pressure and temperature conditions

  • If we determine the minerals in the metamorphic rock and its protolith

  • We can determine its metamorphic facies

  • We will know the range of temperature and pressure conditions the rock was metamorphosed

• named after dominant amphibole in metamorphosed basalts

  • They determine the colour

  • E.g. greenschist has actinolite, blueschist has glaucophane, amphibolite has hornblende

what are index materials

• Materials that do change stability easily are index materials

  • Metamorphic zones = rock in an area that consists of a very specific minerology -> characterised by specific index material

  • Can map metamorphic conditions over areas

• Not all materials are useful in identifying metamorphic environments

  • E.g. quartz, feldspar -> it is stable in all conditions

Metamorphic processes

Neo crystallisation / Metamorphic reaction

Phase change

Recrystallisation

Plastic deformation

Pressure solution

Neo crystallisation / Metamorphic reaction

Growth of new minerals that are different to protolith

• Chemical reaction occurs that changes chemical composition of rock = new minerals

• Atoms move through solid crystal or dissolve and reprecipitate

 

E.g. Orthopyroxene + plagioclase  = Garnet

• Orthopyroxene and plagioclase react to make garnet

• Reaction occurs due to increase in pressure at high temperature

Phase change

Mineral grain transforms into a new grain with the SAME chemical composition but DIFFERENT crystal structure

 

E.g. Quartz to Coesite

• Both have same SiO2 composition

• At high pressure: quartz -> coesite

• Distance between atomic bonds change

• Atom arrangement is different

Recrystallisation

Size and shape of grains change but composition stays the same

• Recrystallised grains = New grains form in the expense of pre-existing larger grains

• Pressure could squish grains and make them smaller

• Occurs until rock is fully recrystalised

• Same mineral between all stages

Plastic deformation

Minerals flatten or elongate without breaking

• Grains deform due to load

• Do not change mineral or crystal composition

Pressure solution

Rock is squeezed more strongly in one direction than others in the presence of water

• Mineral where pressure is the strongest dissolve

• Dissolved ions precipitate along less squeezed faces

• Pressure solution causes grains to become shorter in one direction and longer in another

 

E.g. Oolithic limestone

what are Agents of metamorphism

Heat

Lithostatic Pressure

Temperature + pressure

Directional stress : Deformation

Protolith

effect of heat in metamorphism

Metamorphism takes place around 200-850°C

• Between temperature of diagenesis and melting

• Temperature increases with depth

  • Called geotherm

  • Geotherm depends on geothermal gradient

• Increase in temperature = atoms in grains vibrate rapidly = stretch and bends chemical bonds

  • Vibrations so large = cause atoms to break bonds and form new bonds either with

    • New atoms within grain

    • New atoms in different grain -> after they migrated

  • Leads to neo crystallisation or recrystallisation

what is a geotherm

rate at which temperature change with depth or pressure

• Different geothermal describe different rates

• Depends on tectonic setting

  • Subduction zone vs mid ocean ridge

  • Subduction zones = cold geotherms

    • Rock moves below ground and introduces coldness from surface

    • Temperature increases with depth slower

  • Where magma forms = warm geotherm

    • Temperature increases with depth faster

• Geotherm determines what minerals will form

  • Hotter geotherm = formation of higher pressure and temperature minerals

  • Colder geotherm = formation of lower pressure and temperature minerals

Lithostatic Pressure

pressure increases with depth -> 270/300 kbar/km

• Most rocks metamorphose at 40km

• put stress on grains and cause them to align in a specific direction = foliation

how to use pressure and temperature of metamorphic conditions to identify minerals

• By knowing when different minerals are stable in what conditions = we can know which metamorphic conditions resulted in this rock

  • Use minerals like kaolinite and pyrophyllite to determine temperature conditions rocks were formed in -> narrow temperature conditions

  • Use kyanite or andalusite to determine pressure conditions -> narrow pressure conditions

Directional stress : Deformation

Stress applied on rock due to an external force causes it to deform

-> this determines preferred grain orientation

• Change in length of one direction is different to the other

• Orientation is perpendicular to maximum stress

  • Preferred orientation is called foliation

  • Horizontal force = grain lengthens in vertical direction

• If direction of force is equal on all surfaces of rock -> grain shrinks but no new foliation occurs

Protolith + impact on metamorphosed rock

The chemical composition and grain texture of the original rock impacts how it metamorphoses

• Final chemical composition depends on the original minerals in the protolith

• Unless magma or fluids per image the rock, chemical composition of the original and final metamorphic rock should be the same

  • Fluids can come take away or bring new atoms

    • E.g. water with dissolved ions

  • Help crystal rearrange

  • Aids in change in structure and mineral composition

Type of metamorphism and tectonic setting

High-pressure metamorphism

Contact / thermal metamorphism

Regional metamorphism

High-pressure metamorphism

A long subduction zones

• Rocks are transferred to great depth

• Metamorphic rocks categorised by

  • HIGH PRESSURES

    • Foliation

  • LOW TEMPERATURE

• Subduction zones are colder compared to surrounding rocks

Contact / thermal metamorphism

Take place near magmatic intrusions

-> E.g. back arc areas (volcanoes) behind subduction zones, diverging plate volcanoes, volcanic hot spots

• Magma is hot

  • Heat is transferred

  • Change in heat causes atoms to rearrange and metamorphism to occur

  • HIGH TEMPERATURES

  • LOW PRESSURE

• Hornfels is dominant rock

  • Not lots of shear/external stresses -> no foliation

Regional metamorphism

Collisional zones between two tectonic plates forming mountains called metamorphic belts

-> E.g. Alps or Himalayas

• Centre of metamorphic belt = more intense metamorphic conditions

  • Metamorphic conditions decrease symmetrically as you go further away from belt

• Rocks are being buried, deformed, exhumed

  • HIGH TEMPERATURE

  • HIGH PRESSURE

• Composition and structure of rock is changed

  • Rock heated by geothermal gradient and plutonic magma intrusions

  • Rock squeezed and heated by being buried

    • Due to weight of new continental lithosphere that crashed to form mountains  = further increased pressure

    • Rocks that were at surface of one continent could end up at great depths

  • Rock experience differential stresses when being smashed/sheared at great pressure

    • Lots of shearing and displacement of rock due to lots of pressure

    • Foliation

      Shale (protolith) + regional metamorphism -> slate, phyllite, schist, gneiss, migmatite