L10 New from old - metamorphism

Subduction, contact and regional metamorphism

• Igneous intrusion → contact metamorphism (in contact with e.g. basalt heat + temp)

• Subduction metamorphism → subduction zone’pre

• Regional metamorphism → continental plate (himalayas outstrecthed)

Metamorphism

‘pre-existing rock (protolith) undergoes solid-state change in response to modification of its environment.’

• change in temperature and/or pressure

• compression

• reaction with ‘‘hydrothermal fluids’’

Different protoliths - different metamorphic rocks

(in this case illustrated by contact metamorphism associated with igneous intrusion)

• Original rock i.e. what you start with (protolith)

• What you do to it (T and P + fluids)

• Outputs can be extremely complex

• Emphasis in this course is on processes not products

N.B. Remember ‘shale to schist’, ‘limestone to marble’, ‘sandstone to quartzite’.

Are there textures in hand specimen that are diagnostic of metamorphism

protolith → metamorphic rock

• metamorphic minerals (indicative of P/T condition)

• metamorphic textures (e.g. foliation - alignment of minerals, in this case micas, has a silvery shine)

• garnet porphyroblasts..

metamorphism often results in foliation

• planar foliation is the parallel alignment of play minerals (e.g. clays/micas)

• minerals oriented perpendicular to compression

• sometimes with alternations of differently coloured layers

non-foliated texture

• no preferred orientation of the minerals in the rock

• when deformation is low and clay/mica content is low

processes involved in forming metamorphic rocks?

1. recrystalization

2. phase change

3. neocrystallization

4. pressure solution

5. plastic deformation

Recrystallization

• changes shape and size of grains

• no change in mineral identity

• when changed large crystals and new grains are made

• solid state process where atoms of mineral are reorganised mainly by solid-state diffusion.

Phase change

• one mineral transformed into another with the same composition but different crystal structure.

• Three polymorphs of Al2SiO5

  • Andalusite- low T and P = contact pressure

  • Kyanite- low T, high P = subduction zone

  • Silimanite- high T and P = lava becoming a rock

• Both T and P change with depth in the Earth

  • Mineral stability, which is highly dependent upon T and P, can be graphed on phase diagrram.

• Andalusite → Killiney

neocrystallization

• growth of new minerals different from protolith

Pressure solution

• minerals grains dissolve where their surfaces are pressed against other grains

• ions produced migrate (water-assisted diffusion)

• precipitation where grains under less pressure

• can shorten grains in one direction and lengthen them in another

• also occurs under non-metamorphic conditions

plastic deformation

• rock sheared/squeezed at high temperature/pressure

• minerals change shape without breaking: plastic behaviour

• can occur without change to composition or crystal structure

• or occur when metamorphic reactions are happening

• During plastic deformation, the grains change shape internally without breaking or dissolving - by internal deformation of the atomic lattice.

metamorphism is the result of heat and/or pressure

• Heating

  • atoms vibrate

  • chemical bonds between them stretch and break

  • atoms detach

  • results in rearrangement of atoms within grains and migration of atoms into/out of grains

  • recrystallisation and/or neocrystallisation

• Pressure

  • greater pressure = closer packing of atoms

  • density increases

  • phase changes and/or neocrystalisation

Role of compression, shear and the development of preferred orientation

differential stress: stress applied unequally in all directions

• (1) normal stress: applied perpendicular to a surface

how does preferred orientation form?

• at elevated temperatures rock changes shape without breaking

• internal texture changes in sympathy with the stresses applied

• preferred orientation develops via alignment of inequant grains.

• inequant grains may be pre-existing (i.e. geometry before metamorphism) or may form during the metamorphic episode

• Elongate (cigar shaped):..

Pressure solution, neocrystallisation, plastic deformation

• pressure solution occurs on faces perpendicular to direction of compression i.e. grains shorten in that direction

• often accompanied by neocrystallization: precipitation occurs where rock is stretching

• plastic deformation at high T accompanied by differential stress

…

plastic deformation and rotation

foliation

role of hydrothermal fluids in metamorphism

• hot water, steam, supercritical fluids*

  • * under high temp and pressure

  • characteristics of fluid and gas

  • very pervasive

• accelerate reactions (atoms migrate faster through fluids than solids

• provides water that becomes absorbed by minerals during metamorphism

• metasomatism: chemical composition of rock is altered by introduction and/or removal of chemical components by water-rich fluids

source of water

Types of metamorphism and their settings

Burial - deep burial in a basin

Dynamic - shearing in a fault zone

Contact (i.e. thermal) - heating by an intrusion

Hydrothermal - alteration by hot water leaching..

Burial metamorphism (diagenesis)

Pressure (P) and Temperature (T) increase as sediments are buried ina basin due to…

Get diagenesis (i.e. cementation/dissolution of sediments rather than classic metamorphism..

Dynamic metamorphism

• Breakage of a rock by shearing within a fault zone

• at deeper levels rocks are ductile. rocks in fault zones smear\ to form mylonite.

Contact (thermal) metamorphism

contact - position adhacent to an igneous intrustion

thermal - develops in response to heat without change in pressure

• magma intrusion affects ‘country rock’

• heat transferred and hydrothermal fluids ciruclated

• metamorphic aureole generated

Example of contact metamorphism in Ireland

• Leinster granite (400 millions year old)

  Metamorphic aureole (up to 500m wide) surrounding granite (high T minerals include andalusite and cordierite)

• Long crystals → andalusite (slightly high temp rock)

  original rock was an oceanic mudstone (deposite on south side of lapetus ocean seperating north and south of Ireland)

Hydrothermal metamorphism in Ireland - EXAMPLE

• Forms mineral deposits along the contact of the granite:

  • North (wicklow) - get Pb deposits

  • South (Wexford) - get Li (lithium)

• Indicated circulation of hydrothermal fluids between country rock and granite.

Hydrothermal metamorphism and mineralization

• Avoca Gold:

  Avoca gold deposit, Wicklow
  Formed in association with 450My volcanic arc.

  Ireland’s gold → was from volcanic arc, the subduction zone in the likes of avoca gold deposits
  convection currents stripping those rocks.

Hydrothermal metamorphism at mid ocean ridges

• cold ocean water seeps into fractured oceanic crust

• heated by magma, hot aggressive water chemically alters the basalt

• hot water from black smokers provide mineral deposits (ZnS, PbS & life)