9.metamorphic rocks

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Metamorphism



Metamorphism

• Metamorphism is a process leading to changes

in mineralogy and/or texture (for example

grain size) and often in chemical composition

.

and/or chemical conditions that differ from

those normally occurring at the surface of

planets and in zones of cementation anddiagenesis below this surface. They may

coexist with partial melting.”



The Limits of Metamorphism

•Diagenetic/weathering processes are

indistinguishable from metamorphic.

•Metamorphism begins in the range of 100-

.



Metamorphic Agents and Changes

• Temperature: typically the most important

factor in metamorphism.

• Pressure: High T/P geotherms in areas of

in subduction zones.

• Chemically active fluids e.g. H2O, CO2 etc.



How Metamorphic Rocks are

Formed?• The underlying principle to understanding all thingsgeological is:

– Minerals and rocks are stable only under theconditions at which they form. Change theconditions and the rocks will change to adapt to thenew conditions.

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•• MetamorphismMetamorphism occurs when any previously existingrock, the parent rock , is buried in the earth underlayers of other rock. The deeper the rock is buriedthe hotter it gets, and the higher the pressurebecomes. Eventually, rock must adjust to the newconditions, whether it is baked, or squeezed, or both,and in the process becomes a metamorphic rock.



Agents of Metamorphism

• Heat• Most important agent

• Two sources of heat

– Contact metamorphism – heat from magma

– An increase in temperature with depth due to the

geothermal gradient



• Pressure and differential stress

• Confining pressure applies forces equally in all

directions; increases with depth

• Rocks may also be subjected to differential stress

which is unequal in different directions



• Chemically active fluids

• Mainly water with other volatile components

• Enhances migration of ions

• Aids in recrystallization of existing minerals

• Sources of fluids

–

– Fractures in igneous rocks

– Hydrated minerals such as clays and micas



WHAT CHANGES DURINGMETAMORPHISM?

• Rock texture changes

– Foliation (alignment of minerals) can develop inresponse to stress e.g. schistosity – Minerals recrystallize e.g. calcite

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ar e s a non o ate metamorp c roc resu t ng romthe metamorphism of limestone , composed mostly ofcalcite (a crystalline form of calcium carbonate , CaCO 3 ).

• Mineralogy changes

– New minerals form that are stable under the newmetamorphic conditions e.g. clinozoisite, andalusite



Foliation is a texture that develops when platy or

elongate minerals are aligned by differential stress

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Foliation• Foliation - any planar set of minerals, or

banding of mineral concentrations, especiallythe planar structure that results from flattening

of the mineral grains, like micas.

• Foliation usually formed planes of weakness in

metamorphic rock because the rock can be easily

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.



Kinds of foliation

Slaty cleavage

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Schistosity

Gneissic banding



Recrystallization – minerals grow and

develop an interlocking texture

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Quartzite –metamorphosedsandstone

Marble –Metamorphosedlimestone



New minerals form – these are stable under the

new high temperature and high pressure conditions.Some minerals are found only in metamorphic rocks

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Garnet schist



Metamorphic Grade

= intensity of metamorphism

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Low

Intermediate High



Kinds of Metamorphic

• Contact or thermal metamorphism• Result from a rise in temperature when magma

invades a host rock



• Hydrothermal metamorphism

• Chemical alteration caused when hot, ion-richfluids, called hydrothermal solutions, circulate

through fissures and cracks that develop in rock



• Regional metamorphism

• Produces the greatest quantity of metamorphic rock

• Associated with mountain building



Impact Metamorphism and Tektites



Metamorphic textures• Texture refers to the size, shape, and arrangement of

mineral grains

• Foliation – any planar arrangement of mineral grains

or structural features within a rock• Foliated textures

• Rock or slaty cleavage

– Closely spaced planar surfaces along which rocks split

• Schistosity

– Rocks having this texture are referred to as schist• Gneissic

– Gneissic rocks exhibit a distinctive banded appearance



ommon e amorp c oc s



Foliated Metamorphic Rocks(Regional Metamorphism)

• Gneiss – highest grade ofmetamorphism, coarse

grained; generally banded(segregation bands of lightand dark-coloured minerals);augen texture.

• –

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metamorphism; sand size,schistocity cleavage.• Phyllite – low grade of

metamorphism; fine grained(silt-sized); phyllitic cleavage.

• Slate – lowest grade ofmetamorphism; very fine-grained; mud-sized; slatycleavage; smooth surface.



Non-Foliated

Metamorphic Rocks(Contact/Thermal Metamorphism)

• Granulite (granite / acid

igneous rocks origin)

• Amphibolite (basicigneous rocks origin)

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san s one

origin)

• Marble (limestone origin)

• Hornfels(mudstone/shale origin)



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9.metamorphic rocks

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