geol 13 53-met lec 1 - environments and...

1

Environments of Metamorphism

and Associated Textures

GEOL 13.53

Metamorphic Lecture 1

Metamorphism

•Literally translates to “change of form

”

•In geology it refers to solid-state changes

in m

ineral assemblages of a rock, and/or

the texture of these m

inerals

•Due to changes in temperature

and/or

pressure

Sources of Heat for Metamorphism

•Heat from Earth’s interior

•Geotherm

al gradient is the

increase in temperature with

depth

–Typical continental geotherm

al

gradient is 25-30°C

/km

–Volcanically active areas have

geotherm

al gradients of 30-

50°C

/km

–Oceanic trenches have

geotherm

al gradients as low as

5-10°C

/km

Sources of Heat for Metamorphism

•Heat from m

agma

•Emplacement of magma chambers will add heat to the

immediately surrounding rock

–Gabbroic m

agma ~1300°C

–Granitic m

agma ~700°C

2

Pressure Associated with M

etamorphism

•Lithostaticpressure:the confining pressure created by

the m

aterial that sits above a particular location.

Lithostaticpressure is equal in all directions and

compresses the volume of rock.

–Basalt: 3 g/cm

3(3000 kg/m

3)

–Granite: 2.7 g/cm

3(2700 kg/m

3)

–The lithostaticpressure at a 10 km

depth is≈3 kbar= 0.3 G

Pa

Pressure Associated with M

etamorphism

•Directed pressure:pressure is imposed in a particular

direction due to a regional stress field.

•Directed pressure affects

the shape and arrangement

of the

minerals

•Directed pressure varies w

ith tectonic environment

–Compressionalenvironments: Horz

> VertPressure

–Extensional environments: Vert> Horz

Pressure

Essentially Three Environment-Based Variables that

Control the Character of Metamorphism

•Depth of Burial

•Temperature (usually a function of depth)

–P and T control mineral stability

•Lithostaticversus Directed Pressure

–Controls textures

Types of Metamorphism

•Contact Metamorphism

–Therm

al variation controls processes

•Regional Metamorphism

–OrogenicMetamorphism

•Combination of temperature and

directed pressure

–Burial Metamorphism

•Combination of temperature and

lithostaticpressure

•Fault-Zone Metamorphism

–Directed pressure controls

processes (GEOL 41.1)

3

Contact Metamorphism

•Occurs adjacent to

igneous intrusions

•Temperature contrast

between m

agma

chamber and host rock

•Most evident in low-

pressure (near-surface)

environments

Contact Metamorphism

•Steep therm

al gradient in

host rock around the

intrusion

•Size of aureole depends

on the size of the pluton,

the rate at which it cools,

and tim

e since intrusion

•Rapid relative to m

ost

geological processes

Development of GranoblasticTexture

•Pressure increases solubility

•Mineral dissolves along, or

migrates from, high pressure

points to low pressure areas

Development of PoikiloblasticTexture

•Common feature in contact

metamorphic rocks

•Due to rapid porphyroblast

growth (rapid heat increase

in contact aureoles)

–Crystal envelops non-reactive

or excess m

inerals

•High-energy porphyroblast

due to increased surface

area of inclusions

–Inclusions are commonly

rounded to reduce surface

energy

4

Development of Nodular/Spotted Texture

•Anhedral, ovoid, poikoblastic

crystals

•Typically andalusiteor

cordierite

•Due to rapid porphyroblast

growth (rapid heat increase

in contact aureoles)

Progressive thermal

metamorphism of slate. From

Best (1982). Igneous and

Metamorphic Petrology. W. H.

Freeman. San Francisco.

Progressive Contact Metamorphism

Progressive thermal






Progressive thermal






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Regional Metamorphism

•Increase in temperature

is accompanied by an

increase in pressure

•Usually there is directed

pressure, so rock

deform

ation increases

with m

etamorphic grade

Orogenic


•High T/Low P

metamorphism

associated w

ith arc

complex

–Contact metamorphism at

shallow depth

•High T/High P

metamorphism

associated w

ith the fold

and thrust belt

Orogenic


•Low T/High P

metamorphism

associated w

ith oceanic

trench environments

Development of Subgrains

•Minor degree of

deform

ation causes lattice

defects to m

igrate

•Local accumulation of

lattice defects result in

reorientation of the crystal

lattice

–Unduloseextinction

–Subgrain

development

•Higher degree of directed

pressure w

ill result in

elongated subgrains

6

Development of Foliations

Recrystallization

Pressure Solution

Remobilization

Rotation

Progressive syntectonic

metamorphism of a volcanic

graywacke, New Zealand. From



Freeman. San Francisco

Progressive Regional Metamorphism















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