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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
metamorphism of slate. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco.
Progressive Contact Metamorphism
Progressive thermal
metamorphism of slate. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco.
Progressive Contact Metamorphism
5
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
Regional Metamorphism
•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
Regional Metamorphism
•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
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco
Progressive Regional Metamorphism
Progressive syntectonic
metamorphism of a volcanic
graywacke, New Zealand. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco
Progressive Regional Metamorphism
Progressive syntectonic
metamorphism of a volcanic
graywacke, New Zealand. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco
Progressive Regional Metamorphism
7
Progressive syntectonic
metamorphism of a volcanic
graywacke, New Zealand. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco
Progressive Regional Metamorphism
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