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Metamorphism:Alteration of Rocks by Temperature
and Pressure
Petrology – Session 2
About Metamorphism
• Changes in heat, pressure, and the chemical environment of rocks can alter mineral compositions and crystalline textures, making them metamorphic.
• Metamorphic changes occur in the solid state, so there is no melting.
Causes of Metamorphism
● internal heat of Earth
● internal pressure of Earth
● fluid composition inside Earth
Causes of Metamorphism
● temperature increases with depth
● rate = 20º to 60ºC per km
● at 15 km depth: 450ºC
Causes of Metamorphism● pressure and temperature increase
with depth in all regions
Causes of Metamorphism
Causes of Metamorphism
● the role of temperature
● geothermal gradient
● shallow (20ºC / km)
● steep (50ºC / km)
Causes of Metamorphism
● the role of pressure (stress)
● confining pressure
● directed pressure
Causes of Metamorphism
● the role of pressure (stress)
● rate of increase = 0.3 to 0.4 kbar / km
● minerals are geobarometers
Types of Metamorphism
● the role of fluids
● metasomatism
● accelerated chemical reactions
Depth,km
0
35
75
Asthenosphere
Continental mantle lithosphere
Continental mantle lithosphere
Continental crust
Continental crust
Oceaniccrust
Oceaniclithosphere
Types of Metamorphism
Depth,km
0
35
75
Asthenosphere
Continental mantle lithosphere
Continental mantle lithosphere
Continental crust
Continental crust
Oceaniccrust
Oceaniclithosphere
Shockmetamorphism
Depth,km
0
35
75
Asthenosphere
Continental mantle lithosphere
Continental mantle lithosphere
Continental crust
Continental crust
Oceaniccrust
Oceaniclithosphere
Shockmetamorphism
Regionalmetamorphism
Depth,km
0
35
75
Asthenosphere
Continental mantle lithosphere
Continental mantle lithosphere
Continental crust
Continental crust
Oceaniccrust
Oceaniclithosphere
Shockmetamorphism
Regionalmetamorphism High-pressure
metamorphism
Depth,km
0
35
75
Asthenosphere
Continental mantle lithosphere
Continental mantle lithosphere
Continental crust
Continental crust
Oceaniccrust
Oceaniclithosphere
Shockmetamorphism
Regionalmetamorphism High-pressure
metamorphismContactmetamorphism
Depth,km
0
35
75
Asthenosphere
Continental crust
Continental crust
Oceaniccrust
Oceaniclithosphere
Shockmetamorphism
Regionalmetamorphism High-pressure
metamorphismContactmetamorphism
Burialmetamorphism
Continental mantle lithosphere
Continental mantle lithosphere
Depth,km
0
35
75
Asthenosphere
Continental crust
Continental crust
Oceaniccrust
Oceaniclithosphere
Shockmetamorphism
Regionalmetamorphism High-pressure
metamorphismContactmetamorphism
Burialmetamorphism
Continental mantle lithosphere
Continental mantle lithosphere
Water
Seafloormetamorphism
3. Metamorphic Textures
shale
sandstonelayers
5 cm
3. Metamorphic Textures
shale
sandstonelayers
5 cm
The original beddingcan be seen in the thin sandy layers.
shale
sandstonelayers
5 cm
The original beddingcan be seen in the thin sandy layers.
foliationplane
originalbedding
shale
sandstonelayers
5 cm
The original beddingcan be seen in the thin sandy layers.
Regional metamorphism causes cleavage planesto develop.
foliationplane
originalbedding
Low grade Intermediate grade
3. Metamorphic Textures
Slate Phyllite Schist(abundantmicaceousminerals)
Gneiss(fewer
micaceousminerals)
Migmatite
Slaty cleavage Schistosity Banding Banding
Diagenesis Low grade Intermediategrade
High grade
3. Metamorphic Textures
Diagenesis Low grade Intermediategrade
High grade
Slate Phyllite Schist(abundantmicaceousminerals)
Gneiss(fewer
micaceousminerals)
Migmatite
Slaty cleavage Schistosity Banding Banding
Foliated rocks are classified by the degreeof cleavage, schistosity, and banding.
Metamorphic Textures
● classification of foliated rocks
● metamorphic grade● crystal size● type of foliation● banding
Metamorphic Textures
Nomenclature of foliated rocks● slate● phyllite● schist● gneiss● migmatite
Foliatedtexture:
schist with garnetporphroblasts
Metamorphic Textures
Nomenclature of granoblastic(non-foliated) metamorphic rocks
● hornfels● quartzites● marbles● greenstones● amphibolites● granulites (granofels)
Granoblastic Texture
Grades of metamorphism
● low
● intermediate
● high
Regional Metamorphism and Metamorphic Grade
Regional Metamorphism and Metamorphic Grade
Mineral isograds (zones of change)
● index minerals reflect pressure and temperature conditions
● groups of 2 to 3 index minerals form an isograd
Canada
NY
VT
ME
NH
MA
CT
RI
Isograds
Key: Notmetamorphosed
Chlorite zone
Biotite zone
Garnet zone
Staurolite zone
Sillimanite zone
Lowgrade
Medium grade
High grade
4. Regional Metamorphism
Canada
NY
VT
ME
NH
MA
CT
RI
Isograds
Key: Notmetamorphosed
Chlorite zone
Biotite zone
Garnet zone
Staurolite zone
Sillimanite zone
Lowgrade
Medium grade
High grade
Index minerals define metamorphic zones.
Canada
NY
VT
ME
NH
MA
CT
RI
Isograds
Key: Notmetamorphosed
Chlorite zone
Biotite zone
Garnet zone
Staurolite zone
Sillimanite zone
Lowgrade
Medium grade
High grade
Index minerals define metamorphic zones. Isograds can be
used to plot thelevel or degree of metamorphism.
Pre
ssu
re (
kilo
bar
s)
Temperature (°C)
Dep
th (
km)
LowGrade
IntermediateGrade
HighGrade
Increasing metamorphic grade Slate
Phyllite
Schist
Blueschist Gneiss
Migmatite
Pre
ssu
re (
kil
ob
ars
)
Temperature (°C)
Dep
th (
km
)
20
15
10
5
0
0 200 400 600 800 100040
35
30
25
20
15
10
5
0HornfelsHornfels
EclogiteEclogite
Zeolite
Zeolite
Blu
eschist
Blu
eschist
Gre
en
sc hist
Gre
en
sc hist
Am
ph
ibo
liteA
mp
hib
olite
Gra
nu
lit eG
ran
ulit e
Part
ial
melt
ing
beg
ins
Contact metamorphismConditions beneath
mountain belts
Su
bd
uctio
n zo
ne
13.5
Metamorphic facies correspondto particular combinations ofpressure and temperature...
… and can be used to indicate specific tectonic environments.
Plate Tectonics and Metamorphism
Metamorphism occurs in or near● plate interiors● divergent plate margins● convergent plate margins● transform plate margins
Low P,Low T
High P,High THigh P,High T
Tectonic transport moves rocks through different pressure-temperature zones, …
Low P,Low T
High P,High THigh P,High T
Tectonic transport moves rocks through different pressure-temperature zones, …
…and then transports themback to the shallow crust or the surface.
Plate Tectonics and Metamorphism
Metamorphic pressure-temperature paths
● history of burial and exhumation
● prograde and retrograde paths
Plate Tectonics and Metamorphism
● rapid erosion (exhumation) rates of mountain ranges show a relationship between
● tectonics (orogeny)● climate
● controls the flow of metamorphic rocks to the surface