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METAMORPHISM:
Modification of Rocks by
Temperature and Pressure
Chapter 6
Continental
lithosphere
Oceanic crust
Continental
crust
Mantle lithosphere
Asthenosphere
Pressure
(kilobars)
Depth
Volcanic arc
(subduction zone)
Zone of continental
plate extension
0
0 km
30
30
Ancient stable
continental lithosphere
0
1300°C
50
50
50
1300°C
isotherm
150
Pressure increases with
depth at about the same
rate everywhere,...
...but temperature
increases at different
rates in different regions.
Pressure
(kilobars)
Depth
Volcanic arc
(subduction zone)
Zone of continental
plate extension
0
0 km
30
30
Ancient stable
continental lithosphere
0
1300°C
50
50
50
1300°C
isotherm
150
Regional
Shock
metamorphism metamorphism
Depth,
km
0
Regional
high-pressure
metamorphism
Regional
metamorphism
Oceanic
crust
35
75
Oceanic
lithosphere
Regional
Shock
metamorphism metamorphism
Depth,
km
0
Regional
high-pressure
metamorphism
Contact
metamorphism
Regional
metamorphism
Oceanic
crust
35
Oceanic
lithosphere
75
Water
Burial
metamorphism
Seafloor
metamorphism
Metamorphism of sedimentary rocks —shales undergo the
most spectacular metamorphosis of all the rocks. The reason is
that the clay minerals form at Earth surface temperature (T) (030 C) and pressure (P) (1 bar) by weathering of pre-existing
rocks. Thus, clay minerals are grossly out of equilibrium with the
high temperature and pressure conditions found at depth in the
Earth’s crust. When shale is buried to deeper crustal levels
beneath, for example, mountain belts, the original clay is
transformed to chlorite, and then to biotite/muscovite, with
increasing T and P. These micaceous minerals are platy; they
grow large and become aligned in the metamorphic stress field,
giving the rock a foliated appearance.
slate
(phyllite) schist
gneiss
slate - phyllite
schist
gneiss
Increasing intensity of metamorphism
Low grade
Intermediate grade
Increasing crystal size
Increasing coarseness of foliation
High grade
Foliation is the result
of compressive forces.
Mineral crystals elongate
perpendicular to the
compressive force.
Feldspar
Quartz
Mica
Pyrite
Staurolite
Staurolite
crystal
Mica
Foliated rocks are classified by the degree
of cleavage, schistosity, and banding.
Diagenesis
Slate
Slaty cleavage
Low grade Intermediate
grade
Phyllite
High grade
Schist
(abundant
micaceous
minerals)
Gneiss
(fewer
micaceous
minerals)
Migmatite
Schistosity
Banding
Banding
Rocks without clay minerals do not form micaceous minerals during
metamorphism and, thus, tend not to be foliated. Examples are limestone
and sandstone, which form marble and quartzite, respectively. In the
limestone, the pre-existing grains of calcite and skeletal fossils
(bioclasts) are obliterated and new larger crystals of calcite form in the
high P and T conditions. In the sandstone, the quartz grains become
welded together by quartz cements. Granoblastic texture results.
Schists have the tendency to grow one mineral much
larger than the others. The large mineral is called a
porphyroblast. The mineralogy of the porphyroblast
changes with increasing T and P. A common porphyroblast
is garnet. A series of porphyroblast polymorphs denoting
increasing temperature is staurolite, kyanite, silliminite.
The fact that certain
minerals will grow only
within a restricted range
of temperatures and
pressures gives way to
the concept of using
mineral assemblages to
determine metamorphic
conditions–--the
minerals behave as
paleothermometers and
paleobarometers.
Degree of metamorphism
Diagenesis
Low
Greenschists
Intermediate
High
Amphibolites
Granulites
Chlorite
White mica (mainly muscovite)
Biotite
Garnet
Staurolite
Kyanite
Sillimanite
Albite (sodium plagioclase)
With increasing metamorphic Mineral suites define
grade, mineral composition
metamorphic facies.
changes.
A metamorphic facies is a set of metamorphic mineral assemblages that were formed under similar
pressures and temperatures.[1] The assemblage is typical of what is formed in conditions
corresponding to an area on the two dimensional graph of temperature vs. pressure (See diagram at
right).[1] Rocks which contain certain minerals can therefore be linked to certain tectonic settings,
times and places in geological history of the area.[1] The boundaries between facies (and
corresponding areas on the temperature v. pressure graph), are wide, because they are gradational
and approximate.[1] The area on the graph corresponding to rock formation at the lowest values of
temperature and pressure, is the range of formation of sedimentary rocks, as opposed to
metamorphic rocks, in a process called diagenesis.[1]
Low
Grade
Intermediate
Grade
Phyllite
High
Grade
Schist
Blueschist
Gneiss
Migmatite
Temperature (°C)
Depth (km)
Pressure (kilobars)
Slate
0
0
Hornfels
20
0
15
20
25
13.5
Depth (km)
10
Granulite
10
15
Amphibolite
5
Greenschist
Pressure (kilobars)
5
30
200
Eclogite
35
600
40
1000
400
800
Temperature (°C)
Metamorphic facies correspond
to particular combinations of
pressure and temperature...
… and can be used
to indicate specific
tectonic environments.
Index minerals define
metamorphic zones.
Isograds can be
used to plot the
degree of
metamorphism.
Canada
ME
NY
Isograds
VT
NH
Key:
MA
CT
RI
Low
grade
Medium
grade
High grade
Not
metamorphosed
Chlorite zone
Biotite zone
Garnet zone
Staurolite zone
Sillimanite zone
Tectonic transport moves
rocks through different
pressure-temperature
zones, …
Low P,
Low T
High P,
High T
…and then transports them
back to the shallow crust or
the surface.
Intermediate
Grade
Depth (km)
Pressure (kilobars)
Low
Grade
High
Grade
Temperature (°C)
Pressure-Temperature Paths
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