1. Modification of Rocks by Temperature and Pressure
METAMORPHISM:
Modification of Rocks by Temperature and Pressure
Chapter 6

2. continental lithosphere
Oceanic crust
Oceanic
lithosphere
Continental
crust
Mantle lithosphere
Asthenosphere
Volcanic arc
(subduction zone)
Zone of continental
plate extension
Ancient stable
continental lithosphere
Pressure
(kilobars)
Depth
0 km 50 30 30
1300°C 50 50
150
1300°C
1300°C
isotherm

3. Pressure increases with depth at about the same rate everywhere,...
...but temperature
increases at different
rates in different regions.
Volcanic arc
(subduction zone)
Zone of continental
plate extension
Ancient stable
continental lithosphere
Pressure
(kilobars)
Depth
0 km 50 30 30
1300°C 50 50
150
1300°C
1300°C
isotherm

4. Regional high-pressure metamorphism Shock metamorphism Regional
Depth, km
Oceanic
crust 35
Continental crust
Oceanic
lithosphere 75
Continental mantle lithosphere
Asthenosphere

6. Regional high-pressure metamorphism Shock metamorphism Regional
Contact
metamorphism
Regional
metamorphism
Depth, km
Oceanic
crust 35
Continental crust
Oceanic
lithosphere 75
Continental mantle lithosphere
Asthenosphere
Water
Seafloor
metamorphism
Burial
metamorphism

9. slate (phyllite) schist gneiss
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) (0-30 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

10. Increasing intensity of metamorphism
slate - phyllite
schist
gneiss
Increasing intensity of metamorphism
Increasing crystal size
Increasing coarseness of foliation
Low grade
Intermediate grade
High grade

11. Foliation is the result of compressive forces.
Mineral crystals elongate
perpendicular to the
compressive force.
Feldspar
Quartz
Mica
Pyrite
Staurolite

12. Staurolite
crystal
Mica

13. Foliated rocks are classified by the degree
of cleavage, schistosity, and banding.
Diagenesis
Low grade
Intermediate
grade
High grade
Slate
Phyllite
Schist
(abundant
micaceous
minerals)
Gneiss
(fewer
micaceous
minerals)
Migmatite
Slaty cleavage
Schistosity
Banding
Banding

14. 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.

16. 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.

17. With increasing metamorphic grade, mineral composition changes.
Degree of metamorphism
Diagenesis
Low
Intermediate
High
Greenschists
Amphibolites
Granulites
Chlorite
White mica (mainly muscovite)
Biotite
Garnet
Staurolite
Kyanite
Sillimanite
Albite (sodium plagioclase)
With increasing metamorphic
grade, mineral composition
changes.
Mineral suites define
metamorphic facies.

18. 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]

20. Low
Grade
Increasing metamorphic grade
Slate
Intermediate
Grade
Phyllite
High
Grade
Pressure (kilobars)
Depth (km)
Schist
Gneiss
Blueschist
Migmatite
Temperature (°C)

21. Metamorphic facies correspond to particular combinations of
Hornfels
Contact metamorphism
Zeolite 5
Conditions beneath
mountain belts 5 10
Blueschist 15
Partial melting begins
Amphibolite
Granulite
Pressure (kilobars)
Greenschist
Depth (km) 10 20 25
13.5 15
Subduction zone 30
Eclogite 35 20 40
200
400
600
800
1000
Temperature (°C)
Metamorphic facies correspond
to particular combinations of
pressure and temperature...
… and can be used
to indicate specific
tectonic environments.

22. Index minerals define metamorphic zones. Isograds can be
used to plot the
degree of
metamorphism.
Canada ME NY
Isograds VT NH MA
Key:
Not
metamorphosed CT
Chlorite zone
Low
grade RI
Biotite zone
Garnet zone
Medium
grade
Staurolite zone
High grade
Sillimanite zone

23. 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.

24. Pressure (kilobars) Depth (km) Temperature (°C) Low Grade Slate
RETROGRADE PATH
Intermediate
Grade
Phyllite
Pressure (kilobars)
Depth (km)
PROGRADE PATH
Schist
Gneiss
High
Grade
Temperature (°C)

25. Pressure-Temperature Paths
Click here to view the Flash animation in your web browser
To return to this slide after viewing the animation, close the animation window in your web browser application.