Metamorphism
revised 2/11/04
Metamorphism handout (Index minerals, Regional metamorphic zones, facies,
and plate tectonics)
Metamorphism
- Metamorphism - process whereby
preexisting rock undergoes pronounced changes in texture and mineralogy
at T > 200-300ºC while in the solid state
- Agents of metamorphism are heat,
pressure, chemically active fluids
Agents of Metamorphism
- Heat - temperature increases with
depth
- Geothermal Gradient - variable;
30ºC/km on continent
- Effect of Heat
- Increased ductility
- Change in mineral assemblage
- Pressure (Stress) - Increases with
depth (0.3 kbars/km)
- _________________
pressure - felt equally in all directions
- Differential stress - added pressure
from tectonic activity (tension, compression, shear)
- Effect of pressure
- Favors dense, compact structures
- Minerals grow perpendicular
to maximum pressure
- Produces a parallel arrangement
of platy minerals called foliation
- Gives rock layered or banded
appearance
Chemically Active Fluids
Aqueous; important
for ion transport
Metamorphic Textures
- Foliation
- _________________
- parallel alignment of microscopic platy minerals
- _________________
- parallel alignment of visible platy minerals
- _________________
- banded; layers of light and dark minerals
- Lineation - parallel alignment
of needle-like grains
- Porphyroblastic - larger crystals
(porphyroblasts) in a finer matrix
Metamorphic Rocks
- Rock formed depends on:
- Temperature - affects
mineral stability
- Pressure
- Confining pressure - affects
mineral stability
- Directed pressure - causes
foliation
- _________________
- Parent rock; for example,
shale versus basalt
- Major control on overall
composition
- Fluids
- Supplies ions/atoms; can
alter composition inherited from protolith
Common Metamorphic Rocks
- Foliated Metamorphic Rocks
- _________________
- very fine grained, clay rich
- Slaty cleavage
- Bedding often present
- Metamorphic grade: low
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- Cleavage surface has sheen
- Mica and/or chlorite grains
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- > 50% visible platy
minerals; may have layers or qtz and/or feldspar
- Bedding rare
- Grade: medium to high
- Rock name based on predominant
mineral
- Ex: muscovite schist (clay
rich sed. rock commonly parent)
- _________________
- Light bands of qtz and
feldspars, dark bands of biotite and hornblende (amphibole)
- Grade: high
- Parent: igneous rock, shale
- Non-Foliated Metamorphic Rocks
- Low Fe-Mg
- Marble
- Fine to coarsely crystalline
calcite
- Parent: limestone or
dolomite
- Quartzite
- Metamorphosed qtz sandstone;
almost all qtz
- Metaconglomerate
- High Fe-Mg (probably basalt
parent)
- Hornfels
- Uniform, fine-grained
- Varied composition
- Associated with contact
metamorphism
- Greenstone - dark green,
generally aphanitic
- Amphibolite - amphibole,
plagioclase
- Pyroxene Granulite - pyx,
garnet, sillimanite
- Eclogite - pyx, garnet.
Metamorphic Rocks and Pressure-Temperature
Diagrams
- Different temperatures and
pressures lead to different minerals and consequently different rocks
Types of Metamorphism
- Types of Metamorphism
- _________________
- associated with plutonism; small scale
- _________________
- occurs within fault; very small scale
- _________________
- associated with mountain-building; large scale
Contact Metamorphism
- Contact
- Magma comes in contact with
cooler rock
- Aureole - zone of metamorphosed
rock around pluton; non-foliated
- Skarn - unique rock formed
from metamorphism of protolith of silicate and carbonate (limestone
or dolomite) rocks
Cataclastic Metamorphism
- Cataclastic
- Grinding and pulverizing
of rock within fault
- Fault breccia - broken and
crushed rock in fault
- Mylonite - hard, fine-grained
rock formed from grinding within fault
Regional Metamorphism
- Regional
- Extensive area
- Associated with subduction
(compression)
- Majority of metamorphic rocks
- Can map pattern of regional
metamorphism using index minerals
- Provide clues to metamorphic
pressure and temperature
- Indicate metamorphic grade
Index Minerals and Phase Diagrams
- Phase diagrams show what phase
will be stable under at a give temperature and pressure
Regional Metamorphism
Index Minerals (Fig. 7.18)
Regional Metamorphism (
see handout
)
Metamorphic Zones (
see handout
)
Metamorphic Facies (
see handout
)
- _________________
- used in an abstract sense for characteristics in rocks typical of a specific
origin (used so that no conclusion as to origin need be made)
- Metamorphic facies - groups
metamorphic rocks formed under identical pressure-temperature conditions
Metamorphic Facies (
see handout
)
Plate Tectonics and Metamorphic
Facies (
see handout)