Classification and Facies
Wikipedia
Today
Updates:
Not today
Lecture outline:
- Rock classification
- Facies
Prograde Metamorphism
•
Normally progress through series of mineral
assemblages, for example:
1. epidote, chlorite, andalusite, muscovite, quartz,
k-spar, and albite
2. sillimanite, plagioclase, cordierite, muscovite,
quartz and k-spar
3. sillimanite, plagioclase, cordierite, quartz, and
k-spar
Prograde Metamorphism
Prograde
Retrograde
Retrograde Metamorphism
Prograde
Retrograde
http://www.und.nodak.edu/instruct/mineral/
320petrology/opticalmin/chlorite.htm
Common Prograde Sequence
Index minerals make zones:
 Chlorite zone. Pelitic rocks are slates or phyllites and typically
contain chlorite, muscovite, quartz and albite
 Biotite zone. Slates give way to phyllites and schists, with biotite,
chlorite, muscovite, quartz, and albite
 Garnet zone. Schists with conspicuous red almandine garnet,
usually with biotite, chlorite, muscovite, quartz, and albite or
oligoclase
 Staurolite zone. Schists with staurolite, biotite, muscovite, quartz,
garnet, and plagioclase. Some chlorite may persist
 Kyanite zone. Schists with kyanite, biotite, muscovite, quartz,
plagioclase, and usually garnet and staurolite
 Sillimanite zone. Schists and gneisses with sillimanite, biotite,
muscovite, quartz, plagioclase, garnet, and perhaps staurolite.
Some kyanite may also be present (although kyanite and
sillimanite are both polymorphs of Al2SiO5)
Sequence of indicator minerals
Sillimanite
Kyanite
Staurolite
Garnet
Biotite
Chlorite
Types of Protolith
Six chemically based-groups to start with
1. Ultramafic - very high Mg, Fe, Ni, Cr
2. Mafic - high Fe, Mg, and Ca
3. Shales (pelitic) - high Al, K, Si
4. Carbonates- high Ca, Mg, CO2
5. Quartz - nearly pure SiO2.
6. Quartzo-feldspathic - high Si, Na, K, Al
A Classification of Metamorphic Rocks
A Classification of Metamorphic Rocks
Compositional Classification
Parent
Clay-rich sediment
Sand-rich sediment
Clay-sand mix
Quartz sand
Marl (limy mud)
Limestone
Basalt
Metamorphic rock
pelite
psammite
semi-pelite
quartzite
metamarls-silicate/calcareous
marble
metabasite
Pelite protolith: Foliated Metamorphic Rocks
-- low grade
a
b
Pelite protolith: Foliated Metamorphic Rocks
--middle grade
Pelite protolith: Foliated Metamorphic Rocks
-- high grade
Non-Foliated Rocks
Specific Rock Types
Specific Rock
Types
Skarn: carbonate + contact
metamorphism + Si added
(metasomatism). Minerals:
grossular, epidote, tremolite,
wollastonite, diopside
Granulite: high-grade rock from pelitic, mafic, or
quartzo-feldspathic protolith,
few OH-bearing minerals.
Specific Rock Types
http://z.about.com/d/geology/1/0/I/z/migmatite1_500.jpg
http://www.earth.edu.waseda.ac.jp/photogeology/
200504rocky/BigornMtS/1K_DSC_1687.jpg
Specific Rock Types
Protolith = mafic rock or
graywacke (dirty sst):
Greenschist: low-grade, contains
chlorite, actinolite, epidote (all
green), and albite.
Amphibolite: mid-grade,
dominated by hornblende +
plagioclase.
http://z.about.com/d/geology/1/0/h/L/greenschist.jpg
Specific Rock Types
http://www.npolar.no/geonet/picture_atlas/SE02-1-25.jpg
Serpentinite: ultramafic rock,
low-grade metamorphosed.
Blueschist: high-P, blue by
glaucophane--amphibole
(mafic rock) or blue by
kyanite (pelite).
Eclogite: green + red due to
clinopyroxene and garnet
(omphacite + pyrope). (basalt)
http://www.dges.tohoku.ac.jp/museum/large/eclogite.jpg
Metamorphic Facies
Wikipedia
Prograde Sequence and Facies
Index minerals make zones, but COMPOSITION DEPENDENT
Change in composition, means change in minerals occurring
 Chlorite zone.
Chlorite
 Biotite zone.
Biotite
 Garnet zone.
Cordierite
 Staurolite zone.
Andalusite
 Kyanite zone.
Sillimanite
 Sillimanite zone.
=> Facies is better to compare different metamorphic rocks
Prefix and mineral texture
High Strain Rocks
High Strain Rocks
Why do we care about metamorphic
rocks?