Download Metamorphism

Metamorphism – changes in mineralogy and texture
brought about by perturbations in heat and pressure
Compositional changes are generally restricted to those
related to devolatilization
Metamorphism involves the recrystallization of a rock
Metasomatism involves mass transfer, not just
devolatilization ; commonly, introduced fluids carry
elemental constituents that react with minerals to form
new minerals. These fluid may be of various origins, but
often they are derived from magmas (which crystallize as
igneous plutonic rocks) which are also supplying heat.
Types of metamorphism
• Contact – high T, low P
• Hydrothermal – involving a fluid; often referred to as
hydrothermal alteration
• Regional – high T, can be high P
• Burial – a continuation of diagenesis; related to burial in
a sedimentary basin without active tectonism or
plutonism
T – P ranges are 100-800 OC and 1
bar(105Pa) to 10 kb (109 Pa)
Contact metamorphic aureole
Chapter 22: Foliated
Metamorphic Rocks
Slate: compact, very finegrained, metamorphic rock
with a well-developed
cleavage. Freshly cleaved
surfaces are dull
Phyllite: a rock with a
schistosity in which very fine
phyllosilicates
(sericite/phengite and/or
chlorite), although rarely
coarse enough to see unaided,
impart a silky sheen to the
foliation surface. Phyllites
with both a foliation and
lineation are very common.
a
b
Figure 22-1. Examples of foliated metamorphic rocks. a. Slate. b. Phyllite. Note the difference in reflectance on the foliation surfaces
between a and b: phyllite is characterized by a satiny sheen. Winter (2001) An Introduction to Igneous and Metamorphic Petrology.
Prentice Hall.
Chapter 22: Foliated
Metamorphic Rocks
Schist: a metamorphic rock
exhibiting a schistosity. By
this definition schist is a
broad term, and slates and
phyllites are also types of
schists. In common usage,
schists are restricted to those
metamorphic rocks in which
the foliated minerals are
coarse enough to see easily in
hand specimen.
Figure 22-1c. Garnet muscovite schist. Muscovite crystals are visible and silvery, garnets occur as large dark porphyroblasts.
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Chapter 22: A Classification of
Metamorphic Rocks
Additional Modifying Terms:
Porphyroblastic means that a metamorphic rock has
one or more metamorphic minerals that grew much
larger than the others. Each individual crystal is a
porphyroblast
Some porphyroblasts, particularly in low-grade contact
metamorphism, occur as ovoid “spots”
If such spots occur in a hornfels or a phyllite
(typically as a contact metamorphic overprint over a
regionally developed phyllite), the terms spotted
hornfels, or spotted phyllite would be appropriate.
Garnet Porphroblasts
Chapter 22: A Classification of
Metamorphic Rocks
Figure 23-14b. Spotted Phyllite. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Chapter 22: Foliated
Metamorphic Rocks
Gneiss: a metamorphic rock
displaying gneissose
structure. Gneisses are
typically layered (also called
banded), generally with
alternating felsic and darker
mineral layers. Gneisses may
also be lineated, but must
also show segregations of
felsic-mineral-rich and darkmineral-rich concentrations.
Figure 22-1d. Quartzo-feldspathic gneiss with obvious layering. Winter (2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
Slate
Schist
Phyllite
Gneiss
Foliation
Gneissic texture
Chapter 22: Specific
Metamorphic Rock Types
Marble: a metamorphic rock composed
predominantly of calcite or dolomite. The
protolith is typically limestone or dolostone.
Quartzite: a metamorphic rock composed
predominantly of quartz. The protolith is
typically sandstone. Some confusion may
result from the use of this term in sedimentary
petrology for a pure quartz sandstone.
Chapter 22: Non-Foliated
Metamorphic Rocks
Simpler than for foliated rocks
Again, this discussion and classification applies only to
rocks that are not produced by high-strain
metamorphism
Granofels: a comprehensive term for any isotropic
rock (a rock with no preferred orientation)
Hornfels is a type of granofels that is typically very
fine-grained and compact, and occurs in contact
aureoles. Hornfelses are tough, and tend to splinter
when broken.
Chapter 22: Specific
Metamorphic Rock Types
Skarn: a contact metamorphosed and silica
metasomatized carbonate rock containing
calc-silicate minerals, such as grossular,
epidote, tremolite, vesuvianite, etc. Tactite is a
synonym.
Granulite: a high grade rock of pelitic, mafic, or
quartzo-feldspathic parentage that is
predominantly composed of OH-free minerals.
Muscovite is absent and plagioclase and
orthopyroxene are common.
Chapter 22: Specific
Metamorphic Rock Types
Greenschist/Greenstone: a low-grade metamorphic
rock that typically contains chlorite, actinolite,
epidote, and albite. Note that the first three minerals
are green, which imparts the color to the rock. Such
a rock is called greenschist if foliated, and
greenstone if not. The protolith is either a mafic
igneous rock or graywacke.
Amphibolite: a metamorphic rock dominated by
hornblende + plagioclase. Amphibolites may be
foliated or non-foliated. The protolith is either a mafic
igneous rock or graywacke.
Chapter 22: Specific
Metamorphic Rock Types
Serpentinite: an ultramafic rock metamorphosed at low
grade, so that it contains mostly serpentine.
Blueschist: a blue amphibole-bearing metamorphosed
mafic igneous rock or mafic graywacke. This term is
so commonly applied to such rocks that it is even
applied to non-schistose rocks.
Eclogite: a green and red metamorphic rock that
contains clinopyroxene and garnet (omphacite +
pyrope). The protolith is typically basaltic.
Eclogite
Chapter 22: Specific
Metamorphic Rock Types
Migmatite: a composite silicate rock that is
heterogeneous on the 1-10 cm scale,
commonly having a dark gneissic matrix
(melanosome) and lighter felsic portions
(leucosome). Migmatites may appear layered,
or the leucosomes may occur as pods or form
a network of cross-cutting veins.
Index Minerals and Metamorphic Grade
Sillimanite
Andalusite
Kyanite
Staurolite
The aluminium silicates : kyanite, andalusite and sillimanite
Al2SiO5
Al2SiO5 ???? Why this formula for a silicate with
isolated tetrahedra?
Could be written Al.AlO(SiO4) because there are two
different Al structural sites as well as isolated [SiO4]
tetrahedra.
They also have been considered as belonging to a
separate silicate subclass – subsaturates, because
there are conceptually too few oxygens, a naïve
viewpoint.
Kyanite, andalusite and sillimanite are polymorphs, because they
have the same chemical composition but can exist with different
crystal structures.
The crystal structures of the Al2SiO5 polymorphs
All three structures have straight chains of edge-sharing AlO6 octahedra
along the c axis.
These chains contain half of the Al in the structural formula.
In kyanite the remaining Al atoms are in 6-fold coordination (octahedra)
In andalusite the remaining Al atoms are in 5-fold coordination
In sillimanite the remaining Al atoms are in 4-fold coordination (tetrahedra)
Density: kyanite > sillimanite > andalusite
Therefore kyanite is stable at the highest pressures and
lowest temperatures, while sillimanite is stable at high
temperatures and lower pressures
The crystal structures of the Al2SiO5 polymorphs
All three structures have straight chains of edge-sharing
AlO6 octahedra along the c axis
The structure of kyanite
1. The AlO6 octahedral chains
The structure of kyanite
1. The AlO6 octahedral chains
2. Add the other Al polyhedra
The structure of kyanite
1. The AlO6 octahedral chains
3. Add the [SiO4] tetrahedra
2. Add the other Al polyhedra
The structure of andalusite
1. The AlO6 octahedral chains
The structure of andalusite
1. The AlO6 octahedral chains
2. Add the other Al polyhedra
The structure of sillimanite
1. The AlO6 octahedral chains
The structure of sillimanite
1. The AlO6 octahedral chains
2. Add the other Al polyhedra
The structure of sillimanite
1. The AlO6 octahedral chains
3. Add the [SiO4] tetrahedra
2. Add the other Al polyhedra
Staurolite, Fe22+Al9O6[SiO4]4(O,OH)2
Monoclinic, 2/m (pseudo-orthorhombic)
Pressure (kbar)
The stability of the Al2SiO5 polymorphs
3.55-3.66
3.16-3.20
3.23
The Al2SiO5
polymorphs
form in
metamorphic
rocks
Effects of different protolith compositions
The Metamorphic Facies Concept
• If P-T conditions were the same, different
mineral assemblages must represent
different starting compositions (i.e.
different protoliths)
• If the starting compositions are identical,
then different mineral assemblages must
represent metamorphism under different
physical conditions
The garnet picture gallery
Garnet compositions
Grandite group
Grossular
Andradite
Uvarovite
Ca3Al2(SiO4)3
Ca3Fe3+2(SiO4)3
Ca3Cr2(SiO4)3
Pyralspite group
Pyrope
Almandine
Spessartine
Mg3Al2(SiO4)3
Fe2+3Al2(SiO4)3
Mn3Al2(SiO4)3
Solid solutions within each group are typical.
At high T (above 700oC) there is also solid solution between the 2 groups.
Note: Solid solutions are always more extensive at higher Temperatures
The garnet minerals
Garnets: A large group of cubic minerals with general
formula
A32+B23+(SiO4)3
A – Ca2+, Mg2+, Fe2+ or Mn2+
B – Al3+,Fe3+ or Cr3+
There are also many other synthetic compositions possible
Metamorphic facies
• High P/T series:
subduction zones
• Medium P/T series:
regional met.
• Low P/T series:
contact met.
Metamorphic facies
• See Table 14.2
• Greenschist,
amphibolite, granulite,
blueschist facies
• Minerals expected in
different rock
compositions.