Metamorphism – changes in mineralogy and texturebrought about by perturbations in heat and pressure

Compositional changes are generally restricted to thoserelated 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 formnew minerals. These fluid may be of various origins, butoften they are derived from magmas (which crystallize asigneous 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

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.

aa

bb

Slate: compact, very fine-grained, 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.

Chapter 22: FoliatedMetamorphic Rocks

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.

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.

Chapter 22: FoliatedMetamorphic 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.

Chapter 22: A Classification of Metamorphic Rocks

Garnet Porphroblasts

Figure 23-14b. Spotted Phyllite. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Chapter 22: A Classification of Metamorphic Rocks

Figure 22-1d. Quartzo-feldspathic gneiss with obvious layering. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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 dark-mineral-rich concentrations.

Chapter 22: FoliatedMetamorphic Rocks

Slate Phyllite

Schist Gneiss

Foliation Gneissic texture

Lineation Foliation plus lineation

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: SpecificMetamorphic Rock Types

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: Non-FoliatedMetamorphic Rocks

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: SpecificMetamorphic 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: SpecificMetamorphic 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.

Chapter 22: SpecificMetamorphic Rock Types

Eclogite

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.

Chapter 22: SpecificMetamorphic Rock Types

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 2. Add the other Al polyhedra

3. Add the [SiO4] tetrahedra

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 2. Add the other Al polyhedra3. Add the [SiO4] tetrahedra

Staurolite, Fe22+Al9O6[SiO4]4(O,OH)2

Monoclinic, 2/m (pseudo-orthorhombic)

The stability of the Al2SiO5 polymorphs

aaa

kyanite sillimanite

andalusite

(b)Temperature (oC)

200 400 600 800

2

4

6

8

10P

ress

ure

(kba

r)

The Al2SiO5 polymorphs form in metamorphic rocks

3.55-3.66

3.16-3.20

3.23

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

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

 Grandite group Grossular Ca3Al2(SiO4)3

Andradite Ca3Fe3+2(SiO4)3

Uvarovite Ca3Cr2(SiO4)3

 Pyralspite group Pyrope Mg3Al2(SiO4)3

Almandine Fe2+3Al2(SiO4)3

Spessartine Mn3Al2(SiO4)3

The garnet minerals

Garnets: A large group of cubic minerals with general formula

A32+B2

3+(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.