Introduction to Metamorphism 2 IN THIS LECTURE Importance of Understanding Metamorphic Mineral...

Introduction to Metamorphism 2

IN THIS LECTURE

• Importance of Understanding Metamorphic Mineral Assemblages

• Progressive Nature of Metamorphism

• Stable Mineral Assemblages

• The Phase Rule in Metamorphic Systems

• The MgO-H2O system

What’s so Important About Metamorphic Mineral

Assemblages• Equilibrium mineral assemblages can tell us

about P-T conditions

• P-T conditions tell us about tectonic environments

• P-T conditions combined with geochronology tells us about how tectonic environments evolve through time

• Optical Mineralogy is the starting point!!

The Progressive Nature of Metamorphism

• A rock at a high metamorphic grade probably progressed through a sequence of mineral assemblages rather than hopping directly from an unmetamorphosed rock to the metamorphic rock that we find today

• All rocks that we now find must also have cooled to surface conditions. Therefore, at what point on its cyclic P-T-t path did its present mineral assemblage last equilibrate?

• The preserved zonal distribution of metamorphic rocks suggests that each rock preserves the conditions of the maximum metamorphic grade (temperature)

The Progressive Nature of Metamorphism

• Prograde reactions are endothermic and easily driven by increasing T

• Devolatilization reactions are easier than reintroducing the volatiles

• Geothermometry indicates that the mineral compositions commonly preserve the maximum temperature

Stable Mineral Assemblages in Metamorphic Rocks

Equilibrium Mineral Assemblages

• At equilibrium, the mineralogy (and the composition of each mineral) is determined by T, P, and X

• “Mineral paragenesis” refers to such an equilibrium mineral assemblage

• Relict minerals or later alteration products are thereby excluded from consideration unless specifically stated

The Phase Rule in Metamorphic Systems

• Phase rule, as applied to systems at equilibrium:

F = C - P + 2 the phase rule

P is the number of phases in the system

C is the number of components: the minimum number of chemical constituents required to specify every phase in the system

F is the number of degrees of freedom: the number of independently variable intensive parameters of state (such as temperature, pressure, the composition of each phase, etc.)

• Remember we’ve seen this already with igneous systems


• Pick a random point anywhere on a phase diagram

– Likely point will be within a divariant field and not on a univariant curve or invariant point

• The most common situation is divariant (F = 2), meaning that P and T are independently variable without affecting the mineral assemblage

Remember this diagram?


• If F 2 is the most common situation, then the phase rule may be adjusted accordingly such that

F = C - P + 2 2

and therefore P C

• This is Goldschmidt’s mineralogical phase rule, or simply the mineralogical phase rule


If C has been determined for a particular rock then there are three potential situations according to the phase rule

1. P=C• This is the standard divariant situation in metamorphic

rocks• The rock probably represents an equilibrium mineral

assemblage from within a metamorphic zone

2. P<C• A situation that commonly arises in systems that

display solid solution.• We’ve seen this already with the binary phase

diagrams for the albite-anorthite system

Albite-Anorthite Phase Diagram


If C has been determined for a particular rock then there are three potential situations according to the phase rule

1. P = C• This is the standard divariant situation in metamorphic

rocks• The rock probably represents an equilibrium mineral

assemblage from within a metamorphic zone

2. P < C• A situation that commonly arises in systems that display

solid solution.• We’ve seen this already with the binary phase diagrams

for the albite-anorthite system3. P > C

• A more interesting situation, and at least one of three situations must be responsible


For P > C then the following three situations could apply

1. F < 2

2. Equilibrium not attained

3. Choice of C not correct

If (1) applies then the sample was

collected from a location right on

a univariant reaction curve or

invariant point

The P-T phase diagram for the system Al2SiO5 calculated using the program TWQ (Berman, 1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


2. Equilibrium is not attained

• The situation with (2) is more important in terms of optical mineralogy.

• The phase rule only applies to systems that are in equilibrium.

• If equilibrium is not attained or maintained then there could be any number of minerals co-existing

• Unfortunately, this is often the case, especially in rocks that have been partially retrogressed or rocks in the blueschist and eclogite facies.

• Optical Mineralogy is the main tool for decided which minerals represent the equilibrium mineral assemblage


3. The number of components was not correct• Some guidelines for an appropriate choice of C

– Begin with a 1-component system, such as CaAl2Si2O8 (anorthite), there are 3 common types of major/minor components that we can adda) Components that generate a new phaseo Adding a component such as CaMgSi2O6 (diopside),

results in an additional phase: in the binary Di-An system diopside coexists with anorthite below the solidus

b) Components that substitute for other componentso Adding a component such as NaAlSi3O8 (albite) to the 1-C

anorthite system would dissolve in the anorthite structure, resulting in a single solid-solution mineral (plagioclase) below the solidus

o Fe and Mn commonly substitute for Mgo Al may substitute for Sio Na may substitute for K


3. The number of components was not correct (cont.)

c) “Perfectly mobile” componentso Either a freely mobile fluid component or a

component that dissolves in a fluid phase and can be transported easily

o The chemical activity of such components is commonly controlled by factors external to the local rock system

o They are commonly ignored in deriving C for metamorphic systems


• Consider the very simple metamorphic system, MgO-H2O

– Possible natural phases in this system are periclase (MgO), aqueous fluid (H2O), and brucite (Mg(OH)2)

– How we deal with H2O depends upon whether water is perfectly mobile or not

– A reaction can occur between the potential phases in this system:

MgO + H2O Mg(OH)2 Per + Fluid = Bru– As written this is a retrograde reaction (occurs as the rock

cools and hydrates)

The System MgO-H2O

Cool to the temperature of the reaction curve, periclase reacts with water to form brucite: MgO + H2O Mg(OH)2

The System MgO-H2O

Reaction: periclase coexists with brucite:

P = C + 1

F = 1 (2nd reason to violate the mineralogical phase rule)

To leave the curve, all the periclase must be consumed by the reaction, and brucite is the solitary remaining phase

F = 1 and C = 1 again


Once the water is gone, the excess periclase remains stable as conditions change into the brucite stability fieldThus periclase can be stable anywhere on the whole diagram, if water is present in insufficient quantities to permit the reaction to brucite to go to completion


At any point (other than on the univariant curve itself) we would expect to find two phases, not one

P = brucite + periclase below the reaction curve (if water is limited), or periclase + water above the curve


How do you know which way is correct?

• The rocks should tell you– The phase rule is an interpretive tool, not a predictive tool,

and does not tell the rocks how to behave– If you only see low-P assemblages (e.g. Per or Bru in the

MgO-H2O system), then some components may be mobile

– If you often observe assemblages that have many phases in an area (e.g. periclase + brucite), it is unlikely that so much of the area is right on a univariant curve, and may require the number of components to include otherwise mobile phases, such as H2O or CO2, in order to apply the phase rule correctly

Metamorphism of Pelites

IN THIS LECTURE

– Types of Protoliths– Examples of Metamorphism– Orogenic Metamorphism of the Scottish Highlands– Barrovian vs Buchan Style Metamorphism– Regional Metamorphism Otago New Zealand– Contact Metamorphism of Pelitic Rocks

Types of Protolith

Lump the common types of sedimentary and igneous rocks into six chemically based-groups

1. Ultramafic - very high Mg, Fe, Ni, Cr2. Mafic - high Fe, Mg, and Ca3. Shales (pelitic) - high Al, K, Si4. Carbonates- high Ca, Mg, CO2

5. Quartz - nearly pure SiO2.

6. Quartzo-feldspathic - high Si, Na, K, Al

Some Examples of Metamorphism

• Interpretation of the conditions and evolution of metamorphic bodies, mountain belts, and ultimately the evolution of the Earth's crust

• Metamorphic rocks may retain enough inherited information from their protolith to allow us to interpret much of the pre-metamorphic history as well

• When combined with geochemical and structural information can be used to reconstruct the tectonic environment

Orogenic Regional Metamorphism of the Scottish

Highlands• George Barrow (1893, 1912)

• SE Highlands of Scotland– In Europe Caledonian orogeny ~ 500 Ma– In Africa and other parts of Gondwana Pan-African Orogeny

• Nappes

• Granites

Barrow’s Area

Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London.


Highlands


Highlands• Barrow studied the pelitic rocks

• Could subdivide the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increased

The sequence of zones now recognized, and the typical metamorphic mineral assemblage in each, are:

1. Chlorite zone. Pelitic rocks are slates or phyllites and typically contain chlorite, muscovite, quartz and albite

2. Biotite zone. Slates give way to phyllites and schists, with biotite, chlorite, muscovite, quartz, and albite

3. Garnet zone. Schists with conspicuous red almandine garnet, usually with biotite, chlorite, muscovite, quartz, and albite or oligoclase

4. Staurolite zone. Schists with staurolite, biotite, muscovite, quartz, garnet, and plagioclase. Some chlorite may persist

5. Kyanite zone. Schists with kyanite, biotite, muscovite, quartz, plagioclase, and usually garnet and staurolite

6. 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)


Highlands

• Sequence = Barrovian zones

• The P-T conditions referred to as Barrovian-type metamorphism (fairly typical of many belts)

• Now extended to a much larger area of the Highlands

• Isograd = line that separates the zones (a line in the field of constant metamorphic grade)

Barrovian Metamorphism of Pelites

Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London.

Barrovian Zones in the Scottish Highlands

To Summarise

• An isograd (in this classical sense) represents the first appearance of a particular metamorphic index mineral in the field as one progresses up metamorphic grade

• When one crosses an isograd, such as the biotite isograd, one enters the biotite zone

• Zones thus have the same name as the isograd that forms the low-grade boundary of that zone

• Since classic isograds are based on the first appearance of a mineral, and not its disappearance, an index mineral may still be stable in higher grade zones

Barrovian Zones in the Scottish Highlands

• A variation occurs in the area just to the north of Barrow’s, in the Banff and Buchan district

• Here the pelitic compositions are similar, but the sequence of isograds is: – chlorite– biotite– cordierite– andalusite– sillimanite

Variations on the Barrovian Zones in the Scottish Highlands

The P-T phase diagram for the system Al2SiO5 showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite. Also shown is the hydration of Al2SiO5 to pyrophyllite, which limits the occurrence of an Al2SiO5 polymorph at low grades in the presence of excess silica and water. The diagram was calculated using the program TWQ (Berman, 1988, 1990, 1991).

The stability field of andalusite occurs at pressures less than 0.37 GPa (~ 10 km), while kyanite sillimanite at the sillimanite isograd only above this pressure

Barrovian vs Buchan Metamorphism

Contact Metamorphism of Pelitic Rocks in the Skiddaw

Aureole, UK

• Ordovician Skiddaw Slates (English Lake District) intruded by several granitic bodies

• Intrusions are shallow, and contact effects overprinted on an earlier low-grade regional orogenic metamorphism


Aureole, UK

• The aureole around the Skiddaw granite was sub-divided into three zones, principally on the basis of textures:

o Unaltered slateso Outer zone of spotted slateso Middle zone of andalusite slateso Inner zone of hornfelso Skiddaw granite

Increasing Metamorphic Grade

Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. After Eastwood et al (1968). Geology of the Country around Cockermouth and Caldbeck. Explanation accompanying the 1-inch Geological Sheet 23, New Series. Institute of Geological Sciences. London.


Aureole, UK


Aureole, UK• Middle zone: slates more thoroughly recrystallized, contain biotite

+ muscovite + cordierite + andalusite + quartz

Cordierite-andalusite slate from the middle zone of the Skiddaw aureole. From Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London.

1 mm


Aureole, UKInner zone:

•Thoroughly recrystallized

•Lose foliation

Andalusite-cordierite schist from the inner zone of the Skiddaw aureole. Note the chiastolite cross in andalusite (see also Figure 22-49). From Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London.

1 mm


Aureole, UK• The zones determined on a textural basis

• Better to use the sequential appearance of minerals and isograds to define the zones

• But low-P isograds converge in P-T

• Skiddaw sequence of mineral development with grade is difficult to determine accurately

Pelites in Southern Africa

• Barberton Granite-Greenstone Belt, Mpumalanga

• Damara Orogen, Namibia

• Contact metamorphism associated with Bushveld Complex, Limpopo Province

Introduction to Metamorphism 2 IN THIS LECTURE Importance of Understanding Metamorphic Mineral...

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