Download Lecture A - Aspects of Metamorphism

Introduction to Metamorphism 2
IN THIS LECTURE
• Importance of Understanding Metamorphic Mineral
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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
The Phase Rule in Metamorphic 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?
The Phase Rule in Metamorphic Systems
• 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
The Phase Rule in Metamorphic Systems
If C has been determined for a particular rock then
there are three potential situations according to the
phase rule
1. P=C
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2. P<C
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This is the standard divariant situation in metamorphic
rocks
The rock probably represents an equilibrium mineral
assemblage from within a metamorphic zone
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
The Phase Rule in Metamorphic Systems
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
3. P > C
• A more interesting situation, and at least one of three
situations must be responsible
The Phase Rule in Metamorphic Systems
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.
The Phase Rule in Metamorphic Systems
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
The Phase Rule in Metamorphic Systems
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
add
a) Components that generate a new phase
o
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 components
o
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 Mg
o
Al may substitute for Si
o
Na may substitute for K
The Phase Rule in Metamorphic Systems
3. The number of components was not correct
(cont.)
c) “Perfectly mobile” components
o
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
The Phase Rule in Metamorphic Systems
• Consider the very simple metamorphic system, MgOH2O
– 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
The Phase Rule in Metamorphic Systems
Once the water is gone, the excess periclase remains stable
as conditions change into the brucite stability field
Thus 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
The Phase Rule in Metamorphic Systems
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
The Phase Rule in Metamorphic Systems
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 MgOH2O 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
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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, 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
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
Orogenic Regional Metamorphism of
the Scottish Highlands
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.
Barrow’s
Area
Orogenic Regional Metamorphism of
the Scottish 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
Orogenic Regional Metamorphism of
the Scottish Highlands
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)
Barrovian Metamorphism of Pelites
• Sequence = Barrovian zones
• The P-T conditions referred to as Barrovian-type
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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 Zones in the Scottish
Highlands
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
Variations on the Barrovian Zones in
the Scottish Highlands
• A variation occurs in the area just to the north of
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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
Barrovian vs Buchan Metamorphism
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
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).
Contact Metamorphism of Pelitic
Rocks in the Skiddaw Aureole, UK
• Ordovician Skiddaw Slates (English Lake District) intruded
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by several granitic bodies
Intrusions are shallow, and contact effects overprinted on
an earlier low-grade regional orogenic metamorphism
Contact Metamorphism of Pelitic
Rocks in the Skiddaw Aureole, UK
• The aureole around the Skiddaw granite was sub-divided
into three zones, principally on the basis of textures:
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Increasing
Metamorphic
Grade
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o
o
o
Unaltered slates
Outer zone of spotted slates
Middle zone of andalusite slates
Inner zone of hornfels
Skiddaw granite
Contact Metamorphism of Pelitic
Rocks in the Skiddaw Aureole, UK
Geologic Map and crosssection 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.
Contact Metamorphism of Pelitic
Rocks in the Skiddaw 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
Contact Metamorphism of Pelitic
Rocks in the Skiddaw Aureole, UK
Inner zone:
•Thoroughly recrystallized
•Lose foliation
1 mm
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.
Contact Metamorphism of Pelitic
Rocks in the Skiddaw Aureole, UK
• The zones determined on a textural basis
• Better to use the sequential appearance of minerals
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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