Chapter 21: Metamorphism

1
Chapter 21 Metamorphism

• Fresh basalt and weathered basalt

2
Chapter 21 Metamorphism

• The IUGS-SCMR proposed this definition
• Metamorphism is a subsolidus process leading to
  changes in mineralogy and/or texture (for example
  grain size) and often in chemical composition in
  a rock. These changes are due to physical and/or
  chemical conditions that differ from those
  normally occurring at the surface of planets and
  in zones of cementation and diagenesis below this
  surface. They may coexist with partial melting.

3
The Limits of Metamorphism

• Low-temperature limit grades into diagenesis
• Processes are indistinguishable
• Metamorphism begins in the range of 100-150oC for
  the more unstable types of protolith
• Some zeolites are considered diagenetic and
  others metamorphic pretty arbitrary

4
The Limits of Metamorphism

• High-temperature limit grades into melting
• Over the melting range solids and liquids coexist
• Xenoliths, restites, and other enclaves?
• Migmatites (mixed rocks) are gradational

5
Metamorphic Agents and Changes

• Temperature typically the most important factor
  in metamorphism

Figure 1.9. Estimated ranges of oceanic and
continental steady-state geotherms to a depth of
100 km using upper and lower limits based on heat
flows measured near the surface. After Sclater et
al. (1980), Earth. Rev. Geophys. Space Sci., 18,
269-311.
6
Metamorphic Agents and Changes

• Increasing temperature has several effects
• 1) Promotes recrystallization ? increased grain
  size
• 2) Drive reactions (endothermic)
• 3) Overcomes kinetic barriers

7
Metamorphic Agents and Changes

• Pressure

• Normal gradients perturbed in several ways,
  most commonly
• High T/P geotherms in areas of plutonic activity
  or rifting
• Low T/P geotherms in subduction zones

8
Figure 21.1. Metamorphic field gradients
(estimated P-T conditions along surface traverses
directly up metamorphic grade) for several
metamorphic areas. After Turner (1981).
Metamorphic Petrology Mineralogical, Field, and
Tectonic Aspects. McGraw-Hill.
9
Metamorphic Agents and Changes

• Metamorphic grade a general increase in degree
  of metamorphism without specifying the exact
  relationship between temperature and pressure

10
Metamorphic Agents and Changes

• Lithostatic pressure - uniform stress
  (hydrostatic)
• Deviatoric stress pressure unequal in different
  directions
• Resolved into three mutually perpendicular stress
  (s) components
• s1 is the maximum principal stress
• s2 is an intermediate principal stress
• s3 is the minimum principal stress
• In hydrostatic situations all three are equal

11
Metamorphic Agents and Changes

• Stress
• Strain deformation
• Deviatoric stress affects the textures and
  structures, but not the equilibrium mineral
  assemblage
• Strain energy may overcome kinetic barriers to
  reactions

12

• Foliation is a common result, which allows us to
  estimate the orientation of s1

s1
Strain ellipsoid

• s1 gt s2 s3 ? foliation and no lineation
• s1 s2 gt s3 ? lineation and no foliation
• s1 gt s2 gt s3 ? both foliation and lineation

Figure 21.3. Flattening of a ductile homogeneous
sphere (a) containing randomly oriented flat
disks or flakes. In (b), the matrix flows with
progressive flattening, and the flakes are
rotated toward parallelism normal to the
predominant stress. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
13
Metamorphic Agents and Changes
Shear motion occurs along planes at an angle to s1
s1
Figure 21.2. The three main types of deviatoric
stress with an example of possible resulting
structures. b. Shear, causing slip along parallel
planes and rotation. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
14
Metamorphic Agents and Changes
Fluids

• Evidence for the existence of a metamorphic
  fluid
• Fluid inclusions
• Fluids are required for hydrous or carbonate
  phases
• Volatile-involving reactions occur at
  temperatures and pressures that require finite
  fluid pressures

15
Metamorphic Agents and Changes

• Pfluid S partial pressures of each component
  (Pfluid pH2O pCO2 )
• Mole fractions of components must sum to 1.0
  (XH2O XCO2 1.0)
• pH2O XH2O x Pfluid
• Gradients in T, P, Xfluid
• Zonation in mineral assemblages

16
The Types of Metamorphism
Different approaches to classification

• 1. Based on principal process or agent
• Dynamic Metamorphism
• Thermal Metamorphism
• Dynamo-thermal Metamorphism

17
The Types of Metamorphism
Different approaches to classification

• 2. Based on setting
• Contact Metamorphism
• Pyrometamorphism
• Regional Metamorphism
• Orogenic Metamorphism
• Burial Metamorphism
• Ocean Floor Metamorphism
• Hydrothermal Metamorphism
• Fault-Zone Metamorphism
• Impact or Shock Metamorphism

18
The Types of Metamorphism
Contact Metamorphism The size and shape of an
aureole is controlled by

• The nature of the pluton

• Size
• Shape
• Orientation

• Temperature
• Composition

• The nature of the country rocks

• Composition
• Depth and metamorphic grade prior to intrusion
• Permeability

19
Contact Metamorphism

• Adjacent to igneous intrusions
• Thermal ( metasomatic) effects of hot magma
  intruding cooler shallow rocks
• Occurs over a wide range of pressures, including
  very low
• Contact aureole

20
The Types of Metamorphism
Contact Metamorphism Most easily recognized
where a pluton is introduced into shallow rocks
in a static environment
Hornfelses (granofelses) commonly with relict
textures and structures
21
The Types of Metamorphism
Contact Metamorphism Polymetamorphic rocks are
common, usually representing an orogenic event
followed by a contact one

• Spotted phyllite (or slate)
• Overprint may be due to
• Lag time for magma migration
• A separate phase of post-orogenic collapse
  magmatism (Chapter 18)

22
The Types of Metamorphism
Pyrometamorphism
Very high temperatures at low pressures,
generated by a volcanic or sub-volcanic body
Also developed in xenoliths
23
The Types of Metamorphism
Regional Metamorphism sensu lato metamorphism
that affects a large body of rock, and thus
covers a great lateral extent

• Three principal types
• Orogenic metamorphism
• Burial metamorphism
• Ocean-floor metamorphism

24
The Types of Metamorphism
Orogenic Metamorphism is the type of
metamorphism associated with convergent plate
margins

• Dynamo-thermal one or more episodes of orogeny
  with combined elevated geothermal gradients and
  deformation (deviatoric stress)
• Foliated rocks are a characteristic product

25
The Types of Metamorphism
Orogenic Metamorphism
Figure 21.6. Schematic model for the sequential
(a ? c) development of a Cordilleran-type or
active continental margin orogen. The dashed and
black layers on the right represent the basaltic
and gabbroic layers of the oceanic crust. From
Dewey and Bird (1970) J. Geophys. Res., 75,
2625-2647 and Miyashiro et al. (1979) Orogeny.
John Wiley Sons.
26
The Types of Metamorphism
Orogenic Metamorphism
27
The Types of Metamorphism
Orogenic Metamorphism

• Uplift and erosion
• Metamorphism often continues after major
  deformation ceases
• Metamorphic pattern is simpler than the
  structural one
• Pattern of increasing metamorphic grade from both
  directions toward the core area

28
The Types of Metamorphism
Orogenic Metamorphism

• Polymetamorphic patterns
• Continental collision
• Batholiths are usually present in the highest
  grade areas
• If plentiful and closely spaced, may be called
  regional contact metamorphism

29
The Types of Metamorphism
Burial metamorphism

• Southland Syncline in New Zealand thick pile (gt
  10 km) of Mesozoic volcaniclastics
• Mild deformation, no igneous intrusions
  discovered
• Fine-grained, high-temperature phases, glassy
  ash very susceptible to metamorphic alteration
• Metamorphic effects attributed to increased
  temperature and pressure due to burial
• Diagenesis grades into the formation of zeolites,
  prehnite, pumpellyite, laumontite, etc.

30
The Types of Metamorphism

• Hydrothermal metamorphism
• Hot H2O-rich fluids
• Usually involves metasomatism
• Difficult type to constrain hydrothermal effects
  often play some role in most of the other types
  of metamorphism

31
The Types of Metamorphism
Burial metamorphism occurs in areas that have not
experienced significant deformation or orogeny

• Restricted to large, relatively undisturbed
  sedimentary piles away from active plate margins
• The Gulf of Mexico?
• Bengal Fan?

32
The Types of Metamorphism
Burial metamorphism occurs in areas that have not
experienced significant deformation or orogeny

• Bengal Fan ? sedimentary pile gt 22 km
• Extrap. ? 250-300oC at the base (P 0.6 GPa)
• Passive margins often become active
• Areas of burial metamorphism may thus become
  areas of orogenic metamorphism

33
The Types of Metamorphism
Ocean-Floor Metamorphism affects the oceanic
crust at ocean ridge spreading centers

• Considerable metasomatic alteration, notably loss
  of Ca and Si and gain of Mg and Na
• Highly altered chlorite-quartz rocks- distinctive
  high-Mg, low-Ca composition
• Exchange between basalt and hot seawater
• Another example of hydrothermal metamorphism

34
The Types of Metamorphism

• Fault-Zone and Impact Metamorphism
• High rates of deformation and strain with only
  minor recrystallization

• Impact metamorphism at meteorite (or other
  bolide) impact craters
• Both correlate with dynamic metamorphism, based
  on process

35
(a) Shallow fault zone with fault breccia (b)
Slightly deeper fault zone (exposed by erosion)
with some ductile flow and fault mylonite
Figure 21.7. Schematic cross section across fault
zones. After Mason (1978) Petrology of the
Metamorphic Rocks. George Allen Unwin. London.
36
Prograde Metamorphism

• Prograde increase in metamorphic grade with time
  as a rock is subjected to gradually more severe
  conditions
• Prograde metamorphism changes in a rock that
  accompany increasing metamorphic grade
• Retrograde decreasing grade as rock cools and
  recovers from a metamorphic or igneous event
• Retrograde metamorphism any accompanying changes

37
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
38
The Progressive Nature of Metamorphism
Retrograde metamorphism typically of minor
significance

• 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

39
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

40
Why Study Metamorphism?

• Interpretation of the conditions and evolution of
  metamorphic bodies, mountain belts, and
  ultimately the state and 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

41
Orogenic Regional Metamorphism of the Scottish
Highlands

• George Barrow (1893, 1912)
• SE Highlands of Scotland - Caledonian Orogeny
  500 Ma
• Nappes
• Granites

42
Barrows Area
Figure 21.8. 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.
43
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

44
The sequence of zones now recognized, and the
typical metamorphic mineral assemblage in each,
are

• 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, uartz,
  plagioclase, garnet, and perhaps staurolite. Some
  kyanite may also be present (although kyanite and
  sillimanite are both polymorphs of Al2SiO5)

45

• 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)

46
Figure 21.8. 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.
47
To summarize

• An isograd 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
• Because 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

48
A variation occurs in the area just to the north
of Barrows, in the Banff and Buchan district

• Pelitic compositions are similar, but the
  sequence of isograds is
• chlorite
• biotite
• cordierite
• andalusite
• sillimanite

49
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
Figure 21.9. 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).
50
Regional Burial MetamorphismOtago, New Zealand

• Jurassic graywackes, tuffs, and volcanics in a
  deep trough metamorphosed in the Cretaceous
• Fine grain size and immature material is highly
  susceptible to alteration (even at low grades)

51
Regional Burial MetamorphismOtago, New Zealand
Section X-Y shows more detail
Figure 21.10. Geologic sketch map of the South
Island of New Zealand showing the Mesozoic
metamorphic rocks east of the older Tasman Belt
and the Alpine Fault. The Torlese Group is
metamorphosed predominantly in the
prehnite-pumpellyite zone, and the Otago Schist
in higher grade zones. X-Y is the Haast River
Section of Figure 21-11. From Turner (1981)
Metamorphic Petrology Mineralogical, Field, and
Tectonic Aspects. McGraw-Hill.
52
Regional Burial MetamorphismOtago, New Zealand

• Isograds mapped at the lower grades
• 1) Zeolite
• 2) Prehnite-Pumpellyite
• 3) Pumpellyite (-actinolite)
• 4) Chlorite (-clinozoisite)
• 5) Biotite
• 6) Almandine (garnet)
• 7) Oligoclase (albite at lower grades is
  replaced by a more calcic plagioclase)

53
Regional Burial Metamorphism
Figure 21.11. Metamorphic zones of the Haast
Group (along section X-Y in Figure 21-10). After
Cooper and Lovering (1970) Contrib. Mineral.
Petrol., 27, 11-24.
54
Paired Metamorphic Belts of Japan
Figure 21.12. The Sanbagawa and Ryoke metamorphic
belts of Japan. From Turner (1981) Metamorphic
Petrology Mineralogical, Field, and Tectonic
Aspects. McGraw-Hill and Miyashiro (1994)
Metamorphic Petrology. Oxford University Press.
55
Paired Metamorphic Belts of Japan
56
Figure 21.13. Some of the paired metamorphic
belts in the circum-Pacific region. From
Miyashiro (1994) Metamorphic Petrology. Oxford
University Press.
57
Contact Metamorphism of Pelitic Rocks in the
Skiddaw Aureole, UK

• Ordovician Skiddaw Slates (English Lake District)
  intruded by several granitic bodies
• Intrusions are shallow
• Contact effects overprinted on an earlier
  low-grade regional orogenic metamorphism

58
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
• Unaltered slates
• Outer zone of spotted slates
• Middle zone of andalusite slates
• Inner zone of hornfels
• Skiddaw granite

Increasing Metamorphic Grade
Contact
59
Figure 21.14. 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.
60
Contact Metamorphism of Pelitic Rocks in the
Skiddaw Aureole, UK

• Middle zone slates more thoroughly
  recrystallized, contain biotite muscovite
  cordierite andalusite quartz

Figure 21.15. 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
61
(No Transcript)
62
Contact Metamorphism of Pelitic Rocks in the
Skiddaw Aureole, UK
Inner zone Thoroughly recrystallized Lose
foliation
1 mm
Figure 21.16. 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.
63
Contact Metamorphism of Pelitic Rocks in the
Skiddaw Aureole, UK

• The zones determined on a textural basis

• Prefer to use the sequential appearance of
  minerals and isograds to define zones
• But low-P isograds converge in P-T
• Skiddaw sequence of mineral development with
  grade is difficult to determine accurately

64
Contact Metamorphism and Skarn Formation at
Crestmore, CA, USA

• Crestmore quarry in the Los Angeles basin
• Quartz monzonite porphry intrudes Mg-bearing
  carbonates (either late Paleozoic or Triassic)

• Burnham (1959) mapped the following zones and the
  mineral assemblages in each (listed in order of
  increasing grade)

65

• Forsterite Zone
• calcite brucite clinohumite spinel
• calcite clinohumite forsterite spinel
• calcite forsterite spinel clintonite
• Monticellite Zone
• calcite forsterite monticellite clintonite
• calcite monticellite melilite clintonite
• calcite monticellite spurrite (or tilleyite)
  clintonite
• monticellite spurrite merwinite melilite
• Vesuvianite Zone
• vesuvianite monticellite spurrite merwinite
  melilite
• vesuvianite monticellite diopside
  wollastonite
• Garnet Zone
• grossular diopside wollastonite

66
Contact Metamorphism and Skarn Formation at
Crestmore, CA, USA
An idealized cross-section through the aureole
Figure 21.17. Idealized N-S cross section (not to
scale) through the quartz monzonite and the
aureole at Crestmore, CA. From Burnham (1959)
Geol. Soc. Amer. Bull., 70, 879-920.
67
Contact Metamorphism and Skarn Formation at
Crestmore, CA, USA

1. The mineral associations in successive zones (in
   all metamorphic terranes) vary by the formation
   of new minerals as grade increases

This can only occur by a chemical reaction in
which some minerals are consumed and others
produced
68
Contact Metamorphism and Skarn Formation at
Crestmore, CA, USA
a) Calcite brucite clinohumite spinel zone
to the Calcite clinohumite forsterite
spinel sub-zone involves the reaction 2
Clinohumite SiO2 ? 9 Forsterite 2 H2O

• b) Formation of the vesuvianite zone involves the
  reaction
• Monticellite 2 Spurrite 3 Merwinite
  4 Melilite
• 15 SiO2 12 H2O ? 6 Vesuvianite
  2 CO2

69
Contact Metamorphism and Skarn Formation at
Crestmore, CA, USA
2) Find a way to display data in simple, yet
useful ways
If we think of the aureole as a chemical system,
we note that most of the minerals consist of the
components CaO-MgO-SiO2-CO2-H2O (with minor Al2O3)
70
Figure 21.18. CaO-MgO-SiO2 diagram at a fixed
pressure and temperature showing the
compositional relationships among the minerals
and zones at Crestmore. Numbers correspond to
zones listed in the text. After Burnham (1959)
Geol. Soc. Amer. Bull., 70, 879-920 and Best
(1982) Igneous and Metamorphic Petrology. W. H.
Freeman.
Zones are numbered (from outside inward)
71
Figures not used
Figure 21.4. A situation in which lithostatic
pressure (Plith) exerted by the mineral grains is
greater than the intergranular fluid pressure
(Pfluid). At a depth around 10 km (or T around
300oC) minerals begin to yield or dissolve at the
contact points and shift toward or precipitate in
the fluid-filled areas, allowing the rock to
compress. The decreased volume of the pore spaces
will raise Pfluid until it equals Plith. Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
72
Figures not used
Figure 21.5. Temperature distribution within a
1-km thick vertical dike and in the country rocks
(initially at 0oC) as a function of time. Curves
are labeled in years. The model assumes an
initial intrusion temperature of 1200oC and
cooling by conduction only. After Jaeger, (1968)
Cooling and solidification of igneous rocks. In
H. H. Hess and A. Poldervaart (eds.), Basalts,
vol. 2. John Wiley Sons. New York, pp. 503-536.