Chapter 21: Metamorphism

1
Metamorphic Zones, Index Minerals, Isograds,
Facies and Facies Seriesthe onslaught of
terminology to understand how we categorize
metamorphic rocks and their conditions of
formation!
2
What textures do you see?
3
Review Types of Protolith
1.Pelitic/mudrocks - high Al, K,
Si 2. Quartzo-feldspathic - high Si, Na, K,
Al 3. Calcareous- high Ca, Mg, CO2 4.
Mafic/ultramafic- very high to high Mg, Fe, Ni,
Cr, Ca
WHY IS CHEMICAL COMPOSITION OF PROTOLITH
IMPORTANT?
4
Orogenic Regional Metamorphism of the Scottish
Highlands Development of the Index Mineral
Concept

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

5
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.
6
Orogenic Regional Metamorphism of the Scottish
Highlands

• Barrow studied 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

7
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, quartz,
  plagioclase, garnet, and perhaps staurolite. Some
  kyanite may also be present (although kyanite and
  sillimanite are both polymorphs of Al2SiO5)

8
Each of these minerals is an INDEX mineral.

• Chlorite zone
• Biotite zone
• Garnet zone
• Staurolite zone
• Kyanite zone
• Sillimanite zone

WHAT IS AN INDEX MINERAL
9

• 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
• ANOTHER DEFINTION
• Isograd

line that separates the zones (a line in the
field of constant metamorphic grade). Also
reflects the FIRST APPEARANCE of the index
mineral.
10
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.
11
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

12
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
• garnet
• andalusite
• kyanite
• sillimanite

13
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
1 GPa 10kbars
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).
14
Metamorphic Facies

• Eskola (1915) developed the concept of
  metamorphic facies
• What is a metamorphic facies?

15
Metamorphic Facies
Fig. 25-2. Temperature-pressure diagram showing
the generally accepted limits of the various
facies used in this text. Boundaries are
approximate and gradational. The typical or
average continental geotherm is from Brown and
Mussett (1993). Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
16
Metamorphic Facies

• The range of temperature and pressure conditions
  represented by each facies
• Eskola aware of the P-T implications and
  correctly deduced the relative temperatures and
  pressures of facies he proposed
• Can now assign relatively accurate temperature
  and pressure limits to individual facies

17
Metamorphic Facies

• Eskola (1920) proposed 5 original facies
• Greenschist
• Amphibolite
• Hornfels
• Sanidinite
• Eclogite
• Easily defined on the basis of mineral
  assemblages that develop in mafic rocks
• More facies have been added since original
  designations

18
Metamorphic Facies

• Fig. 25-1 The metamorphic facies proposed by
  Eskola and their relative temperature-pressure
  relationships. After Eskola (1939) Die Entstehung
  der Gesteine. Julius Springer. Berlin.

19
Metamorphic Facies
Fig. 25-2. Temperature-pressure diagram showing
the generally accepted limits of the various
facies used in this text. Boundaries are
approximate and gradational. The typical or
average continental geotherm is from Brown and
Mussett (1993). Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
20
Metamorphic Facies defined for mafic protolith

• The definitive mineral assemblages that
  characterize each facies (for mafic rocks).

21

• It is convenient to consider metamorphic facies
  in 4 groups
• 1) Facies of high pressure
• The blueschist and eclogite facies low molar
  volume phases under conditions of high pressure
• Blueschist facies occurs in areas of low T/P
  gradients, characteristically developed in
  subduction zones
• Eclogites are stable under normal geothermal
  conditions
• May develop wherever mafic magmas solidify in the
  deep crust or mantle crustal chambers or dikes,
  sub-crustal magmatic underplates, subducted crust
  that is redistributed into the mantle

22
Metamorphic Facies

• 2) Facies of medium pressure
• Most metamorphic rocks now exposed belong to the
  greenschist, amphibolite, or granulite facies
• The greenschist and amphibolite facies conform to
  the typical geothermal gradient

23
Metamorphic Facies

• 3) Facies of low pressure
• Albite-epidote hornfels, hornblende hornfels, and
  pyroxene hornfels facies contact metamorphic
  terranes and regional terranes with very high
  geothermal gradient.

• Sanidinite facies is rare- limited to xenoliths
  in basic magmas and the innermost portions of
  some contact aureoles adjacent to hot basic
  intrusives

24
Metamorphic Facies

• 4) Facies of low grades
• Rocks often fail to recrystallize thoroughly at
  very low grades, and equilibrium is not always
  attained

• Zeolite and prehnite-pumpellyite facies are thus
  not always represented, and the greenschist
  facies is the lowest grade developed in many
  regional terranes

25
Metamorphic Facies

• Review
• Metamorphic zone (e.g., chlorite zone)
• Index Mineral
• Isograd
• Metamorphic Facies

26
Example of Contact Metamorphism
What does this diagram show? ------------------gt
Explain how the metamorphic grade and
assemblages MIGHT change with distance from this
dike.
27
Facies Series/Field Gradient

• A traverse up grade through a metamorphic terrane
  should follow one of several possible metamorphic
  field gradients, and, if extensive enough, cross
  through a sequence of facies

28
Fig. 25-3. Temperature-pressure diagram showing
the three major types of metamorphic facies
series proposed by Miyashiro (1973, 1994). Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
29
Pressure-Temperature Time Paths

• Facies concept leads to idea that metamorphic
  petrologists try to reconstruct CONDITIONS of
  metamorphism.
• Also important is TIME. Time tells us about the
  RATES of processes.

30
Regional Metamorphism
3 stages Burial/crustal thickening--why does
trajectory have steep slope? Heating
stage Uplift stage
31
Regional Metamorphism
What are prograde vs. retrograde metamorphic
paths or reactions?
32
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.
33
Fig. 25-9. Typical mineral changes that take
place in metabasic rocks during progressive
metamorphism in the medium P/T facies series. The
approximate location of the pelitic zones of
Barrovian metamorphism are included for
comparison. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
34
Regional Burial MetamorphismOtago, New Zealand

• Jurassic graywackes, tuffs, and volcanics in a
  deep trough metamorphosed in the Cretaceous

35
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.
36
Regional Burial MetamorphismOtago, New Zealand

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

37
Contact Metamorphism and Skarn Formation at
Crestmore, CA, USA

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

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

38

• 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

39
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.
40
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