Chapter%2021:%20Metamorphism

1
Chapter 21 Metamorphism

• Rocks as chemical systems (Ch. 5)
• a particular assemblage of coexisting phases
  (thermodynamic equilibrium and the phase rule)
• A basaltic composition can be either
• Melt
• Cpx plag (? olivine, ilmenite)
• Or any combination of melt minerals along the
  liquid line of descent
• If uplifted and eroded ? surface, will weather ?
  a combinations of clays, oxides

2
Chapter 21 Metamorphism

• The IUGS-SCMR has proposed the following
  definition of metamorphism
• 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
• The boundary is somewhat arbitrary
• Diagenetic/weathering processes are
  indistinguishable from metamorphic
• 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
• If we heat a metamorphic rock until it melts, at
  what point in the melting process does it become
  igneous?
• Xenoliths, restites, and other enclaves are
  considered part of the igneous realm because melt
  is dominant, but the distinction is certainly
  vague and disputable
• 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
• Larger surface/volume ratio of a mineral ? lower
  stability
• Increasing temperature eventually overcomes
  kinetic barriers to recrystallization, and fine
  aggregates coalesce to larger grains

7
Metamorphic Agents and Changes

• Increasing temperature has several effects
• 2) Drive reactions that consume unstable
  mineral(s) and produces new minerals that are
  stable under the new conditions

3) Overcomes kinetic barriers that might
otherwise preclude the attainment of equilibrium
8
Metamorphic Agents and Changes

• Pressure

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

9
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.
10
Metamorphic Agents and Changes

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

11
Metamorphic Agents and Changes

• Lithostatic pressure is uniform stress
  (hydrostatic)
• Deviatoric stress unequal pressure in different
  directions
• Deviatoric stress can be 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

12
Metamorphic Agents and Changes

• Stress is an applied force acting on a rock (over
  a particular cross-sectional area)
• Strain is the response of the rock to an applied
  stress ( yielding or deformation)
• Deviatoric stress affects the textures and
  structures, but not the equilibrium mineral
  assemblage
• Strain energy may overcome kinetic barriers to
  reactions

13
Metamorphic Agents and Changes

• Deviatoric stresses come in three principal
  types
• Tension
• Compression
• Shear

14
Tension s3 is negative, and the resulting strain
is extension, or pulling apart
strain ellipsoid
original shape
s1
s3
Figure 21-2. The three main types of deviatoric
stress with an example of possible resulting
structures. a. Tension, in which one stress in
negative. Tension fractures may open normal to
the extension direction and become filled with
mineral precipitates. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
15
Compression s1 is dominant, ? folding or more
homogenous flattening
s3
s1
Figure 21-2. The three main types of deviatoric
stress with an example of possible resulting
structures. b. Compression, causing flattening or
folding. Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
16

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

s1

• 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.
17
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.
18
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

19
Metamorphic Agents and Changes

• Pfluid indicates the total fluid pressure, which
  is the sum of the partial pressures of each
  component (Pfluid pH2O pCO2 )
• May also consider the mole fractions of the
  components, which must sum to 1.0 (XH2O XCO2
  1.0)

20
Metamorphic Agents and Changes

• Gradients in T, P, Xfluid across an area
• Zonation in the mineral assemblages

21
The Types of Metamorphism
Different approaches to classification

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

22
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

23
Contact Metamorphism

• Adjacent to igneous intrusions
• Result of thermal (and possibly metasomatic)
  effects of hot magma intruding cooler shallow
  rocks
• Occur over a wide range of pressures, including
  very low
• Contact aureole

24
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

25
The Types of Metamorphism
Contact Metamorphism Most easily recognized
where a pluton is introduced into shallow rocks
in a static environment

• The rocks near the pluton are often high-grade
  rocks with an isotropic fabric hornfelses (or
  granofelses) in which relict textures and
  structures are common

26
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 between the creation of the magma at
  depth during T maximum, and its migration to the
  lower grade rocks above
• Plutonism may reflect a separate phase of
  post-orogenic collapse magmatism (Chapter 18)

27
The Types of Metamorphism
Pyrometamorphism
Very high temperatures at very low pressures,
generated by a volcanic or subvolcanic body
Also developed in xenoliths
28
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

29
The Types of Metamorphism
Orogenic Metamorphism is the type of
metamorphism associated with convergent plate
margins

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

30
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.
31
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

32
The Types of Metamorphism
Orogenic Metamorphism

• Most orogenic belts have several episodes of
  deformation and metamorphism, creating a more
  complex polymetamorphic pattern
• Continental collision

33
The Types of Metamorphism
Orogenic Metamorphism

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

34
The Types of Metamorphism
Burial metamorphism for low-grade metamorphism
in sedimentary basins due to burial

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

35
The Types of Metamorphism

• Coombs (1961) also proposed hydrothermal
  metamorphism, caused by hot H2O-rich fluids and
  usually involving metasomatism
• Difficult type of metamorphism to constrain,
  since hydrothermal effects often play some role
  in most of the other types of metamorphism

36
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?

37
The Types of Metamorphism
Burial Metamorphism

• Bengal Fan ? sedimentary pile gt 22 km
• Extrapolating ? 250-300oC at the base (P 0.6
  GPa)
• Well into the metamorphic range, and the weight
  of the overlying sediments sufficient to ? impart
  a foliation at depth
• Passive margins often become active
• Areas of burial metamorphism may thus become
  areas of orogenic metamorphism

38
The Types of Metamorphism
Ocean-Floor Metamorphism affects the oceanic
crust at ocean ridge spreading centers

• Wide range of temperatures at relatively low
  pressure
• Metamorphic rocks exhibit considerable
  metasomatic alteration, notably loss of Ca and Si
  and gain of Mg and Na
• These changes can be correlated with exchange
  between basalt and hot seawater

39
The Types of Metamorphism
Ocean-Floor Metamorphism

• May be considered another example of hydrothermal
  metamorphism
• Highly altered chlorite-quartz rocks- distinctive
  high-Mg, low-Ca composition

40
The Types of Metamorphism
Fault-Zone and Impact Metamorphism occur in areas
experiencing relatively high rates of
deform-ation and strain with only minor
recrystallization

• Impact metamorphism (shock metamorphism) occurs
  at meteorite (or other bolide) impact craters
• Both fault-zone and impact metamorphism correlate
  with dynamic metamorphism, based on process

41
(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.
42
The Progressive Nature of 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

43
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

44
The Progressive Nature of Metamorphism
All rocks that we now find must also have cooled
to surface conditions 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)

45
The Progressive Nature of Metamorphism
Retrograde metamorphism is of only 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

46
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

47
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

48
Orogenic Regional Metamorphism of the Scottish
Highlands

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

49
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.
50
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

51
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)

52

• 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)

53
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.
54
To summarize

• 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

55
A variation occurs in the area just to the north
of Barrows, in the Banff and Buchan district

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

56
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).
57
Regional Burial MetamorphismOtago, New Zealand

• Jurassic graywackes, tuffs, and volcanics in a
  deep trough metamorphosed in the Cretaceous
• The fine grain size and immature nature of the
  material is highly susceptible to alteration,
  even at low grades

58
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.
59
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)

60
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.
61
Regional Burial MetamorphismOtago, New Zealand

• Orogenic belts typically proceed directly from
  diagenesis to chlorite or biotite zones
• The development of low-grade zones in New Zealand
  may reflect the highly unstable nature of the
  tuffs and graywackes, and the availability of hot
  water, whereas pelitic sediments may not react
  until higher grades

62
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.
63
Paired Metamorphic Belts of Japan

• The NW belt (inner belt, inward, or away from
  the trench) is the Ryoke (or Abukuma) Belt
• Low P/T Buchan-type of regional orogenic
  metamorphism
• Dominant meta-pelitic sediments, and isograds up
  to the sillimanite zone have been mapped
• A high-temperature-low-pressure belt, and
  granitic plutons are common

64
Paired Metamorphic Belts of Japan

• Outer belt, called the Sanbagawa Belt
• It is of a high-pressure-low-temperature nature
• Only reaches the garnet zone in the pelitic rocks
• Basic rocks are more common than in the Ryoke
  belt, however, and in these glaucophane is
  developed (giving way to hornblende at higher
  grades)
• Rocks are commonly called blueschists

65
Paired Metamorphic Belts of Japan

• Two belts are in contact along their whole length
  across a major fault zone (the Median Line)
• Ryoke-Abukuma lithologies are similar to seds
  derived from a relatively mature volcanic arc
• Sanbagawa lithologies more akin to the oceanward
  accretionary wedge where distal arc-derived
  sediments and volcanics mix with oceanic crust
  and marine sediment

66
Paired Metamorphic Belts of Japan

• Fig. 16-15 suggests that the 600oC isotherm, for
  example, could be as deep as 100 km in the
  trench-subduction zone area, and as shallow as 20
  km beneath the volcanic arc

67
Miyashiro (1961, 1973) suggested that the
occurrence of coeval metamorphic belts, an outer,
high-P/T belt, and an inner, lower-P/T belt ought
to be a common occurrence in a number of
subduction zones, either modern or ancient
Figure 21-13. Some of the paired metamorphic
belts in the circum-Pacific region. From
Miyashiro (1994) Metamorphic Petrology. Oxford
University Press.
68
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

69
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
70
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.
71
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
72
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.
73
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 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

74
Contact Metamorphism of Pelitic Rocks

• Inner aureole at Comrie (a diorite intruded into
  the Dalradian schists back up north in Scotland),
  the intrusion was hotter and the rocks were
  metamorphosed to higher grades than at Skiddaw

• Tilley describes coarse-grained non-foliated
  granofelses containing very high-temperature
  minerals such as orthopyroxene and K-feldspar
  that have formed due to the dehydration of
  biotite and muscovite in the country rocks

75
Contact Metamorphism and Skarn Formation at
Crestmore, CA, USA

• Crestmore quarry in the Los Angeles basin
• Quartz monzonite porphry of unknown age 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)

76

• 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

77
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.
78
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
79
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

80
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)

81
Figure 21-17. 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)
82
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.
83
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.