Transcript Metamorphism - Faculty Server Contact

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…
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.”
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
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
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.
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
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
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
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. McGrawHill.
Metamorphic Agents and Changes
• Metamorphic grade: a general increase in
degree of metamorphism without specifying
the exact relationship between temperature
and pressure
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
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
Metamorphic Agents and Changes
Deviatoric stresses come in three principal types:
 Tension
 Compression
 Shear
Tension: s3 is negative, and the resulting strain is
extension, or pulling apart
strain
original shape
ellipsoid
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.
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.
• Foliation is a common result, which allows us to
estimate the orientation of s1
s1
 s1 > s2 = s3  foliation and no lineation
 s1 = s2 > s3  lineation and no foliation
 s1 > s2 > 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.
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.
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
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)
Metamorphic Agents and Changes
• Gradients in T, P, Xfluid across an area
• Zonation in the mineral assemblages
The Types of Metamorphism
Different approaches to classification
1. Based on principal process or agent
 Dynamic Metamorphism
 Thermal Metamorphism
 Dynamo-thermal Metamorphism
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
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
The Types of Metamorphism
Contact Metamorphism
The size and shape of an aureole is controlled by:


The nature of the pluton
 Size
 Temperature
 Shape
 Composition
 Orientation
The nature of the country rocks
 Composition
 Depth and metamorphic grade prior to intrusion
 Permeability
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
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 postorogenic collapse magmatism (Chapter 18)
The Types of Metamorphism
Pyrometamorphism
Very high temperatures at very low pressures,
generated by a volcanic or subvolcanic body
Also developed in xenoliths
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
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
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.
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
The Types of Metamorphism
Orogenic Metamorphism
• Most orogenic belts have several episodes of
deformation and metamorphism, creating a more
complex polymetamorphic pattern
• Continental collision
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
The Types of Metamorphism
Burial metamorphism = for low-grade
metamorphism in sedimentary basins due to burial
• Southland Syncline in New Zealand: a thick pile (> 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.
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
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?
The Types of Metamorphism
Burial Metamorphism
• Bengal Fan  sedimentary pile > 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
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
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
The Types of Metamorphism
Fault-Zone and Impact Metamorphism occur in
areas experiencing relatively high rates of deformation 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
(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.
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
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
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)
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
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
Orogenic Regional Metamorphism of
the Scottish Highlands
•
•
•
•
George Barrow (1893, 1912)
SE Highlands of Scotland - Caledonian orogeny ~ 500 Ma
Nappes
Granites
Barrow’s
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.
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
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)
• 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)
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.
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
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
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).
Regional Burial Metamorphism
Otago, 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
Regional Burial Metamorphism
Otago, 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 prehnitepumpellyite 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.
Regional Burial Metamorphism
Otago, 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)
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.
Regional Burial Metamorphism
Otago, 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
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.
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
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
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
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
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.
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
Contact Metamorphism of Pelitic Rocks
in the Skiddaw Aureole, UK
• The aureole around the Skiddaw granite was subdivided into three zones, principally on the basis of
textures:

Increasing
Metamorphic
Grade




Unaltered slates
Outer zone of spotted slates
Middle zone of andalusite slates
Inner zone of hornfels
Skiddaw granite
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.
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. Cordieriteandalusite 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
Figure 21-16. Andalusite-cordierite
schist from the inner zone of the
Skiddaw aureole. Note the chiastolite
cross in andalusite (see also Figure 2249). 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 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
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
Contact Metamorphism and Skarn
Formation at Crestmore, CA, USA
• Crestmore quarry in the Los Angeles basin
• Quartz monzonite porphry of unknown age intrudes Mgbearing carbonates (either late Paleozoic or Triassic)
• Brunham (1959) mapped the following zones and the
mineral assemblages in each (listed in order of increasing
grade):

Forsterite Zone:




Monticellite Zone:





calcite + forsterite + monticellite + clintonite
calcite + monticellite + melilite + clintonite
calcite + monticellite + spurrite (or tilleyite) + clintonite
monticellite + spurrite + merwinite + melilite
Vesuvianite Zone:



calcite + brucite + clinohumite + spinel
calcite + clinohumite + forsterite + spinel
calcite + forsterite + spinel + clintonite
vesuvianite + monticellite + spurrite + merwinite +
melilite
vesuvianite + monticellite + diopside + wollastonite
Garnet Zone:

grossular + diopside + wollastonite
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, 879920.
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
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
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)
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)
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.
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.