Download Chapter 21: Metamorphism

Chapter 21: Metamorphism
Fresh basalt and
weathered basalt
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.”
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
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
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
2) Drive reactions (endothermic)
3) Overcomes kinetic barriers
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
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 - 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
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
• Foliation is a common result, which allows us to
estimate the orientation of s1
s1
Strain
ellipsoid
 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 = 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
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
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
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
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
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)
The Types of Metamorphism
Pyrometamorphism
Very high temperatures at low pressures,
generated by a volcanic or sub-volcanic 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: 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
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
From Understanding
Earth, Press and Siever.
Freeman.
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
The Types of Metamorphism
Burial metamorphism
• Southland Syncline in New Zealand: thick pile (> 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.
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
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 occurs in areas that have not
experienced significant deformation or orogeny
• Bengal Fan  sedimentary pile > 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
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
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
(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.
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
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
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
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
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
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, uartz, 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 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
A variation occurs in the area just to the north of
Barrow’s, in the Banff and Buchan district
• 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
• Fine grain size and immature 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 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.
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.
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
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
• 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
Contact
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
• 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
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):
• 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
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 21.18. 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)
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.