Introduction to Metamorphism

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Introduction to Metamorphism

• IN THIS LECTURE
• Definition of Metamorphism
• Limits of Metamorphism
• Agents of Metamorphic Change
• Temperature
• Pressure
• Stress and Strain
• Fluids
• Metamorphic Change
• Classification of Metamorphic Rocks
• Types of Metamorphism

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Introduction to Metamorphism

• Rocks as chemical systems represented by a
  particular assemblage of coexisting phases
  (thermodynamic equilibrium and the governed by
  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

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Introduction to 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.

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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

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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

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The Limits of Metamorphism

• Migmatites metamorphic or igneous rocks?

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Limits of Metamorphism
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Agents of Metamorphic Change

• There are several main agents of metamorphic
  change
• Temperature
• Pressure
• Stress and Strain
• Fluids

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Agents of Metamorphic Change1. Temperature

• TEMPERATURE typically the most important factor
  in metamorphism
• 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.

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Agents of Metamorphic Change1. Temperature

• Increasing temperature has several effects
• Promotes recrystallization leading to increased
  grain size
• Larger surface/volume ratio of a mineral means
  lower stability
• Increasing temperature eventually overcomes
  kinetic barriers to recrystallization, and fine
  aggregates coalesce to larger grains
• Drive reactions that consume unstable mineral(s)
  and produces new minerals that are stable under
  the new conditions
• Overcomes kinetic barriers that might otherwise
  preclude the attainment of equilibrium

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Agents of Metamorphic Change2. Pressure

• PRESSURE
• Normal gradients may be perturbed in several ways
• The two main examples are
• High T/P geotherms in areas of plutonic activity
  or rifting
• Low T/P geotherms in subduction zones

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Agents of Metamorphic Change3. Stress and Strain

• 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

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Agents of Metamorphic Change4. 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
• 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)

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Metamorphic Change

• Metamorphic grade
• A general increase in degree of metamorphism
  without specifying the exact relationship between
  temperature and pressure
• Generally both temperature and pressure increase
• Three terms used
• Low Grade
• Medium Grade
• High Grade

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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 changes that
  accompany decreasing metamorphic grade

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Evidence for Metamorphic Change

• How do we see metamorphic change
• Mineral assemblage and grainsize can be used to
  estimate metamorphic grade
• Gradients in T, P, Xfluid across an area
• Zonation in the mineral assemblages

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Classification of Metamorphic Rocks

• Two main ways in which metamorphic rocks are
  classified
• Based on the style or type of metamorphism
• Based on the mineral assemblage or texture of the
  rocks
• Well look first at the types of metamorphism

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The Types of Metamorphism

• Different approaches to classification based on
  type
• Based on principal process or agent
• Dynamic Metamorphism
• Thermal Metamorphism
• Dynamo-thermal Metamorphism

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The Types of Metamorphism

• 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
• Well look at the classifivation based on setting

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Contact Metamorphism

• Adjacent to igneous intrusions
• Result of thermal (and possibly metasomatic
  involvement of fluids) effects of hot magma
  intruding cooler shallow rocks
• Occur over a wide range of pressures, including
  very low pressures
• Usually leads to the formation of a contact
  aureole around the pluton. A contact aureole is a
  zone in which the rocks into which the pluton has
  intruded have been affected by the heat of the
  intrusion and have developed new metamorphic
  mineral assemblages

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Contact Metamorphism

• The size and shape of an aureole is controlled by
• The nature of the pluton, ie
• Size
• Shape
• Orientation
• Composition
• Temperature
• The nature of the country rocks
• Composition
• Depth and metamorphic grade prior to intrusion
• Permeability

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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
• Polymetamorphic rocks are common, usually
  representing an orogenic event followed by a
  contact one, e.g. 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

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Contact Metamorphism

• Pyrometamorphism
• Very high temperatures at very low pressures,
  generated by a volcanic or subvolcanic body
• Also developed in xenoliths
• Not very common and wont be looked at further in
  this course

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Regional 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

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Regional Metamorphism

• OROGENIC METAMORPHISM
• 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

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Regional 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.
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Regional 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
• Most orogenic belts have several episodes of
  deformation and metamorphism, creating a more
  complex polymetamorphic pattern
• Associated with continental collision

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Regional Metamorphism

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

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Regional Metamorphism

• BURIAL METAMORPHISM
• Low grade metamorphism in sedimentary basins due
  to burial
• Example 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.

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Regional 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

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Regional Metamorphism

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

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Regional Metamorphism

• OCEAN-FLOOR METAMORPHISM
• Affects the oceanic crust at ocean ridge
  spreading centres
• Wide range of temperatures at relatively low
  pressure, beginning in the diagenesis field and
  increasing to lower greenschist facies
• 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
• Weve seen this already when we looked at the
  Cyprus thin-sections!

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Regional Metamorphism

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