Metamorphism and Metamorphic Rocks

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Metamorphism and Metamorphic Rocks

• Processes and Products

Schist
Amphibolite
Eclogite
Gneiss
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Metamorphic Rocks Importance and Occurrence

• Importance
• Together with igneous rocks, make up gt90 of
  earths crust
• Excellent indicator of P-T history and
  deformation history of mountain belts
• Ore deposits (Fe, Au-Ag-Cu, Ag-Pb-Zn, U, garnet)

• Occurrence
• Mantle (metamorphosed ultramafic rocks)
• Oceanic crust (alteration of ocean floor basalt)
• Continental crust (mountain belts)
• Specific assemblages tied to tectonic setting

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Metamorphic Rocks Review

• With your neighbors, discuss and answer
• What is meant by metamorphism? Why do rocks
  undergo metamorphism?
• What processes cause rocks to undergo
  metamorphism? List and describe several.
• Where does metamorphism occur? Describe several
  tectonic settings in which rocks are
  metamorphosed.

Schist
?
Shale
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The Formation of Metamorphic Rocks

• Metamorphism
• Mineralogic and/or textural changes in a rock in
  the solid state
• Occurring between diagenesis and melting
• Protolith
• Parent rock igneous or sedimentary
• Metamorphic agent
• Pressure (uniform, lithostatic, 1-15 kbar)
• Directed stress (non uniform)
• Temperature (100-750 ºC)
• Geothermal gradient, friction, deformation, magma
• Chemically active fluids
• H2O, CO2, other fluids present in most rocks
• Temperature, pressure, or stress changes cause
  fluid migration

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Types of Metamorphism

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Metamorphic Minerals

• Minerals present in metamorphic rocks reflect
• Composition of the protolith
• P-T conditions under which the rock formed

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Metamorphic Rock Names

• Metamorphic rock names are generally based
  entirely on texture OR entirely on composition
• Specifically, rock names are commonly derived
  utilizing any one, or a combination of the
  following criterion (Yardley, 1989)
• the nature of the parent material (bulk
  composition)
• the metamorphic mineralogy
• the rock's texture (grain size and fabric
  development)
• any appropriate special name

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Metamorphic Rock Classification by Bulk
Composition Mineralogy
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Metamorphic Rock Classification by Texture

• Foliated
• Alignment of platy, acicular, or prismatic
  crystals due to directed stress and
  partial/complete neo-crystallization
• strong tendency to split along planar surfaces
  Cleavage
• Deformation under Ductile Conditions
• Cataclastic
• crushing and breaking of mineral grains with
  little/no re- or neo-crystallization
• deformation under Brittle Conditions
• Granoblastic
• equidimensional re- or neo-crystallize mineral
  grains

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Metamorphic Rock Classification by Texture
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Special Metamorphic Rocks
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Metamorphic Structures

• Metamorphic Tectonites
• Rocks with metamorphic fabric (textural and
  structural features) due to a history of
  deformation (one or more episode of directed
  stress)
• Produced by ductile deformation with
  recrystallization
• Characterized by oriented mineral grains
• S- Tectonites contain a strong planar or Surface
  fabric
• Rock cleavage (breakage along parallel surfaces)
• L- Tectonites contain a strong Linear fabric

• Transposition
• Modification of original planar fabric (i.e.
  bedding) through penetrative deformation

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Metamorphic Structures

• Folds
• bends in planar structures
• Boudins
• Sausage- (cylindrical) shaped masses usually
  formed by tension (pulling apart) of planar
  fabric in protolith
• Joints
• fractures (brittle) breakage of rock body no
  movement
• Faults
• Fractures along which movement occurs

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Metamorphic Facies and Mineral Assemblages
Contact and Regional Metamorphism
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Metamorphic Grade

• One of the primary goals of metamorphic petrology
  is to interpret P-T conditions under which a rock
  (or set of rocks) formed
• Metamorphic grade
• relative temperature and pressure conditions
  under which metamorphic rocks form
• Low-grade metamorphism
• T 200 to 320ºC, relatively low pressure
• abundant hydrous minerals
• High-grade metamorphism
• T gt320ºC and relatively high pressure
• Dehydration less hydrous minerals common
• Prograde metamorphism
• T and/or P, grade of metamorphism increases 
• Retrograde metamorphism
• T and/or P, grade of metamorphism decreases 

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Metamorphic Facies

• set of rocks characterized by equilibrium
  mineral assemblage that reflect specific range of
  metamorphic (T-P) conditions

• Mineral assemblage present depends on protolith
  composition and P-T conditions
• Ex marble, metabasalt and schist all in
  amphibolite facies

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Metamorphic Series

• Progression or sequence of facies across a
  metamorphic terrain
• Identified by field and experimental work

3.
2.
1.

• Contact Series Hi T, low P contact
  metamorphism
• Dynamic Regional Series Mod T P Barrovian
• Static Regional Series Lo T, hi P Blueschist

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Metamorphic Reactions and Critical Minerals

• For a given rock composition, some reactions
  occur under specific P-T conditions
• Critical minerals
• minerals with limited P-T stability
• Presence indicates certain metamorphic conditions
• Ex
• Kyanite
• Andalusite
• Sillimanite
• Polymorphs of Al2SiO5

Al2Si4O10(OH)2 ltgt Al2SiO5 3SiO2 H2O 
Pyrophyllite       Ky or Andal   Qtz      fluid
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Metamorphic Isograds

• Reactions and appearance/disappearance of
  critical minerals can be used to determine P-T
  conditions
• Isograd line to indicate same grade of
  metamorphism
• First appearance of mineral of increasing
  metamorphic grade

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Contact Metamorphism

• Result of high geothermal gradient produced
  locally around intruding magma
• Restricted to relatively shallow depths (low
  pressure)
• Rocks generally not foliated
• Produce fine grained rocks called hornfels

• Contact aureole surrounding rocks metamorphosed
  by heat of intrusion
• Size/shape of aureole depends on
• Size, temp, cooling history of the igneous
  intrusion
• Properties of the country rock (conductivity,
  presence of water)

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Contact Metamorphism Hornfels

• Mineral assemblage characterized by hornfels
  facies series
• Albite-epidote hfls
• Hornblende hfls
• Pyroxene hfls
• Sanidinite

1.
Pyroxene hornfels
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Contact Metamorphism Skarn

• Contact metamorphism of carbonate or siliceous
  carbonate
• Usually involves significant metasomatism
  (chemical exchange between magmatic fluids and
  rock)
• Can be hosts of Au, Cu, Fe, Mo, Sn, W, Zn-Pb ore
  minerals

Wollastonite-garnet skarn
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Regional Metamorphism

• Affects large areas of earths crust
• Produced during orogenic (mountain-building) event

4.
3.
2.
1.

• Foliated rocks developed under med-high T and P
• 1. Buchan 2. Barrovian
• Rocks developed under low-med T and high P
• 3. Sanbagawa 4. Franciscan

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Regional Metamorphism Barrovian

• Normal metamorphic series recognized in
  mountain belts world-wide
• Medium to high T low to high P
• Moderate to high geothermal gradient
• Critical mineral sequence
• Kaolinite gt pyrophyllite gt kyanite gt
  sillimanite
• Textural sequence (in pelitic rocks)
• Slate gt phyllite gt schist gt gneiss
• Facies sequence
• Zeolite gt prehnite-pumpellyite gt greenschist gt
  amphibolite gt granulite

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Regional Metamorphism Franciscan

• Low T high P
• Low geothermal gradient
• Little directed stress (little deformation not
  foliated)
• Distinguished by presence of blue, sodic
  amphiboles
• Facies sequence
• Zeolite gt prehnite-pumpellyite gt blueschist gt
  eclogite

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Regional Metamorphism and Tectonics

• Barrovian (mod-high T and P)
• Develop in orogenic belts at convergent margins
• Regional heat flow increased by upward movement
  of magma and migration of fluids
• Thrust faulting thickens crust, increasing
  pressure and directed stress
• Geothermal gradient of 20-40ºC/km
• Blueschist (Franciscan) (low T and high P)
• Occur world-wide in orogenic belts of Phanerozoic
  age
• Associated with subduction zones
• rapid subduction of oceanic crust depresses
  isotherms
• Geothermal gradient of 10-20ºC/km

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Regional Metamorphism and Tectonics

• Paired metamorphic belts specific to subduction
  zones

• Barrovian
• Formed in rocks beneath and around the arc
• Heat due to magmatic activity
• Blueschist
• formed in the mélange of the trench