Metamorphic Facies and Mineral Assemblages

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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

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1. Contact Series Hi T, low P contact
   metamorphism
2. Dynamic Regional Series Mod T P Barrovian
3. Static Regional Series Lo T, hi P Blueschist

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Metamorphic Phase Assemblages

• Specific minerals present within a facies (P-T
  conditions) depends largely on protolith
  composition
• Equilibrium assemblages can be shown on ternary
  phase diagrams (with some assumptions and
  simplifications)

• Hypothetical A-B-C
• 7 possible minerals can occur at this temp
  pressure
• Tie lines show pairs of minerals in equilibrium
• Smaller triangles show regions where sets of 3
  minerals are stable together

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Metamorphic Phase Assemblages

• Changing the P-T conditions changes
• Location of the tie lines
• Regions of coexisting stable mineral assemblages
• Due to chemical reactions between minerals

Change in conditions
AB A2C  gt 2A ABC
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Metamorphic Phase Assemblages

• Actual metamorphic rocks typically contain 10
  common elements (Si, Al, Mg, Fe, Ca, Na, K, H2O,
  CO2)
• Simplify to plot on ternary diagram
• Ignore components (assume present in rock)
• Combine components (e.g., Fe and Mg)
• Limit diagram to specific rock type (e.g.,
  metabasalt, pelite)

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Metamorphic Phase Assemblages

• Useful for plotting most common metamorphic rocks
• Limitations
• Assumes quartz present
• Assumes Mg Fe freely substitute

• ACF
• A Al2O3
• C CaO
• F FeO MgO

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Metamorphic Phase Assemblages

• Useful for plotting mafic (metabasalt) rocks,
  pelites, and some meta-sandstones
• Discriminates between Fe and Mg and allows for
  solid solution (tie lines)
• Limitations
• Assumes quartz present

• AFM
• A Al2O3
• F FeO
• M MgO

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Metamorphic Phase Assemblages

• Minerals present reflect equilibrium P-T
  conditions for different protoliths
• As P-T conditions increase (prograde
  metamorphism) minerals react with each other and
  with fluids
• Some minerals no longer stable new minerals
  appear

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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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Assemblages, Facies, and Series

• For a given parent rock composition, mineral
  assemblage reflects P-T conditions

• A variety of different rocks metamorphosed under
  the same conditions indicate facies
• Regional changes between facies indicate series
  (and tectonic setting)

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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

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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

Wollastonite-garnet skarn
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Regional Metamorphism

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

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• 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)
• 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

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Collaborative Activity

• Begin working on maps and metamorphic phase
  diagrams for the homework!

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Collaborative Activity
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Collaborative Activity