Transcript Metamorphism and Metamorphic Rocks

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 >320ºC 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
•
3.
2.
•
1.
1.
2.
3.
Progression
or sequence
of facies
across a
metamorphic
terrain
Identified by
field and
experimental
work
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 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 => 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 metasandstones
• Discriminates between Fe
and Mg and allows for
solid solution (tie lines)
• Limitations:
• AFM
– A = Al2O3
– F = FeO
– M = MgO
– Assumes quartz present
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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 <=> 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
1.
Pyroxene hornfels
– Albite-epidote
hfls
– Hornblende hfls
– Pyroxene hfls
– Sanidinite
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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
4.
3.
2.
1.
• Affects large
areas of earth’s
crust
• Produced during
orogenic
(mountainbuilding) event
• 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 => pyrophyllite =>
kyanite => sillimanite
• Textural sequence (in
pelitic rocks):
– Slate => phyllite => schist => gneiss
• Facies sequence:
– Zeolite => prehnite-pumpellyite => greenschist => amphibolite =>
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 => prehnitepumpellyite => blueschist
=> 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
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