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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 >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

• Progression or sequence of facies across a metamorphic terrain

• Identified by field and experimental work

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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2.

3.

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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 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 <=> 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

• 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)

• 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

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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 earth’s 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

3.2.

4.

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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 => prehnite-

pumpellyite => 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