1. Metamorphic Facies and Mineral Assemblages
Contact and Regional Metamorphism

2. 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 

3. 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

4. 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

5. 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

6. 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

7. 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)

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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)

14. 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)

15. Contact Metamorphism: Hornfels
Mineral assemblage characterized by hornfels facies series
Albite-epidote hfls
Hornblende hfls
Pyroxene hfls
Sanidinite 1.
Pyroxene hornfels

16. 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

17. Regional Metamorphism
Affects large areas of earth’s 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

18. 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

19. 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

20. 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

21. 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

22. Collaborative Activity
Begin working on maps and metamorphic phase diagrams for the homework!

23. Collaborative Activity

24. Collaborative Activity