Metamorphic Facies and Mineral Assemblages

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