Metamorphism and Metamorphic Rocks

1. Metamorphism and Metamorphic Rocks Processes and Products Schist Amphibolite Eclogite Gneiss

2. Metamorphic Rocks: Importance and Occurrence • Importance: • Together with igneous rocks, make up >90% of earth’s crust • Excellent indicator of P-T history and deformation history of mountain belts • Ore deposits (Fe, Au-Ag-Cu, Ag-Pb-Zn, U, garnet) • Occurrence: • Mantle (metamorphosed ultramafic rocks) • Oceanic crust (alteration of ocean floor basalt) • Continental crust (mountain belts) • Specific assemblages tied to tectonic setting

3. Metamorphic Rocks: Review With your neighbors, discuss and answer: • What is meant by “metamorphism”? Why do rocks undergo metamorphism? • What processes cause rocks to undergo metamorphism? List and describe several. • Where does metamorphism occur? Describe several tectonic settings in which rocks are metamorphosed. Schist ? Shale

4. The Formation of Metamorphic Rocks • Metamorphism: • Mineralogic and/or textural changes in a rock in the solid state • Occurring between diagenesis and melting • Protolith • “Parent” rock – igneous or sedimentary • Metamorphic agent • Pressure (uniform, lithostatic, 1-15 kbar) • Directed stress (non uniform) • Temperature (~100-750 ºC) • Geothermal gradient, friction, deformation, magma • Chemically active fluids • H2O, CO2, other fluids present in most rocks • Temperature, pressure, or stress changes cause fluid migration

5. Types of Metamorphism

6. Metamorphic Minerals • Minerals present in metamorphic rocks reflect: • Composition of the protolith • P-T conditions under which the rock formed

7. Metamorphic Rock Names • Metamorphic rock names are generally based entirely on texture OR entirely on composition • Specifically, rock names are commonly derived utilizing any one, or a combination of the following criterion (Yardley, 1989): • the nature of the parent material (bulk composition) • the metamorphic mineralogy • the rock's texture (grain size and fabric development) • any appropriate special name

8. Metamorphic Rock Classification by Bulk Composition & Mineralogy

9. Foliated: Alignment of platy, acicular, or prismatic crystals due to directed stress and partial/complete neo-crystallization strong tendency to split along planar surfaces: Cleavage Deformation under Ductile Conditions Cataclastic: crushing and breaking of mineral grains with little/no re- or neo-crystallization deformation under Brittle Conditions Granoblastic: equidimensional re- or neo-crystallize mineral grains Metamorphic Rock Classification by Texture

10. Metamorphic Rock Classification by Texture

11. “Special” Metamorphic Rocks

12. Metamorphic Tectonites Rocks with metamorphic fabric (textural and structural features) due to a history of deformation (one or more episode of directed stress) Produced by ductile deformation with recrystallization Characterized by oriented mineral grains S- Tectonites: contain a strong planar or Surface fabric Rock cleavage (breakage along parallel surfaces) L- Tectonites: contain a strong Linear fabric Transposition Modification of original planar fabric (i.e. bedding) through penetrative deformation Metamorphic Structures

13. Metamorphic Structures • Folds • bends in planar structures • Boudins • Sausage- (cylindrical) shaped masses usually formed by tension (pulling apart) of planar fabric in protolith • Joints • fractures (brittle) breakage of rock body; no movement • Faults • Fractures along which movement occurs

14. Metamorphic Facies and Mineral Assemblages Contact and Regional Metamorphism

15. 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 

16. 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

17. 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

18. 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

19. 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

20. 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)

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

22. 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 ore minerals Wollastonite-garnet skarn

23. 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

24. 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

25. Regional Metamorphism: Franciscan • Low T; high P • Low geothermal gradient • Little directed stress (little deformation = not foliated) • Distinguished by presence of blue, sodic amphiboles • Facies sequence: • Zeolite => prehnite-pumpellyite => blueschist => eclogite

26. 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

27. 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