Metamorphic petrology

1. Chapter 14, 15, 16, 18 Metamorphic rock: aggregate of minerals; composition and fabric reflect changes to new states to adjust to changes in P, T and X and stress. Parent rock: protolith Metamorphism: path from protolith to final rock. Driving force: increasing P and T Upper limit: dependent on composition; igneous and metamorphic processes overlap in heterogeneous bodies. Lower limit: distinction between metamorphism and diagenetic changes or alteration is also blurred. Metamorphic petrology Kaolinite and smectite clay minerals formed during alteration of feldspars: H2O addition Rock made of clays or their equivalents: pelites As T increases low-T clay minerals are replaced by less water-rich variaties: illites (<100oC) at 300oC chlorite and sericite. Sericite: white mica that can include muscovite, paragonite, pyrophyllite and phengite. These harder less hydrous minerals tarnsfer a shale into a aphanitic platy metamorphic rock: slate

2. Role of water Water is agent of change Dry rocks are more “resistant” to metamorphism. Water stabilizes new phases and catalyzes reactions, enhancing diffusion rates In open system in the presence of water hydration reactions: Biotite+ H2O  Chlorite + Rutile Hornblende+ H2O  Chlorite + Rutile Clinopyrxene+ H2O  Actinolite + Epidote Olivine/opx+ H2O  Serpentinite+ Fe-oxides Plagioclase+ Ca +Fe + H2O  epidote Feldspars + H2O  sericite + Si +K (high T) Feldspars + H2O  clay minerals + Si+Ca+Na

3. Recrystallization Two distinct processes: 1: Boundaries of existing crystals are modified, no new phases. 2: Solid state crystallization: new phases are created due to changing metamorphic conditions. Example of 1. Conversion of limestone to marble: Granoblastic texture:isotropic agregate of polygonal grains of roughly similar size

4. Recrystallization cont’d Growth of new phases Prograde metamorphism of diabase Metadiabase: Actinolite (with chlorite and epidote inclusions) replaces pyroxene, sphene, labradorite replaced by albite, epidote and mica Greenstone: isotropic, original texture disappeared. Amphibolite: Granoblastic textur, micas and most hydrous minerals disappeared Granoblastic plagioclase pyroxene granofels, complete dehydration

5. Recrystallization cont’d Epitaxial growth: new rowth on substrate with similar atomic structure (amphibole on pyroxene). Limited nucleation: porphyroblastic rocks: large euhedral-subhedral crystals: metamorphosed Al-rick rocks, porphyroblast: garnet, staurolite, andalusite Porphyroblast results in local metamorphic differentiation. Porphyroblasts with inclusions: poikiloblasts Inclusions are relics of previous state provide insight in metamorphic history

6. Tectonite fabric Recrystallization under non-hydrostatic pressure. At shallow levels: brittle behavior: rock flour, fault gauge will be cemented by water percolation: cement a cataclastic fabric: sharp and angular grain shapes, poly granular Greater dept: ductile deformation, ductile flow. During ductile flow rock remanins cohesive. Results in foliation: linear fabric: schists Strain ellipse: deformation of a sphere, foliations paralel to the plane of flattening. Often multiple stage of deformation

7. Tectonite fabric cont’d Mylonite: grain reduction due to shear often marks faults and shear zones. Non hydrostatic stress makes grain instable and results in dynamic recrystallization Increasing mylonitization

8. Greywacke to schist Greywacke: sandstone Protolith: clasts of Qtz, fsp and Fe-Mg mineral in clay matrix Phyllite, foliated, relict clasts of Qtz, grain size reduction Growth of mineral under non-hydrostatic pressure, crystallization of new mineral aligned in the stress field Fine grained schist, schistosity due to metamorphic segregation in felsic and mafic bands. Developmant of granoblasts

9. Protoliths • Recognition of protoliths through: • Relict fabrics • Field relations • Bulk composition • Ultramafic: high T: olivine, pyroxene limited feldspar, no Qtz, low T: serpentinite, chlorite, tremolite, magnetite; with CO2 magnesite and dolomite • Mafic, relatively high Mg, Fe and Ca (gabbro) actinolite, hornblende, pyroxene, garnet, epidote, plagioclase, chlorite, pumpellyite • Felsic, qtzofeldspatic: felsic magmatic and feldspatic and lithic sandstones: Qtz and fsp bearing, minor mafic minerals. Distinction beweetn protoliths will be difficult • Pelitic: shale-mudstone protolith, Al tich silicates: Al2SiO5 polymorphs, cordierite, staurolite, garnet. Qtz, mica(absent at high T) • Calcareous:limestones and dolestones: in absence of qtz calcite and dolomite are stable over large P-T range • Calc-silicate: impure carbonate protoliths: significant amount of clay and Qtz in addition to carbonate. Carbonates of Fe, Ca and Mg (Mn), Ca-rich silicates: grossular-andradite, vesovianite, epidote group, diopside hedenbergite, wollastonite and tremolite • Ferrugineous: banded iron formations and marine cherts: meta minerals qtz, hematite, magnetite, Fe-chlorite, siderite, ankerite

10. Metamorphic terranes: large scale field relations allows distinction from adjacent rock masses. Types of metamorphism Regional metamorphism: orogen related Burial metamorphism: little or no deformation Contact metamorphism: steep thermal gradients: metamorphic aureole

11. Contact metamorphism Granodiorite intrusion in slate If a hydrothermal system develops: skarn formation, silicate rich fluids percolate through rock. Formation of reaction zones. hornfels Semi hornfels

12. Metamorphic grades and zones • Grade: corresponds to equilibration T, independent of P • Distinguished by mineral assembledge • Lower grade has more hydrous minerals • Prograde metamorphism: increasing T • Retrograde metamorphism: metamorphism after maximum T has been reached • Water enhances metamorphic reaction rates: therefore retrograde metamorphism less extensive • Metamorphic zones: • Distinctive fabric • Distinctive mineral assemblage (often indicator mineral)

13. Barovian zones in pelites Mappable line often recognized through an index mineral: isograd

14. Facies: suite of mineral assemblages, repeatedly found in terranes of all ages and possesses a regular variation between mineral composition and bulk chemical composition Metamorphic facies Reactions: Analcite+ Qtzalbite+H2O 2 Laws+ 5 glautrem+10 alb+ 2 chlo 6trem+50alb+9chlo25 glau+6zoi+7Qtz+14H2O 25pump+2chlo+29Qtz7trem+43zoi+67H2O 4chlo+18zoi+21qtz5Al-amph+26An+20H2O Also:7chlo+13trem+12zoi+14Qtz25Al-amph+22H2O Also:alb+trem=Al-amph+Qtz hblcpx+opx+Ca-plag+H2O

15. Facies and assemblages

16. P-T-t paths

17. Metamorphic fabrics Anisotropic fabrics: Penetrative i.e. throughout the rock: tectonite Most common anisotropic fabric: foliation: S-surface. Multiple foliations indicated as S1, S2 etc. Foliations often indicated by alignment of minerals: long axis paralel to the foliation. Tectonite with one or more foliations: S-tectonite L-tectonite: only lineated (line) Most common foliation: compositional layering and preferred orientation Aligned platy grains (like mica and chlorites in phyllites and schists) called lepidoblastic texture and can show slaty cleavage Hornfels with slaty cleavage

18. Metamorphic fabrics cont’d Further developed foliations: formation of laminae or lenses of contrasting texture and/or composition. Individual domains are called microlithons Cleavage is called spaced cleavage. Two categories of spaced cleavage: Crenulated cleavage: cuts across pre-existing S-surfaces Disjunctive cleavage: occurs in rocks lacking foliation: seams of minerals Disjunctive cleavage Crenulated foliation

19. Lineations Mineral lineation: nematoblastic: aligned acicular, columnar and prismatic grains of amphibole, sillimanite and kyanite Stretching lineation: streaked appearance of foliation, elongated agregates of minerals Boudins: segments of once intact layer that has been pulled apart:sausage links. Boudins are less deformed than their surroundings Intersection of two oblique foliations Nematoblastic hornbvlende-plagioclase-epidote schist Lineated and weakly foliated feldspar-quartz-biotite gneiss

20. Augen: ovoidal crystals, typically of feldspar • Cataclastic: isotropic rock of angular rock and mineral fragments with through going cracks • Corona: mantle surrounding a mineral grain, reaction • Decussate: aggregate of interpenetrating grains • Epitaxial: oriented overgrowth on substrate • Flaser: texture of mylonites where large crystals (porphyroclasts) survived ductile deformation and are in fine grained matrix • Lepidoblastic: Platy minerals with preferred orientation imparting schistosity and cleavage • Megacrystic: large crystals in fine matrix • Nematoblastic: acicular or columnar grains imparting lineation • Poikiloblastic: Porphyroblasts containing inclusions • Porphyroblastic: large subhedral to euhedral grains porphyroblasts in fine grained matrix Metamorphic textures

21. Metamorphic textures cont’d • Strain shadow; cone shaped domains adjacent to rigid object, filled with mineral aggregate • Symplectite: intimita, vermicular intergroowth of two mineralsthat nucleated and grew together. Can occur as corona. If very fine grained called kelyphytic rim Pressure shadow Symplectite

22. Classification and description Several bases: Fabric Protolith Mineralogical names, like marble, serpentinite Geological setting: nature of metamorphism Grade Chemical composition Fabric

23. Metasomatic rock types • Skarn: calc-silicate rock produced by replacement of carbonate rock • Jasperoid: Like skarn, but fluid more silic-rich • Greisen: metasomatized granite (often due to hydrothermal solutions • Fenite: syenite produced by alkali metasomatism, Na-K rick solution desilicate the protolith • Rodingite: Infiltration of Ca-bearing solutions • Spilite: metasomatized basalt due to hydrothermal processes

24. The phase rule applies Representation in graph: only two dimensions Rock has far more components: Reduce the number of components to the three most relevant Ignore components that occur in one phase: Ti; Titanite or ilmenite Ignore component that only occurs as pure phase: Qtz-SiO2, hematite Fe2O3. Ignore those dictated by external conditions : H2O CO2. Restrict the range of compositions considered Combine those with widespread substitution: Fe, Mn and Mg Project composition from a phae common in all facies Graphical representation of assemblages

25. Composition diagrams No solid solution Three components: h, k and l At equilibrium P and T, number of stable phases cannot exceed three. Tielines connect phases that are stable together Composition within any of the five sub-triangles: triangle depicts the phases stable for that composition, P and T Solid solution Tielines indicate the two phase compositions in equilibrium with each other. Because of solid solution the extent of the 2 phase fields is enlarged In the two phase field specification of one component fraction and P and T defines the system

26. Compatibility diagrams Diagrams to depicts compositional relationships in metamorphic rocks ACF diagram: F=FeO+MgO+MnO: anthophyllie, cummingtonite, hyperstene, olivine Molar proportions of oxides A=Al2O3+Fe2O3-Na2O-K2O Al in excess of that needed for alk fsp C=CaO-3.3P2O5-CO2: Ca in excess of what is needed for apatite and calcite F=FeO+MgO+MnO-TiO2-Fe2O3: excess over what is needed to make ilmenite and magnetite AKF diagrams (for potassic minerals) A=Al2O3+Fe2O3-(Na2O+K2O+CaO) eliminates plag K=K2O F=FeO+MgO+MnO

27. Compatibility diagrams cont’d AFM projection: Projection from either Kfsp or muscovite on AFM Simplifications: SiO2 is always present, H2O is always present Fe2O3, MnO, CaO, Na2O and TiO2 are present In small enough quantities that they occur in one Mineral. Remaining: Al2O3, FeO, MgO, K2O Calculation: A=Al2O3-3K2O: KAl2AlSi3O10(OH)2 (musc), from Kfsp: A=Al2O3-K2O 2. F=FeO (FeO-TiO2) 3. M=MgO