Metamorphic petrology
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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

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Metamorphic petrology

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Metamorphic petrology

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


Role of water

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


Recrystallization

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


Recrystallization cont d

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


Recrystallization cont d1

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


Tectonite fabric

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


Tectonite fabric cont d

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


Greywacke to schist

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


Protoliths

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


Types of metamorphism

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


Contact metamorphism

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


Metamorphic grades and zones

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)


Barovian zones in pelites

Barovian zones in pelites

Mappable line often recognized through an index mineral: isograd


Metamorphic facies

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


Facies and assemblages

Facies and assemblages


P t t paths

P-T-t paths


Metamorphic fabrics

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


Metamorphic fabrics cont d

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


Lineations

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


Metamorphic textures

  • 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


Metamorphic textures cont d

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


Classification and description

Classification and description

Several bases:

Fabric

Protolith

Mineralogical names, like marble, serpentinite

Geological setting: nature of metamorphism

Grade

Chemical composition

Fabric


Metasomatic rock types

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


Graphical representation of assemblages

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


Composition diagrams

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


Compatibility diagrams

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


Compatibility diagrams cont d

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


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