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.
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 (lt100oC) 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
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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 SiCaNa
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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 textureisotropic agregate of
polygonal grains of roughly similar size
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Recrystallization contd
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

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Recrystallization contd
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
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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
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Tectonite fabric contd
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
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Greywacke to schist
Greywacke sandstone

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

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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)
• Calcareouslimestones 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

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

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Barovian zones in pelites
Mappable line often recognized through an index
mineral isograd
14
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

• Reactions
• Analcite Qtz?albiteH2O
• 2 Laws 5 glau?trem10 alb 2 chlo
• 6trem50alb9chlo?25 glau6zoi7Qtz14H2O
• 25pump2chlo29Qtz?7trem43zoi67H2O
• 4chlo18zoi21qtz?5Al-amph26An20H2O
  Also7chlo13trem12zoi14Qtz?25Al-amph22H2O
  AlsoalbtremAl-amphQtz
• hbl?cpxopxCa-plagH2O

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Facies and assemblages
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P-T-t paths
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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
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Metamorphic fabrics contd

• 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
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Lineations

1. Mineral lineation nematoblastic aligned
   acicular, columnar and prismatic grains of
   amphibole, sillimanite and kyanite
2. Stretching lineation streaked appearance of
   foliation, elongated agregates of minerals
3. Boudins segments of once intact layer that has
   been pulled apartsausage links. Boudins are
   less deformed than their surroundings
4. Intersection of two oblique foliations

Nematoblastic hornbvlende-plagioclase-epidote
schist
Lineated and weakly foliated feldspar-quartz-bioti
te gneiss
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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

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Metamorphic textures contd

• 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
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Classification and description

• Several bases
• Fabric
• Protolith
• Mineralogical names, like marble, serpentinite
• Geological setting nature of metamorphism
• Grade
• Chemical composition

Fabric
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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

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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

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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
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Compatibility diagrams

• Diagrams to depicts compositional relationships
  in metamorphic rocks
• ACF diagram
• FFeOMgOMnO anthophyllie, cummingtonite,
  hyperstene, olivine
• Molar proportions of oxides
• AAl2O3Fe2O3-Na2O-K2O Al in excess of that
  needed for alk fsp
• CCaO-3.3P2O5-CO2 Ca in excess of what is needed
  for apatite and calcite
• FFeOMgOMnO-TiO2-Fe2O3 excess over what is
  needed to make ilmenite and magnetite
• AKF diagrams (for potassic minerals)
• AAl2O3Fe2O3-(Na2OK2OCaO) eliminates plag
• KK2O
• FFeOMgOMnO

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Compatibility diagrams contd

• 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
• AAl2O3-3K2O KAl2AlSi3O10(OH)2 (musc),
• from Kfsp AAl2O3-K2O
• 2. FFeO (FeO-TiO2)
• 3. MMgO