Metamorphic petrology by wanghonghx


									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
                                 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
                                 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 (<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
 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
 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
Recrystallization cont’d
Growth of new phases

                       Prograde metamorphism of diabase
                       a. Metadiabase: Actinolite (with
                           chlorite and epidote inclusions)
                           replaces pyroxene, sphene,
                           labradorite replaced by albite,
                           epidote and mica
                       b. Greenstone: isotropic, original
                           texture disappeared.
                       c. Amphibolite: Granoblastic textur,
                           micas and most hydrous minerals
                       d. Granoblastic plagioclase pyroxene
                           granofels, complete dehydration
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
               Porphyroblasts with inclusions: poikiloblasts
               Inclusions are relics of previous state provide insight
               in metamorphic history
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
Results in foliation: linear fabric: schists

                             Strain ellipse:
                             deformation of
                             a sphere,
                             paralel to the
                             plane of
                             Often multiple
                             stage of
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
                   Increasing mylonitization
Greywacke to schist
 Greywacke: sandstone   1.   Protolith: clasts of Qtz, fsp
                             and Fe-Mg mineral in clay
                        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
Recognition of protoliths through:
1. Relict fabrics
2. Field relations
3. Bulk composition
   a) Ultramafic: high T: olivine, pyroxene limited feldspar, no Qtz, low T:
        serpentinite, chlorite, tremolite, magnetite; with CO2 magnesite and dolomite
   b) Mafic, relatively high Mg, Fe and Ca (gabbro) actinolite, hornblende,
        pyroxene, garnet, epidote, plagioclase, chlorite, pumpellyite
   c) Felsic, qtzofeldspatic: felsic magmatic and feldspatic and lithic sandstones:
        Qtz and fsp bearing, minor mafic minerals. Distinction beweetn protoliths
        will be difficult
   d) Pelitic: shale-mudstone protolith, Al tich silicates: Al2SiO5 polymorphs,
        cordierite, staurolite, garnet. Qtz, mica(absent at high T)
   e) Calcareous:limestones and dolestones: in absence of qtz calcite and
        dolomite are stable over large P-T range
   f) 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
   g) 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.

                                               Regional metamorphism: orogen
                                               Burial metamorphism: little or no
                                               Contact metamorphism: steep thermal
                                               gradients: metamorphic aureole
Contact metamorphism

                            Granodiorite intrusion in slate

                            If a hydrothermal system
                            develops: skarn formation,
                            silicate rich fluids percolate
                            through rock. Formation of
                            reaction zones.

Semi hornfels
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

                            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

1. Analcite+ Qtzalbite+H2O
2. 2 Laws+ 5 glautrem+10 alb+ 2 chlo
3. 6trem+50alb+9chlo25 glau+6zoi+7Qtz+14H2O
4. 25pump+2chlo+29Qtz7trem+43zoi+67H2O
5. 4chlo+18zoi+21qtz5Al-amph+26An+20H2O
6. hblcpx+opx+Ca-plag+H2O
Facies and assemblages
P-T-t paths
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
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
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:
1. Crenulated cleavage: cuts across pre-existing S-surfaces
2.   Disjunctive cleavage: occurs in rocks lacking foliation: seams of minerals

                                                     Disjunctive cleavage
Crenulated foliation
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
     apart:sausage links. Boudins are
     less deformed than their
     surroundings                                  Nematoblastic hornbvlende-
4.   Intersection of two oblique foliations        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
• Poikiloblastic: Porphyroblasts containing inclusions
• Porphyroblastic: large subhedral to euhedral grains
porphyroblasts in fine grained matrix
Metamorphic textures cont’d
• Strain shadow; cone shaped domains adjacent to rigid object, filled with mineral
•Symplectite: intimita, vermicular intergroowth of two mineralsthat nucleated and
grew together. Can occur as corona. If very fine grained called kelyphytic rim

            Pressure shadow
Classification and description
Several bases:
1. Fabric
2. Protolith
3. Mineralogical names, like marble, serpentinite
4. Geological setting: nature of metamorphism
5. Grade
6. Chemical composition

       Strongly         Weakly          Non-foliated
       foliated         foliated
       Slate            Gneiss          Greenstone
       Phyllite         Migmatite       Amphibo lite
       Schist           Myloni te       Eclogite
                                        Charn okite
                                        Hornfe ls
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
 1. Ignore components that occur in one phase: Ti; Titanite or ilmenite
 2. Ignore component that only occurs as pure phase: Qtz-SiO2, hematite Fe2O3.
 3. Ignore those dictated by external conditions : H2O CO2.
 4. Restrict the range of compositions considered
 5. Combine those with widespread substitution: Fe, Mn and Mg
 6. Project composition from a phae common in all facies
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
 Diagrams to depicts compositional relationships in metamorphic rocks
 ACF diagram:
     F=FeO+MgO+MnO: anthophyllie, cummingtonite, hyperstene, olivine
 1. Molar proportions of oxides
 2. A=Al2O3+Fe2O3-Na2O-K2O Al in excess of that needed for alk fsp
 3. C=CaO-3.3P2O5-CO2: Ca in excess of what is needed for apatite and calcite
 4. F=FeO+MgO+MnO-TiO2-Fe2O3: excess over what is needed to make ilmenite
     and magnetite

 AKF diagrams (for potassic minerals)
 1. A=Al2O3+Fe2O3-(Na2O+K2O+CaO) eliminates plag
 2. K=K2O
 3. F=FeO+MgO+MnO
Compatibility diagrams cont’d
 AFM projection:
 Projection from either Kfsp or muscovite on AFM
 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
 Remaining: Al2O3, FeO, MgO, K2O
 1. A=Al2O3-3K2O: KAl2AlSi3O10(OH)2 (musc),
     from Kfsp: A=Al2O3-K2O
 2. F=FeO (FeO-TiO2)
 3. M=MgO

To top