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
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 (<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
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 size
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
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
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
Mylonite: grain reduction due to shear often marks faults and shear zones. Non hydrostatic stress makes grain instable and results in dynamic recrystallization
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
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
Granodiorite intrusion in slate
If a hydrothermal system develops: skarn formation, silicate rich fluids percolate through rock. Formation of reaction zones.
Mappable line often recognized through an index mineral: isograd
Facies: suite of mineral assemblages, repeatedly found in terranes of all ages and possesses a regular variation between mineral composition and bulk chemical composition
2 Laws+ 5 glautrem+10 alb+ 2 chlo
4chlo+18zoi+21qtz5Al-amph+26An+20H2O Also:7chlo+13trem+12zoi+14Qtz25Al-amph+22H2O Also:alb+trem=Al-amph+Qtz
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
Aligned platy grains (like mica and chlorites in phyllites
and schists) called lepidoblastic texture and can show
Hornfels with slaty cleavage
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
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
Mineralogical names, like marble, serpentinite
Geological setting: nature of metamorphism
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
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
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
Diagrams to depicts compositional relationships in metamorphic rocks
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
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
A=Al2O3-3K2O: KAl2AlSi3O10(OH)2 (musc),
from Kfsp: A=Al2O3-K2O
2. F=FeO (FeO-TiO2)