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Classification & Textures
Classification of Metamorphic Rocks
Metamorphic Classification
• Classified on the basis of texture and composition (either
mineralogical or chemical)
Foliated Rocks
Non-foliated Rocks
Etc.
• Much simpler classification than that for igneous rocks
Metamorphic Textures
Relict textures
Contact metamorphism
• Prefix-type modifiers used to indicate important or unusual textural
or mineralogical aspects
Deformation & Recrystallization
Dynamic/Regional metamorphism
• The key purpose is to convey information in a concise way
Foliations, Lineations, Sense of Shear
Progressive development of textures
Sequence of deformations, Metamorphic events
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Foliated Metamorphic Rocks
Foliated Metamorphic Rocks
• Foliation: and planar fabric element
Schistosity
• Lineation: any linear fabric elements
• A preferred orientation of inequaint mineral grains or grain
aggregates produced by metamorphic processes
• Aligned minerals are coarse grained enough to see with the
unaided eye
• The orientation is generally planar, but linear orientations are
not excluded
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• Cleavage
 Traditionally: the property of a rock to split along a
regular set of sub-parallel, closely-spaced planes
 A more general concept adopted by some geologists is
to consider cleavage to be any type of foliation in
which the aligned sheet silicates are too fine grained to
see individually with the unaided eye
Gneissose structure
• Either a poorly-developed schistosity or segregated into layers
by metamorphic processes
• Gneissose rocks are generally coarse grained
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Foliated Metamorphic Rocks
Foliated Metamorphic Rocks
Slate: compact, very finegrained, metamorphic rock
with a well-developed
cleavage. Freshly cleaved
surfaces are dull
Phyllite: a rock with a
schistosity in which very fine
phyllosilicates
(sericite/phengite and/or
chlorite), although rarely
coarse enough to see unaided,
impart a silky sheen to the
foliation surface. Phyllites
with both a foliation and
lineation are very common.
a: Slate
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Schist: a metamorphic rock
exhibiting a schistosity. By this
definition schist is a broad term, and
slates and phyllites are also types of
schists. In common usage, schists are
restricted to those metamorphic
rocks in which the foliated minerals
are coarse enough to see easily in
hand specimen.
b: Phyllite
Figure 22.1. Examples of foliated metamorphic rocks. a. Slate. b. Phyllite. Note the difference in reflectance on the foliation surfaces
between a and b: phyllite is characterized by a satiny sheen. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice
Hall.
Figure 22.1c. Garnet muscovite schist.
Muscovite crystals are visible and silvery,
garnets occur as large dark
porphyroblasts. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
S. Kuehn 2005
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Foliated Metamorphic Rocks
Non-Foliated Metamorphic Rocks
Gneiss: a metamorphic
rock displaying
gneissose structure.
Gneisses are typically
layered (also called
banded), generally with
alternating felsic and
darker mineral layers.
Gneisses may also be
lineated, but must also
show segregations of
felsic-mineral-rich and
dark-mineral-rich
concentrations.
Simpler than for foliated rocks
Again, this discussion and classification applies only to
rocks that are not produced by high-strain metamorphism
Granofels: a comprehensive term for any isotropic rock (a
rock with no preferred orientation)
Hornfels is a type of granofels that is typically very finegrained and compact, and occurs in contact aureoles.
Hornfelses are tough, and tend to splinter when broken.
Figure 22.1d. Quartzo-feldspathic gneiss with obvious layering. Winter (2001) An Introduction to Igneous and Metamorphic Petrology.
Prentice Hall.
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Quartzite
Specific Metamorphic Rock Types
Fig 8.9 Understanding Earth
Marble: a metamorphic rock composed predominantly of
calcite or dolomite. The protolith is typically limestone or
dolostone.
Quartzite: a metamorphic rock composed predominantly of
quartz. The protolith is typically sandstone. Some
confusion may result from the use of this term in
sedimentary petrology for a pure quartz sandstone.
Marble
Fig 8.9 Understanding Earth
typically have little to no foliation
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Specific Metamorphic Rock Types
Figure 23.11. Drawings of quartz-mica schists. a. Closer
spacing of micas in the lower half causes quartz grains to
passively elongate in order for quartz-quartz boundaries to
meet mica (001) faces at 90o. From Shelley (1993). b. Layered
rock in which the growth of quartz has been retarded by
grain boundary “pinning” by finer micas in the upper layer.
Greenschist/Greenstone: a low-grade metamorphic rock that
typically contains chlorite, actinolite, epidote, and albite.
From Vernon, 1976) Metamorphic Processes:
Reactions and Microstructure Development. Allen &
Unwin, London.
Note that the first three minerals are green, which imparts the color
to the rock. Such a rock is called greenschist if foliated, and
b
greenstone if not. The protolith is either a mafic igneous
rock or graywacke.
Amphibolite: a metamorphic rock dominated by hornblende
+ plagioclase. Amphibolites may be foliated or nonfoliated. The protolith is either a mafic igneous rock or
graywacke.
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Specific Metamorphic Rock Types
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Specific Metamorphic Rock Types
Serpentinite: an ultramafic rock metamorphosed at low
grade, so that it contains mostly serpentine.
Skarn: a contact metamorphosed and silica metasomatized
carbonate rock containing calc-silicate minerals, such as
grossular, epidote, tremolite, vesuvianite, wollastonite, etc.
Blueschist: a blue amphibole-bearing metamorphosed mafic
igneous rock or mafic graywacke. This term is so
commonly applied to such rocks that it is even applied to
non-schistose rocks.
Granulite: a high grade rock of pelitic, mafic, or quartzofeldspathic parentage that is predominantly composed of
OH-free minerals. Muscovite is absent and plagioclase
and orthopyroxene are common.
Eclogite: a green and red metamorphic rock that contains
clinopyroxene and garnet (omphacite + pyrope). The
protolith is typically basaltic.
Specific Metamorphic Rock Types
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Classification of Metamorphic Rocks
Migmatite: a composite silicate rock that is heterogeneous on the 1-10
cm scale, commonly having a dark gneissic matrix (melanosome) and
lighter felsic portions (leucosome). Migmatites may appear layered, or
the leucosomes may occur as pods or form a network of cross-cutting
veins.
Additional Modifying Terms:
Porphyroblastic means that a metamorphic rock has
one or more metamorphic minerals that grew much
larger than the others. Each individual crystal is a
porphyroblast
Some porphyroblasts, particularly in low-grade
contact metamorphism, occur as ovoid “spots”
If such spots occur in a hornfels or a phyllite (typically as a contact
metamorphic overprint over a regionally developed phyllite), the
terms spotted hornfels, or spotted phyllite would be appropriate.
Geology - Chernicoff
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Large garnet porphyroblasts in schist
Fig 8.10 Understanding Earth
Figure 23.14b. Spotted Phyllite. Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
S. Kuehn 2005
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Classification of Metamorphic Rocks
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Classification of Metamorphic Rocks
Additional Modifying Terms:ugen structure.
Augens
Some gneisses have large eyeshaped grains (commonly
feldspar) that are derived
from pre-existing large
crystals by shear. Individual
grains of this sort are called
auge (German for eye), and
the (German) plural is
augen.
An augen gneiss is a gneiss
with augen structure.
Additional Modifying Terms:
Other modifying terms that we may want to add as a
means of emphasizing some aspect of a rock may
concern such features as grain-size, color,
chemical aspects, (aluminous, calcareous, mafic,
felsic, etc.). As a general rule we use these when
the aspect is unusual. Obviously a calcareous
marble or mafic greenschist is redundant, as is a
fine grained slate.
Figure 23.18. Augen Gneiss. Winter (2010) An Introduction
to Igneous and Metamorphic Petrology. Prentice Hall.
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Classification of Metamorphic Rocks
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High Strain Rocks
Additional Modifying Terms:
Ortho- a prefix indicating an igneous parent, and
Para- a prefix indicating a sedimentary parent
The terms are used only when they serve to dissipate doubt. For
example, many quartzo-feldspathic gneisses could easily be derived
from either an impure arkose or a granitoid rock. If some
mineralogical, chemical, or field-derived clue permits the
distinction, terms such as orthogneiss, paragneiss, or
orthoamphibolite may be useful.
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High Strain Rocks
Mylonite
Ultramylonite
Mylonite Produced by shearing in the deepest parts of fault zones.
Arrows indicate the shear direction in this sample Understanding Earth
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Scaleindependence
(structure)
(structure)
Figure 22.2. Schematic cross section through a shear zone, showing the vertical distribution of fault-related rock types, ranging from noncohesive gouge and breccia near the surface through progressively more cohesive and foliated rocks. Note that the width of the shear zone
increases with depth as the shear is distributed over a larger area and becomes more ductile. Circles on the right represent microscopic
views or textures. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag. Berlin.
(texture)
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Diagram showing that
structural and fabric
elements are
generally consistent
in style and
orientation at all
scales. From Best
(1982). Igneous and
Metamorphic
Petrology. W. H.
Freeman. San
Francisco.
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Metamorphic Textures
Textures of Contact Metamorphism
Textures are small-scale penetrative features
• Typically shallow pluton aureoles (low-P)
• Crystallization/recrystallization is near-static
 Monomineralic with low  surface energy 
granoblastic polygonal
 Larger  S.E.  decussate
• Isotropic textures (hornfels, granofels)
• Relict textures are common
Relict Textures
•
•
•
•
Inherited from original rock
“Blasto-” = relict
Any degree of preservation
Pseudomorphs of minerals or pre-metamorphic
textures/structures (bedding, pillows, etc.)
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Progressive thermal
metamorphism of a diabase
(coarse basalt). From Best
(1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco.
Sandstone
texture
Quartzite
texture
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Progressive thermal
metamorphism of slate. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco.
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Progressive thermal
metamorphism of slate. From
Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco.
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Metamorphic Textures
• Contact overprint on earlier regional events are
common
 Thermal maximum later than deformational
 Separate post-orogenic (collapse) event
• Nodular overprints
Figure 23.14. Overprint of contact metamorphism on regional:
• Spotted slates and phyllites
Spotted phyllite in which small porphyroblasts of cordierite develop
in a preexisting phyllite. Winter (2010) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
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Depletion haloes
Depletion haloes
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Figure 23.13. Light
colored depletion haloes
around cm-sized garnets
in amphibolite. Fe and
Mg were less plentiful, so
that hornblende was
consumed to a greater
extent than was
plagioclase as the garnets
grew, leaving
hornblende-depleted
zones.
Sample courtesy of Peter Misch.
Winter (2010) An Introduction to
Igneous and Metamorphic
Petrology. Prentice Hall.
Depletion halo around garnet porphyroblast. Boehls Butte area, Idaho
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Depletion haloes
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Pressure Solution
Progressive development of
a depletion halo about a
growing porphyroblast.
From Best (1982). Igneous
and Metamorphic Petrology.
W. H. Freeman. San
Francisco.
Figure 23.2 a. Highest strain in areas near grain contacts (orange hatch pattern). b. High-strain
areas dissolve and material precipitates in adjacent low-strain areas (shaded). The process is
accompanied by vertical shortening. c. Pressure solution of a quartz crystal in a deformed
quartzite (1 is vertical). Pressure solution results in a serrated solution surface in high-strain
areas (small arrows) and precipitation in low-strain areas (large arrow). ~ 0.5 mm across. The
faint line within the grain is a hematite stain along the original clast surface. After Hibbard (1995)
Petrography to Petrogenesis. Prentice Hall.
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Dislocation migration forms two strain-free
subgrains
Recrystallization by grain boundary migration
and sub-grain rotation
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Figure 23.5. Illustration of a recovery process in which
dislocations migrate to form a subgrain boundary. Winter (2010)
An Introduction to Igneous and Metamorphic Petrology. Prentice
Hall.
Figure 23.7a. Recrystallized quartz with irregular
(sutured) boundaries, formed by grain boundary
migration. Width 0.2 mm. From Borradaile et al. (1982).
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Fig 23.21 Types of foliations
a. Compositional layering
b. Preferred orientation of platy
minerals
c. Shape of deformed grains
d. Grain size variation
e. Preferred orientation of platy
minerals in a matrix without
preferred orientation
f. Preferred orientation of
lenticular mineral aggregates
g. Preferred orientation of
fractures
h. Combinations of the above
Progressive syntectonic
metamorphism of a volcanic
graywacke, New Zealand.
From Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco.
Figure 23.21. Types of fabric elements that may define a foliation. From
Turner and Weiss (1963) and Passchier and Trouw (1996).
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Progressive syntectonic
metamorphism of a volcanic
graywacke, New Zealand.
From Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco.
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Progressive syntectonic
metamorphism of a volcanic
graywacke, New Zealand.
From Best (1982). Igneous and
Metamorphic Petrology. W. H.
Freeman. San Francisco.
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Progressive development (a  c)
of a crenulation cleavage for both
asymmetric (top) and symmetric
(bottom) situations. From Spry
(1969) Metamorphic Textures.
Pergamon. Oxford.
Figure 23.24a. Symmetrical crenulation cleavages in amphibole-quartz-rich schist. Note concentration of quartz in hinge areas. From
Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag.
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Figure 23.24b. Asymmetric crenulation cleavages in mica-quartz-rich schist. Note horizontal compositional layering (relict bedding)
and preferential dissolution of quartz from one limb of the folds. From Borradaile et al. (1982) Atlas of Deformational and
Metamorphic Rock Fabrics. Springer-Verlag.
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Development of an axial-planar cleavage in folded
metasediments. Circular images are microscopic views
showing that the axial-planar cleavage is a crenulation
cleavage, and is developed preferentially in the micaceous
layers. From Gilluly, Waters and Woodford (1959)
Principles of Geology, W.H. Freeman; and Best (1982).
Igneous and Metamorphic Petrology. W. H. Freeman. San
Francisco.
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Types of lineations
a. Preferred orientation
of elongated
mineral aggregates
b. Preferred
orientation of
elongate minerals
c. Lineation defined by
platy minerals
d. Fold axes
(especially of
crenulations)
e. Intersecting planar
elements.
Figure 23.26. Types of fabric elements that define a
lineation. From Turner and Weiss (1963) Structural
Analysis of Metamorphic Tectonites. McGraw Hill.
Figure 23.27. Proposed mechanisms for the development of foliations. After Passchier
and Trouw (1996) Microtectonics. Springer-Verlag.
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Pre-kinematic crystals
a. Helicitic folds b. Randomly oriented crystals c. Polygonal arcs
d. Chiastolite e. Late, inclusion-free rim on a poikiloblast (?)
f. Random aggregate pseudomorph
a. Bent crystal with
undulose
extinction
b. Foliation
wrapped around
a porphyroblast
c. Pressure shadow
or fringe
d. Kink bands or
folds
e. Microboudinage
f. Deformation
twins
Figure 23.35.
Typical textures
of postkinematic
crystals. From
Spry (1969)
Metamorphic
Textures.
Pergamon.
Oxford.
Figure 23.34. Typical textures of prekinematic crystals. From Spry (1969)
Metamorphic Textures. Pergamon.
Oxford.
Syn-kinematic crystals
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Post-kinematic crystals
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Syn-kinematic crystals
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Spiral Porphyroblast
Figure 23.38. Spiral Si
train in garnet,
Connemara, Ireland.
Magnification ~20X.
From Yardley et al.
(1990) Atlas of
Metamorphic Rocks and
their Textures.
Longmans.
Figure 23.38. Traditional interpretation of spiral Si train in which a porphyroblast is
rotated by shear as it grows. From Spry (1969) Metamorphic Textures. Pergamon.
Oxford.
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Syn-kinematic crystals
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Figure 23.38.
“Snowball garnet”
with highly rotated
spiral Si.
Porphyroblast is ~ 5
mm in diameter.
From Yardley et al.
(1990) Atlas of
Metamorphic Rocks
and their Textures.
Longmans.
Figure 23.48c. Interpreted sequential development of a polymetamorphic rock.
From Spry (1969) Metamorphic Textures. Pergamon. Oxford.
Sense of Shear Indicators
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Figure 23.19. Mantled porphyroclasts and “mica fish” as sense-of-shear indicators. After Passchier and Simpson (1986)
Porphyroclast systems as kinematic indicators. J. Struct. Geol., 8, 831-843.
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