Classification of Metamorphic Rocks
(C
(Chapter
22)
Granitic augen gneiss– NW Scotland. Image: Darrell Henry
Classification – Metamorphic rocks
IUGS-SCMR
IUGS
SCMR – general principles
Terminology of classification (root names) based on:
• Structural features (as seen in hand sample or outcrop)
• Schist
• Gniess
G ess
• Granofels
p
• Likelyy protolith
• Specific names (historically well-known and commonly
used terms)
The root names is modified by:
• Mineralogical prefixes features (with progressively greater modal
amounts next to root name) e.g. qtz-bt schist
• Additional textural features e.g. lineated pl-qtz-hbl schist
Classification – Metamorphic rocks
Foliated Metamorphic Rocks (structural terms/names)
Types of foliations
a. Compositional layering on less than cm scale
b. Preferred orientation of platy minerals
c. Shape of deformed grains
d G
d.
Grain
i size
i variation
i ti
e. Preferred orientation of platy minerals in matrix
without preferred orientation
f. Preferred orientation of lenticular mineral
aggregates
g. Preferred orientation of fractures
h. Combinations of the above
More than one kind of foliation, with more than one
orientation
i
i may b
be present iin a rock.
k
The surface to which the foliations are parallel are
called s-surfaces - s0, s1, s2 ...
Classification – Metamorphic rocks
Lineation: any repetitively-occurring or penetrative visible linear
f
feature
iin a rock
kb
body.
d IIt may b
be d
defined
fi d b
by:
Types of lineations
a. Preferred orientation of elongated
g
mineral
aggregates
b. Preferred orientation of elongate minerals
(mineral lineation)
c. Lineation defined by platy minerals
d. Parallelism of hinge lines or small scale folds
(crenulation lineation)
e. Intersecting planar elements (intersection
lineation)
also:
l :
slickenside striations or fibers
More than one kind of lineation, with more than
one orientation may be present in a rock.
rock
Image: Winter (2001)
The lines to which lineations are parallel are called
l-lines - l0, l1, l2, ..
Classification – Metamorphic rocks
IUGS-SCMR
IUGS
SCMR Flowchart for naming metamorphic rocks
“The scheme must
provide a consistent
set of names to cover
the spectrum of rock
types and their
ch r cteri tic
characteristics
without any
terminology gaps.
gaps.”
Classification – Metamorphic rocks
SCMR general guidelines for deriving an appropriate
metamorphic rock name
GL1. If the rock features are dominated by those of the protolith
or the protolith may be determined by the context of the rock
(generally field criteria), then a protolith name may be applied.
• This is p
particularlyy recommended for weaklyy metamorphosed
p
rocks.
• E.g. for a metamorphosed sandstone a name such as biotite-quartz-feldspar
metasandstone should take precedence of biotite-quartz-feldspar gneiss
Classification – Metamorphic rocks
SCMR general guidelines for deriving an appropriate
metamorphic rock name
GL 2. If the rock contains >75% modally of one mineral then it
may be named by adding the suffix "ite" to the dominant
mineral name
• E.g. biotitite, epidotite, chloritite, ...
• Exceptions include amphibolite (specific name for rock with >75% amphibole
+ plagioclase); hornblendite, pyroxenite and olivinite (igneous rock names);
and carbonate mineral dominated rocks ((i.e. various marbles such as calcite
marble)
Classification – Metamorphic rocks
SCMR general guidelines for deriving an appropriate
metamorphic rock name
GL 3a: If the rock fits the definition of one of the well-known and
commonly-used specific names then it is more appropriate to use
the specific term
• E.g. amphibolite, eclogite, marble, slate, calc-silicate rock, ...
• In general a specific name is preferred over
o er the equivalent
eq i alent
systematic/structural root name (such as the use of marble over calcite
granofels).
Classification – Metamorphic rocks
SCMR general guidelines for deriving an appropriate
metamorphic rock name
More about prefixes to root names
• Mineral prefixes to rock names should consider all the major
((>5%)) mineral constituents ((except
p those implied
p
byy the rock
name) and possibly some of the minor mineral constituents
(<5%) if they convey critical information.
• For major mineral constituents prefixes should be hyphenated and placed
in order of increasing abundance (such as biotite-quartz-plagioclase
gneiss).
• For minor constituents convey critical information, prefix the beginning
of the name with "mineral"-bearing (such as rutile-bearing biotitequartz-plagioclase gneiss.)
gneiss )
• Mineral abbreviations are encouraged for long prefixes
Classification – Metamorphic rocks
Cleavage: property to split along regular set of sub-parallel, closelyspaced surfaces. More than one cleavage may be present in rock.
Slaty cleavage: Well-developed
planar schistosity in a rock in which
individual grains are too small to be
seen by unaided eye and the
schistosity is developed on grain
scale.
I
Image:
Wi
Winter
(2001)
Slate: An ultrafine
ultrafine- or very fine-grained
fine grained
metamorphic rock displaying slaty cleavage.
Freshly cleaved surfaces are dull.
Ph lli A fine- to medium-grained
Phyllite:
metamorphic rock characterized by a
lustrous sheen and well-developed
schistosity
hi
i resulting
l i ffrom the
h parallel
ll l
arrangement of phyllosilicates.
http://csmres.jmu.edu/geollab/fichter/MetaRx/Rocks/phyllite1.html
Classification – Metamorphic rocks
Cleavage: property to split along regular set of sub-parallel, closelyspaced surfaces. More than one cleavage may be present in rock.
Schistosity: A preferred orientation
of inequant mineral grains or grain
aggregates produced by
metamorphic processes - rock will
split on a scale of < 1 cm.
Image: Winter (2001)
Schist: A metamorphic rock displaying
schistose texture. In common usage, schists
are restricted to those metamorphic rocks in
which the foliated minerals are coarse enough to
see easily in hand specimen.
(top left) Bt-bearing qtz-pl-grt-ms schist. Muscovite crystals are
visible
i ibl and
d silvery,
il
garnets occur as llarge ddarkk porphyroblasts.
h bl
Image: Barb Dutrow
(bottom left) pl-qtz-grt-st-ms schist. Large porphyroblasts of
staurolite (elongate) and garnet (round).
Classification – Metamorphic rocks
Gneissose structure: Either a poorly
poorly-
developed schistosity or, if
developed, occurs in broadly spaced
zones, such that rock will split on a
scale of more than one cm. It is
Image: Darrell Henry
commonlyy segregated
g g
into layers
y byy
metamorphic processes, but does
not have to be to fit definition.
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
d rk min r l ri h concentrations.
dark-mineral-rich
n ntr ti n
Image: Darrell Henry
Bt-Qtz-fsp gneisses. Layering due to metamorphic differentiation. Quad Creek,
Beartooth Mtns, MT.
Classification – Metamorphic rocks
Non-Foliated Metamorphic Rocks
(structural terms/names)
Simpler than for foliated rocks
This classification applies only to rocks that are
not produced by high-strain metamorphism
Granofels structure: A type of structure
Preserved magmatic layering with a high grade
metamorphic overprint . SW Sweden. Image:
Darrell Henryy
resulting from absence of schistosity
such
h th
thatt mineral
in l grains
in and
nd aggregates
t
of mineral grains are equant, or if
inequant have a random orientation.
Mineralogical and lithological layering
may be present.
Granofels: A metamorphic
hi rock
k di
displaying
l i
www.earth.ox.ac.uk/~oesis/micro/index.html
granofelsic texture (a rock with no preferred
orientation).
Classification – Metamorphic rocks
Specific
p
rock names
Amphibolite: is a gneissose or
lineated Garnet amphibolite from Alberta
granofelsic metamorphic rock mainly
consisting
i i off green, b
brown or bl
black
k
amphibole and plagioclase (including
albite), which combined form ≥75%
of the rock and both of which are
present as major constituents; the
amphibole constitutes ≥50% of the
total mafic constituents and is present
in an amount of ≥30%.
Other common minerals
Oth
i l include
i l d quartz,
t clinopyroxene,
li p
garnet, epidote-group minerals, biotite, titanite and
scapolite.
lineated titanite-bearing amphibolite
www.earth.ox.ac.uk/~oesis/micro/index.html
The protolith is either a mafic
T
f igneous rock or
graywacke
Classification – Metamorphic rocks
Specific rock names
Granulite: a high grade rock in which
Fe-Mg
g silicates are dominantlyy OHfree. The presence of feldspar and
absence of muscovite are critical,
cordierite may also be present.
present
Conversion of felsic gneiss (left) to charnockite
(right -type of the granulite - opx granitoid).
Kabbaldurga
g quarry,
q
y, India Image:
g Darrell Henryy
Cordierite-biotite- plagioclase gneiss. Madagascar
Mineral composition is to be indicated by
prefixing the major constituents. Rocks with
>30% mafic minerals (dominantly pyroxene)
may be called mafic granulites, and those with
<30% mafic minerals ((dominantlyy pyroxene)
py
)
may be called felsic granulites.
Classification – Metamorphic rocks
Specific rock names
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
(l
(leucosome).
)
Migmatite with felsic leucosome and garnet-biotite-sillimanite neosome.
Hellroaring Plateau, Beartooth Mtns, MT. Image: Darrell Henry
Migmatites may appear layered,
or the
h lleucosomes may occur as
pods or form a network of crosscutting veins
Classification – Metamorphic rocks
Specific rock names
Eclogite: Plagioclase-free metamorphic
rock composed of >75% of omphacite
and garnet, both of which are present as
major constituents.
E l i W
Eclogite,
W. N
Norway. IImage: D
Darrellll H
Henry
Generally green and red metamorphic rock
Generally,
(Christmas tree rock) that contains clinopyroxene and
garnet (omphacite + pyrope). The protolith is
typically basaltic
Blueschist: A schist whose blue color is
due to the p
presence of alkali
amphiboles.
Generally, it is 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.
Classification – Metamorphic rocks
Specific rock names
Serpentinite: an ultramafic rock
metamorphosed at low grade,
so that it contains mostly
serpentine.
chromite-bearing serpentinite. Image: Darrell Henry
Metamorphosed iron
formation: Bedded Fe-rich
rocks whose bulk composition
contains at least 15% Fe (19.3%
FeO or 21.4%
21 4% Fe2O3).
)
Folded banded iron formation (BIF) with quartz, hematite and
crocidolite (Tiger's eye). Images: Darrell Henry
Classification – Metamorphic rocks
Specific names: Metacarbonate and related rocks
Marble: a metamorphic rock
composed
p
of >50% carbonate
minerals (calcite, aragonite, or
dolomite).
• Pure marble contains >95%
carbonate minerals.
• Impure marble contains 50-
95% carbonate minerals.
Classification – Metamorphic rocks
Specific names: Metacarbonate and related rocks
Carbonate-silicate rock:
Metamorphic
p rock mainlyy
composed of silicate minerals
and containing 5-50% carbonate
minerals.
minerals
Calc-silicate rock: Metamorphic
rock
k primarily
i
il composed
d off C
Carich silicates and <5% of
carbonate minerals.
layers of vesuvianite (green-brown), grossular garnet
(orange), diopside (green) and calcite (gray). from
contact zone of the Alta Stock, Alta, UT – image:
Darrell Henry
Classification – Metamorphic rocks
Specific names: Metacarbonate and related rocks
Skarn: a contact
metamorphosed
p
and silica
metasomatized carbonate
rock containing calc-silicate
minerals such as grossular
minerals,
grossular,
epidote, tremolite,
vesuvianite, etc. Tactite is a
synonym.
Wollastonite - grossular garnet - diopside skarn
from the Adirondacks (NY) – image: Darrell
Henry
Classification – Metamorphic rocks
Specific names
Quartzite: a metamorphic rock
composed predominantly of quartz
(>75%) The protolith is typically
(>75%).
sandstone. Some confusion may result
from the use of this term in
sedimentary
di
t
petrology
p t l
for
f ap
pure
quartz sandstone.
http://www.union.edu/PUBLIC/GEODEPT/COURSES/pet
rology/fieldtrips/fieldtrips2004.htm
www.geo.umn.edu/mgs/virt_egg/greenstone.htm
Greenschist/Greenstone:
/
a lowgrade metamorphic rock that typically
contains chlorite, actinolite, epidote,
and albite. Note that first three
minerals are green, which imparts the
color to the rock. Such a rock is called
greenschist if foliated, and greenstone
if not. The protolith is either a mafic
igneous rock or graywacke.
Classification – Metamorphic rocks
Specific names: high strain rocks
Schematic cross section through a shear zone
Classification – Metamorphic rocks
Specific names: high strain rocks
Classification – Metamorphic rocks
Specific names: high strain rocks
Granitic mylonite (protomylonite)
with alignment of amphiboles
Mojave Desert, near Barstow, CA. image: Darrell Henry
Granitic mylonite gneiss
(ultramylonite) with white
plagioclase porhyroclasts
behaving
g ductilelyy and the
matrix quartz and biotite
behaving ductilely.
Mojave Desert, near Barstow, CA. image: Darrell
Henry
Classification – Metamorphic rocks
Additional modifying terms
Porphyroblastic means that a metamorphic
rock has one or more metamorphic minerals
that grew much larger than the others.
others Each
individual crystal is a porphyroblast
image: Darrell Henry
Some porphyroblasts, particularly in lowgrade contact metamorphism, occur as
ovoid "spots“
p
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
Nodular texture of cordierite porphyroblasts developed
during a thermal overprinting of previous regional
metamorphism . image: Winter (2001)
Classification – Metamorphic rocks
Additional modifying terms
Some gneisses have large eye-shaped grains
(commonly feldspar) that are derived from
pre existing large crystals by shear (as
pre-existing
described in Section 23.1). Individual grains
of this sort are called auge (German for
eye), and the (German) plural is augen.
image: Winter (2001)
Granitic augen gneiss– NW
Scotland. Image: Darrell Henry