Facies and Description
of Metamorphic Rocks
Nugroho Imam Setiawan, Ph.D
Laboratory of Optical Geology, Dept. of Geological Engineering
Gadjah Mada University
2014
Classification of Metamorphic Rocks
• Metamorphic rocks are classified on the basis of
texture and composition (either mineralogical or
chemical)
• Unlike igneous rocks, which have been plagued by
a proliferation of local and specific names,
metamorphic rock names are surprisingly simple
and flexible
• May choose some prefix-type modifiers to attach
to names if care to stress some important or
unusual textural or mineralogical aspects
Metamorphic Structure
Struktur foliasi
Metamorphic Structure
• Slate: a strongly cleaved rock in a) Slate
which the cleavage planes are
pervasively developed
throughout the rock, due to
orientation of very fine
phyllosilicate grains. The
individual aligned grains are too
small to be seen with the naked
b) Phyllite
eye and the rock has a dull
appearance on fresh surfaces.
• Phyllite: similar to slate but
slightly coarser phyllosilicate
grains and give silky appearance
to cleaved surfaces.
Metamorphic Structure
• Schist: characterized by
a) Grt-Mica schist
parallel alignment of
moderately to coarse grains
usually clearly visible with the
naked eye. The fabric is
known as schistosity.
• Gneiss: gneisses are coarse,
b) Bt-Ms gneiss
with grain size of several
millimeters and foliated. The
term of orthogneiss is used
for gneisses of igneous
parentage, paragneiss for
metasedimentary gneisses.
Metamorphic Structure
• Mylonite: term used for a) Mylonite
fine-grained rocks produced
in zones of intense ductile
deformation where pre-
existing grains have been b) Mylonite thin section
deformed and recrystalized
as finer grains
Metamorphic rock names
• Metamorphic structure and texture spotted
schist, gneiss, phyllite, augen-mylonite
• Metamorphic protolith metabasic,
metatonalite
• Spesific name Facies, Cataclasite, Quartzite,
Marble
• Combination garnet-mica-quartz schist,
Ind: amfibolit garnet-biotit
Metamorphic Facies
Specific rock names by its facies
• Greenschist: a low-grade metamorphic
rock that typically contains chlorite, a) Greenschist
actinolite, epidote, and albite. Such a
rock is called greenschist if foliated, and
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 non- b) Amphibolite c) Granulite
foliated. The protolith is either a mafic
igneous rock or graywacke.
• Granulite: a high grade rock of pelitic,
mafic, or quartzo-feldspathic parentage
that is predominantly composed of OH-
free minerals. Muscovite is absent and
plagioclase and orthopyroxene are
common.
• Blueschist: a blue amphibole-bearing d) Blueschist e) Eclogite
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.
• Eclogite: a green and red metamorphic
rock that contains clinopyroxene and
garnet (omphacite + pyrope). The
protolith is typically basaltic.
Special names for metamorphic rocks
• Migmatite: a composite silicate rock that is a) Migmatite
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.
• Serpentinite: an ultramafic rock
metamorphosed at low grade, so that it b) Serpentinite
contains mostly serpentine.
• Hornfels: is a type of granofels that is typically
very fine-grained and compact, and occurs in
contact aureoles. Hornfelses are tough, and
tend to splinter when broken. Skarn: a contact c) Grt-Wo skarn
metamorphosed and silica metasomatized
carbonate rock containing calc-silicate
minerals, such as grossular, epidote, tremolite,
vesuvianite, etc. Tactite is a synonym.
Special Terms for metamorphic names
Spotted phyllite
• Spot: porphyroblast minerals; 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.
• Augen: Some gneisses have Augen gneiss
large eye-shaped 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.
Protolith Name
• Meta-: used for metamorphic rock based on
protolith. Reason: 1) Determination of the
original nature consideration in geological
history; 2) In weakly metamorphosed rocks
and particularly those subjected to little
deformation.
Metamorphic Facies
vs Mineral
Assemblages
Table 25-1. Definitive Mineral Assemblages of Metamorphic Facies
Facies Definitive Mineral Assemblage in Mafic Rocks
Zeolite zeolites: especially laumontite, wairakite, analcime
Prehnite-Pumpellyite prehnite + pumpellyite (+ chlorite + albite)
Greenschist chlorite + albite + epidote (or zoisite) + quartz ± actinolite
Amphibolite hornblende + plagioclase (oligoclase-andesine) ± garnet
Granulite orthopyroxene (+ clinopyrixene + plagioclase ± garnet ±
hornblende)
Blueschist glaucophane + lawsonite or epidote (+albite ± chlorite)
Eclogite pyrope garnet + omphacitic pyroxene (± kyanite)
Mineral assemblages in mafic rocks of the facies of contact meta-
Contact Facies
morphism do not differ substantially from that of the corresponding
regional facies at higher pressure.
After Spear (1993)
Metamorphic Facies vs Mineral
Assemblages
Variation of mineral assemblages in
the metamorphic rocks
1. Differences of whole rock chemistries
2. Differences of metamorphism (P, T, H2O)
3. Differences of protolith:
1. Ultramafic - very high Mg, Fe, Ni, Cr serpentinite
2. Mafic - high Fe, Mg, and Ca metabasic
3. Shales (pelitic) - high Al, K, Si metapelite
4. Carbonates - high Ca, Mg, CO2 calc-silicate/marble
5. Quartz - nearly pure SiO2 quartzite
6. Quartzo-feldspathic - high Si, Na, K, Al metapsammite
IUGS Classification
GL 1. If the rock features are dominated by those of the protolith or
the protolith may be determined by the context of the rock then a
protolith name may be applied. Protolith-based names are
particularly recommended for weakly metamorphosed rocks,
especially where the use of a structural root name would be
considered contrary to established practice For example, with a
metamorphosed sandstone the name 'biotite-quartz-feldspar
metasandstone' should take precedence over 'biotite-quartz-feldspar
gneiss (or granofels)'.
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 (for
example, biotitite, epidotite, glaucophanite).
GL 3a. If the rock fits the definition of one of the well-known and
commonly used specific names then it is generally appropriate to use
that specific term There is no absolute rule on when to use or not to
use a specific name. However a specific name will generally take
preference over the equivalent systematic/structural root name if
the specific name is well established or understood or if it is more
concise or gives greater detail than the systematic alternative (for
example marble rather than calcite granofels, amphibolite rather
than hornblende-plagioclase granofels, slate and phyllite as types of
schist). Conversely, a systematic name is more appropriate where
there is no specific name or a possible specific name is little used,
ambiguous or poorly defined.
GL 3b. If the context or genesis (that is, the metamorphic processes
forming the rock) of the rock is known and particularly if it is
desirable to emphasise this or give additional or detailed information
about the context or genesis of the rock then the appropriate
specific name should be used (for example, nebulite, blastomylonite,
tektite, hornfels). In this case the names should conform to those in
the relevant SCMR paper
METAMORPHISM
