Metamorphic Rock Classification

Classification - Metamorphic rocks

(Chapter 22)

last update:10/30/06

Classifications

	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 - take care to stress important or unusual textural or mineralogical aspects
	when possible, use the recommendations of the Subcommission on the Systematics of Metamorphic Rocks (SCMR) - see handout "How to Name a Metamorphic Rock"

SCMR general guidelines for deriving an appropriate metamorphic rock name

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 particularly recommended for weakly metamorphosed rocks.
	E.g. for a metamorphosed sandstone a name such as biotite-quartz-feldspar metasandstone should take precedence of biotite-quartz-feldspar gneiss

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)

If there is a structural fabric that a metamorphic rock exhibits (schist, gneiss or granofels), the structural term becomes the rootname with the appropriate modifiers unless there is a specific name that will take precedence.

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 the equivalent systematic/structural root name (such as the use of marble over calcite granofels).

If the context or genesis of the rock is known or desirable to emphasize, then the appropriate specific name should be used.

E.g. nebulite, blastomylonite, tektite,...

Mineral prefixes to rock names should consider all the major (>5%) mineral constituents (except those implied by the rock name) and possibly some of the minor mineral constituents (<5%) if they convey critical information.

	For major mineral constituents the 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 biotite-quartz-plagioclase gneiss.)

Foliated Metamorphic Rocks (structural terms/names)

Foliation: any repetitively-occurring or penetrative planar feature in a rock body. Examples are:

Types of foliations

a. Compositional layering on less than a cm scale

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

More than one kind of foliation, with more than one orientation may be present in a rock.

The surface to which the foliations are parallel are called s-surfaces - s₀, s₁, s₂ ...

Lineation: any repetitively-occurring or penetrative visible linear feature in a rock body. It may be defined by:

Types of lineations

a. Preferred orientation of elongated 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:

slickenside striations or fibers

More than one kind of lineation, with more than one orientation may be present in a rock.

The lines to which lineations are parallel are called l-lines - l₀, l₁, l₂, ...

Cleavage: The property of a rock to split along a regular set of sub-parallel, closely-spaced surfaces. More than one cleavage may be present in a rock.

	Slaty cleavage: A well-developed planar schistosity in a rock in which the individual grains are too small to be seen by the unaided eye and the schistosity is developed on the grain scale. Slate: An ultrafine- or very fine-grained metamorphic rock displaying slaty cleavage. Freshly cleaved surfaces are dull. Phyllite: *A fine- to medium-grained metamorphic rock characterized by a lustrous sheen and well-developed schistosity resulting from the parallel arrangement of phyllosilicates.*
	Schistosity: A preferred orientation of inequant mineral grains or grain aggregates produced by metamorphic processes - rock will split on a scale of < 1 cm. 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. Garnet muscovite schist. Muscovite crystals are visible and silvery, garnets occur as large dark porphyroblasts.
	Gneissose structure: Either a poorly-developed schistosity or, if developed, occurs in broadly spaced zones, such that the rock will split on a scale of more than one cm. It is commonly segregated into layers by metamorphic processes, but does not have to be to fit the 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 dark-mineral-rich concentrations. Quartzo-feldspathic gneiss with obvious layering. Quad Creek, Beartooth Mtns, MT.

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 resulting from the absence of schistosity such that the mineral grains and aggregates of mineral grains are equant, or if inequant have a random orientation. Mineralogical and lithological layering may be present.

Granofels: A metamorphic rock displaying granofelsic texture (a rock with no preferred orientation).

Marble: a metamorphic rock composed of >50% carbonate minerals (calcite, aragonite, or dolomite).

Pure marble contains >95% carbonate minerals.

Impure marble contains 50-95% carbonate minerals.

Carbonate-silicate rock: Metamorphic rock mainly composed of silicate minerals and containing 5-50% carbonate minerals.

Calc-silicate rock: Metamorphic rock primarily composed of Ca-rich 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

Skarn: a contact metamorphosed and silica metasomatized carbonate rock containing calc-silicate minerals, such as grossular, epidote, tremolite, vesuvianite, etc. Tactite is a synonym.

Wollastonite - grossular garnet - diopside skarn from the Adirondacks (NY)

Quartzite: a metamorphic rock composed predominantly of quartz (>75%). The protolith is typically sandstone. Some confusion may result from the use of this term in sedimentary petrology for a pure quartz sandstone.

Greenschist/Greenstone: a low-grade metamorphic rock that typically contains chlorite, actinolite, epidote, and albite. Note that the 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.

Amphibolite: a gneissose or granofelsic metamorphic rock consisting of green, brown of black amphibole and plagioaclase which forms >75% of the rock, and both of which are present as major constituents. The amphibole forms more than 50% of the total mafic constituents and is present at greater than 30% modally.

The protolith is either a mafic igneous rock or graywacke

Garnet amphibolite from Alberta

Blueschist: A schist whose blue color is due to the 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.

Eclogite: Plagioclase-free metamorphic rock composed of >75% of omphacite and garnet, both of which are present as major constituents.

Generally, a green and red metamorphic rock (Christmas tree rock) that contains clinopyroxene and garnet (omphacite + pyrope). The protolith is typically basaltic

Eclogite from Norway

Conversion of felsic gneiss (left) to charnockite (right -type of the granulite - opx granitoid). Kabbaldurga quarry, India

Cordierite-gneiss. Madagascar

Granulite: a high grade rock in which Fe-Mg silicates are dominantly OH-free. The presence of feldspar and absence of muscovite are critical, cordierite may also be present.

The 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 (dominantly pyroxene) may be called felsic granulites.

Migmatite with felsic leucosome and garnet-biotite-sillimanite neosome. Hellroaring Plateau, Beartooth Mtns, MT

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

Serpentinite: an ultramafic rock metamorphosed at low grade, so that it contains mostly serpentine.

Metamorphosed iron formation: Bedded Fe-rich rocks whose bulk composition contains at least 15% Fe (19.3% FeO or 21.4% Fe2O3).

Folded banded iron formation (BIF) with quartz, hematite and crocidolite (Tiger's eye).

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.

Overprint of contact metamorphism on regional. a. Nodular texture of cordierite porphyroblasts developed during a thermal overprinting of previous regional metamorphism (note the foliation in the opaques). Approx. 1.5 x 2 mm. From Bard (1986) Microtextures of Igneous and Metamorphic Rocks. Reidel. Dordrecht.
b. Spotted phyllite in which small porphyroblasts of cordierite develop in a preexisting phyllite. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Some gneisses have large eye-shaped grains (commonly feldspar) that are derived from pre-existing large crystals by shear (as described in Section 23.1). Individual grains of this sort are called auge (German for eye), and the (German) plural is augen.

Augen Gneiss

High Strain Rocks

Schematic cross section through a shear zone, showing the vertical distribution of fault-related rock types, ranging from non-cohesive 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.

	Granitic mylonite (protomylonite) with alignment of amphiboles Mojave Desert, near Barstow, CA
	Granitic mylonite gneiss (ultramylonite) with white plagioclase porhyroclasts behaving ductilely and the matrix quartz and biotite behaving ductilely. Mojave Desert, near Barstow, CA
	Marble mylonite Mojave Desert, near Barstow, CA