Granitic augen gneiss NW Scotland. Image: Darrell Henry

Transcription

1 Classification of Metamorphic Rocks (Chapter 22) Granitic augen gneiss NW Scotland. Image: Darrell Henry

2 IUGS-SCMR SCMR general principles Terminology of classification (root names) based on: Structural features (as seen in hand sample or outcrop) Schist Gniess Granofels Likely yprotolith 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

3 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. Grain size variation 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 may be present in a rock. The surface to which the foliations are parallel are called s-surfaces - s 0, s 1, s 2...

4 Lineation: any repetitively-occurring or penetrative visible linear feature in a rock kbody. It may be defined dby: Types of lineations Image: Winter (2001) 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 - l 0, l 1, l 2,..

5 IUGS-SCMR SCMR Flowchart for naming metamorphic rocks The scheme must provide a consistent set of names to cover the spectrum of rock types and their characteristics cteri tic without any terminology gaps.

6 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 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

7 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)

8 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 o er the eq i alent In general a specific name is preferred over the equivalent systematic/structural root name (such as the use of marble over calcite granofels).

9 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 those implied by 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 biotite- quartz-plagioclase gneiss.) Mineral abbreviations are encouraged for long prefixes

10 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. Image: Winter (2001) 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 i resulting from the parallel l arrangement of phyllosilicates.

11 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 iibl and silvery, garnets occur as large dark porphyroblasts. Image: Barb Dutrow (bottom left) pl-qtz-grt-st-ms schist. Large porphyroblasts of staurolite (elongate) and garnet (round).

12 Image: Darrell Henry Gneissose structure: Either a poorlydeveloped schistosity or, if developed, occurs in broadly spaced zones, such that rock will split on a scale of more than one cm. It is commonly segregated g into layers by 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 drkmin dark-mineral-rich r ri concentrations. ntr ti n Image: Darrell Henry Bt-Qtz-fsp gneisses. Layering due to metamorphic differentiation. Quad Creek, Beartooth Mtns, MT.

13 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 Preserved magmatic layering with a high grade metamorphic overprint. SW Sweden. Image: Darrell Henry Granofels structure: A type of structure resulting from absence of schistosity such hthat tmineral 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 i granofelsic texture (a rock with no preferred orientation).

14 lineated Garnet amphibolite from Alberta lineated titanite-bearing amphibolite Specific rock names Amphibolite: is a gneissose or granofelsic metamorphic rock mainly consisting i of green, brown or black 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 include quartz, clinopyroxene, garnet, epidote-group minerals, biotite, titanite and scapolite. The protolith is either a mafic igneous rock or graywacke

15 Specific rock names Granulite: a high grade rock in which Fe-Mg silicates are dominantly OHfree. The presence of feldspar and absence of muscovite are critical, cordierite may also be present. Conversion of felsic gneiss (left) to charnockite (right -type of the granulite - opx granitoid). Kabbaldurga quarry, India Image: Darrell Henry 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 ypy pyroxene) may be called felsic granulites. Cordierite-biotite- plagioclase gneiss. Madagascar

16 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 (leucosome). Migmatite with felsic leucosome and garnet-biotite-sillimanite neosome. Hellroaring Plateau, Beartooth Mtns, MT. Image: Darrell Henry Migmatites may appear layered, or the leucosomes may occur as pods or form a network of crosscutting veins

17 El Eclogite, WN W. Norway. Image: Darrell llhenry Specific rock names Eclogite: Plagioclase-free metamorphic rock composed of >75% of omphacite and garnet, both of which are present as major constituents. Generally, green and red metamorphic rock (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 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.

18 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% Fe 2 O 3 ). Folded banded iron formation (BIF) with quartz, hematite and crocidolite (Tiger's eye). Images: Darrell Henry

19 Specific names: Metacarbonate and related rocks 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.

20 Specific names: Metacarbonate and related rocks Carbonate-silicate rock: Metamorphic rock mainly composed of silicate minerals and containing 5-50% carbonate minerals. Calc-silicate rock: Metamorphic rock primarily composed of 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

21 Specific names: Metacarbonate and related rocks 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) image: Darrell Henry

22 Specific names Greenschist/Greenstone: a low- grade 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. / rology/fieldtrips/fieldtrips2004.htm 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.

23 Specific names: high strain rocks Schematic cross section through a shear zone

24 Specific names: high strain rocks

25 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 ductilely and the matrix quartz and biotite behaving ductilely. Mojave Desert, near Barstow, CA. image: Darrell Henry

26 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 image: Darrell Henry Some porphyroblasts, particularly in lowgrade 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 Nodular texture of cordierite porphyroblasts developed during a thermal overprinting of previous regional metamorphism. image: Winter (2001)

27 Additional modifying terms Some gneisses have large eye-shaped grains (commonly feldspar) that are derived from pre-existing 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. image: Winter (2001) Granitic augen gneiss NW Scotland. Image: Darrell Henry