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1Classification of Metamorphic Classification of Metamorphic RocksRocksReading: Winter Chapter 22:Classification of Metamorphic Classification of Metamorphic RocksRocks• 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 aspectsFoliated Metamorphic RocksFoliated Metamorphic Rocks• Foliation: and planar fabric element• Lineation: any linear fabric elements–They have no genetic connotations–Some high-strain rocks may be foliated, but they are treated separatelyFoliated Metamorphic RocksFoliated Metamorphic RocksCleavage– 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 platy phyllosilicates are too fine grained to see individually with the unaided eye Schistosity– 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 Foliated Metamorphic RocksFoliated Metamorphic RocksGneissosestructure– Either a poorly-developed schistosity or segregated into layers by metamorphic processes– Gneissose rocks are generally coarse grained Foliated Metamorphic RocksFoliated Metamorphic Rocks2Figure 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 Petrolo gy. Prentice Hall.aabbSlate: compact, very fine-grained, 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. Foliated Metamorphic RocksFoliated Metamorphic RocksFigure 22-1c. Garnet muscovite schist. Muscovite crystals are visible and s ilvery, garnets occur as large dark porphyroblasts. Winter (2001) An Introduction to Igneous a nd Metamorphic Petrology. Prentice Hall.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. Foliated Metamorphic RocksFoliated Metamorphic RocksFigure 22-1d. Quartzo-feldspathic gneiss with obvious layering. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.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. Foliated Metamorphic RocksFoliated Metamorphic RocksSimpler than for foliated rocksAgain, this discussion and classification applies only to rocks that are not produced by high-strain metamorphismGranofels: a comprehensive term for any isotropic rock (a rock with no preferred orientation)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. NonNon--Foliated Metamorphic RocksFoliated Metamorphic RocksMarble: 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.Specific Metamorphic Rock Specific Metamorphic Rock TypesTypesGreenschist/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 metamorphic rock dominated by hornblende + plagioclase. Amphibolites may be foliated or non-foliated. The protolith is either a mafic igneous rock or graywacke.Specific Metamorphic Rock Specific Metamorphic Rock TypesTypes3Serpentinite: an ultramafic rock metamorphosed at low grade, so that it contains mostly serpentine.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.Eclogite: a green and red metamorphic rock that contains clinopyroxene and garnet (omphacite + pyrope). The protolith is typically Specific Metamorphic Rock Specific Metamorphic Rock TypesTypesSkarn: a contact metamorphosed and silica metasomatized carbonate rock containing calc-silicate minerals, such as grossular, epidote, tremolite, vesuvianite, etc. Tactiteis a synonym.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.Specific Metamorphic Rock Specific Metamorphic Rock TypesTypesMigmatite: 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. Specific Metamorphic Rock Specific Metamorphic Rock TypesTypesAdditional Modifying Terms:Porphyroblasticmeans that a metamorphic rock has one or more metamorphic minerals that grew much larger than the others. Each individual crystal is a porphyroblastSome porphyroblasts, particularly in low-grade contact metamorphism, occur as


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UB GLY 206 - Classification of Metamorphic Rocks

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