Metamorphic RocksBy: Waqas JavaidTable of Contents:Types of Metamorphic RocksMetamorphic Textures Foliated Metamorphic RocksNon-Foliated Metamorphic Rocks

Type of Metamorphic Rocks:On the basis of texture, metamorphic rocks are classified into two main categories:Foliated Metamorphic RocksNon- Foliated Metamorphic RocksMetamorphic TexturesFoliation:when platy, lamellar or flaky minerals (e.g.. sheet silicate minerals the micas: biotite and muscovite, chlorite, talc, and serpentine), occurring in rock orient themselves parallel to one another (i.e. perpendicular to the direction of maximum pressure or stress).

Platy MineralsNeedle-like MineralsFoliation:Non-FoliatedFoliated

Slaty Cleavage: usually formed during the early stage of Low-grade Metamorphism due to lithostatic stress.

New sheet-structure minerals tends to be parallel to the bedding planes during metamorphism. however, further deep burial along the continental margin; compressional forces will cause deformation (folding).

hence, the sheet minerals as well as foliation will no longer be parallel to the bedding planes, such type of foliation in fine grained rocks is called slaty cleavage.

Schistosity:usually formed during intermediate and high grade metamorphism Grain size increases and can be seen by naked eye; grains tends to enlarge with increasing grade of metamorphism; the coarse grained sheet-structure minerals show preferred orientationgrain size is the main difference between the slaty structure and schistose structure.

Gneissosity:usually associated with high-grade regional metamorphism (where differential stress prevails I.e. tectonic forces)distinct bands of light and dark color in the rocks- known as gneissic banding. Dark color layer is due to ferromagnesian minerals like biotite, amphibole.Light color layer is due to silicic minerals like quartz and feldspar.Light color layer is thicker as compared to dark color layer.

Granoblastic texture:Characteristics:Grains visible with naked eye.EquigranularEquidimensionalNon FoliatedFormed due to thermal metamorphism.Typically found in marble, quartz and non foliated metamorphic rocks

Porphyroblastic:

Large mineral crystal in metamorphic rocksEmbedded in fine ground massPorphyroblasts are commonly euhedral crystalsMost common porphyroblasts in pelitic metamorphic rocks are:Garnets and Staurollites

Foliated Metamorphic RocksGeneral Properties:

Slate:Physical Properties:Mostly greyish colorFine grain rockLow grade metamorphismTendency to break into thin platesSlaty cleavage is presentFormed by regional (orogenic) metamorphism

Chemical PropertiesRich in colorless micaProtolith is fine grained sedimentary rocksProtolith can also be pyroclastic tuff or basic igneous rocksIncase of basic igneous protolith, large crystalls of mica, andalusite, cardierite are present as porphyroblast.Slate:

Phyllite:Properties:Fine grained rockCrystals coarser than slateFormed from slate when P/T risesBreaks into sheets (thinner than slate)Lustrous, silk sheen on fracture surfacesLow grade regional metamorphism

Schist:Physical PropertiesMedium to coarse grained Well crystallized strongly schistoseSchistosity is the diagnostic fabricBreaks into lenticlesMedium grade regional metamorphism

Chemical Properties:Protolith is fine grained sedimantery rocks or basic igneous rocksAbundance of mica associated with quartz and garnetOn basis of different minerals present it is divided into:Green SchistBlue SchistSchist:

Schist:Green Schist:Low grade regional metamorphismGenreraly green colorDue to presence of green color mineralsLike chlorite, epidote, actinolite, albite, quartz etc.These minerals indicate that protolith was fine grained basic igneous rocks i.e. basaltCan also form from pelitic sedimantry rocks i.e silicious dolostone, marl etcBlue Schist:Form under high pressure and low temperatureBlue in colorMainly because presence of blue color minerals i.e. glaucophene. lawsonite quartz, crossite, zoisite are also foundProtolith is basic igneous rocks, mainly basaltForm in orogenic and subduction envronment

Schist:Green Schist:

Blue Schist:

Gneiss:Physical Properties:Coarse grained rockHigh grade regional metamorphismShows gneissosityLight and dark bandinglight minerals are always equidimensional, they form granoblastic fabric.Dark minerals are mostly flaky, they show preferred orientation

Chemical Properties:Divided into two types on basis of composition of protolithOrthogneiss: derived from igneous rocks. Mainly granite with Qtz, feldspar, mica, biotite.Paragneiss: derived form clastic sedimentary rocks i.e. greywackes, sandstone etc.Gneiss:

Non-Foliated Metamorphic RocksMarble:Properties:Any metamoprhic rock rich in carbonates.Protolith: limestone, dolostone etcFormed due to confining pressure.No layering or foliationInterlocking grains of Co3Texture is mosaic: a type of granoblastic texture.

Quartzite:Properties:Any metamorphic rock with greater than 80% quartzHave mosaic granoblastic textureCan not split easily: very hardEquidimensional interlocking grains of quartz and feldspar.

Hornfels:Physical properties:Metamorhpic rocks with no prefered orientation.Mostly asociated with contact metamorphismEquigranular fabric equidimensional granoblastic texture.

Chemical properties:In the basis of composition hornfels are of these types:Biotite hornfelsQuartz-Albite felsPlagioclase felsCalc-silicate fels

Hornfels:

Phyllonite:Properties:Macroscopically resembles phylliteFine grainedFormed due to mechanical degradation: severe deformation (mylonization)Reconstruction of minerals (different orientation)Posses mica and crushed mineralsProduct of cataclastic metamorphism of graywacke, granite or gneiss.

Eclogite:Physical properties:Form at very high pressure up to 30kbarUnusually dense rock comparatively low temperature.Found at subduction zonesDepth greater than 35kmChemical properties:Protolith is basaltAbsence of plagioclaseTwo most essential minerals are:::Omphacite green variety of clinopyroxenesGarnet almandine Eclogite:

Buchite:Properties:Vitreous: composed mostly of glassHigh temp, low pressure rockDue to pyrometamorphismHigh temp minerals: Tridymite, cardierite, sanidine, serpentine.Protolith is quartz felspathatic or pelitic rocks.

The End.

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