Granite

Granite is a medium-tocoarse-grained acid igneous rock with essential quartz (>20%) and feldspar, where alkali feldspar constitutes between 100 and 35% of the feldspars, and minor mafic minerals. Hornblende and biotite are the commonest mafic minerals, however, muscovite is also frequently encountered. Pyroxene, andalusite, corundum, tourmaline, garnet and topaz are also relatively common mafic minerals. Rhyolite is the fine-grained equivalent of granite.

Granites are subdivided on the basis of the relative proportion of alkali feldspar to plagioclase feldspar into monzogranites (65-35% alkali feldspar), syenogranites (90-65% alkali feldspar), and alkali granites (>90% alkali feldspar). Around 70% of granites are monzogranites. Aplites are equigranular granitic rocks with little mafic component occurring in veins or dykes. Granophyres are granitic rocks in which quartz and feldspar has a granophyric intergrowth.

Granites can be subdivided on the basis of their chemistry into peralkaline, metaluminous, and peraluminous on the basis of the ratio Al2O3/(Na2O + K2O + CaO). The related terminology is:

• peraluminous: Al2O3 > (Na2O + K2O + CaO) or Al2O3/(Na2O + K2O + CaO) > 1.1; Rock will probably have muscovite and may have garnet; will be corundum-normative.
• metaluminous: Al2O3 < (Na2O + K2O + CaO) but Al2O3 > (Na2O + K2O) or Al2O3/(Na2O + K2O + CaO) > 1.0; rock may have hornblende
• subaluminous: Al2O3 < (Na2O + K2O + CaO) but Al2O3 = (Na2O + K2O); Al2O3/(Na2O + K2O + CaO) < 1.0; rock will probably have hornblende; will have Di in the norm.
• peralkaline: Al2O3 < (Na2O + K2O) or Al2O3/(Na2O + K2O + CaO) << 1.0; rock will probably have lot of K-feldspar in the norm, probably a feldspathoid or very little, if any quartz.

But Granites can be subdivided also on the basis of the infered genesis into I-type, S-type, M-type and A-type. Chappell & White classification system was proposed initially to divide granites into I-type granite (or igneous protolith) granite and S-type or sedimentary protolith granite. Both of these types of granite are formed by melting of high grade metamorphic rocks, either other granite or intrusive mafic rocks, or buried sediment, respectively.

S-type Granites: S-type granites are thought to originate by melting (or perhaps by ultrametamorphism) of a pre-exiting metasedimentary or sedimentary source rock. These are peraluminous granites [i.e. they have molecular Al2O3 > (Na2O + K2O)]. Mineralogically this chemical condition is expressed by the presence of a peraluminous mineral, commonly muscovite, although other minerals such as the Al2SiO5 minerals and corundum may also occur. Since many sedimentary rocks are enriched in Al2O3 as a result of their constituents having been exposed to chemical weathering near the Earth's surface (particularly rocks such as shales that contain clay minerals), melting of these rocks is a simple way of achieving the peraluminous condition.
Many S-type granitoids are found in the deeply eroded cores of fold-thrust mountain belts formed as a result of continent-continent collisions, such as the Himalayas and the Appalachians, and would thus be considered orogenic granites.

I-type Granites: I-type granites are granites considered to have formed by melting of an original igneous type source. These are generally metaluminous granites, expressed mineralogically by the absence of peraluminous minerals and the absence of peralkaline minerals, as discussed below. Instead these rocks contain biotite and hornblende as the major mafic minerals.

Mesozoic or younger examples of I-type granites are found along continental margins such as the Sierra Nevada batholith of California and Nevada, and the Idaho batholith of Montana. In these regions the plutonism may have been related to active subduction beneath the western U.S. during the Meszoic. I-type granites are also found in the Himalayas, which are related to continent-continent collisions.

M-type or mantle derived granite was proposed later, to cover those granites which were clearly sourced from crystallized mafic magmas, generally sourced from the mantle. These are rare, because it is difficult to turn basalt into granite via fractional crystallisation.

A-type Granites: A-type granites are generally peralkaline in composition [molecular (Na2O + K2O) > Al2O3]. Minerals like the sodic amphiboles - riebeckite and arfvedsonite, and the sodic pyroxene - aegerine, are commonly found in these rocks. In addition, they tend to be relatively Fe-rich and thus fayalitic olivine sometimes occurs.

They are considered anorogenic granites because they are generally found in areas that have not undergone mountain building events. Instead, they appear to be related to continental rifting events wherein continental lithosphere is thinned as a result of upwelling asthenosphere. The upwelling raises the geothermal gradient resulting in melting. Young peralkaline granites are found in the Basin & Range Province of the western U.S., and older examples are found throughout southeastern Australia.

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QAPF Diagram Granite field in blue





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"Normal" Granite with alkali feldspar (pink), plagioclase (white), quartz (colorless) and biotite (brown). From (Sandatlas). Lødingen, Hinnøya, Norway.



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Peraluminous granite with red garnet phenocrysts. From (Sandatlas). Estonia.



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Peraluminous granite with light-colored muscovite. From (Sandatlas). Estonia.



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Peralkaline granite with fayalite (iron-rich end-member of olivine group). From (Sandatlas). Norway.



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Rapakivi granite with plagioclase rims around ovoidal phenocrysts of alkali feldspar. From (Sandatlas). Luumäki, Finland.



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Peralkaline granite with aegirine (grass-green) and arfvedsonite (blue-balck).



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Peralkaline granite with riebeckite (blue-balck). From (Virtual microscope). sle of Ailsa Craig lies in the Firth of Clyde, Scotland.


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Xenolith of diorite in granite from near Wickenburg, Arizona




Bibliography



• Eric A.K.Middlemost (1985): Magmas and Magmatic Rocks. Longman, London
• Ron H. Vernon (2004): A pratical guide to rock microstructure. Cambridge editore
• K.G.Cox, J.D.Bell & R.J Pankhurst (1979): The interpretetion of igneous rocks. George Allen&Unwin editori.
• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori


Photo
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Plagioclase, Biotite and Quartz in a Granite. XPL image, 2x (Field of view = 7mm)/td>
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Plagioclase, Biotite and Quartz in a Granite. PPL image, 2x (Field of view = 7mm)/td>
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Plagioclase, Biotite and Quartz in a Granite. XPL image, 2x (Field of view = 7mm)/td>
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Plagioclase, Biotite and Quartz in a Granite. PPL image, 2x (Field of view = 7mm)/td>
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Plagioclase, Biotite and Quartz in a Granite. XPL image, 2x (Field of view = 7mm)/td>
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Plagioclase, Biotite and Quartz in a Granite. XPL image, 2x (Field of view = 7mm)/td>
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Plagioclase, Biotite and Quartz in a Granite. XPL image, 2x (Field of view = 7mm)/td>
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Plagioclase, Biotite and Quartz in a Granite. PPL image, 2x (Field of view = 7mm)/td>
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Plagioclase, Biotite, Microcline and Quartz in a Granite. XPL image, 2x (Field of view = 7mm)/td>
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Plagioclase, Biotite and Quartz in a Granite. PPL image, 2x (Field of view = 7mm)/td>
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Plagioclase, Biotite, Microcline and Quartz in a Granite. XPL image, 2x (Field of view = 7mm)/td>
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Biotite rich-layer in a Granito. Antarctica. PPL image, 2x (Field of view = 7mm)/td>
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Biotite rich-layer in a Granito. Antarctica. PPL image, 2x (Field of view = 7mm)/td>
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Green Hornblende and Biotite in a Granito. Antarctica. PPL image, 2x (Field of view = 7mm)/td>
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Green Hornblende and Biotite in a Granito. Antarctica. PPL image, 2x (Field of view = 7mm)/td>
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Green Hornblende and Biotite in a Granito. Antarctica. XPL image, 2x (Field of view = 7mm)/td>
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Green Hornblende and Biotite in a Granito. Antarctica. PPL image, 2x (Field of view = 7mm)/td>
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Green Hornblende and Biotite in a Granito. Antarctica. PPL image, 2x (Field of view = 7mm)/td>
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Green Hornblende and Biotite in a Granito. Antarctica. PPL image, 2x (Field of view = 7mm)/td>
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Green Hornblende and Biotite in a Granito. Antarctica. XPL image, 2x (Field of view = 7mm)/td>