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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"Normal" Granite with alkali feldspar (pink), plagioclase (white), quartz (glassy-grey) and biotite (brown). From Sandatlas.

Peraluminous granite with red garnet phenocrysts. Estonia. From Sandatlas

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

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

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

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

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