Granite
Granite is a plutonic igneous rock consisting essentially of quartz, alkali feldspar, and sodic plagioclase. Mafic minerals, in general biotite or hornblende, normally constitute less than 20% of the rock. The name derive from the Latin word granum, grain, a reference to its granular appearance. Granites are some of the most abundant igneous rocks in the continental crust, to the point that the average composition of the upper continental crust is generally considered that of a granite. Granites occur in a wide range of geodynamic settings from magmatic arcs, orogenic belts, cratonic settings, and rift zones.
Classification
The term ‘granite’ in the geological literature has largely been used broadly to refer to granitoids, all rocks from alkali feldspar granite to tonalite containing more than 20% quartz among felsic minerals. Strictly speaking, however, ‘true’ granites are only the plutonic rocks with quartz = 20 – 60% of the felsic minerals and plagioclase representing 10 to 65% of the feldspars, according to the QAPF classification. This wide field is further divided in two main varieties, based on the plagioclase/feldspar ratio: syenogranite (plagioclase ratio < 35%) and monzogranite (plagioclase ratio > 35%).
Peraluminous vs peralkaline granites
In granites, Al and alkali elements (K, Na, Ca) are normally combined together with silica to produce feldspars. This is the case of most granites, which are known as metaluminous and are characterized by K2O + Na2O + CaO > Al2O3 > K2O + Na2O. Some granites, however, contain excess alkali elements with respect to Al (K2O + Na2O + CaO > Al2O3) and are said to be peralkaline. On the opposite end of the spectrum, peraluminous granites contain excess Al with respect to alkali elements (K2O + Na2O + CaO < Al2O3). This difference has a profound effect on granite mineralogy:
• peralkaline granites contain minerals that accommodate K, Na, and Ca in their structure consuming less Al than feldspars, like sodium amphibole (riebeckite) or pyroxene (aegirine);
• in peraluminous granites the ‘excess’ Al produces Al-rich minerals such as muscovite, garnet, cordierite, and Al2SiO5 (e.g. andalusite). A granite with both muscovite and biotite is also known as two-mica granite or binary granite;
• in metaluminous granites Al and alkali elements are balanced to produce feldspars. Excess Ca results in the formation of hornblende.
Granite
Plutonic igneous rock
Felsic minerals:
• quartz
• alkali feldspar
• sodic plagioclase
Mafic minerals:
• biotite
• hornblende
QAPF classification:
Q = 20 – 60%
Plagioclase/feldspars = 10 – 35% (syenogranite); 35 – 65% (monzogranite)
Colored varieties:
• leucogranite (M < 5%)
• melagranite (M > 20%)
Other varieties:
peralkaline granite, peraluminous granite, two-mica granite
Extrusive equivalent: rhyolite
Genetic classification
Granites are also divided based on their inferred genesis in I-type, S-type, M-type, A-type, and H-type granites where the letters stand for: I = igneous, S = sedimentary, M = mantle, A = anorogenic, and H = hybrid. This classification system is based on a series of observations that include granite mineralogy, its geologic setting and geochemistry. I-type granites are thought to derive from the melting of a lower crustal igneous source and are typically metaluminous, containing biotite and/or hornblende. S-type granites are peraluminous granites, which are considered the product of melting of a sedimentary source (i.e. metapelites), since sedimentary rocks are commonly enriched in Al. M-type or mantle-derived granites are thought to be the product of fractionation of basaltic magma derived from the melting of the mantle. As such, M-type granites are very limited in volume and occur in association with mafic rocks, since their formation requires the crystallization of large volumes of basaltic magma. A-type (anorogenic) granites are found in cratonic or intra-continental settings (e.g. rifts) and are generally peralkaline. Finally, H-type granites are considered to be the result of mixing of two different magma sources.
Leucogranites
Leucogranites or light-colored granites, characterized by very low modal content or absence of mafic minerals (M < 5%), are commonly found as dykes around or within larger granitic intrusions. They are usually representative of the crystallization of a very evolved magma. An aplite is a fine-grained variety of leucogranite containing only quartz and feldspar with grain size < 1 mm.
References
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