A-type

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...

is a particular category of the S-I-A-M or 'alphabet' system which classifies granitoids and granitic rock by their protoliths or source.1. Winter, John D. (2014). ''Principles of igneous and metamorphic petrology'' (Second; Pearson new international ed.), p. 402. . The 'A' stands for Anorogenic or

Anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...

, as these granites are characterized by low water content and a lack of orogenic or transitional tectonic fabric.Clemens, J. D., Holloway, John R., White, A. J.R. (1986), Origin of an A-type granite: Experimental Constraints, ''American Mineralogist'', Volume 71, pages 317-324. Other SIAM categories are S, I, and M types.

Alphabet Classification System

In mid 1970's Chappell and White established 2 fundamentally distinctive types of granite: rocks with attributes that could be derived from metasedimentary rock, “S-type” granites and those whose attributes derived from metaigneous rock, “I-type” granites. The addition of the A-type granitoids was proposed by Loiselle and Wones in 1979 however this type was based on tectonic regime and geochemical characteristics. The later M-type granitoids were based on their mantle-sourced protoliths and of having particular chemical characteristics.

Occurrence

The A-type granites dominantly form within continental intraplate rifting or uplifting or at regional post-orogeny uplift or collapse. Their formation could be either anorogenic, meaning far from any

orogeny Orogeny () is a mountain-mountain formation, building process that takes place at a convergent boundary, convergent plate margin when plate motion compresses the margin. An or develops as the compressed plate crumples and is tectonic uplift, u ...

, or after orogeny is completed.

Geochemistry

Chemical characteristics of A-type granites include high silica, alkalis, zirconium, niobium, gallium, yttrium and cerium.Clarke, D. B. (1992) Granitoid Rocks, Chapman & Hall, London. The ratio of gallium to aluminium is high, as is the ratio of iron to magnesium. There are lower levels of calcium and strontium. By using Ga/Al ratio, fractionated felsic I or S-type granites can overlap in apparent composition. Enriched alkalis include sodium, potassium, rubidium and caesium. Other depleted elements include barium, phosphorus, titanium and europium.

Subtypes

Subtypes include A₁, anorogenic, derived from

ocean island basalt Ocean island basalt (OIB) is a volcanic rock, usually basaltic in composition, erupted in oceans away from tectonic plate boundaries. Although ocean island basaltic magma is mainly erupted as basalt lava, the basaltic magma is sometimes modified ...

; and A₂ post-orogenic, derived by crustal melting or crust and mantle mixing.

Sources

The source could be dry

granulite Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated ...

left over from the loss of wet magma during orogenies.

References

{{Reflist Granite