A-type
granite
Granite () is a coarse-grained (phaneritic) 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 cools and solidifies undergro ...
is a particular category of the S-I-A-M or 'alphabet' system which classifies granitoids and granitic rock by their photoliths or source.
[1. Winter, John D. (2014). ''Principles of igneous and metamorphic petrology'' (Second ; Pearson new international ed.), p. 402. .][1. Eby, G. (1990). The A-type granitoids: A review of their occurrence and chemical characteristics and speculations on their petrogenesis. ''Lithos'', ''26''(1), 115–134. https://doi.org/10.1016/0024-4937(90)90043-Z] 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.
[1. 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.
[1. Barbarin, B. (1990). Granitoids: Main petrogenetic classifications in relation to origin and tectonic setting. ''Geological Journal (Chichester, England)'', ''25''(3‐4), 227–238. https://doi.org/10.1002/gj.3350250306] Their formation could be either anorogenic, meaning far from any
orogeny
Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted t ...
, or after orogeny is completed.
Geochemistry
Chemical characteristics of A-type granites include high silica, alkalis, zirconium, niobium, gallium, yttrium and cerium.
[1. 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 A
1, 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 b ...
; and A
2 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 w ...
left over from the loss of wet magma during orogenies.
References
{{Reflist
Granite