A-type granite

A-type granite 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][2] The 'A' stands for Anorogenic or Anhydrous, as these granites are characterized by low water content and a lack of orogenic or transitional tectonic fabric.^[3] Other SIAM categories are S, I, and M types.^[1]

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.^[4]    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.^[4]  The later M-type granitoids were based on their mantle-sourced protoliths and of having particular chemical characteristics.^[1]

Occurrence

The A-type granites dominantly form within continental intraplate rifting or uplifting or at regional post-orogeny uplift or collapse.^[5] Their formation could be either anorogenic, meaning far from any orogeny, or after orogeny is completed.^[5]

Geochemistry

Chemical characteristics of A-type granites include high silica, alkalis, zirconium, niobium, gallium, yttrium and cerium.^[2][6] The ratio of gallium to aluminium is high, as is the ratio of iron to magnesium.^[2] There are lower levels of calcium and strontium.^[6] By using Ga/Al ratio, fractionated felsic I or S-type granites can overlap in apparent composition.^[2] Enriched alkalis include sodium, potassium, rubidium and caesium.^[2][6] Other depleted elements include barium, phosphorus, titanium and europium.^[2]

Subtypes

Subtypes include A₁, anorogenic, derived from ocean island basalt; and A₂ post-orogenic, derived by crustal melting or crust and mantle mixing.^[2]

Sources

The source could be dry granulite left over from the loss of wet magma during orogenies.^[7]