A-type granite
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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]
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
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]