A-type granite explained

A-type granite 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] [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.

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.  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.[5] Their formation could be either anorogenic, meaning far from any orogeny, or after orogeny is completed.

Geochemistry

Chemical characteristics of A-type granites include high silica, alkalis, zirconium, niobium, gallium, yttrium and cerium.[6] 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; and A2 post-orogenic, derived by crustal melting or crust and mantle mixing.

Sources

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

Notes and References

  1. 1.     Winter, John D. (2014). Principles of igneous and metamorphic petrology (Second; Pearson new international ed.), p. 402. .
  2. Eby . G. Nelson . 1990-12-01 . The A-type granitoids: A review of their occurrence and chemical characteristics and speculations on their petrogenesis . Lithos . Alkaline Igneous Rocks and Carbonatites . en . 26 . 1 . 115–134 . 10.1016/0024-4937(90)90043-Z . 0024-4937.
  3. 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.
  4. Chappell. B. W.. White. A. J. R.. August 2001. Two contrasting granite types: 25 years later. Australian Journal of Earth Sciences. 48. 4. 489–499. 2001AuJES..48..489C. 10.1046/j.1440-0952.2001.00882.x. 0812-0099.
  5. Barbarin . Bernard . July–December 1990 . Granitoids: Main petrogenetic classifications in relation to origin and tectonic setting . Geological Journal . en . 25 . 3-4 . 227–238 . 10.1002/gj.3350250306. free .
  6. Clarke, D. B. (1992) Granitoid Rocks, Chapman  & Hall, London.
  7. Whalen. Joseph B.. Currie. Kenneth L.. Chappell. Bruce W.. April 1987. A-type granites: geochemical characteristics, discrimination and petrogenesis. Contributions to Mineralogy and Petrology. 95. 4. 407–419. 10.1007/BF00402202.