A-type granite

A-type granites are a class of granite. They are characterised by low water and a lack of tectonic fabric.[1] A stands for anorogenic or anhydrous. The granite class was proposed by Loiselle and Wones in 1979. It is one category of the "alphabet" classification system of granites[2] with other categories including I-, S- and M-type granites.[1]

Occurrence

The A-type granites are considered to dominantly form along tectonic rifts or within stable continent interiors. Their formation could either be far from any orogeny, or after orogeny is completed.[1]

Geochemistry

Chemical characteristics of A-type granites include high silica, alkalis, zirconium, niobium, gallium, yttrium and cerium. 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.[3] Enriched alkalis include sodium, potassium, rubidium and caesium.[1] Other depleted elements include barium, phosphorus, titanium and europium.[1]

Subtypes

Subtypes include A1, anorogenic, derived from ocean island basalt; and A2 post-orogenic, derived by crustal melting or crust and mantle mixing.[1]

Source

The source could be dry granulite left over from the loss of wet magma during orogenies.[3]

References
  1. Nelson Eby. "A-type Granitoids".
  2. 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. Bibcode:2001AuJES..48..489C. doi:10.1046/j.1440-0952.2001.00882.x. ISSN 0812-0099.
  3. 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. doi:10.1007/BF00402202.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.