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.[1]
Alphabet Classification System edit
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 edit
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 edit
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 edit
Subtypes include A1, anorogenic, derived from ocean island basalt; and A2 post-orogenic, derived by crustal melting or crust and mantle mixing.[2]
Sources edit
The source could be dry granulite left over from the loss of wet magma during orogenies.[7]
References edit
- ^ a b c 1. Winter, John D. (2014). Principles of igneous and metamorphic petrology (Second ; Pearson new international ed.), p. 402. ISBN 9781292021539.
- ^ a b c d e f g 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. 26 (1): 115–134. doi:10.1016/0024-4937(90)90043-Z. ISSN 0024-4937.
- ^ 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.
- ^ a b 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.
- ^ a b Barbarin, Bernard (July–December 1990). "Granitoids: Main petrogenetic classifications in relation to origin and tectonic setting". Geological Journal. 25 (3–4): 227–238. doi:10.1002/gj.3350250306.
- ^ a b c Clarke, D. B. (1992) Granitoid Rocks, Chapman & Hall, London.
- ^ 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.