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
[edit | edit source]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 | edit source]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 | edit source]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 | edit source]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 | edit source]The source could be dry granulite left over from the loss of wet magma during orogenies.[7]
References
[edit | edit source]- ^ a b c 1. Winter, John D. (2014). Principles of igneous and metamorphic petrology (Second; Pearson new international ed.), p. 402. Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value)..
- ^ a b c d e f g Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ 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 Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ a b Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ a b c Clarke, D. B. (1992) Granitoid Rocks, Chapman & Hall, London.
- ^ Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).