词条 | A-type granite |
释义 |
The A-type granites form in an extensional tectonic setting. This could either be far from any orogeny, or after orogeny is completed.[1] 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.[2] Enriched alkalis include sodium, potassium, rubidium and caesium.[1] Other depleted elements include barium, phosphorus, titanium and europium.[1] Subtypes include A1, anorogenic, derived from ocean island basalt; and A2 post-orogenic, derived by crustal melting or crust and mantle mixing.[1] The source could be dry granulite left over from the loss of wet magma during orogenies.[2] References1. ^1 2 3 4 5 {{cite web|url=http://faculty.uml.edu/Nelson_Eby/Research/A-type%20granites/A-type%20granites.htm|title=A-type Granitoids|author=Nelson Eby}} 2. ^1 {{cite journal|last1=Whalen|first1=Joseph B.|last2=Currie|first2=Kenneth L.|last3=Chappell|first3=Bruce W.|title=A-type granites: geochemical characteristics, discrimination and petrogenesis|journal=Contributions to Mineralogy and Petrology|date=April 1987|volume=95|issue=4|pages=407–419|doi=10.1007/BF00402202}} 1 : Granite |
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