Granite may have more alkali feldspar, whereas a granite derived from partial melting of metaigneous rocks may be richer in plagioclase. It is on this basis that the modern "alphabet" classification schemes are based. The letter-based Chappell & White classification system was proposed initially to divide granites into I-type (igneous source) granite and S-type (sedimentary sources). Both types are produced by partial melting of crustal rocks, either metaigneous rocks or metasedimentary rocks. M-type granite was later proposed to cover those granites that were clearly sourced from crystallized mafic magmas, generally sourced from the mantle. However, this proposal has been rejected by
A-type granite 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 include A, anorogenic, derived from ocean island basalt; and A post-orogenic, derived by crustal melting or crust and mantle mixing. The source could be dry granulite left over from the loss of wet magma during orogenies. A-type granite A-type granites are a class of granite. They are characterised by low water and a lack of tectonic fabric. A