On Ferroan (A-type) Granitoids: their Compositional Variability and Modes of Origin

We recognize eight types of ferroan granitoid that can be distinguished on the basis of major element chemistry. These include alkalic granitoids that may be metaluminous or peralkaline, alkali-calcic granitoids that may be metaluminous, peraluminous or peralkaline, calc-alkalic granitoids that may be metaluminous or peraluminous, and rare calcic ferroan granitoids. These granitoids may form by two distinct end-member processes. Extreme differentiation of basaltic melts results in ferroan granitoids that are either peralkaline alkalic and alkali-calcic, or metaluminous alkalic, alkali-calcic, and calc-alkalic, with alkalinity increasing with increasing pressure of differentiation. Partial melting of tonalitic…