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

Abstract

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 to granodioritic crust produces alkali-calcic to calc-alkalic granitoids that are metaluminous at low pressures and peraluminous at high pressures. It is likely that a combination of these two processes plays some role in the formation of most ferroan granitoids. Most granitoids that are referred to as 'A-type' are ferroan. However, the term 'A-type' has become more confusing than edifying because it has been applied to a broad spectrum of granitoid compositions with varying petrogenesis. For this reason we recommend that the term 'A-type' be discontinued and 'ferroan' used in its stead.