Peraluminous

Peraluminous is a term used in geology to describe a class of aluminous igneous rocks in which there is an excess of aluminum relative to alkalis and lime. This relation is quantified by the aluminum saturation index (ASI), defined as ASI = Al2O3 − (Na2O + K2O + CaO) in molar proportions. When ASI is greater than zero, the rock is described as peraluminous; when ASI is less than or equal to zero, the rock is considered metaluminous or, in cases where alkalis exceed alumina, peralkaline.

Peraluminous rocks commonly form through crustal melting processes, such as the partial melting of aluminum-rich source rocks. They are often associated with S-type granites, which typically contain aluminous minerals such as muscovite and may host other alumina-bearing phases like garnet, cordierite, or sillimanite. The mineral assemblage reflects the excess aluminum that must be accommodated by Al-bearing minerals beyond those formed into feldspars and other alkali-bearing phases.

Distinctions among the related categories are based on the same elemental balance. Metaluminous rocks have ASI near or below zero, indicating enough calcium, sodium, and/or potassium to bind alumina without forming surplus aluminum minerals. Peralkaline rocks have excess alkalis relative to alumina, with Na2O + K2O exceeding Al2O3, often producing highly alkali-rich compositions and characteristic accessory minerals.

Peraluminous derivates occur in various tectonic settings, especially in regions of crustal thickening or collision, and are important for understanding the crustal contribution to magma genesis and differentiation.