Metaluminous

Metaluminous is a term used in igneous petrology to describe rocks whose aluminum saturation is balanced, such that aluminum is not in excess relative to calcium, sodium, and potassium. The concept is most often quantified with the Aluminum Saturation Index (ASI) or A/CNK, defined as A/CNK = Al2O3 / (CaO + Na2O + K2O). In this scheme, metaluminous rocks have A/CNK less than or equal to 1.0, peraluminous rocks have A/CNK greater than 1.0, and rocks near A/CNK ≈ 1.0 lie at the metaluminous boundary. A related distinction for alkaline rocks uses the condition Na2O + K2O > Al2O3 to indicate peralkaline compositions.

Mineralogically, metaluminous rocks typically contain minerals such as plagioclase, alkali feldspar, quartz, and aluminous ferromagnesian minerals like biotite or hornblende, but they generally lack excess aluminous minerals such as muscovite. In contrast, peraluminous rocks often show minerals like muscovite, garnet, or cordierite that reflect excess aluminum.

Geochemically, the metaluminous/peraluminous classification helps interpret magma genesis and tectonic setting. Metaluminous signatures are common in calc-alkaline series associated with convergent-margin crustal melting and mantle involvement, whereas peraluminous signatures can indicate significant crustal melting, metasedimentary contributions, or strong aluminous enrichment.

Limitations include alteration, fractional crystallization, and regional variation in oxide activities, all of which can modify the apparent A/CNK value. Nevertheless, the metaluminous classification remains a widely used tool for exploring the petrogenesis and tectonic context of igneous rocks.