felsischintermediair

Felsischintermediair is a term used in igneous petrology to describe rocks whose chemical composition and mineral assemblage lie between felsic and intermediate categories. The expression is more common in some languages (such as Dutch and German) and in regional petrographic or geochemical discussions than as a formal IUGS category. In practice, felsischintermediair rocks are those that plot between granite/granodiorite on the felsic side and diorite/tonalite on the intermediate side.

Mineralogy and composition are the main criteria. These rocks typically contain quartz and plagioclase together with alkali feldspar, but in proportions that are intermediate between granitic and dioritic rocks. Ferromagnesian minerals such as biotite, amphibole or pyroxene occur in moderate amounts. Silica contents are generally high, roughly in the mid to upper 50s to low 60s weight percent, with chemical indices that place them between felsic and intermediate suites. They often show calc-alkaline affinities and may form through processes such as fractional crystallization of a mafic magma or partial melting of crustal rocks.

Common rock types that are described as felsischintermediair include granodiorite, tonalite and quartz-diorite, as well as related transitional granitoid rocks. These lithologies are typical of continental crust magmatism and are often associated with subduction-related tectonic settings, where magmas differentiate to yield intermediate-to-felsic compositions.

In summary, felsischintermediair denotes transitional igneous rocks whose mineralogy and chemistry bridge felsic and intermediate rock groups, reflecting particular magmatic histories and tectonic environments. See also felsic, intermediate, granodiorite, tonalite, quartz-diorite.