feldspartomafic

Feldspathomafic is a descriptive term used in petrology to characterize igneous rocks that lie between felsic and mafic in their mineralogy. The name highlights the prominent presence of feldspar minerals (both plagioclase and alkali feldspar) together with a significant amount of ferromagnesian minerals such as pyroxene, amphibole, biotite, or olivine. It is not a formal IUGS category but a way to convey a rock’s mixed character when precise geochemical data are limited or when describing textures and mineral assemblages in the field.

In practice, rocks described as feldspathomafic include members of the diorite–granodiorite spectrum and related compositions that are richer in feldspar than typical mafic rocks but contain notable ferromagnesian minerals. Tonalites and related granitoid rocks may also be described as feldspathomafic when they exhibit a substantial feldspar component alongside mafic minerals, rather than a strictly felsic or strictly mafic modal make-up. The term helps distinguish rocks with significant but not dominant felsic minerals from purely felsic granites or from strictly mafic rocks.

Geochemically, feldspathomafic rocks tend to show intermediate silica content and a mixed alkali-feldspar plus ferromagnesian mineral assemblage. They commonly form in continental crust settings and in subduction-related magmatism, where melting or differentiation yields a blend of felsic and mafic minerals.

See also: felsic, mafic, intermediate igneous rocks; diorite; granodiorite; tonalite.