Maficrich

Mafic-rich is a geological descriptor used to characterize rocks or magmas with a relatively high content of mafic minerals—ferromagnesian minerals such as olivine, pyroxene, and amphibole. The term contrasts with felsic-dominated compositions, which are richer in light-colored minerals and silica. Mafic-rich rocks are typically darker in color and denser than felsic rocks.

Common mafic-rich rock types include basalt and gabbro, which are basaltic in composition, as well as diabase (intermediate in grain size). At the ultramafic end are peridotite and dunite, which contain very high olivine and pyroxene. Mafic-rich rocks can form as volcanic rocks from low-silica basaltic magmas or as plutonic rocks that crystallize underground from the same magmas.

Origins and settings: Mafic-rich magmas originate mainly from partial melting of the mantle and evolve through crystallization and differentiation. They are characteristic of tectonic settings such as mid-ocean ridges and oceanic hotspot volcanism, as well as island arcs with mantle-derived magmas. The composition yields relatively low silica contents compared with felsic rocks.

Geochemical properties: They commonly display higher MgO and FeO contents and lower silica (SiO2) than felsic rocks. Magnesium number (Mg#) is often high, reflecting the dominance of olivine and pyroxene. Textures range from fine-grained basalts to coarse-grained gabbros, depending on cooling history.