monzodiorite

Monzodiorite is an intrusive igneous rock of intermediate composition in the diorite family. It is chemically and mineralogically intermediate between diorite and granodiorite, characterized by a feldspar-rich assemblage and relatively low quartz content. The dominant minerals are plagioclase feldspar and alkali feldspar, with ferromagnesian minerals such as hornblende or pyroxene commonly present; biotite may occur. Quartz, if present, is usually minor.

The texture of monzodiorite is typically phaneritic, with crystals visible to the naked eye; some specimens are porphyritic, containing larger feldspar phenocrysts. Monzodiorite forms when magmas crystallize at crustal depths in magma chambers, often in continental arc settings. It is associated with subduction-related magmatism and calc-alkaline rock series, and may derive by fractional crystallization from basaltic to andesitic magmas.

In igneous rock classification, monzodiorite is placed in the diorite family and often lies near the boundary with granodiorite on QAPF diagrams, reflecting its admixture of alkali feldspar with plagioclase. The term is used mainly by field geologists and petrologists to describe rocks transitional between diorite and granodiorite rather than to denote a fixed, exact composition. See also diorite, granodiorite, and quartz diorite.