An intermediate igneous rock from the middle of Bowen's Reaction Series. Typically intermediate colored with a subequal mixture of light colored Sodium Plagioclase and dark colored Amphibole (although Biotite may also be present). Appearance is often described as "salt and pepper" because of the mix. The mafic/plagioclase mix can vary widely, however. This specimen is low on the mafic side, but the next specimenruns high. 

 

What keeps this from then being a monzonite is the high mafic content; monzonites have less than 5% mafics. orthoclase is universally absent.

Na/ca plagioclase & MAFICS (amphibole) about 50/50 quartz - absent to trace diorite is the result of fractional melting of a mafic parent rock above a subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain building (subduction along the edge of a continent, such as with the Andes Mountains). It emplaces in large batholiths (many thousands of square miles) and sends magma to the surface to produce composite volcanoes with andesite lavas. 

 

 

 

In this detail we see the mixed Amphibole and translucent, white Sodium Plagioclase. Some quartz is present (clear, glassy, grayish - as near the top center of the photo) but it is only about 5%. Note that the plagioclase and amphibole are near 50/50, with plagioclase being more abundant. It is this near mix that puts the rock in the diorite range. 

 

orthoclase is absent.