Diorite is an igneous rock formed from the partial melting of the crust caused from by basaltic magma from the mantle. It is associated with batholiths, dikes and stocks usually around volcanic arcs. Its volcanic equivalent is andesite which is formed from strombolian type volcanoes.
Diorite is a phaneritic rock that is speckled black and white/grey with a very variable colour index of between 40 and 90 over short distances. The rock has an equigranular or porphyritic texture and has a generally coarse grain size but can be pegmatitic. Phenocrysts of feldspar and horneblende can be found in the porphyritic variety. The equigranular and porphyritic varieties may vary in texture between each other within a few centimetres. A foliated appearance can develop from the roughly parallel alignment of the minerals.
The rock has an intermediate composition between plagioclase feldspar (oligoclase or andesine) and amphibole (usually hornblende) with various other minor minerals like pyroxene, biotite. Quartz and alkali feldspar may also be present. When the amount of quartz increases, the rock is named quartz diorite. The rock grades into granodiorite as the amount of quartz and alkali feldspar increases.
Alkali feldspar increase. Some common accessory minerals are apatite, titanite and various iron oxides.
Xenoliths from the partially melted margins of the magma chamber are common.
Diorite usually forms along with granite in continental crust above subduction