Petrography and Petrogenesis of Diorites

Diorites

  • Diorite is a coarse-grained, intrusive igneous rock that is intermediate in composition between granite and gabbro.
  • Diorite is composed primarily of plagioclase feldspar, amphibole, and pyroxene minerals with small amounts of biotite mica.
  • It typically contains very little quartz. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory minerals.

  • Varieties deficient in hornblende and other dark minerals are called leucodiorite.
  • When olivine and more iron-rich augite are present, the rock grades into ferrodiorite, which is transitional to gabbro.
  • The presence of significant quartz makes the rock type quartz-diorite (>5% quartz) or tonalite (>20% quartz), and if orthoclase (potassium feldspar) is present at greater than ten percent the rock type grades into monzodiorite or granodiorite.
  • Diorites may be associated with either granite or gabbro intrusions, into which they may subtly merge. Diorite results from partial melting of a mafic rock above a subduction zone.
  • It is commonly produced in volcanic arcs, and in cordilleran mountain building such as in the Andes Mountains as large batholiths.

Formation of diorite

  • Partial melting of the oceanic plate produces a basaltic magma that rises and intrudes the granitic rock of the continental plate.
  • There, the basaltic magma mixes with granitic magmas or melts granitic rock as it ascends through the continental plate.
  • This produces a melt that is intermediate in composition between basalt and granite. Diorite forms if this type of melt crystallizes below the surface.