Dunite

An intrusive igneous rock of ultramafic composition with a coarse grained, phaneritic texture, made up of more than 90 percent olivine along with trace amounts of pyroxene, chromite, magnetite and pyrope. It is rare in continental settings but occurs in specific geological contexts, including Twin Sisters Mountain near Mount Baker in Washington's Cascade Range, the Troodos Mountains of Cyprus, and an ultramafic complex near Tulameen in British Columbia that is enriched in platinum group metals, chromite and magnetite. Dunite and the related rock peridotite are the major constituents of the Earth's mantle above a depth of about 400 kilometers, and where dunite is found on land it typically forms part of an ophiolite sequence, a slice of oceanic mantle thrust onto continental crust during a tectonic collision; near the surface it commonly undergoes retrograde metamorphism into serpentinite or soapstone. It may represent the solid residue left after basaltic magma has been extracted from the mantle, or it can form through the accumulation of olivine crystals settling within a large magma chamber, producing layered deposits within intrusive complexes of varying ages.