DUNITE & PYROXENITE  
     
  Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with minor pyroxene and chromite. Dunite is the olivine-rich end-member of the peridotite group of mantle-derived rocks. Dunite and other peridotite rocks are considered to represent the Earth’s mantle. Dunite is rarely found within continental rocks, but where it is found, it typically occurs at the base of ophiolite sequences where slabs of mantle rock from a subduction zone have been thrust onto continental crust by obduction during continental or island arc collisions (orogeny). It is also found in alpine peridotite massifs that represent slivers of sub-continental mantle exposed during collisional orogeny. Dunite typically undergoes retrograde metamorphism in near-surface environments and is altered to serpentinite and soapstone.

Dunite may represent the refractory residue left after the extraction of basaltic magmas in the upper mantle. This is the type of dunite found in the lowermost parts of ophiolites, alpine peridotite massifs, and xenoliths. Dunite may also form by the accumulation of olivine crystals on the floor of large basaltic or picritic magma chambers. These “cumulate” dunites typically occur in thick layers in ultramafic to mafic layered intrusions, associated with cumulate layers of wehrlite, olivine pyroxenite, harzburgite, and even chromitite (a cumulate rock consisting largely of chromite).

 
     
  Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite and diopside, hypersthene, bronzite or enstatite. They are classified (see diagram below) into clinopyroxenites, orthopyroxenites, and the websterites which contain both pyroxenes. Closely allied to this group are the hornblendites, consisting essentially of hornblende and other amphiboles.

They are essentially of igneous origin, though some pyroxenites are included in the metamorphic complex of the Lewisian of Scotland. The pyroxene-rich rocks which result from the contact metamorphism of impure limestones are described as pyroxene hornfelses (calc-silicate hornfelses).