ABSTRACT
Ultramafic rocks must have more than 90% of mafic minerals, which are Mg-and Fe-rich minerals that include olivine, Mg-, Fe-, and Ca-rich pyroxene, garnet, biotite, chlorite, talc, serpentine, amphiboles, and others. Eclogites, blueschists, and other rocks that have more than 90% of minerals that are arguably mafic (omphacite and glaucophane, respectively, plus garnet) have their own special designations, and are not considered ultramafic. The largest potential source for ultramafic rocks globally is Earth’s mantle. Parts of the mantle, typically that associated with oceanic lithosphere, have been brought to observable levels along thrust faults during volcanic arc-continent collisions. Ultramafic rocks of the oceanic lithosphere typically occur as fresh or altered rock in the lower parts of ophiolites. Such rocks can also be found exposed on the ocean floor in transform faults, or along very slowly spreading ocean ridges. Sub-continental mantle can also be found exposed in some orogenic belts, and as xenoliths in some alkali basalts and kimberlites. While most mantle rock can be thought of as metamorphic, metamorphism can also occur during and after emplacement in the crust.
