Ultramafic rock

Ultramafic (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). The Earth's mantle is composed of ultramafic rocks. Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks.

• http://en.wikipedia.org/wiki/Ultramafic rock
• Page 192 - Schmidt, K. L., et al., 2016, Mesozoic tectonics west of the accretionary boundary in west-central Idaho: A road log along U.S. Highway 95 between Moscow and New Meadows, Idaho: Geol. Soc. Am., Field Guide 41, p. 175-209.