Ultrapotassic igneous rocks

Ultrapotassic igneous rocks are a class of rare, volumetrically minor and generally ultramafic or mafic silica-depleted igneous rocks.

Ultrapotassic rocks are defined by molar K₂O/Na₂O >3 in much of the scientific literature. In other papers written as recently as 2005, they are defined as rocks with weight percents K₂O/Na₂O >2. Hence, caution is indicated in interpreting use of the term "ultrapotassic", and the nomenclature of these rocks continues to be debated.

Genesis of these ultrapotassic rocks has been much discussed. The magmas probably are produced by a variety of mechanisms and from a variety of sources. The magma production may be favored by the following:[1]

• great depth of partial melting
• low degrees of partial melting
• lithophile element (K, Ba, Cs, Rb) enrichment in sources
• peridotite (variety harzburgite) so enriched, especially in potassium
• pyroxene and phlogopite-rich volumes within the mantle, not from peridotite alone
• carbon dioxide or water in sources (each condition leading to a distinctive magma);
• reaction of melts with surrounding rock as they rise from their sources

Mantle sources of ultrapotassic magmas may contain subducted sediments, or the sources may have been enriched in potassium by melts or fluids partly derived from subducted sediments. Phlogopite and/or potassic amphibole probably are typical in the sources from which many such magmas have been derived. Ultrapotassic granites are uncommon and may be produced by melting of the continental crust above upwelling mafic magma, such as at rift zones.