The magmas that produce ultrapotassic rocks are produced by a variety of mechanisms and from a variety of sources, but generally occur in a heterogenous, anomalous, phlogopite-bearing upper mantle.
The following conditions are favorable for the formation of ultrapotassic magmas.
* partial melting at a great depth
* low degrees of partial melting
* lithophile element (K, Ba, Cs, Rb) enrichment in sources
* enriched peridotite (variety harzburgite), especially in potassium
* pyroxene and phlogopite-rich volumes within the mantle, not from peridotite alone
* carbon dioxide or water (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 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.