POTASSIUM-RICH ROCKS: PHASE ANALYSIS AND HETEROMORPHIC RELATIONS

Abstract

A large number of potassium-rich igneous rocks can be displayed as a contiguous set of mineral assemblages in a modified normative tetrahedron kalsilite-forsterite-larnite-quartz. Pairs of phases in subtetrahedra undergo reaction relations that result in new subtetrahedra representative of observed igneous rock assemblages. Some of the heteromorphic relations recognized are: Jumillite-Shonkinite⇄Missourite-Absarokite Alnoite-Italite⇄Venanzite-Melilite Ugandite Leucite Katungite-Melilite Ugandite⇄Melilite Mafurite-Leucite Mafurite Alnoite-Vesecite⇄Monticellite Peridotite-Monticellite Melilitite Other subtetrahedra are generated by the breakdown of phlogopite, leucite, and monticellite. The relationships of the various magma types are outlined in a flow sheet for 1 atm, and some of the critical changes in thermal barriers are described for elevated pressures. The high-potassium content remains the principal problem; however, the shift of invariant points with pressure suggests generation by partial melting at high pressures. The complex mineralogy and diversity of the potassium-rich rocks are attributed mainly to incomplete reactions, with or without volatiles, between critical phases of limited range of stability.