Evolution of nepheline from mafic to highly differentiated members of the alkaline series: the Messum complex, Namibia

Abstract

AbstractThe change in chemical composition trend of magmatic nepheline through magma evolution has been characterized from the alkaline series of the Messum complex in which nepheline occurs in a succession of different mineral parageneses from mafic-rich (theralites) to strongly evolved felsic-rich rock types (nepheline syenites). The nepheline compositions are dependent on those of coexisting feldspar(s). They record an evolution parallel to that of the melt schematized according to experimental phase diagrams, from initially Ca-rich compositions in equilibrium with calcic plagioclase towards increasingly Ca-poor, Na-rich and Si-rich compositions. The K contents show a maximum that corresponds to the appearance of alkali feldspar in the parageneses. This evolution is qualitatively preserved in spite of the low-T Na/K re-equilibration typical of plutonic nephelines. Although a slight increase in the silica content of nepheline is consistent with the experimentally defined magmatic trend, several high-silica nephelines from the Messum rocks as well as from other reported occurrences, cannot be reconciled with the experimental data. The nepheline solid-solution model available suggests that such ‘abnormal’ compositions might be related to different crystallization mechanisms between natural nephelines and some synthetic analogues.