Lithosphere metasomatism and the petrogenesis of the Chilwa Province of alkaline igneous rocks and carbonatites, Malawi

Rocks of the Chilwa Alkaline Igneous Province fall into three principal groups, namely: (1) syenite-quartz syenite-granite, (2) syenite-nepheline syenite, and (3) carbonatite-nephelinite-nepheline syenite. Dykes of all the principal rock types are abundant and there are numerous small agglomerate-filled volcanic vents. The nephelinites, which are minor, are the only basic silicate rocks found in the province, which is characterized by its overwhelmingly felsic character. The petrochemistry indicates that three fundamental magmas of trachytic, phonolitic and nephelinitic/carbonatitic composition are required to encompass all the rock types present. It is suggested that these three magma types were produced by partial melting at different depths of strongly metasomatized lithosphere, the whole igneous event having been preceded by a long period of focused metasomatism which produced anomalous lithosphere mantle, and lower crust, leading to isostatic uplift, rift faulting, and eventually volcanism. The trachytic melt was generated towards the base of the crust from rocks akin to fenites, and the phonolitic melt also directly, but at greater depths. It is suggested that although the carbonatite may have formed by immiscible separation from carbonate-rich nephelinite, it may also have been generated directly from a relatively carbonate-rich zone in the upper mantle concentrated by gradual upward creep of low viscosity carbonate melt.