Metasomatic Nepheline-Bearing Gneisses from Darkainle Somali Republic

The Darkainle nepheline-syenite complex, consisting essentially of intrusive magmatic nepheline syenites and syenites also includes small amounts of associated metasomatic nepheline-bearing biotite gneisses. The metasomatic rocks are characteristically richer in biotite than the magmatic types and are commonly migmatitic. They differ texturally in having interstitial nephelines and disoriented feldspars, whereas the magmatic types have large subhedral nephelines and show parallel-flow alignment of feldspars. Formation of the nepheline-bearing gneisses is ascribed to the process of nephelinization. This process has involved addition of $$Na_{2}O, Al_{2}O_{3}$$, and $$SiO_{2}$$ to pelitic and semipelitic schists that border the complex and removal of cafemic oxides. The solutions causing the metasomatism probably originated partly through hydrothermal alteration of nepheline in the magmatic nepheline syenites and partly as a residuum of the nepheline-syenite magma.