The complex is composed of alkaline plutonic rocks, pyroclastics and carbonatites, whose outcrop pattern is concentric. The alkaline plutonics consist of a. central body of biotite uncompahgrite, turjaite and ijolite, only part of which survives, and small outlying intrusions of carbonatite and nepheline syenite and areas of brecciation, fenitisation and felspatliisation of the basement. The biotite uncompahgrite is formed by the metasomatism of the inner part of a mass of pyroxenite and melteigite, the outer part of which is intruded by ijolitic conesheets and dykes. The pyroclastics, which were emplaced later than the central alkaline plutonics, consist of banded agglomerate, which was formed by fluidisation, and tuffs, which were deposited subaerially and under water. Most of the pyroclastics dip inwards, and they have been downfaulted along their outer boundary. The main carbonatites have been emplaced in several phases, all of which are related to central foci. They consist of small cone sheets and dykes, a partial ring of coarse micaceous carbonatite, and a plug of banded, magnetite-rich carbonatite. They are associated with brecciation and felspathisation. The complex is intruded by late alkaline silicate dykes. Rangwa is at the centre of a large dissected nephelinite volcano, beneath which the basement has been domed. The central plutonic complex was emplaced before most of the nephelinite eruptions. The pyroclastics on Rangwa are tentatively correlated with conglomerates and tuffs, which occur beneath the main nephelinites the north and northwest of the volcano, and many of the nephelinite lavas and agglomerates of the volcano are thought to have been erupted from a centrally-placed vent on Rangwa.