The structure and petrology of the carbonatitic complexes of North Ruri and Okuge, Kenya.

The volcanic centre of North Ruri is situated in the Kavirondo Rift Valley about 1 Km north of its counterpart, South Ruri# The complex, emplaced in updomed Pre-Cambrian Nyanzian metabasalt, consists of numerous steeply inclined of alvikite conesheets/which form short arcs in a much larger coneshaped structure. Smaller pipes and dykes of sSvite with plates of ferroan calcite, and ferruginous carbonatite also occur. The alvikite usually has rhombs of calcite, due both to micro- brecciation of the earlier sflvite and primary crystallisation. Further brecciation and recrystallisation may modify the textures. Associated carbonatitic volcanoclasts show phlogo-pitisation and feldspathisation due to reaction with the carbonatite*s volatile constituents. Alkaline silicate rocks are represented by several intrusions of nepheline-microsyenite within the complex and numerous outlying plugs of phonolitic nephelinite, two of which contain biotite xenocrysts. Ijolite is widespread as xenoliths and fragments and is believed to underlie the whole complex. The matrix between the feldspar plates of the nepheline-microsyenite is essentially micro-ijolitic, and suggests that development of the interstitial orthoclase has occurred in the upper part of an ijolite mass. Limited fenitisation of the Nyanzian metabasalt by the adjacent nepheline-microsyenite has produced a narrow zone of alkali- syenite fenite. Blocks of augite-glimmerite, hornblende-pyroxenite and biotite-pyroxenite in the carbonatitic agglomerate indicate the presence of these rocks at depth. North Ruri is suggested to be later than the nearby Kisingiri complex of Miocene nephelinite lavas and agglomerates, which almost surround North Ruri and Okuge, on the relationship between the agglomerates and the phonolitic plugs of Ruri, Age determinations of the carbonatites also suggest a post-Miocene age. The much smaller complex of Okuge consists of a horseshoe shaped mass of carbonatite conesheets, with scattered outcrops of melanephelinite lava and agglomerate (from the Kisingiri Volcano), and phonolitic nephelinite lava surrounding it