The structure and petrology of the Kisingiri nephelinitic volcano, western Kenya.

The Kisingiri volcano of Tertiary age, consists of a pile of lavas and agglomerates 60-70 Km. across underlain by Precambrian basement and early Miocene alkaline silicate and carbonatite intrusions. A pyroxenite-ijolite-urtite complex and a micro-melteigite ring dyke are differentiation products of a melanephelinite magma. An uncompahgrite-turjaite complex formed by the reaction of a layered accumulative peridotite body with its own residual fluids. Micro-ijolite/foyaite intrusions originated by differentiation from melanephelinite magma and metasomatism during intrusion. Emplacement of alkaline silicate complexes resulted in doming of the basement to a height exceeding 900 metres and in fenitisation. Four zones have been distinguished in fenites derived from granodiorite, with variation from fenitised granodiorite with minor replacement of quartz by aegirine-augite to a syenitic fenite with all relic features destroyed by granulation consisting of perthite and aegirine-augite. Leucoadamellite, due to high mechanical strength, was less prone to intense granulation and fenitisation. Sodic amphiboles formed in fenitised diorite. A consequence of fenitisation was metasomatism of ijolite to nepheline syenite by diffusion of silicon ions back, from the country rock. Emplacement of sovites was proceeded by extensive brecciation of the basement with potash feldspathisation at depth, and accompanied by intrusion of potash trachytes, tuffs and agglomerates by fluidisation. A considerable time interval followed before nephelinitic volcanism commenced about 19.6 million years ago. Lacustrine sedimentation occurred on the flanks of the dome. Six hundred metres of lavas (49%) and aggomerates (51%) are preserved. In addition to melanephelinite, nephelinite, and mugearite, melilite- and olivine-bearing lavas are common. Phonolite and alnoite only occur as dykes. No overall differentiation with time is present. The final events were rift faulting and the formation of the Rangwa pyroclastic/carbonatite cauldron.