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Department of Mineralogy, British Museum (Natural History), Cromwell Road, London SW7 5BD, U.K.
The Chilwa province of alkaline igneous rocks and carbonatites lies at the southern end of the East African rift, and is unique within the rift for its essentially intrusive nature. The province comprises numerous carbonatite centres (some with nepheline syenite and nephelinite), large complexes of nepheline syenite and syenite, and plutons of peralkaline syenite, quartz syenite and granite, together with dykes compositionally equivalent to all the major rock types. 196 rock samples from the northern half of the province have been chemically analysed and the data indicate that three rock series are present: (1) syenite-quartz syenite-granite, (2) nepheline syenite-syenite and (3) nephelinite-carbonatite-nepheline syenite. It has not proved possible to relate these series to a single parental magma, and it is concluded that three parental magmas of trachytic, phonolitic and nephelinitic-carbonatitic composition were involved.
A model is suggested for the generation of these three magmas involving production of a zoned metasomatized lithosphere wedge by lithosphere focussing. Fracturing of the crust above the growing wedge led to pressure release and to the uprise of geotherms and volatiles with consequent partial melting at different levels throughout the metasomatized wedge. The metasomatism may have reached the base of the crust and it was from this region that trachyte magmas were produced by melting of rocks similar to syenitic fenites. Phonolitic magmas were generated directly at intermediate levels, and nephelinites were generated at the base of the metasomatized lithosphere.
