ABSTRACT
A carbonatite is an igneous rock that contains more than 50 per cent primary carbonate minerals. Vesicular calcite-carbonatite lava flows have been described from the Fort Portal area of Western Uganda. The primary magmas responsible for most carbonatite complexes are probably sodium-rich melanephelinites, or potassium-rich aillikites, but there are a wide range of other magmas that belong to the alnoite-polzenite, aillikite-damtjernite and kimberlite-lamproite families that also may evolve to produce carbonatites. Partial melting is likely to be triggered by deep-seated fracturing that promotes decompression melting and the release and focusing of previously trapped volatiles. The segregation and ascent of these extremely reactive magmas is likely to be rapid. On moving up through the lithosphere, the drop in confining pressure probably induces volatile-rich carbonatitic fluid to separate from the silicate primary magmas. Wall-rock reaction, fracturing and fractional crystallisation would occur during ascent of both silicate and carbonatite magmas.
