The island of Brava, Cape Verdes, eshibits a Quaternary nepheline-phonolite-carbonatite pyroclastic sequence. Eruptive styles of the dominant phonolites depart markedly from those typical of felsic magmas, and mapping the deposits will add to our knowledge of the eruptive behavior of highly alkaline magmas. Phonolites are typically strongly depleted in rare-earth elements, and bear a complementary relationship to associated rare-earth-enriched carbonatite volcanics. The most probable mechanism for this is the immiscible separation of a carbonatite liquid from silicate magma, the carbonate being represented in the volcanic pile as carbonatite lavas and tuffs. The phonolites appear to be products of fractional crystallization via the line of descent, olivine nephelinite - nephelinine - phonolite, but major questions remain, especially regarding the role of the sulphate feldspathoid nosean in phonolite geneses. Petrogenetic modelling will be based on mineral chemistry and major, trace element, and isotopic determinations of a suite of whole rocks and mineral separates from fresh pumice clasts in carefully studied pyroclastic units. The close association of nephelinitic, basanitic, phonolitic and carbonatitic volcanic units at a well-exposed Quaternary center provides an unusual opportunity to advance understanding of the genesis and behavior of carbonatite magmas, and their relationship to associated alkaline silicate magmas.
