Carbonatitic Lavas

Abstract

RECENTLY, von Knorring and du Bois1 described a vesicular lava of high carbonate content from western Uganda which shows close geochemical affinities with intrusive carbonatites. They stated: “It must be emphasized that the lavas examined are fresh and unaltered and that the carbonated nature cannot be attributed to alteration after solidification”. Descriptions of extrusive equivalents of carbonatites are rare, but two other examples have been reported recently from Africa. Bailey2 concluded that certain carbonate-rich tuffs in Northern Rhodesia were primary, and not alteration products of more normal rocks. On p. 58 of his report, he wrote: “Indigestible though the fact may be to some philosophical systems—the pyroclastics around the Rufunsa vents are effusive carbonatite”. Reference is made, in both papers, to the 1954 eruption of Oldonyo Lengai in Tanganyika, during which showers of dust and ashes, largely composed of calcium and sodium carbonates, were observed to fall on the flanks of the volcano3.