LIQUID IMMISCIBILITY BETWEEN TRACHYTE AND CARBONATE IN ASH-FLOW TUFFSFROM KENYA

Three thin, syn-caldera ash flow tuffs of the Suswa volcano, Kenya, co ntain pumiceous clasts and globules of trachytic glass, and clasts ric h in carbonate globules, in a carbonate ash matrix. Petrographic and t extural evidence indicates that the carbonate was magmatic. The trachy te is metaluminous to mildly peralkaline and varies from nepheline- to quartz-normative. The carbonate is calcium-rich, with high REE and F contents. The silicate and carbonate fractions have similar Nd-143/Nd- 144 values, suggesting a common parental magma. Chondrite-normalized R EE patterns are consistent with a carbonate liquid being exsolved from a silicate liquid after alkali feldspar fractionation. Sr isotopic an d REE data show that the carbonate matrix of even the freshest tuffs i nteracted to some degree with hydrothermal and/or meteoric water. A li quid immiscibility relationship between the trachyte and carbonate is indicated by the presence of sharp, curved menisci between them, the p resence of carbonate globules in silicate glass and of fiamme rich in carbonate globules separated by silicate glass, and by the fact that s imilar phenocryst phases occur in both melts. It is inferred that the carbonate liquid separated from a carbonated trachyte magma prior to, or during, caldera collapse. Viscosity differences segregated the magm a into a fraction comprising silicate magma with scattered carbonate g lobules, and a fraction comprising carbonate globules in a silicate ma gmatic host.Explosive disruption of the magma generated silicate- and carbonate-rich clasts in a carbonate matrix. The silicate liquid was d isaggregated by explosive disruption and texturally appears to have be en budding-off into the carbonate matrix. After emplacement, the basal parts of the flows welded slightly and flattened. The Suswa rocks rep resent a rare and clear example of a liquid immiscibility relationship between trachyte and carbonate melts.