PHLOGOPITE CRYSTALLIZATION IN CARBONATITIC MAGMAS FROM UGANDA

We have determined the composition of zoned phlogopitic mica from two carbonatite complexes in Uganda by electron-microprobe analysis, and c ompared our findings with results for carbonatites in Arkansas and the Transvaal. The mica phenocrysts and xenocrysts, as well as calcite-co tectic phlogopite, are good indicators of the evolving chemistry of th e carbonatitic magma and fenitizing fluids. Early-crystallizing phlogo pite commonly shows evidence of decreasing Fe/(Mg + Fe) value, reflect ing incipient precipitation of magnetite as the Fe3+/Fe2+ ratio rises in the magma. The level of Al decreases in the mica owing to its low a vailability in the carbonatitic magma, which drives the mica compositi on toward ''ferriphlogopite''. Cessation of magnetite precipitation al lows the ''ferriphlogopite'' to zone toward ''ferribiotite''. At this stage, crystallization of mica ceases, only to be renewed in some comp lexes when wallrock assimilation adds Al, alkalis and Si to the magma. This high-Al phlogopite incorporates Ba. Micas in individual complexe s show properties peculiar to that complex, but the general indication s are that the availability of Al and the ratio Fe3+/Fe2+ in the carbo natitic magma are the main factors governing the composition of the ph logopitic mica.