PHLOGOPITE CRYSTALLIZATION IN CARBONATITIC MAGMAS FROM UGANDA

GEORGE R. McCORMICK
Department of Geology, The University of Iowa, Iowa City, Iowa 52242, U.S.A.

MICHAEL J. LE BAS
Department of Geology, The University of Leicester, Leicester LE1 7RH, U.K.


Abstract

We have determined the composition of zoned phlogopitic mica from two carbonatite complexes in Uganda by electron- microprobe analysis, and compared our findings with results for carbonatites in Arkansas and the Traansval. The mica phenocrysts and xenocrysts, as well as calcite-cotectic phlogopite, are good indicators of the evolving chemistry of the carbonatitic magma and fenitizing fluids. Early crystallizing phlogopite commonly shows evidence of decreasing Fe/(Mg + Fe) value, reflecting 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 availability in the carbonatitic magma, which drives the mica composition toward "ferriphlogopite". Cessation of magnetite precipitation allows the "ferriphlogopite" to zone toward "ferribiotite". At this stage, crystallization of mica ceases, only to be renewed in some complexes when wallrock assimilation adds Al, alkalis and Si to the magma. This high-Al phlogopite incorporates Ba. Micas in individual complexes show properties peculiar to that complex, but the general indications are that the availability of Al, and the ratio Fe3+/Fe2+ in the carbonatitic magma are the main factors governing the composition of the phlogopitic mica.