Abstract

The volcanic fields of Southwest Uganda and the adjoining part of the Belgian Congo are characterized by ultrabasic potassic rocks. The principal rock types are katungite, ugandite, and mafurite. Katungite is rich in melilite but free from augite; its potash is mainly in glass. Ugandite and mafurite contain augite, but their dominant feldspathoids are leucite and kalsilite respectively. Olivine is present in these three types. The investigated rocks also include members of the potash ankaratrite-mela-leucitite series, olivine melilitite, kivite or leucite basanite, and limburgite.

The constituents of the pyroclasts include fragments of prevolcanic crustal rocks and a subvolcanic suite of rocks consisting of various combinations of augite, biotite, and olivine. Representatives of both groups occur in the lavas as xenoliths and xenocrysts.

The chemical compositions and trace-element contents of the different types are discussed. They are relatively rich in K2O, TiO2, and P2O5 and characterized by having K2O > Na2O and Al2O3 > (K2O + Na2O). They possess relatively high abundance of trace elements uncommon in ultrabasic rocks—namely, Sr, Ba, Rb, and Zr.

It is shown that neither the crystallization differentiation nor the limestone-assimilation hypothesis can explain the peculiar geochemical features of the examined rocks. It is suggested that they originated by reactions between carbonatite magma which contributed Ca with Sr, La, and Y; Mg with Cr and Ni; Fe with V and Ti; and P; and sialic crustal rocks which provided Si; Al with Ga; and K with Rb and Ba. The variations in the proportions of the reacting materials and in the physical conditions under which the reactions took place gave rise to differences in the compositions of the various rock types.

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