Geochemical evidence for a non-alkalic origin for the carbonatic bodies of Kibuye, Rwanda

Abstract

The coarse-grained carbonatic bodies of Kibuye have been interpreted as magmatic carbonatites (spathic crystal from, uncomformable contacts, situation along the rift), metamorphozed limestones (occurrence within a sedimentary sequence containing a.o. limestone horizons) or limestones mobilized through magmatic activity (abundance of gabbro-doleritic dykes and sills in the vicinity). These bodies are made up of nearly pure carbonate, either calcic or calco-magnesian. They are R.E.E.-, Ba-and Sr-poor, thereby precluding any magmatic alkalic affinity. Their 87Sr/ 86Sr ratio of 0.71141 is likewise too high for a mantellic filiation. This ratio is however intermediate between those shown by the gabbro-dolerites and some dolomitic phyllites of the Burundian Sequence (Middle Proterozoic), which have respectively yielded 0.70991 and 0.71250 (when corrected for 1400 M.a. of 87Rb decay). The latter values, together with the 18O and 13C ratios, suggest that the origin of the carbonatic bodies must be searched for in a process where sedimentary dolomites have been remobilized, probably under the influence of the magmatic intrusions. Although situated in the vicinity of the Western Rift, as many other alkalic complexes, the carbonatics rocks of Kibuye are not representatives of this magmatic kindred.