The present-day Sr87/Sr86 ratios of 117 representative samples from the Birunga and Toro-Ankole regions vary significantly and range between 0.7036 and 0.7111. The feldspar-bearing lavas have higher ratios (average = 0.707) than the melilite- and nepheline-bearing varieties (average = 0.705). Samples of carbonated lavas have slightly lower Sr87/Sr86 ratios.The Sr87/Sr86 ratios show a highly significant, positive, linear correlation with Rb/Sr ratio, and a negative correlation with Sr, Nb, and Zr abundances. Graphs of Sr87/Sr86 ratios against elemental abundances in some cases give hyperbolic patterns. Such relationships are true, not only for the volcanic field as a whole, but also for lava flows from one extensively sampled volcano.Hypotheses involving simple fractional crystallization or limestone syntexis are inconsistent with the isotopic data. The elemental and isotopic abundance patterns are most easily explained by the mixing of two end members of quite different Sr isotopic and chemical compositions. If mixing is assumed, approximate limits can be set for the compositions of the two end members. These limits are consistent with the hypothesis of assimilation of sialic material by either a carbonatitic or nephelinitic parent magma. © 1969 Oxford University Press.
