Strontium Isotopic Studies of Alkalic Rocks: The Potassium-rich Lavas of the Birunga and Toro--Ankole Regions, East and Central Equatorial Africa

The present-day Sr⁸⁷/Sr⁸⁶ ratios of 117 representative samplesfrom the Birunga and Toro-Ankole regions vary significantlyand range between 0.7036 and 0.7111. The feldspar-bearing lavashave higher ratios (average = 0.707) than the melilite- andnepheline-bearing varieties (average = 0.705). Samples of carbonatedlavas have slightly lower Sr⁸⁷/Sr⁸⁶ ratios. The Sr⁸⁷/Sr⁸⁶ ratios show a highly significant, positive, linearcorrelation with Rb/Sr ratio, and a negative correlation withSr, Nb, and Zr abundances. Graphs of Sr⁸⁷/Sr⁸⁶ ratios againstelemental abundances in some cases give hyperbolic patterns.Such relationships are true, not only for the volcanic fieldas a whole, but also for lava flows from one extensively sampledvolcano. Hypotheses involving simple fractional crystallization or limestonesyntexis are inconsistent with the isotopic data. The elementaland isotopic abundance patterns are most easily explained bythe mixing of two end members of quite different Sr isotopicand chemical compositions. If mixing is assumed, approximatelimits can be set for the compositions of the two end members.These limits are consistent with the hypothesis of assimilationof sialic material by either a carbonatitic or nepheliniticparent magma.