Abstract

Strontium isotopic ratios and rubidium and strontium abundances are reported for 34 representative samples from eight alkalic complexes in eastern Uganda, East Africa. The alkalic rocks and carbonatites exhibit a significant range of initial Sr87/Sr86 ratios from 0.7029 to 0.7061. The carbonatites have the lowest Sr87/Sr86 ratios, averaging 0.7034, although the Sr87/Sr86 ratios of nephelinites and ijolites overlap those of carbonatites. Strontium averages 5000 ppm in carbonatite, 1200 ppm in nephelinite, and 500 ppm in ijolite.

Initial Sr87/Sr86 ratios of 8 specimens of fenite associated with three of the complexes vary between 0.7058 and 0.7527. These high initial ratios show that the material of the fenites, in part at least, predates the alkalic activity. The inverse correlation between initial Sr87/Sr86 ratio and total strontium content in the fenites is interpreted as the result of migration of nonradiogenic strontium from the fenitizing solutions into the country rock.

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