Nd and Sr isotope compositions of nephelinites, carbonatites, and phonolites from Shombole, a Pliocene volcano in East Africa, show that the phonolites cannot be derived by simple fractional crystallization of nephelinite magma. For a given initial 87Sr/86Sr ratio, 143Nd/144Nd is lower in most phonolites than in the nephelinites and carbonatites. Interaction between nephelinitic magma and lower-crustal granulites can account for these differences. The similar ranges in isotopic composition of the carbonatites and nephefinites are consistent with repeated melting events involving heterogeneous mantle. The carbonatites could have formed by immiscibility with nephelinite magma or by direct partial melting of the same mantle source(s) as the nephelinites.

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