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Sr and Nd isotopic compositions are reported for minerals separated from the five distinct intrusions which comprise the central carbonatite body of the Jacupiranga alkaline complex. The calculated Rb-Sr age of 131 ± 3 m.y. is consistent with previously reported K-Ar ages. Calculated initial Sr and Nd isotopic ratios (εSr = 1.5 to 10.6; εNd = 0.1 to 1.6) plot near those estimated for the bulk earth at 130 m.y. ago. The carbonatite plug is isotopically heterogeneous with respect to initial ⁸⁷Sr/⁸⁶Sr ratios. Consequently the data do not lead to an unequivocal interpretation regarding source composition. We propose that the parental alkaline magma was isotopically similar to some Walvis Ridge basalts and that this parental magma was contaminated by continental crust during the differentiation process which culminated with the formation of the carbonatite.
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Roden et al. (1985) studied this question.
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