K/Ar age determinations on biotite from the carbonated biotite pyroxenite of the Musensi Hill intrusion gave 96 + or - 9 and 101 + or - 12 million years, or middle Cretaceous. The nearby Panda Hill carbonatite, on stratigraphic evidence, is considered pre-Karroo and earlier than the Musensi Hill intrusion. Feldspar from feldspathized carbonate rock of the Mbeya range gives a K/Ar age of 100 + or - 10 m.y., indicating its contemporaneity with the Musensi Hill intrusion and affinity of the dike rocks in the fault zone of the rift valley with the carbonatite suite.

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