PALEOMAGNETIC AND 40AR 39AR STUDIES OF LATE KIBARAN INTRUSIVES IN BURUNDI, EAST-AFRICA - IMPLICATIONS FOR LATE PROTEROZOIC SUPERCONTINENTS

The Kibaran tectonothermal event in East Africa (1400-1200 Ma) peaked at 1300 Ma and was followed by the emplacement of a linear belt of mafic/ultramafic and felsic plutons between 1275-1220 Ma. A series of the mafic/ultramafic plutons in Burundi from the Congo craton carry a stable paleomagnetic remanence with a mean direction of D = 180-degrees, I = + 16-degrees (N = 10 sites, k = 37, alpha95 = 8-degrees) with a corresponding paleomagnetic pole at 17-degrees S, 113-degrees E. Biotite samples from the Mukanda-Buhoro massif yield well-defined 40Ar/39Ar plateau ages averaging 1223 +/- 4 Ma and slightly younger integrated ages of 1211 +/- 2 Ma. The 40Ar/39Ar ages, combined with previously published U-Pb and Rb-Sr ages, suggest that the magnetic directions were acquired during cooling of the massif between 1260-1210 Ma (1236 +/- 24 Ma). One pluton at Nyabikere yields an anomalous direction of D = 46-degrees, I = 29-degrees and a corresponding virtual geomagnetic pole at 43-degrees N, 137-degrees E. Hornblendes and biotites from Nyabikere yield younger 40Ar/39Ar ages of c. 950 Ma. The magnetization and K-Ar ages in the Nyabikere rocks were reset during a c. 950 Ma thermal event known elsewhere in East Africa. Paleomagnetic poles for the interval 1200-1250 Ma from the Kalahari craton, Congo craton, Laurentia, Baltica, and Australia suggest that the Neoproterozoic supercontinent of Rodinia was not yet fully formed at 1200 Ma. Alternative, previously published supercontinental configurations are not supported by these new paleomagnetic data. Assuming the validity of Rodinia for Late Proterozoic times, this implies that our paleomagnetic results provide evidence for mid-Proterozoic plate movements leading to Grenville-aged collisions (c. 110-1000 Ma) and the assembly of Rodinia.