Palaeomagnetism of a Pan-African diorite: a Late Precambrian pole for western Africa

A study of a Late Pan-African diorite (Adma diorite; 18.3°N, 1.2° E) gives a mean direction of magnetization D = 317.3°, I=78.6° (13 sites, 62 samples, α₉₅ = 9.0°) based on alternating field and thermal cleaning. The corresponding palaeopole is at 32.5° N, 344.7° E (A₉₅=15.9°). The diorite is dated by U/Pb method at 616 ± 11 Myr. The age of this magnetization is believed to be somewhat younger, between 590 and 616 Myr. Thermal treatment shows that above 675°C (Curie point of synthetic hematite and the highest value usually observed in rocks) the specimens retain a magnetization whose intensity and direction are approximately the same across the collection. In some specimens, the directions continue to change between 670 and 720°C. The carrier of this ‘high-temperature’ magnetization is not known; it could be hematite with blocking temperature higher than 675°C, or iron, or an iron-rich mineral such as sphene. Comparison of the palaeopole with others from rocks of about the same age from Gondwana shows a good agreement. There is an excellent agreement with a result from southern Africa (Ntonya Ring Structure) which suggests that there have been no important motions between the different blocks which form Africa, at least since that time. This pole places western Africa in high latitudes and is consistent with the presence of upper Precambrian tillites.