Paleomagnetism of mesozoic kimberlites in southern Africa and the cretaceous apparent polar wander curve for Africa

Abstract

Including those previously published, good paleomagnetic data have been obtained from 48 of 51 sites in 28 separate kimberlites. Radiometric dates (by one or more methods) are available for 17 of these bodies and range from 81 to 145 Ma, with maxima around 90 and 120 Ma. These ages correlate with geochemical contrasts distinguishing what have been designated Group I (younger) and II (older) kimberlites [Smith et al., 1985b]. Stable magnetization appears to be associated with magnetite developed deuterically, after emplacement, but in most bodies, secular variation has not been completely averaged in the remanence acquired. The distribution of “VGPs” calculated is distinctly, but not sequentially, streaked yet correlates broadly with the older and younger petrochemical types. Mean North poles calculated on this basis yield KP1 (N = 14), 81–100 Ma, 64.1°N, 226.1°E, A95 = 5.2 and KP2 (N = 10), 113–145 Ma, 47.6°N, 269.9°E, A95 = 9.7. With the exception of one undated pipe (Meltonwold), which was not included in the averages, and the melilititic Male fissure at Swartruggens (in Group II average), all the kimberlites sampled are normally magnetized. Including these new data, 10 of 17 published African Cretaceous poles meet acceptability criteria specified, and define an apparent polar wander (APW) curve from ∼130 to 75 Ma. The curve appears to consist of two segments, trending latitudinally before ∼100 Ma, and meridionally thereafter. The latter trend is evident even in the distribution of the individual KP1 poles, and may be related to the true polar wander speculated by Gordon and Livermore (1987) to have occurred in that interval.