A New Palaeomagnetic Result from the Lower Cretaceous of East-Central Africa

Abstract

Summary The mean direction of natural remanent magnetization (NRM) in eight samples of syenite of Lower Cretaceous age from the Mlanje Massif, Malawi, has declination 333 and inclination - 54 (upward) after magnetic cleaning. The stability of remanence in alternating magnetic fields suggests that the NRM is primary. On the assumption of a geocentric dipole field, the palaeomagnetic pole is calculated at 60 N, 98 W. This is in agreement with previous results from volcanics of the Lupata Series nearby in Mozambique, which are of similar age. Combination of results from the Mlanje Massif and the Lupata volcanics yields an estimate of the palaeomagnetic pole for the lower Cretaceous of this part of Africa at 609 N, 993 W, with circular standard deviation (related to palaeosecular variation) of 10.0. Directions of natural remanent magnetization in eight samples from the Lauderdale erosion crater (16 1' S, 35 329 E) on the southern side of the Mlanje Massif in south-eastern Malawi are reported. The massif is a syenite-granite complex and has been mapped by Garson & Walshaw (in preparation). The samples were collected from the outer syenite ring of the complex, along about 150 metres of a stream running at right-angles to its contact with the Palaeozoic basement. The boundary of the complex is transitional, the adjacent basement having been syenitized. Only the intrusive body has been sampled. It is several kilometres in diameter, and it is likely that the section which has been sampled represents a long period in the cooling history of the body. Therefore, in spite of the small collection there is reason to believe that palaeosecular variation may have been effectively averaged out in calculating the mean direction, and that the dispersion may be a fair estimate of the palaeosecular variation. Two recent K-Ar age determinations by Snelling (1966) indicate the Cretaceous age of the complex : biotite from perthosite (sample WllOS) from the outer ring at Chambe indicated 1 16T 6 m.y., and biotite-hornblende mixture from quartz-syenite (sample W1112) from the Nanchidwa Hills, some 19 km east of the Lauderdale Crater gave 128 T 6 m.y. Gough & Opdyke (1963) have reported the palaeomagnetism of the Lupata alkaline volcanics and also of one site in redbeds of the Lupata Series, some 160 km south-west of Mlanje, in Mozambique. McDougall made two determinations on anorthoclases from the Lupata volcanics at 110.5 and 106.7m.y. (Gough et al. 1964). Because Gough & Opdyke sampled seven sites in single lava flows, and hence sampled no more