Palaeomagnetism and magnetic fabric in the Freetown Complex, Sierra Leone

About six separately orientated cores were collected at each of 14 sites di stributed throughout the arcuate, west-dipping, 6 km thick, Freetown layere d igneous complex. Alternating field and thermal demagnetization both isola te a stable component of remanent magnetism which corresponds to a palaeoma gnetic south pole from 13 sites (nine reverse, four normal polarity) at 82. 9 degrees S, + 32.7 degrees E (alpha(95) = 5.6 degrees). This is indistingu ishable from that reported in 1971 based on alternating field demagnetizati on of cores from 10 orientated hand samples. The difference between the Freetown pole (age: 193 +/- 3 Ma) and other mid- Jurassic poles from West Africa could be due to its greater age. The differ ence between the whole West African Jurassic pole group and the Karoo pole from southern Africa, however, suggests moderate (similar to 10 degrees) di fferential rotation of West Africa relative to the Kaapvaal craton. A prevalent magnetic foliation fabric coincides generally with the petrolog ical layering, as might be expected, but a ubiquitous magnetic lineation is predominantly down-dip. This is compatible with a down-dip pyroxene lineat ion reported to be present in some field outcrops, and interpreted in terms of late-stage deformation during the slow crystallization and cooling of t he large igneous body. However, a fold test shows that the igneous layering had already achieved its present attitude before the Complex cooled to sim ilar to 570 degrees C (the maximum blocking temperature of the characterist ic remanence).