Abstract:
Palaeomagnetic and petrological investigations were undertaken near Roossenekal, at the contact between subzones A and B of the main zone in the eastern Bushveld Complex. Three rock-types were identified at the site; the upper mottled anorthosite, a two-pyroxene gabbro and a critical zone-associated noritic sill. Twenty orientated core samples, which yielded fifty eight specimens, were obtained from the three rock-types for palaeomagnetic purposes. Additional samples were collected for petrographical and mineralogical studies. Palaeomagnetic investigations comprised measurement of magnetization, magnetic susceptibility and anisotropy of susceptibilty of the specimens. Additionally, specimens were subjected to alternating field and continuous thermal demagnetization, to isolate the components of magnetization. Data obtained from Zijderveld stability diagrams, difference vectors, and great circle techniques facilitated the identification of magnetization components. Petrographic procedures included microscopy under transmitted light, ore microscopy, electron microprobe and scanning electron microscope investigations. Electron microprobe work involved quantitative analysis of silicates and microscopic ore minerals, as well as qualitative analysis of sub-microscopic ore grains by using X-ray and backscattered electron i~ages. Mineralogical and palaeomagnetic data suggest that the differences in ore mineralogy, and in particular the relative quantities of the respective ore minerals, are responsible for the observed polarity differences in these rocks. The spurious behaviour of certain specimens, and the r2versed polarity of the norite to that of the anorthosite/gabbro unit, is associated with hematite exsolutions in ilmenite. Ilmeno-hematite is thus linked to a partial self-reversal of magnetization at this site
