A Palaeomagnetic investigation of the layered Mafic Sequence of the Bushveld Complex

Abstract

Orientated samples were collected from 88 sites 1n the marginal, lower, critical, main and upper zones of the layered sequence of the Bushveld Complex. Apart from measuring the remanent magnetization of these samples, the magnetization of the samples was subjected to detailed analyses, using alternating field and thermal demagnetization techniques. This was supplemented by various heating experiment and tests to establish the stability and origin of the observed magnetization. The critical zone yielded six sites with stable and consistent magnetization directions, forming a group with a corresponding palaeomagnetic pole at 37°S, 135°W. With the igneous layering in a horizontal position, the group statistics improve, but not significantly at the 95 per cent confidence limit. However, the mixed polarity of this group, which becomes significantly more antipodal after correction for the dip of the layering, is considered to be an indicator that the critical zone could have acquired its remanent magnetization with the igneous layering in a horizontal position. The pole position corresponding to this corrected group is situated at 39,5°S, 133°W. Heating experiments and mineralogical evidence suggest the possibility that the magnetic polarity of the critical zone could be due to a self-reversal of magnetization. This is further supported by the position and polarity of the palaeomagnetic pole of the critical zone on the apparent polar wander (APW) path for Africa. Magnetization directions from the main zone in the eastern Bushveld Complex form two groups, approximately antipodal to each other. The majority of sites are situated in subzone B, forming a group with a reversed magnetization which improves significantly with the igneous layering in a horizontal position. The palaeomagnetic pole of this group is situated at 17,3°N, 35,7°E. The second group represents sites from the top of the main zone (subzone C) which again yield greatly improved results when a fold test is applied. The group is magnetized in a normal direction with the corresponding palaeomagnetic pole at 28°S, 161 ,7°W. The polarity difference between the two groups is considered to be due to a geomagnetic field reversal towards the end of formation of the main zone. With the exception of two sites, the magnetization directions from all the sites in the main zone in the western Bushveld Complex group together and are reversely magnetized. Rotation of the layering to a horizontal position improves the grouping, which indicates that the main zone could have acquired its remanent magnetization with the igneous layering in a horizontal position. The corresponding pole position is at 9,2°N, 27,3°E. The two sites situated in the lower part of the main zone have normal magnetization directions similar to those of the critical zone. Consistent normal magnetization directions from sites 1n the upper zone was obtained after alternating field and thermal demagnetization of specimens. Structural folding failed to improve group statistics, suggesting that the upper zone acquired its remanent magnetization with the igneous layering in its present attitude. The palaeomagnetic pole representing the upper zone is situated at 16,1°S, 148,5°W. The layered sequence exhibits a magnetic polarity pattern, which allows the subdivision of the sequence into three magnetic polarity zones. The lower- and uppermost zones have normal magnetization directions with respect to the present-day geomagnetic field. The third, situated between the former two, is reversely magnetized.