The paleomagnetism of the precambrian dolerites of Eastern Southern Rhodesia, an example of geologic correlation by rock magnetism

Abstract

Thirty sites have been sampled in dolerites which intrude the Precambrian shield and the Umkondo system of eastern Southern Rhodesia. The natural remanent magnetization is random at sixteen of the twenty-five sites used in the final analysis. In many cases it has been necessary to resort to the use of high alternating fields to successfully remove later components of magnetization, which are thought to be the result of isothermal remanent magnetization due to lightning currents. The directions of magnetization after treatment in alternating fields form two distinct groups about 90° apart, which indicates that these dolerites represent two quite distinct magmatic episodes. These directions are given by D = 292°, I = +51° (pole position 7°N, 20°W, A95 = 7½°), and D = 182½°, I = −10½° (pole position 65°N, 42½°E, A95 = 6°). The former group, which intrudes the Rhodesian Precambrian shield, are termed the Mashonaland dolerites, and the latter, which are associated with the Umkondo system, are termed the Umkondo dolerites. Paleomagnetic evidence is presented to show that the Mashonaland dolerites have intruded the Umkondo dolerites, which are therefore the older of the two sets of intrusions. Potassium-argon age determinations on some of the samples of Mashonaland dolerite used in this study indicate that this group of intrusions is at least 1600 m.y. old. This would indicate that the Umkondo dolerites and therefore the Umkondo system itself are at least this old.