Paleomagnetism and age of the Archean Usushwana Complex, southern Africa

Abstract

Paleomagnetic and 40 Ar/ 39 Ar age data are reported from the Usushwana Complex, a layered gabbro and pyroxenite intrusive exposed in Swaziland and eastern South Africa (26.5°S, 31.0° E). The main gabbro body and an associated granophyre return a mean Rb‐Sr whole‐rock and Sm‐Nd mineral age of 2875±40 Ma. Alternating field and thermal analysis of specimens from 18 sites (70 samples) yields a mean direction D = 304°, I = 53°, ( k = 32, α 95 = 6°). This direction is significantly different from that obtained from the ∼ 2100 Ma Bushveld Complex. Rare dikes of probable Bushveld age cut the Usushwana Complex, and a low coercivity overprint seen in specimens from a few sites has a Bushveld direction and almost certainly reflects this event. Thermal overprinting is recorded by 40 Ar/ 39 Ar hornblende and pyroxene ages between 2094±54 Ma and 2386±58 Ma that we interpret to have occurred at low temperature but over a significant period of time. Isothermal remanent magnetization acquisition experiments and laboratory unblocking temperatures suggest that the primary remanence carrier is single domain magnetite. The paleomagnetic pole position calculated from the Usushwana Complex (9° N, 347° E) is significantly different from other late Archean poles. Although the Usushwana pole is close to the pole from the Palabora Igneous Complex of South Africa (1900 Ma), there is no isotopic evidence indicating a thermal event of that age in the Usushwana Complex.