Palaeomagnetism and40Ar/39Ar geochronology of mafic dykes from the eastern Bushveld Complex (South Africa)

Abstract

SUMMARY We report palaeomagnetic and 40Ar/39Ar age data for dykes that intrude the ∼2 Ga eastern Bushveld Igneous Complex (BIC). The dykes were previously assumed to be of Karroo age (Jurassic ∼ 180 Ma) based on their NE–SW orientation. Palaeomagnetic data (pole position 8.7°N, 22°E; dp/dm= 18/20.6°), however, clearly demonstrate that these dykes are Precambrian in age, either ∼1.9 Ga and close to the Early Proterozoic Bushveld age, or 1649 ± 10 Ma based on 40Ar/39Ar plagioclase laser fusion ages from one of the dykes. Both normal and reverse polarity dykes are identified, and a positive reversal test together with a semi-conclusive contact test attests to a primary magnetization. If the 40Ar/39Ar age represents a primary cooling age then palaeomagnetic poles from South Africa (Kalahari) at ∼1.9 to ∼2 Ga and ∼1650 Ma are virtually identical, and suggest an apparent polar wander loop; alternatively, the Kalahari Craton drifted from high southerly (>50°) to high northerly latitudes (or vice versa) during this interval. Conversely, if we assign a ∼1.9 Ga age for dyke emplacement as suggested from a comparison with Kalahari palaeomagnetic poles (e.g. Waterberg–Soutpansberg pole), the ∼1650 Ma 40Ar/39Ar age must relate to a thermal disturbance that did not erase the primary magnetic signature.