Palaeomagnetism and 40Ar/39Ar geochronology of mafic dykes from the eastern Bushveld Complex (South Africa)

Details Palaeomagnetism and 40Ar/39Ar geochronology of mafic dykes from the eastern Bushveld Complex (South Africa) Palaeomagnetism and 40Ar/39Ar geochronology of mafic dykes from the eastern Bushveld Complex (South Africa)

Jul 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005geoji.162...36l&link_type=abstract

Geophysical Journal International, Volume 162, Issue 4, pp. 36-48.

Physics

1

Scientific paper

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.

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