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

Abstract

S U M M A R YWe report palaeomagnetic and 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar age must relate to a thermal disturbance that did not erase the primary magnetic signature.

Late Paleoproterozoic mafic magmatism and the Kalahari craton during Columbia assembly

“…1). Paleoproterozoic magnetizations that pass reversal and baked-contact tests were reported for the BHDS (Letts et al, 2005(Letts et al, , 2011Lubnina et al, 2010) and are confirmed by combined U-Pb geochronology, geochemistry, and paleomagnetic data sets (Olsson et al, 2016;Wabo et al, 2019). Baddeleyite U-Pb crystallization ages of 12 dikes range between ca.…”

Variations in geomagnetic reversal frequency during the Earth's middle age

“…Recent paleomagnetic and magnetostratigraphic results dated to different time intervals of the Late Archean and of the Proterozoic also provided positive reversal tests, which again contradict the persistent or even the temporary occurrence of an asymmetrical geomagnetic field during this period. Among those studies, there are the data obtained from the ∼2.7 Ga flood basalts in the Pilbara craton (Western Australia) [ Strik et al , 2003], from the ∼2.0–1.6 Ga mafic dykes of the eastern Bushveld Complex (South Africa) [ Letts et al , 2005], from red beds in the ∼1.8 Ga Shoksha Formation (Russia) [ Pisarevsky and Sokolov , 2001], from the ∼1.4 Ga Belt‐Purcell Supergroup (North America) [ Elston et al , 2002], from the ∼1.1 Ga Umkondo dolerites in the Kalahari craton (southern Africa) [ Gose et al , 2006] and from the ∼0.8 Ga Aksu dyke swarm (Tarim basin) [ Chen et al , 2004]. We further note that positive reversal tests were also obtained from eastern Canadian formations and carbonatite complexes coeval with the ∼1.1 Ga Keweenawan lava flows [e.g., Costanzo‐Alvarez et al , 1993; Symons , 1994], which argues against a regional consistency of the asymmetrical geomagnetic reversals found in the Keweenawan lavas.…”

The downward continuation of aeromagnetic data from magnetic source ensembles

“…The dykes that produce negative anomalies are remanently magnetized (Letts et al. ). Figure (a) shows the original aeromagnetic data, whereas Fig.…”