The palaeomagnetism of some Upper Cretaceous kimberlite occurrences in South Africa

Palaeomagnetic results are reported from the De Beers, DuToitspan and Wesselton kimberlite pipes (86 ± 3Ma) at Kimberley and from the Finsch and Koffyfontein pipes. The latter are both less than 150 km from Kimberley and although undated are geologically correlated with the Kimberley pipes. Eighty blocks, oriented without recourse to magnetic methods, have been collected from the five pipes. The samples comprise kimberlite, accidental inclusions of widely varying lithologies and wall rock thus enabling inclusion and baked contact tests to be performed. Extensive alternating field and thermal demagnetization experiments show that the magnetization of the kimberlite is primary. Secular variation is not averaged out in any one pipe (with the possible exception of Finsch) and it is suggested the three Kimberley pipes were emplaced simultaneously. The mean pole position of the five pipes (57.2°E, 58.2°S withK = 25.8 andA₉₅ = 15.3°) is believed to be a good estimate of the palaeomagnetic pole86 ± 3Ma ago. It is now possible to state that the African Mesozoic palaeomagnetic pole remained essentially in the same position until at least86 ± 3Ma ago.