The palaeomagnetism of 2400 m.y. old rocks from the Australian Pilbara craton and its relation to Archaean-Proterozoic tectonics

Abstract

Palaeomagnetic results are described from 16 sites distributed in two traverses across the Black Range dyke. The dyke, a quartz gabbro dated radiogenically at 2329 ± 89 m.y., is one of the principal members of the NNE trending Black Range Suite which intruded the Archaean assemblage of granite plutons and greenstone belts of the Pilbara craton at around the time of the Archaean-Lower Proterozoic boundary. Results are also presented from 9 sites in the Cajuput dyke. The field evidence is consistent with this dyke having acted as a feeder for the overlying Mount Roe Basalt, the basal member of the Lower Proterozoic Fortescue Group. The Cajuput dyke also trends NNE and is thought to belong to the Black Range Suite. Samples of metamorphosed granite collected in the baked contact zone of the Black Range dyke exhibit various degrees of remagnetization. Directions of magnetization in material from the remelted granite margins are close to the mean direction of magnetization for the dyke. Beyond about 30 m from the contact, the effects of remagnetization are absent. The pole positions computed from site-mean virtual geomagnetic poles for the Black Range dyke and the Cajuput dyke lie at lat. 32°S, long. 154°E (A95 = 9°) and lat. 46°S, long. 146°E (A95 = 22°), respectively. They fall on the Australian apparent polar wander curve drawn for the period pre-2000 m.y. which until now was based primarily on data from the Yilgarn craton. These data suggest that it is improbable that the younger, intervening mobile belt between the Yilgarn and Pilbara cratons was generated through the process of a plate tectonic, continent/continent-type convergence.