Palaeomagnetism of the 2054 Ma Bushveld Complex (South Africa): implications for emplacement and cooling

Abstract

SUMMARY The Kaapvaal Craton (South Africa) was the host of several major magmatic events around 2000 Ma, including the Bushveld Complex, the world's largest known layered mafic intrusion (∼0.5–1 × 106 km3). The Bushveld Complex has been the subject of numerous palaeomagnetic studies, which yielded a large spread in palaeomagnetic pole positions for the different Zones, and interpreted to indicate that the Bushveld Complex was emplaced and cooled over a time span of ∼50 million years. New palaeomagnetic data collected from 100 sites (996 drill-cores) from all Zones of the Rustenburg Layered Suite of the Bushveld Complex, yield exceptional palaeomagnetic results with high unblocking (HB) components carried by magnetite. Comparable palaeomagnetic poles from all Zones (mean pole: latitude = 19.2°N, longitude = 030.8°E, A95 = 5.8°) [correction made after online publication 22 September 2009: the mean pole values have been corrected] eliminates the previously noted large spread in poles, and this observation concurs with precise age data that constrain the time period of emplacement of the Bushveld Complex to a few million years at around 2054 Ma. Bedding-corrected HB components from all zones produced better directional groupings, which together with at least seven reversals, strongly points to a primary magnetic signature. This imply that cooling of the Bushveld Complex below the blocking-temperature of magnetite (<580 °C) occurred in a near-horizontal position, and based on the maximum observed reversal rates in the geological history (∼5 Ma−1), we estimate a minimum cooling-interval for the Bushveld Complex of 1.4 million years.