S U M M A R YThe 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 × 10 6 km 3 ). 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 nearhorizontal 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.
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.
Multiple surveys using a Full Tensor Magnetic Gradient (FTGM) signal instrument from IPHT, have been made in Southern Africa. Traditional Fourier domain and minimum least squares residual of the linear differential tensor relationships have been adapted. The result is a full tensor grid representation of the curvature gradients that is coherent and compliant with the physics at all points. Superior anomaly interpretation regarding the full magnetic history and inferences can then be made. There is more directly inferable structural geology in this tensor signal than can be found in a conventional TMI signal.
Palaeomagnetic data from the well-dated 2060.6±0.5 Ma Phalaborwa Complex in South Africa (Kaapvaal Craton) are of excellent quality. High unblocking components are carried by magnetite and single polarity remanence directions (mean declination 5.0°, inclination 57.3°, α95 = 5.2°) yield a palaeomagnetic pole (latitude 27.7°N, longitude 35.8°E, A95 = 6.6°) that overlaps with existing poles from the near coeval 2054.4±1.3 Ma Bushveld Complex. The Phalaborwa and Bushveld complex poles, along with poles from the well-dated Vredefort impact (2023±4 Ma) and Post-Waterberg Dolerites (1874.6±3.9 Ma), define the most reliable poles for the Kaapvaal Craton during this time interval (c. 2060–1875 Ma) and witness low rates of Mid-Palaeoproterozoic apparent polar wander. Poorly dated NE–NNE-trending dyke swarms that intrude the Phalaborwa and Bushveld complexes both yield dual-polarity remanence components that share a common mean at the 95% confidence level. Primary palaeomagnetic poles (Phalaborwa dykes pole latitude 7.6°, longitude 12.1°, A95 = 11.8°; Bushveld dykes pole latitude 12.6°, longitude 24.1°, A95 = 10.8°) suggest that they are of the same age as the Post-Waterberg dolerites (c. 1875 Ma). They could also be as old as the Phalaborwa and Bushveld Complexes, however; high-precision geochronology is required to resolve this issue and to enlarge the number of Palaeoproterozoic key poles for the Kaapvaal Craton.
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