CONTINUITY BETWEEN EASTERN AND WESTERN BUSHVELD COMPLEX, SOUTH AFRICA, CONFIRMED BY XENOLITHS FROM KIMBERLITE

The eastern and western limbs of the Bushveld Complex, South Africa have been interpreted by some as dipping sheets of mafic cumulate rocks ever since the modeling of gravity and geoelectrical data by Meyer & de Beer (Nature 325, 610, 1987). However, re-interpretation of the regional gravity data by Cawthorn & Webb (Tectonophys. 330, 195, 2001), with consideration of isostatic crustal flexure and depressing of the Moho, allows Bushveld to be modeled as a single, connected body. This is consistent with the anomalously thick sub-Bushveld crust (to 48 km) as revealed by seismic data (Nguuri et al, Geophys. Res. Lett 28, 2501, 2001). Here we provide direct evidence from xenoliths in kimberlite for a regionally interconnected Bushveld Complex, implying its emplacement as a single sill-like body ~400 km across and ~8 km thick. The Cretaceous Palmietgat Group 1 kimberlites, located mid-way between the eastern and western lobes, about 70 km N of Pretoria, form a cluster of 6 pipes linked by dikes, over a distance of ~3 km. The K15W pipe is actively mined for diamonds. We recovered 5 small (~4 cm across) xenoliths of pyroxenitic rocks in the waste pile representing the high density portion of crushed material that was rejected for diamond recovery by a Sortex apparatus. The xenoliths are medium-grained orthopyroxene cumulates (80-90% opx) with interstitial zoned plagioclase (8-15%), clinopyroxene (<1-2%) and phlogopite. One sample contains ~2% of small, euhedral chromite grains. Orthopyroxenes have mg# = 75-80, with Al2O3 = 0.4-0.6 wt%, Cr2O3 = 0.4-0.5 wt% and TiO2 = 0.19-0.22 wt% and clinopyroxenes have mg# = 82-85. Disseminated chromites show a limited compositional range, with average Fe3+:Al:Cr = 0.55:0.25:1.03 and mg# = 12.6. These compositions match well with Bushveld Complex cumulate rocks, particularly with those from the Upper Critical Zone, which shows orthopyroxenes with mg# = 78-82, and similar Ti, Al and Cr concentrations to those measured here. Likewise, Upper Critical Zone chromites have similar trivalent chemistry and mg# to our measured xenoliths from Palmietgat. We suggest, therefore, that the recovered xenoliths provide strong evidence that significant thicknesses of Bushveld rocks occur at depth between the eastern and western lobes, hence supporting connectivity of the Complex, at least at the level of the Upper Critical Zone. The presence of a prominent gravity high near the Palmietgat kimberlites may indicate that high-density Bushveld cumulate rocks reside at shallow depths (~1-3 km?) due to diapiric doming or folding of the floor. In any case, our results imply that the Bushveld Complex must be perceived as a saucer-shaped body of layered mafic cumulates that is continuous over at least 400 km in horizontal extent, and can be confirmed as the largest surviving layered mafic intrusion on Earth.