Rapid emplacement of the one of the world's greatest continental magmatic provinces - precise age constraints on the Bushveld Complex

Richard A. Armstrong 1 , Sandra Kamo 2 , R.E. Harmer 3

1 Research School of Earth Sciences. The Australian National University , Canberra 0200, Australia
2 Jack Satterly Geochronology Laboratory, Department of Geology, University of Toronto , Toronto , Ontario , M5S 3B1 , Canada
3 African Platinum, Building 4 (Greystone), Fourways Golf Park , 2 Roos Street , Fourways 2191, South Africa .

The Bushveld Complex ( sensu lato ) in southern Africa represents one of the largest examples of continental magmatism on Earth. If the precursor felsic volcanic rocks of the Rooiberg Group are included, the volume of magma generated is estimated to be up to a staggering 1,000,000 km 3 . The layered mafic phase of the Bushveld Complex is the largest such igneous intrusion on Earth, with estimates of magma volume as high as 400,000 km 3 . Geophysical evidence also shows that the mantle beneath the Kaapvaal Craton still records a possible Bushveld-related seismic anomaly today (Fouch, et al., 2004). The timing and the origin of the various components of the complex are, however, poorly constrained with some work suggesting thermal activity could have continued for up to a billion years (McNaughton, et al., 1993). This research is aimed at determining a precise chronology of events covering the entire history of the Bushveld Complex, using combined TIMS and SHRIMP zircon U-Pb dating, with the aim at establishing some constraints on the possible origin of this massive and economically important event.

In the broadest sense, the Bushveld Complex is generally considered to include the intermediate to felsic volcanic rocks of the Rooiberg Group, the mafic layered rocks of the Rustenburg Layered Suite, the felsic intrusive rocks of the Lebowa Granite Suite, plus the enigmatic Rashoop Granophyres. Representatives of all these major components were sampled for dating (note that some of the dates quoted below are still subject to a final assessment once the last analyses are completed, but it is anticipated that this should not affect the absolute ages, but might affect the final precision quoted). TIMS dating of felsites of the roofing Rooiberg Group show that these precursors to the main phase of the Bushveld Complex were emplaced 2059.9 ± 1 Ma ago. This is significantly prior to intrusion of the main phase of the complex – as shown by dates obtained on zircons from the famous PGE-bearing Merensky Reef, and from a late-stage basic pegmatoid. These gave statistically identical ages of 2055.3 ± 1.2 Ma and 2056.3 ± 0.7 Ma respectively. Dating of granites of the Lebowa Suite that demonstrably intrude and post-date the mafic rocks, shows they were intruded at 2054 ± 2 Ma (a mean of several dates obtained on a variety of granites from this suite). A date of 2054 ± 4 Ma recently published by Dorland et al., 2006 on a rhyolite within the overlying sedimentary sequence of the Waterberg Group shows that the Bushveld Complex had cooled and had undergone significant erosion short time after intrusion of the mafic phase.

This high-precision geochronological study established for the first time that the whole event occurred over a very short time interval of approximately 4 Ma. The emplacement of the mafic Rustenburg Layered Complex and the felsic Lebowa Granite Complex was within 1-2 Ma, a time interval similar to that measured for large igneous volcanic provinces such as the Karoo or Deccan . Extensive recent geochronological investigations of large parts of southern Africa have shown that Bushveld-aged igneous rocks occur over a vast region of the subcontinent. These are currently the focus of a larger study aimed at discovering the full areal extent of rocks of this age and to establishing a possible causal link between the Bushveld Complex and a larger regional event. It certainly seems probable that the Bushveld Complex, unique as it is, was a part of some larger “Bushveld event”, rather than an isolated igneous event of unknown origin. Certainly some origins can now be discounted – it is unlikely that an extraterrestrial (impact) origin can be reconciled with the distinctly different ages now established for the Rooiberg and Bushveld events. The origin of this enormous and economically important complex is still uncertain, but the rapid emplacement and erosion does provide some clues. Further research on the age of the late-stage cassiterite-bearing granites associated with the Bushveld Complex is in progress.

References: Dorland, H.C., Beukes , N.J. , Gutzmer, J., Evans, D.A.D and Armstrong, R.A. (2006) Precise SHRIMP U-Pb zircon age constraints on the lower Waterberg and Soutpansberg Groups, South Africa . South African Journal of Geology , 109, 139-156.

Fouch, M.J., James, D.E., Vandecar, J.C., van der Lee, S and Kaapvaal Seismic Group (2004). Mantle seismic structure beneath the Kaapvaal and Zimbabwe Cratons. South African Journal of Geology , 107, 33-44.

McNaughton, N.J,, Pollard. P.J., Groves , D.I. and Taylor , R.G. (1993). A long-lived hydrothermal system in Bushveld Granites at the Zaaiplaats Tin Mine: lead isotope evidence. Economic Geology , 88. 27-43.