PRELIMINARY STUDIES OF COEXISTING MICRON-SCALE ZIRCON AND BADDELEYITE IN SILICEOUS ROCKS FROM THE BASAL ROOIBERG GROUP, BUSHVELD COMPLEX, SOUTH AFRICA

Rhodes (1975) cited evidence that the ≤5 km siliceous Rooiberg Group of the ~2.6 Ga Bushveld Complex, widely accepted as an epicrustal volcanic succession, originated as an impact melt. This proposal was widely rejected because French (1990) found no evidence for shock. This study revisits the impact hypothesis with focus on Zr-bearing phases in the basal ~130 m of the Rooiberg Group (for setting, see Elston and Tegtmeier, this session). The rocks contain ~75 wt % SiO₂ (in rhyolite range) and 200-300 ppm Zr (Schweitzer, unpubl. Pretoria PhD diss. 1998), but lack optically resolvable and extractable zircon. Characteristics which suggest quenching from abnormally high temperature include unusual quench textures and an absence of phenocrysts.

WDS Zr X-ray maps of samples from the basal Rooiberg, obtained by EM, located numerous <10 µm, unzoned, unshocked, and subhedral-anhedral zircon grains. Of 50 grains examined in ~2 cm² of sample, 8 were irregularly shaped, 3-8 µm long, and composed of intergrown zircon and baddeleyite (ZrO₂). A FEGSEM and FIB-TEM investigation of one of these composite grains has revealed several distinct subgrains of ZrO₂ with the same crystallographic orientation, ± 2-3°. Dark field STEM imaging shows that the ZrO₂ subgrains are irregularly-shaped, and are either fully or partially enclosed by metamict zircon, suggesting that zircon partially replaced the ZrO₂. EM analyses found > 2,000 ppm U in zircon, accounting for its metamict state in a > 2 Ga rock (Ewing et al. 2000).

Zircon-ZrO₂ associations have been cited as evidence for “impact induced high temperatures” (French and Koeberl, 2010). In all previous reports of this association in impactites, zircon is the primary phase, inherited from the target rock and dissociated to ZrO₂ and SiO₂ at ~1680°C. These preliminary results suggest that ZrO₂ is the primary phase in basal Rooiberg. Zircon may have replaced ZrO₂ by reaction, T ≤1680°C, or by later alteration. If confirmed, this would be the first example of primary ZrO₂ crystallized from a SiO₂-oversaturated melt at T >1680°C. All of our conclusions are preliminary. Ongoing work will examine additional grains, determine ZrO₂ crystal structure, and test alternative interpretations. For example, there is a possibility that the ZrO₂ was inherited, but, unlike zircon, ZrO₂ is a rare accessory phase.