Studies of the Rooiberg Group, Bushveld Complex, South Africa: No evidence for an impact origin

It has been suggested that the Bushveld Complex of South Africa could be the result of multiple large meteorite or comet impacts. According to this hypothesis, part of the lower Rooiberg Group, which forms the roof of the Complex, represents a sheet of impact melt breccia and other impact breccias. The present study is an attempt to test the viability of the impact hypothesis for the Bushveld Complex by interpreting newly acquired field, geochemical, petrographic, and textural data for Rooiberg Group and associated rocks. Extensive field work throughout the Rooiberg Group and, particularly, at the contact between this unit and the underlying Pretoria Group metasediments has failed to identify any material that could be interpreted as impact-related. The Rooiberg Group is predominantly composed of individual volcanic flows and pyroclastic units representing several geochemically distinct magma types. These volcanic units are interbedded with thin, laterally extensive, sedimentary units, a few of which are sedimentary breccias. The presence of needles of quartz that may represent paramorphs after tridymite in some Rooiberg Group units has been used as evidence to support the contention that these rocks represent superheated impact melt. However, quartz paramorphs after tridymite have been recognized in terrestrial volcanic provinces (e.g., the North Shore Volcanic Group in northeastern Minnesota). Structural data, including dips of Rooiberg Group strata, suggest that the lobate shape of the Complex, which resembles several closely-spaced ring features, is the result of post-Rooiberg Group deformation. Microdeformation features in quartz from Bushveld-related rocks do not satisfy the criteria of shock metamorphic planar deformation features (PDFs) which would be characteristic of impact-induced shock pressures between ∼10 and ∼30 GPa. These data, especially the absence of macroscopic and microscopic evidence of shock deformation in pre-Bushveld rocks, are inconsistent with formation of the Bushveld Complex by impact.