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 oth er impact breccias. The present study is an attempt to test the viabil ity 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 identif y any material that could be interpreted as impact-related. The Rooibe rg Group is predominantly composed of individual volcanic flows and py roclastic units representing several geochemically distinct magma type s. These volcanic units are interbedded with thin, laterally extensive , sedimentary units, a few of which are sedimentary breccias. The pres ence of needles of quartz that map represent paramorphs after tridymit e in some Rooiberg Group units has been used as evidence to support th e contention that these rocks represent superheated impact melt. Howev er, quartz paramorphs after tridymite have been recognized in terrestr ial volcanic provinces (e.g., the North Shore Volcanic Group in northe astern Minnesota). Structural data, including dips of Rooiberg Group s trata, suggest that the lobate shape of the Complex, which resembles s everal closely-spaced ring features, is the result of post-Rooiberg Gr oup deformation. Microdeformation features in quartz from Bushveld-rel ated rocks do not satisfy the criteria of shock metamorphic planar def ormation features (PDFs) which would be characteristic of impact-induc ed shock pressures between similar to 10 and similar to 30 GPa. These data, especially the absence of macroscopic and microscopic evidence o f shock deformation in pre-Bushveld rocks, are inconsistent with forma tion of the Bushveld Complex by impact. (C) 1998 Elsevier Science B.V.