Abstract
To assess the causes and extent of isotopic resetting of zircon, U-Pb and Lu-Hf analyses have been performed on zircons from Archean and Bushveld granites as well as Transvaal sediments which have been intruded by magmas of the Bushveld Complex, Pilanesberg Complex, Spitskop Complex, Goudini Complex, and Karoo dolerites in the vicinity of Bushveld Complex using laser ablation multi-collector inductively coupled plasma source mass spectrometry (LA-MC-ICPMS). U-Pb results for zircon analyses from both Archean granites and Transvaal sedimentary rocks indicate that they have undergone partial Pb loss and age resetting. Pb loss was influenced by radiation damage in zircons and metamorphism. Contributions from the Bushveld Complex thermal overprint cannot be excluded, based on evidence from concordia diagrams and 207Pb/206Pb ages; two samples of Magaliesberg sandstones (MEMG 3, 4), closest to the contact with the Marginal Zone, are affected the most. The presence of common Pb and its correction also contributed in shifting the data towards younger 207Pb/206Pb ages. Younger alkaline intrusions as well as Karoo dolerites did not have much influence on zircon signatures. There are only small differences in initial Hf ratios of analyses within the same sample, indicating that the Lu-Hf isotope system remained undisturbed. There are no appreciable effects on trace element concentrations to support that disturbance in U-Pb signatures of zircons is accompanied by their modifications. ..
M.Sc. (Geology)