An Archaean arc-arc collisional event:: a short-lived (ca 3 Myr) episode, Weltevreden area, Barberton greenstone belt, South Africa

Detailed mapping, structural analysis and high precision U-Pb zircon ages from hypabyssal porphyries and tonalites has enabled the timing of the major collisional episode of the Barberton greenstone belt to be determined. This episode, known as the D-2 deformation, can be divided into D-2-early and D-2-late. D-2-early deformation, bracketed as having occurred between 3229 and 3227 Ma, was typified by isoclinal folding (F-2) of shares and associated cherts (Fig Tree Group), but not of the overlying quartzose elastic rocks (Moodies Group). D-2-late deformation was at 3226 Ma, or later, with large F-3 open-tight folds dominating the map pattern in the Weltevreden study area. Late D-2 faults, such as the Moodies Fault, that define the boundaries of the major structural domains converge near the Saddleback Fault, a major boundary within the Barberton greenstone belt. Strain calculations suggest that plane strain was dominant, with at least 60% shortening occurring during the D-2 deformational events. Although an early and a late stage of D-2 deformation can be distinguished in the field, precise U-Pb zircon ages indicate that these events represent a continuum, ranging from the onset of an arc-arc assembly event (D-2-early), through to major collisional and post-amalgamation events (D-2-late), all of which spanned similar to 3 million years. (C) 2000 Elsevier Science Limited. All rights reserved.