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 f rom hypabyssal porphyries and tonalites has enabled the timing of the major collisional episode of the Barberton greenstone belt to be determined. Thi s 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 M a, 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 fo lds dominating the map pattern in the Weltevreden study area. Late D-2 faul ts, such as the Moodies Fault, that define the boundaries of the major stru ctural domains converge near the Saddleback Fault, a major boundary within the Barberton greenstone belt. Strain calculations suggest that plane strai n was dominant, with at least 60% shortening occurring during the D-2 defor mational 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-earl y), through to major collisional and post-amalgamation events (D-2-late), a ll of which spanned similar to 3 million years. (C) 2000 Elsevier Science L imited. All rights reserved.