Timing of deformation in the Norseman-Wiluna Belt, Yilgarn Craton, Western Australia

Abstract

Establishing relative and absolute time frameworks for the sedimentary, magmatic, tectonic and gold mineralisation events in the Norseman-Wiluna Belt of the Archean Yilgarn Craton of Western Australia, has long been the main aim of research efforts. Recently published constraints on the timing of sedimentation and absolute granite ages have emphasized the shortcomings of the established rationale used for interpreting the timing of deformation events. In this paper the assumptions underlying this rationale are scrutinized, and it is shown that they are the source of significant misinterpretations. A revised time chart for the deformation events of the belt is established. The first shortening phase to affect the belt, D 1 , was preceded by an extensional event D 1e and accompanied by a change from volcanic-dominated to plutonic-dominated magmatism at approximately 2685-2675 Ma. Later extension (D 2e ) controlled deposition of the ca 2655 Ma Kurrawang Sequence and was followed by D 2 , a major shortening event, which folded this sequence. D 2 must therefore have started after 2655 Ma - at least 20 Ma later than previously thought and after the voluminous 2670-2655 Ma high-Ca granite intrusion. Younger transcurrent deformation, D 3 -D 4 , waned at around 2630 Ma, suggesting that the crustal shortening deformation cycle D 2 -D 4 lasted approximately 20-30 Ma, contemporaneous with low-volume 2650-2630 Ma low-Ca granites and alkaline intrusions. Time constraints on gold deposits suggest a late mineralisation event between 2640-2630 Ma. Thus, D 2 -D 4 deformation cycle and late felsic magmatism define a 20-30 Ma long tectonothermal event, which culminated with gold mineralisation. The finding that D 2 folding took place after voluminous high-Ca granite intrusion led to research into the role of competent bodies during folding by means of numerical models. Results suggest that buoyancy-driven doming of pre-tectonic competent bodies trigger growth of antiforms, whereas non-buoyant, competent granite bodies trigger growth of synforms. The conspicuous presence of pre-folding granites in the cores of anticlines may be a result from active buoyancy doming during folding.