Cambrian backarc-basin basalt in western Victoria related to evolution of a continent-dipping subduction zone

Abstract

Lower Cambrian basalts in the Stawell Zone (e.g. Magdala Basalt) are intercalated with a thick package of continentally derived quartz-rich turbidites, whereas similar basalts in the Heathcote and Mt Wellington Greenstone Belts are underlain by broadly coeval boninites and overlain by Upper Cambrian cherts. These marked differences in the magmatic – stratigraphic associations of the Early Cambrian basalts in Victoria have led to highly varied interpretations of their tectonic setting, which include subduction zones that are east-dipping, west-dipping and even a combination of east- and west-dipping systems. Here, we present new geochemical data for Cambrian basalts from the Stawell Zone and reassess the palaeogeographic and palaeotectonic setting of the region. Our geochemical data show that basalts in the Stawell Zone may be broadly divided into two units: basalts that are LREE-depleted relative to N-MORB and have low (≤3.0 ppm) Nb contents; and basalts that are slightly LREE-enriched relative to N-MORB and have relatively high (>3.0 ppm) Nb contents. The LREE-depleted Nb-poor basalts are the most abundant type in the Stawell Zone and generally occur at lower stratigraphic levels than the Nb-rich basalts. While the slightly LREE-enriched basalts are most similar to the backarc-basin basalts in the Heathcote and Mt Wellington Greenstone Belts, all basalts from the Stawell Zone display characteristics similar to modern backarc-basin basalts from the West Pacific. We propose that basaltic magmatism in the Stawell Zone was generated during a widespread backarc extension event that affected much of western Victoria. However, important stratigraphic – magmatic associations in the Stawell Zone, including the absence of forearc-generated boninites and the presence of a thick (>300 m) package of quartz-rich sandstone and mudstone underlying the Early Cambrian basalts, indicate that the Stawell Zone was relatively close to (or possibly at the outer margins of) the East Gondwanan continental margin and thus always distal to the forearc (cf. Heathcote and Mt Wellington Greenstone Belts). These characteristics indicate that basaltic magmatism in the Stawell Zone occurred in the backarc of a west-dipping subduction zone that initiated (i.e. generated) boninites farther east and southeast before undergoing rapid east-directed rollback.