The Late Archaean Melita Complex, Eastern Goldfields, Western Australia: shallow submarine bimodal volcanism in a rifted arc environment

Abstract

The Melita Volcanic Complex is a Late Archaean bimodal rhyolite/basalt volcanic succession within the Gindalbie Terrane in the Eastern Goldfields Province of the Yilgarn Craton. The Melita Complex has been dated by ion probe at 2683±3 Ma (95%) and forms part of a distinctive 2681–2692-Ma volcanic association that records bimodal (basalt/rhyolite) and calc–alkaline intermediate-silicic volcanism at several discrete volcanic centres, and which locally hosts volcanic massive sulphide mineralisation (Teutonic Bore). Approximately 3 km of stratigraphic thickness is exposed in the Melita area. The upper 1–1.5 km of the exposed succession is dominated by subaqueously resedimented volcaniclastic sandstones and breccias, rhyolite flows and sills. Primary subaerial pyroclastic deposits including ignimbrites have not been identified in this study, although subaerial explosive activity is indicated by the occurrence of accretionary lapilli and the abundance of vitric material (shards) and pumice fragments in resedimented deposits. The lower part of the succession is dominated by pillowed to massive basalt lavas, and in situ and resedimented mafic hyaloclastites. Mafic extrusive and intrusive rocks are tholeiitic with trace element concentrations similar to modern arc tholeiites. Felsic volcanic rocks at Melita are dacite to high-silica rhyolite. They are highly enriched in incompatible elements (particularly high field strength element-enriched), compared to other felsic associations in the Eastern Goldfields Province, representing evolved partial melts of heterogeneous intermediate arc-type crust. The volcanic facies and geochemistry of volcanic rocks at Melita are consistent with those observed in modern intra-arc or arc-rift settings, and the succession is interpreted to represent the initial stages of back-arc rifting within a complex convergent margin.