Geochemistry and tectonic setting of basalts from the Eastern Goldfields Superterrane

Abstract

A database of more than 650 whole-rock analyses on basaltic rocks from the Eastern Goldfields Superterrane has been compiled from the literature and from public-domain datasets. The data fall into three distinct geochemical categories: a High-Th Siliceous Basalt group, a Low-Th Basalt group and an Intermediate-Th Basalt group. The Low-Th Basalt group shows SiO2 values between 50 and 53 wt%; Al2O3 around 15 wt%; elevated Cr and Ni; MgO mostly between 5.5 and 8 wt%; and flat REE–HFSE patterns with slight depletion in Th and minor positive and negative Nb anomalies. The High-Th Siliceous Basalt group has high SiO2, commonly >54 wt%; MgO between 6 and 9 wt% with higher values reflecting presence of accumulated olivine or pyroxene; low Fe and Ti compared with Low-Th Basalt group; depleted Ni for given Mg#; enriched LREE and Th, combined with strongly negative Nb anomalies and mantle-like Zr/Nb, Nb/Y, Al/Ti and HREE ratios. The Intermediate-Th Basalt group is intermediate between these two end-members in almost all respects. All three groups are represented across the entire Eastern Goldfields Superterrane. The most widespread group, the Low-Th Basalt, is remarkably homogeneous across terranes and domains. It is interpreted as the Archean analogue of plume-head related Large Igneous Province basalts, showing a close match to flood basalts associated with late-stage continental rifting. The High-Th Siliceous Basalt group displays a distinctive geochemical signature, which is evidently unique to Archean greenstone terranes. Derivation by contamination during fractionation of komatiites, probably deep in the crust, and followed by mid-crustal homogenisation in magma chambers, is the most likely hypothesis. Basalts with characteristic island arc signatures such as Nb depletion and low Ni and Cr contents, have not been recorded in the Eastern Goldfields Superterrane. This poses a significant challenge to uniformitarian models, which attempt to explain the evolution of the entire east Yilgarn craton in terms of modern arc accretion tectonics. Distribution of mafic and ultramafic mafic magmatism across the superterrane at ca 2700 Ma can be explained by emplacement of a major driving plume under the ‘lid’ of the Youanmi Craton. The keel of thickened, buoyant lithosphere under the archon diverted the plume head towards the craton margin, where it induced continental rifting. Voluminous eruption of plume-tail komatiite was concentrated and focused through this zone of rifting along the eastern margin of the Youanmi craton in the Kalgoorlie Terrane, while plume-head basalts and less voluminous komatiites were erupted over a much wider area.