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 falls 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 SiO 2 values between 50 and 53 wt%; Al 2 O 3 around 15 wt%; elevated Cr and Ni; MgO mostly between 5.5 and 8 wt%; and flat REE-HFSE

more »

... with slight depletion in Th and minor positive and negative Nb anomalies. The High-Th Siliceous Basalt group has high SiO 2 , 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. 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.