Continental setting inferred for emplacement of the 2.9–2.7 Ga Belingwe Greenstone Belt, Zimbabwe

Abstract

Late Archean volcanic rocks of the Belingwe Greenstone Belt have long been known to contain a small continental crustal component evident in their radiogenic isotope and trace element signatures (Chauvel et al., 1993). Recently, we discovered new geochemical evidence suggesting that all volcanic units contain members with a distinctive crustal fingerprint. It was concluded that the volcanic rocks were erupted through pre-existing continental crust, although a specific tectonic interpretation was deliberately avoided. Hofmann and Dirks’ claim that the new data are also compatible with an oceanic deposition, provided that enriched mantle sources were melted or that dismembered continental crust was present in the oceanic lithosphere. We wish to draw the reader’s attention to the fact that when all available evidence is considered, continental emplacement of the rocks is an unavoidable conclusion.In view of this overwhelming body of evidence, we remain unconvinced by Hofmann and Dirks’ suggestion that these greenstones formed in the oceanic realm. We re-emphasize that the existence of a major metamorphic and structural disconformity has not been verified by subsequent studies (Blenkinsop et al., 1993). The validity of inferring horizontal accretion on the basis of sheared contacts alone appears problematic too, since the extent of tectonic displacement cannot be quantified. Even tectonic transport involving large-scale displacement does not preclude extrusion onto pre-existing crust, because interaction of asthenospheric melts with crust could have occurred prior to horizontal accretion.Finally, we refute Hofmann and Dirks’ suggestion that lithological variation provides a more reliable discriminator of tectonic setting than trace element geochemistry. The remarkably coherent geochemical variations between different stratigraphic units require very similar processes of magma genesis. Once magmas are formed, lithological variation can be imposed by a variety of environmental factors that are common to many tectonic settings.