Mesoproterozoic oceanic subduction, island-arc formation and the initiation of back-arc spreading in the Kibaran Belt of central, southern Africa: evidence from the Ophiolite Terrane, Chewore Inliers, northern Zimbabwe

Abstract

The Ophiolite Terrane of the Southern Chewore Inliers is comprised of two related, but lithologically- and geochemically-distinct groups. The Maunde Ophiolite Group comprises a suite of lithologies similar to those within Phanerozoic ophiolites. The Kaourera Island Arc Group contains a suite of silica-variable, extrusive lithologies similar to those within present-day island-arcs. Geochemical analyses (HFSE and REE) of meta-basalts from the Maunde Ophiolite Group coupled with the ophiolite-type lithostratigraphy, indicate that this group represents an ophiolite which formed as part of an immature, back-arc marginal basin (the Chewore Ophiolite). Meta-basalts of the Kaourera Island Arc Group predominantly display tholeiitic, island-arc geochemical signatures, indicating that this group represents an associated island-arc (the Kaourera Arc); however, some meta-basalts display oceanic within-plate geochemistries. These oceanic within-plate meta-basalts are interleaved with both arc-lavas and marginal basin-lavas and are interpreted to result either from the interleaving of a within-plate seamount with the arc/marginal basin during accretion/Pan African tectonism or from the contamination of the arc-/marginal basin-lavas by the subduction and dehydration of a within-plate seamount under the arc/marginal basin. The Chewore Ophiolite has been dated at 1393±22 Ma and is at present the oldest dated ophiolite (senso stricto) in Africa. The Mesoproterozoic age of this marginal basin and island-arc complex make this the first description of recognised oceanic-type crust of this age within the Kibaran orogenic system of central, southern Africa. The location of the Ophiolite Terrane (on the northern margin of the Zimbabwe Craton and between that of the Congo Craton), indicates that Kibaran-aged crust extends further south than had previously been indicated by other authors and that a regional scenario at this time must involve ocean–ocean collision, oceanic subduction, the formation of an island-arc and the initiation of juvenile back-arc basin spreading. At 1393 Ma the two cratons must have therefore been separated by an ocean (the Chewore Ocean) of unknown extent.