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

The Ophiolite Terrane of the Southern Chewore Inliers is comprised of two r elated, but lithologically- and geochemically-distinct groups. The Maunde O phiolite Group comprises a suite of lithologies similar to those within Pha nerozoic ophiolites. The Kaourera Island Are Group contains a suite of sili ca-variable, extrusive lithologies similar to those within present-day isla nd-arcs. Geochemical analyses (HFSE and REE) of meta-basalts from the Maund e Ophiolite Group coupled with the ophiolite-type lithostratigraphy, indica te that this group represents an ophiolite which formed as part of an immat ure, back-are marginal basin (the Chewore Ophiolite). Meta-basalts of the K aourera Island Are Group predominantly display tholeiitic, island-are geoch emical signatures, indicating that this group represents an associated isla nd-are (the Kaourera Arc); however, some meta-basalts display oceanic withi n-plate geochemistries. These oceanic within-plate meta-basalts are interle aved with both are-lavas and marginal basin-lavas and are interpreted to re sult either from the interleaving of a within-plate seamount with the arc/m arginal basin during accretion/Pan African tectonism or from the contaminat ion 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 ophioli te (senso stricto) in Africa. The Mesoproterozoic age of this marginal basi n and island-are complex make this the first description of recognised ocea nic-type crust of this age within the Kibaran orogenic system of central, s outhern Africa. The location of the Ophiolite Terrane (on the northern marg in of the Zimbabwe Craton and between that of the Congo Craton), indicates that Kibaran-aged crust extends further south than had previously been indi cated by other authors and that a regional scenario at this time must invol ve ocean-ocean collision, oceanic subduction, the formation of an island-ar e and the initiation of juvenile back-are basin spreading. At 1393 Ma the t wo cratons must have therefore been separated by an ocean (the Chewore Ocea n) of unknown extent. (C) 2000 Elsevier Science B.V. All rights reserved.