Zircon geochronology of late archean komatiitic sills and their felsic country rocks, south-central Zimbabwe: A revised age for the reliance komatiitic event and its implications

The 2.8-2.6 Ga Bulawayan Supergroup of the Zimbabwe craton is inferred to be a passive margin- and plume-related conformable sequence comprising (in part) the felsic volcanic Koodoovale→sedimentary Manjeri→komatiitic Reliance→basaltic Zeederbergs units. The Reliance unit is spatially associated with subjacent komatiitic sills of the Mashaba Ultramafic Suite, the inferred subvolcanic intrusive phase of the Reliance flow-fields. We report U-Pb zircon ages for several units. The Pangani sill, in the Filabusi belt, yielded an age of 2912 Ma, indicating it may belong to the 3.0-2.9 Ga Belingwean Supergroup. The Koodoovale unit appears to range in age from 2866 to 2788 Ma. Inferred syn-eruptive mass flow deposits within the Manjeri basinal shallow-water sub-facies in the Midlands SE belt yield ages of 2877 and 2870 Ma. Sills of the Mashaba Ultramafic Suite, together with a previous result of 2743 Ma for the Mashaba Igneous Complex, yield a mean age of 2746 Ma, which we propose as a proxy age for the Reliance unit and for the Reliance komatiitic event as a whole. The 2.80 Ga age for the top of the Koodoovale unit in the western Filabusi and GweruNWbelts confirms (with greater stratigraphic control) earlier precise geochronology and provides an age for the start of sedimentation in the overlying Manjeri basinal deep-water sub-facies. The 2.79 Ga felsic sample from within the Reliance unit in the Shangani N belt is likely from a Koodoovale-derived, post-eruptive mass flow deposit. In the Midlands SE belt, the new felsic rock ages support the conformable relationship between the uppermost Koodoovale unit and overlying Manjeri unit but place the transition at c. 2.87 Ga, significantly earlier than in belts further west. The new ages for the Koodoovale unit and the revised age for the Reliance unit introduce new perspectives on the development of the Bulawayan Supergroup: (1) The Koodoovale-Manjeri transition is diachronous across the Zimbabwe craton, (2) the duration of Manjeri sedimentation varied from 50Ma to 120Ma in the basinal deep-water and shallow-water sub-facies, respectively, to 80Ma in the platformal facies in the east where sedimentation is estimated to have begun at 2.83 Ga; such prolonged sedimentation is controversial but can be explained by extended periods of non-deposition in the basinal deep-water sub-facies, (3) using all published precise ages, Reliance-Zeederbergs volcanism may have lasted 70Ma (2.75-2.68 Ga), much longer than Phanerozoic plume systems, and (4) the previously proposed correlation and late Archean 'nearest neighbour' relationship between the Zimbabwe and Slave cratons are provided further support. © 2012 Published by Elsevier B.V.