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Contact relations between the Koodoovale- and Manjeri-type lithostratigraphic units of the late Archaean Bulawayan Supergroup at Hunters Road, central Zimbabwe

1, Carson Close, Greendale, Harare, Zimbabwe Present address: Guesachan, Shielhill Road, Northmuir, Kirriemuir, Angus DD8 4PA, Scotland, U.K., email: marprend{at}hotmail.com

Central to current controversies concerning the tectonic origin of the late Archaean Bulawayan Supergroup in the Zimbabwe craton is the nature of the basal Upper Bulawayan Manjeri-type lithostratigraphic unit (exclusively sedimentary or at least partly tectonic?) in the north and west parts of the craton and its structural relationship (conformable or unconformable?) to the underlying Lower Bulawayan felsic Koodoovale-type lithostratigraphic unit. A ca 265m-thick composite stratigraphic drill-section at the Hunters Road nickel deposit in central Zimbabwe provides critical new information on the Koodoovale–Manjeri contact zone in the west part of the craton. Largely pristine, with often well preserved primary mineralogy and textures, the lithologies in this section comprise felsic (rhyolitic to rhyodacitic) lavas and volcaniclastic rocks of the Koodoovale unit and volcaniclastic rocks and mixed facies iron formations of the Manjeri unit. No significant layer-parallel shearing occurs either within or at the upper and lower contacts of the Manjeri unit. The volcaniclastic rocks of both units are lithologically very similar and predominantly made up of non-welded, angular to subangular, cognate lithic and crystal fragments of limited compositional range and pumiceous glass shards in cyclically repeated, sharply separate, normal size-graded units from a few centimetres to many metres thick. Coarse- and fine-grained lithofacies can be distinguished, the fine lithofacies containing chert and oxide laminae and dominating the lower Manjeri unit. These rocks are interpreted as resedimented syn-eruptive volcaniclastic deposits formed during shallow water pyroclastic eruptions and transported and laid down in proximal and moderately deep water distal marine environments by mass flow and suspension sedimentation processes. At Hunters Road, the Manjeri unit is evidently exclusively sedimentary (non-tectonic) in origin and the Koodoovale–Manjeri contact essentially gradational and conformable with no depositional break. Contrary to some previous views, the volcaniclastic and chemical sedimentary Manjeri unit in the north and west represents not a basal Upper Bulawayan thrust zone but the terminal exhalative phase of Koodoovale felsic volcanism and the basinal facies of craton-wide Manjeri sedimentation. Thus, Koodoovale felsic volcanism was part of, and not separate from, the Bulawayan tectonomagmatic event as a whole.