The Bulawayan Supergroup: a late Archaean passive margin-related large igneous province in the Zimbabwe craton

The four principal lithostratigraphic units (Koodoovale, Manjeri, Reliance and Zeederbergs) of the widespread, late Archaean Bulawayan Supergroup in the Zimbabwe craton record the early to middle-stage development of a major (c. 250 000 km(2)) volcanosedimentary basin. The craton-wide c. 2.70 Ga Manjeri unit varies systematically from a tens of metres thick, dominantly chemogenic, basinal sediment in gradational, conformable contact with the underlying, several kilometres thick, diachronous, 2.83-2.71 Ga felsic Koodoovale unit in the north and west, to an up to hundreds of metres thick, dominantly siliciclastic, retrogradational, platformal sediment unconformably overlying protocratonic basement in the south and east. The intermittent, c. 2.70 Ga Reliance unit consists of the flow fields of several discrete komatiitic sill-flow complexes scattered throughout the craton and fed by an interlinked system of mid- to upper-crustal 'flow-through' feeder sills. The c. 2.70 Ga Zeederbergs unit comprises a submarine, flood basalt sequence of several kilometres thickness that erupted from ENE-trending, parallel mafic dykes preserved in the south and east. The basalts form a craton-wide blanket to the Reliance flow fields and intervening Manjeri sediments. Certain structural-stratigraphic features rule out primary construction of the Bulawayan Supergroup by early lateral accretion as proposed by several previous workers. Instead, evolution of the Bulawayan basin can best be explained by vertical accretion involving (1) abundant, episodic and bimodal, plume-related volcanism and (2) extension and rapid subsidence of continental lithosphere. Many aspects of the Bulawayan Supergroup provide important insights into Archaean tectonics, mantle plume activity, magma generation and lithospheric plumbing.