Onverwacht Group, Barberton Greenstone Belt, South Africa: 300 m.y. development of a Paleoarchean stagnant lid

Abstract

The 3.55−3.26 Ga Onverwacht Group, Barberton Greenstone Belt, South Africa and Eswatini, records nearly 300 m.y. of Paleoarchean history dominated by basaltic, komatiitic, and felsic volcanism. It provides the oldest well-preserved record of crustal development and the nature and evolution of associated surface environments, ocean, atmosphere, and biosphere on early Earth. Sedimentary layers within this 10- to 12-km-thick sequence represent a range of proximal to distal sedimentary environments relative to the centers of volcanism. Proximal deposits include coarse felsic breccias, conglomerates, and sandstones and mafic to komatiitic lapillistones that often show evidence for deposition in relatively shallow water. Distal deposits are composed of fine pyroclastic debris, chemical sediments, and biogenic materials deposited under subaqueous conditions during local volcanic quiescence. They show abundant current-produced features and are interpreted to have formed at water depths of a few hundred meters or less under the influence of tidal and/or ocean-circulation currents. Terrigenous clastic sediments formed by the weathering and erosion of older rocks are essentially absent. The Onverwacht Group was deposited under marine conditions on what appears to have been a water world with little evidence of large land areas and no evidence of active tectonism. It is interpreted to represent the upper part of a Paleoarchean stagnant lid overlying but decoupled from an active mantle. The Barberton Greenstone Belt and other Paleoarchean terranes offer a view of this lid over the past 300 m.y. of its development and during the early stages of its fragmentation and disruption.