The non-glacial and non-cratonic origin of an early Archean felsic volcaniclastic unit, Barberton Greenstone Belt, South Africa

Abstract

Recent studies have suggested that ~3.445 Ga felsic volcaniclastic rocks of member H6 of the Hooggeneoeg Formation in the Barberton Greenstone Belt, South Africa, show evidence for deposition above a major erosional unconformity under glacial to periglacial conditions and contain debris eroded from older, granitic continental crust. These conclusions represent fundamental re-interpretations of this sequence, its crustal setting, and processes of deposition. We here present evidence that these felsic strata represent terrestrial to shallow water mass flows deposits, storm-influenced shallow-marine units, and turbidity-current-like, storm-driven submarine flows, all of which lack any evidence of a glacial influence during sedimentation. They accumulated as part of a conformable stratigraphic sequence and show no evidence of mantling an erosional surface. The debris consists entirely of volcanic-derived materials. No clasts of plutonic rock are found in the conglomerates. Low-temperature diagenesis and potash metasomatism have altered primary volcanic feldspar in the volcaniclastic debris to tartan twinned microcline, previously thought to be characteristic of granitic plutonic rocks. This potassium feldspar is quite distinctive and unlike the microcline found in plutons surrounding the greenstone belt. This succession is part of a classic felsic volcaniclastic sequence deposited as an apron flanking a major felsic volcanic center.