Recently discovered 3.42-3.23 Ga impact layers, Barberton Belt, South Africa: 3.8 Ga detrital zircons, Archean impact history, and tectonic implications

Abstract

The Barberton greenstone belt (BGB) includes eight known layers containing spherical particles (spherules) that condensed from rock vapor clouds formed by the impact of large meteorites or asteroids 3.47–3.23 Ga. Previous studies have inferred that the spherules represent bolides at least 20–70 km across. Spherule beds S1–S4 have been previously characterized in detail: we provide here the first detailed analysis of more recently discovered beds S5–S8. All eight beds are composed of the same basic compositional and textural spherule types, including nearly pure silica spherules representing nonaluminous melt precursors, nearly pure phyllosilicate spherules representing mostly mafic and ultramafic liquids, and compositionally mixed spherules. Evidence of spherule amalgamation and surface corrosion within the rock vapor clouds is developed in some beds. Bed S6, which occurs within a thick sequence of ultramafic volcanic rocks, is overlain by a tsunami layer containing zircons as old as 3811 ± 7 Ma, suggesting deep, possibly impact-related crustal uplift and erosion in distant areas. The formation of at least 8 major impact layers representing bolides 20–70 km in diameter over an interval of ∼240 m.y. suggests impact rates greatly exceeding those of later geologic time, and provides direct evidence that terrestrial bombardment by large bolides did not end abruptly at 3.8 Ga, but waned gradually until 3.0 Ga or even later. The coincidence of at least four large impact layers and the initiation of BGB deformation at 3.26–3.23 Ga suggests that an impact cluster at this time may have disrupted a long-lived earlier geodynamic system and triggered the development of a contrasting, more modern plate tectonic regime.