Cool greenstone drips and the role of partial convective overturn in Barberton greenstone belt evolution

A thick, upward-younging stratigraphy and presence of crustal contamination in even the oldest rocks of the Barberton greenstone belt precludes crust formation through tectonic stacking of oceanic lithosphere above a subduction zone. Rather, structural and geochronological evidence for simultaneous formation of dome-and-keel structure in the footwall of the belt, greenstone-down extensional shearing and radially inward-plunging lobe–cusp folds around the belt margins, and recumbent folds and thrusts in the core of the belt is uniquely explained by sinking of thick, dense greenstones into partially molten granitic middle crust during partial convective overturn at 3.26–3.22 Ga. High-pressure (P), moderate-temperature (T) metamorphism in vertically-lineated greenstone septae around granite domes contrasts with moderate P–T assemblages in the cores of the same domes in the footwall, reflecting initial sinking of cool greenstone drips from the base of the belt. These data suggest crust formation as a long-lived volcanic plateau affected by intracrustal modification.

► Re-assessment of geological data from Barberton greenstone belt.
► Subduction–accretion did not contribute to crust formation.
► Structural geology and metamorphism result from partial convective overturn.
► Crust formed as a thick volcanic plateau, modified by intracrustal melting.