Windblown Hadean Zircons Derived By Erosion Of Impact-Generated 3.3 Ga Uplifts, Barberton Greenstone Belt, South Africa

In the absence of a rock record older than about 4.03 Ga, Hadean (>4.0 billion-years-old) zircons provide a unique window into crustal development and surface conditions on the Earth within a few hundred million years of its formation. Recently discovered Hadean detrital zircons, 4.0-4.2 Ga, in the Barberton Greenstone Belt, South Africa, occur in a 3 to 5 m thick layer termed the Green Sandstone Bed (GSB) that is composed largely of mafic to komatiitic volcaniclastic debris and trace amounts of sand-sized quartz, chromite, and zircon. The youngest concordant zircons in the GSB and ages of zircons in immediately underlying strata suggest a depositional age of about 3.306 +/- 6 Ga. Hadean zircons are concentrated in the lower 200 cm of the unit: higher parts of the GSB lack Hadean zircons and the uppermost 150 cm contain zircons 3.312 +/- 5 Ga. The quartz and heavy minerals in the lowest 200 cm of the GSB appear to have been transported as windblown sediments in a subaerial environment. They were subsequently incorporated into mass flows represented locally by the lowest 200 cm of the GSB. The bulk of the overlying GSB sediments were worked and deposited in wave- and current-active, shallow-marine to intertidal settings. Cathodoluminescence studies suggest that a high proportion of the detrital GSB quartz is of plutonic origin although the GSB is underlain by 10-12 km of volcanic and volcaniclastic silicified sedimentary rocks with no evidence of uplift or terrigenous sediment sources. The GSB lies <2 m above impact layer S6 and we infer that much of the windblown sediment was derived by erosion of crustal rocks that were uplifted and emplaced at the surface during the S6 impact and of komatiites exposed subaerially between the impact and depositional sites.