Provenance and tectonic implications of the 3.28–3.23 Ga Fig Tree Group, central Barberton greenstone belt, South Africa

Abstract

Sedimentary rocks provide a valuable record of non-uniformitarian Paleoarchean tectonic processes. The rocks of the 3.28–3.23 Ga Mapepe Formation mark the initiation of orogenesis in the Barberton Greenstone Belt, South Africa. However, reconstruction of the Mapepe basin(s) is challenging because the rocks occur in fault-bounded structural belts that are characterized by lateral heterogeneity and whose correlation remains uncertain. We have studied the petrography, geochemistry, and detrital zircon geochronology from outcrop and drill core samples from two adjacent structural belts, the Manzimnyama Syncline and the Eastern Barite Valley area, to evaluate the timing of basin formation, correlate between these two adjacent areas, and provide constraints on the character of tectonic uplift. In the Eastern Barite Valley area sedimentation was initiated at about 3260 Ma. The section includes a lower, deep-water unit of mainly mudstone overlain by a shoaling-upward sequence of fan-delta sandstone and conglomerate. The sandstone throughout the formation was derived by erosion of local uplifts of the underlying greenstone-belt sequence except at the top of the section where sedimentation was dominated by dacitic volcaniclastic sediments dated at 3239 Ma. In contrast, rocks of the Mapepe Formation in the Manzimnyama Syncline area, 1–2 km south of the Eastern Barite Valley area, record the initiation of sedimentation at about 3277 Ma with deposition of a rhyolitic tuffaceous unit followed by persistent deep-water sedimentation of banded iron formation, banded ferruginous chert, and turbiditic sandstone. Siliciclastic sediments are immature and were derived by erosion of older supracrustal greenstone-belt rocks. No Fig Tree age volcanism younger than 3260 Ma is recorded in this belt. Overall, Mapepe sedimentary rocks record the formation of numerous local uplifts of underlying supracrustal rocks with periodic volcanic activity, and the dispersal of debris to form mainly small fan-deltas flanked by deep-water systems. The contrasting ages, lithologies, depositional settings, and provenance of the sedimentary rocks in the now adjacent Eastern Barite Valley and Manzimnyama Syncline belts suggest that they were deposited in either different parts of a large, complex Fig Tree basin or represent separate basins that have been juxtaposed during post-Fig Tree tectonism and shortening.