Using sedimentology and provenance studies to determine depositional relationships between three structural belts of the ca. 3.22 Ga Fig Tree Group, Barberton Greenstone Belt, South Africa

The Barberton Greenstone Belt of South Africa and Swaziland preserves some of the oldest (3.2–3.5 Ga) building blocks of continental crust. The Fig Tree Group (3.26–3.22 Ga) represents the earliest orogenic siliciclastic sedimentation in the belt after the abrupt end of a 300 myr-long period of predominantly mafic to ultramafic volcanism. In the southeastern part of the belt, the Mapepe Formation of the Fig Tree Group outcrops in three parallel northeast-striking synclinal structural belts: from northwest to southeast, the Manzimnyama Syncline, Paulus Syncline, and Emlembe Syncline. Mapepe sediments vary from deep-water turbidites and banded iron formation in the northwestern belt, the Manzimnyama Syncline, to shallow-water fan-delta conglomerates in the southeast in the Emlembe Syncline, suggesting that these structural belts may represent a distal to proximal cross-section of a single large, now-dismembered depositional basin. We here combine new field mapping with quantitative sandstone petrography, shale geochemistry, and U-Pb detrital zircon geochronology across the Manzimnyama, Paulus, and Emlembe Synclines to test this hypothesis.
Mapping confirms the initial observations that these structural belts display different depositional environments in what might be an offshore (northwest) to onshore (southeast) sequence. However, the area is more faulted than previously interpreted, complicating the stratigraphic and spatial relationships.
In the Manzimnyama Syncline, lithic fragments composed predominantly of chert and dacitic volcanic rocks derived from uplifted, eroded Onverwacht Group units dominate sandstone composition, combining for more than 95% of the framework grains on average. To the east in the Paulus Syncline, lithic fragments, especially chert, are most abundant; however, more sedimentary lithic grains are present (more than 15% compared to less than 4% in the Manzimnyama Syncline), derived from underlying strata. One fault block in the Paulus Syncline has sandstones with a high proportion of potassium-rich feldspars, both as single grains and in volcanic fragments, which are likely a diagenetic overprint of the original grain but still reflect a post-depositional process unique to this block for the study area. The southeastern Emlembe Syncline is rich in monocrystalline quartz, about 25% of the total composition compared to less than 10% in all other areas, yet retains a high abundance of chert and sedimentary lithic fragments.
Immobile-element geochemistry appears to provide a relatively useful means of comparing the shale compositions in the belts and for comparing their source areas. In the Manzimnyama Syncline, a strong felsic influence is indicated by high LREE to HREE ratios, fractionated REE patterns, and enrichments in La and Th relative to Sc. In contrast, shales in the Paulus Syncline display a strong mafic provenance signal with nearly flat REE patterns and trace element chemistries matching those of komatiites. A single shale sample from the Emlembe Syncline shows a more felsic influence on the fine-grained fraction, consistent with the high monocrystalline quartz content.
The detrital zircon contents of the Mapepe Formation demonstrate clearly the role of older parts of the greenstone terrain in providing sediment. Older Fig Tree felsic volcanic rocks, the Mendon Formation, and the upper Hooggenoeg Formation are the dominant zircon sources, as well as an unknown 3.40 Ga source. However, the relative abundance of these age populations differs significantly among the belts. Sandstones in the Manzimnyama Syncline appear to lack Mapepe age felsic volcanic sources whereas such sources provided the bulk of detrital zircons to sandstones in the Paulus and Emlembe Synclines.
These findings do not support the interpretation that these structural belts are parts of a single Mapepe Basin. Rather, while each of these structural blocks appears to include components derived largely from underlying volcanic and sedimentary rocks of the Onverwacht Group and, in some cases, Fig Tree age volcanic rocks, detailed source information suggests that these sources were not the same and that the blocks do not represent a single large source-to-sink transport and depositional system. Fig Tree deposition in this area may have been dominated by local sources and small tectonic basins that, during subsequent orogensis, were disrupted and juxtaposed against other albeit unrelated basinal sequences.