Detrital Zircon Geochronology and Petrology of Sediments from the Barberton greenstone belt: Implications for Provenance and Tectonic Reconstructions

Abstract:

Paleogeographic and tectonic reconstructions in Paleoarchean rocks are challenged by syn- and post-depositional tectonics. Sediments of the 3.2 Ga Fig Tree Group in the Barberton Greenstone Belt (BGB) represent first significant uplift after almost 300 Ma of tectonic quiescence during the Onverwacht time. The rocks are now exposed in isolated structural belts, which are separated by belts of faulted older Onverwacht and younger Moodies strata. A systematic comparative provenance study of Fig Tree Group utilizing sandstone petrography and detrital zircon geochronology assesses the evolution in the provenance of different isolated structural belts to test a possible correlation and evaluating the nature of tectonic uplifts that sourced the sediments. Samples were obtained from the ICDP drill cores BARB4 and BARB5 of the Mapepe Formation in the Manzimnyama Syncline (MS) and the Barite Valley (BV) structural belts, respectively.

The framework mode of the MS and basal BV sandstones represents a composite of common silicified BGB rocks. In contrast, sandstones from the middle and top of the BV are composed of exclusively sedimentary or intermediate volcanic rock fragments. They were derived locally by shallow erosion of immediately underlying sedimentary and volcanic rocks. Detrital zircon spectra show four main age clusters at 3.26-3.24, 3.00, 3.45, and 3.55 Ga. The 3.26-3.24, 3.45, and 3.55 Ga clusters correspond to major magmatic episodes of volcanism of the Fig Tree Group, as well as Hooggenoeg Formation (H6), and the Theespruit Formation, respectively. A cluster of Mendon Formation-age zircons at 3.30 Ga represents a source that has been eroded or is not exposed on the surface. MS sediments and basal BV sandstones were exclusively sourced from H6. Towards the middle of the BV section, sediments erodeds first into Mendon sediments, and then into Fig Tree Formation felsic volcanics.

The sandstones in the MS and BV structural belts represent first-cycle sediments derived from local uplifts of different parts of the greenstone sequence, as well as, penecontemporaneous volcanism. Different provenance signatures towards the stratigraphic middle and top in the MS and BV belts indicate distinct evolution in the source areas of these structural belts. Erosion did not reach deper-seated plutonic or metamorphic rocks.