Crustal setting, provenance and sedimentation in the Paleoarchean Barberton greenstone belt, South Africa

Surface and crustal conditions during the Paleoarchean were likely very different than today and their exact nature is still poorly understood. The 3.6 to 3.2 Ga Barberton greenstone belt (BGB), southern Africa, represents one of the oldest and best-preserved crustal remnants on Earth: however, complex post-depositional deformation and metasomatism challenge a direct interpretation of tectonic and sedimentary processes. This thesis studies the provenance and sedimentology of siliciclastic rocks within the BGB to gain insights into the nature of the crustal setting in which the BGB formed and the interplay between local tectonic uplifts and sedimentation. Chapter 1 addresses whether the BGB was built on, or adjacent to, older continental crust. Although geologists have been studying the BGB for over 100 years, we still have distressingly little actual knowledge of the relationship between the volcanic-sedimentary supracrustal sequence and the underlying crustal rocks on which it formed. This study utilizes detrital zircon grains in sandstone as a tracer for fractionated rocks in the source. Out of 3,410 analyzed grains only 15 grains (< 0.5%) are older than the oldest rocks in the BGB, about 3.55 Ga. This suggests that no substantial amount of older fractionated rock was present in the source areas of the BGB during its formation and provides compelling evidence for the absence of older continental crust in the vicinity of the BGB. These findings provide further evidence that the large, thick, high-standing, highly evolved blocks of continental crust that characterize the Earth during the Phanerozoic were scarce in the early Archean. Chapters 2 and 3 focus on the 3.26 to 3.22 Ga Mapepe Formation of the Fig Tree Group. This formation records the onset of deformation and siliciclastic sedimentation after nearly 300 million years dominated by basaltic and komatiitic volcanism represented by underlying rocks of the Onverwacht Group. However, paleogeographic and paleotectonic reconstructions are challenged by complex syn- and post-depositional deformation and, as a consequence, the fragmented character of the rocks of the Mapepe Formation. In this chapter we address the local provenance and stratigraphic evolution of the Mapepe Formation in the eastern Barite Valley structural block and compare these to the stratigraphy and evolution of the Mapepe Formation in neighboring structural belts. The results of Chapters 2 and 3 collectively show a contrasting evolution in the source rocks and the stratigraphy from the eastern Barite Valley compared to neighboring structural belts. The high lateral and stratigraphic variability of the Mapepe sequences suggests that the evolution of the rocks was controlled largely by local tectonics, that the structural belts may have formed in unrelated sedimentary systems that were subsequently tectonically amalgamated, and that previous basin reconstructions based on stratigraphic relationships need to be carefully reevaluated.

Keywords: Geological and Environmental Sciences.
Advisor: Lowe Donald R.
Byerly Gary R.
Graham S. A. (Stephan Alan)