The Great Oxygenation Event (GOE) in the Paleoproterozoic coincides with a complex series of glacial episodes preserved in supracrustal successions on the world’s cratons. These Paleoproterozoic glacial rocks are bracketed in age between 2.46 and 2.22 Ga, based on the glacial record in the Huronian Supergroup on the Superior Craton in Canada). In southern Africa, the cover successions of the western Kaapvaal Craton preserve possible glacial correlatives of the Huronian Supergroup in the Griqualand West sub-basin of the Transvaal Supergroup. This potential correlative is the glacial diamictites of the Makganyene Formation. The Makganyene Formation is in turn conformably overlain by the Ongeluk Formation, a series of subaqueous mafic volcanic rocks with a putative age of ca. 2.22 Ga. However, both local and global correlations using glacial units of the Transvaal and Huronian supergroups are hampered by a lack of robust well-placed geochronological data. The timing of the Ongeluk Formation itself has been challenged. Here, the sub-volcanic sills of the Ongeluk volcanic rocks have been dated using U-Pb in-situ SIMS analysis on baddeleyite to ca. 2.43 Ga. This has been coupled with ID TIMS dating and paleomagnetism studies on the coeval Westerberg Sill Suite. This suggests that the Ongeluk Formation volcanic rocks, and the immediately underlying Makganyene Formation glacial deposits, are approximately 200 Myr older than generally assumed. These new ages lend support to three glacial epochs around the GOE, of which the first was low-latitude in the Makganyene Formation. The Makganyene diamictites could correlate to diamictites from the Duitsland Formation in the more eastern Transvaal sub-basin of the Transvaal Supergroup, with a significant revision of regional stratigraphic correlations, that is in itself not without challenges. In such a scenario, the Ongeluk volcanic rocks could be correlated with the Bushy Bend volcanics, and not the ≤ 2.25 Ga Hekpoort Formation volcanics. This new magmatic event may be linked with the evolution of the Vaalbara continent, the Kenorland supercontinent, and global environmental changes related to magmatism and weathering in the Paleoproterozoic Era.
Upprättat; 2015; 20150312 (tobkam)