DETRITAL ZIRCON AGE CONSTRAINT OF THE MAKGANYENE SNOWBALL EARTH EVENT (SOUTH AFRICA) AT 2.25-2.22 GA: THE YOUNGEST OF FOUR HURONIAN GLACIATIONS?

The Makganyene glacial diamictite (Postmasburg Group, Griqualand West outcrop area of the Transvaal Supergroup) has classically been considered slightly older than 2.22 Ga in age. This is based on its conformable contact with overlying Ongeluk lava (Rb-Sr and Pb-Pb whole rock age ~2.22 Ga), a marked erosional unconformity at its base and correlation with the Rietfontein diamictite-Boshoek conglomerate and conformably overlying Hekpoort lava (Rb-Sr, Pb-Pb whole rock and single zircon age ~2.22 Ga) succession of the Pretoria Group in the Transvaal outcrop area. More recently this correlation has been challenged and suggestions made that the Makganyene diamictite-Ongeluk lava succession predates the Rietfontein-Hekpoort succession by as much as ~180 my. This allowed correlation of the Makanyene with either the Lower Duitschland diamictite near base of the Pretoria Group or the sequence boundary at base of the upper Duitschland succession marking the Great Oxidation Event. The latter correlation is based on the assumption that the GOE, defined essentially by the disappearance of sulfur mass independent fractionation, coincided with a Snowball Earth Event but without any paleomagnetic constraints. However, new LA-SF-ICP-MS analyses indicate that the Makanyene diamictite contain detrital zircons as young as 2226 ± 42 my. This new data confirms earlier stratigraphic age estimates of the diamictite and the Ongeluk-Hekpoort lava correlation. A remaining uncertainty remaining is whether the Makganyene diamictite correlates with the Rietfontein-Boshoek succession in total or only with the upper Boshoek conglomerate that has a marked sequence boundary at its base. This newly confirmed age for the Makganyene diamictite implies that it is younger than any of the three Huronian glacial deposits on the Superior Craton. However, the Huronian succession is unroofed as lavas that must have been associated with the 2.2 Ga Nipissing dykes, that intrude the entire succession, are not preserved. It is thus quite possible that four glacial events mark the Huronian period with the Makganyene diamictite representing the youngest but not preserved in the Superior area.