Baddeleyite geochronology and geochemistry of mafic cobbles from the Dwyka diamictite: New insights into the sub-Kalahari basement, South Africa

Abstract

U–Pb baddeleyite geochronology and geochemistry of mafic cobbles from the ~ 300 Ma Dwyka diamictite on the western boundary of the Kalahari Basin provide new insights into the sub-Kalahari basement. Palaeo-ice flow directions indicate that the origin of the Dwyka diamictite deposits on the border between South Africa and Namibia is from beneath the Kalahari sands to the east or north-east. This work shows that the mafic cobbles from three localities near Rietfontein come from intrusions related to the Umkondo Large Igneous Province (LIP), located in the Kheis or Rehoboth Provinces. The lack of Banded Iron Formation (BIF) cobbles in these glacial diamictites indicates that their source region lies west or north of the BIF-dominated Griqualand West Basin of the Kaapvaal Craton. The four dated mafic cobbles give U–Pb ages of 1111–1123 Ma, coeval with dolerites of the Umkondo LIP. The cobbles experienced greenschist facies metamorphism before the Dwyka glaciation, but are unfoliated. The geochemistry of the cobbles indicates that they are genetically similar and have a within-plate setting. They show many similarities to the Umkondo LIP dolerites and the coeval Rouxville Formation, an upper unit of the Koras Group. The most likely sources of the cobbles are the mafic intrusives and lavas along the Kalahari Line, southern equivalents of the ~ 1.11 Ga Tshane and Xade complexes or intrusive and extrusive mafic units related to the Umkondo LIP, now concealed beneath the Kalahari sands.