U-Pb baddeleyite geochronology and geochemistry of the White Mfolozi Dyke Swarm: unravelling the complexities of 2.70-2.66 Ga dyke swarms across the eastern Kaapvaal Craton, South Africa

摘要

On the south-easternmost Kaapvaal Craton, a NE-trending plagioclase-megacrystic dolerite dyke swarm, herein named the White Mfolozi Dyke Swarm (WMDS), has been identified. New U-Pb baddeleyite ages presented here indicate that the WMDS was emplaced within less than 10 million years, with our three most robust results yielding a weighted mean age of 2662 +/- 2Ma. The WMDS is coeval with the youngest dykes of a 2.70-2.66Ga radiating dyke swarm already identified further north on the eastern side of the Kaapvaal Craton. This dyke swarm radiates out from the eastern lobe of the ca. 2.05Ga Bushveld Complex. A clustering of ages from the WMDS and the 2.70-2.66Ga radiating dyke swarm identify potential magmatic peaks at 2701-2692Ma, 2686-2683Ma and 2665-2659Ma. Geochemical signatures of the dykes do not correlate with these age groups, but are rather unique to specific areas. The northern part of the eastern Kaapvaal Craton hosts relatively differentiated 2.70-2.66Ga dolerite dykes that could have been derived from a moderately enriched mantle source, whereas the ca. 2.66Ga WMDS from the southernmost area exhibit much more depleted signatures. In between these two margins, the central area hosts more andesitic 2.70-2.66Ga dykes that may have assimilated substantial amounts of partly digested tonalite-trondhjemite-granodiorite crust from the basement. We investigate the evolution for the Kaapvaal Craton during a highly magmatic period that extends for over 60 million years from extensive Ventersdorp volcanism to the eruption of proto-basinal volcanic rocks at the base of the Transvaal Supergroup.