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

University essay from Lunds universitet/Geologiska institutionen

Abstract: Numerous mafic dykes of different generations and trends intrude the eastern part of the Kaapvaal Cra-ton. On eastern Kaapvaal, there are three previously investigated dyke swarms that appear to radiate out from the eastern lobe of the Bushveld Complex. The NE- and E trending branches comprise dykes dated at ca. 2660 Ma, 2685 Ma and 2700 Ma (Olsson et al. 2011). Further south, a NE-trending swarm, here named the White Mfolozi Dyke Swarm (WMDS), appears to cross cut the ca. 2700 Ma SE trending branch of the radiating swarm. New U-Pb baddeleyite ages for seven dykes of the WMDS are presented. The three most robust results yield a weighted mean age of 2661.8 ± 2.0 Ma (MSWD = 0.3), taken as a mean age of the WMDS. Taking the errors of the U-Pb results into account, the results do not rule out the possibility that emplacement of dykes was of very short duration (1 Myr, or even less). These results are identical in age to the NE- and E-trending dykes of the radiating swarm further north, indicating a possible more extensive event at ca. 2660 Ma. These results also cast some doubt on the inter-pretation of Olsson et al. (2011), i.e. that the radiating dykes were linked to a mantle plume that caused dyking over a 40 Myr time period (i.e. from 2700 to 2660 Ma). Rather, a maximum duration of 15 Myr appears to be a better estimate if assuming the 2660 Ma dykes instead belong to the WMDS, making this swarm up to 400 km in width. Another possibility is that there are two coeval but chemically different events. The ca. 2660 Ma dykes may be fee-ders to the lavas of the protobasinal fill sequence at the bottom of Transvaal Supergroup, dated to 2664 ± 1 Ma (U-Pb zircon; Barton et al. 1995). The geochemistry of the ca. 2660 Ma WMDS are characterized by flat REE-patterns and overall depleted signatu-res, in contrast to its coeval more northerly dykes which are LREE enriched and show increasing crustal contami-nation. The 2660 Ma dyking event records either a spatially extensive common event during which magmas were more contaminated further north than in the south, or two spatially unrelated events.

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