U-PB BADDELEYITE GEOCHRONOLOGY AND GEOCHEMISTRY OF THE WHITE MFOLOZI DYKE SWARM: PART OF A COMPLEX ARRAY OF 2.70-2.66 GA DYKE SWARMS ACROSS THE KAAPVAAL CRATON, SOUTH AFRICA

The barcode of extensional magmatic events affecting the Kaapvaal Craton in southern Africa is progressively being completed. Olsson et al. (2011) showed that dolerite dykes dated between ca. 2700 and 2660 Ma radiate out from beneath the eastern lobe of the ca. 2.05 Ga Bushveld Complex and across the eastern basement of the Kaapvaal Craton. The younger ca. 2660 Ma dykes may be feeders to lavas of the protobasinal fill sequence at the base of the Transvaal Supergroup, whereas the older ages may relate to volcanism within the upper part of the Ventersdorp Supergroup.

A NE-trending and more feldspar-megacrystic dolerite dyke swarm, named the White Mfolozi Dyke Swarm (WMDS), appears to cross-cut the south-eastern branch of the radiating swarm. New high-precision U-Pb baddeleyite ages for seven dykes of the WMDS are presented here. The three most robust age results yielded a weighted mean age of approximately 2662 Ma, which is taken as a mean age of the WMDS. The ages show that the emplacement of WMDS dolerite dykes was of very short duration, possibly ≤1 Myr. The WMDS is therefore coeval with parts of both NE- and E-trending dykes within the radiating swarm, indicating a more extensive magmatic event at ca. 2660 Ma. These results illustrate a greater complexity to the interpretation that radiating dykes were linked to a mantle plume event that caused there emplacement over an extended (ca. 40 Myr) time period from 2700 to 2660 Ma. A maximum duration of 15 Myr appears to be a better estimate, provided all the 2660 Ma dykes belong to the WMDS. Another possibility is that there are two coeval but different events. This is supported by geochemistry of the ca. 2660 Ma WMDS, which are characterized by more depleted signatures in contrast to the radiating dykes. The 2660 Ma magmatic event thus records either a extensive common event during which magmas were more contaminated further to the north, possibly due to thicker continental crust, or that two spatially unrelated magmatic events occurred. Another possibility is that depleted dykes were emplaced towards the end of a more protracted period of emplacement of a 2700-2660 Ma radiating dyke swarm.

[1] Olsson, J.R., Söderlund, U., Hamilton, M.A., Klausen, M.B., Helffrich, G.R. (2011). A late Archaean radiating dyke swarm as possible clue to the origin of the Bushveld Complex. Nature Geoscience 4, 865-869.