Paper No. 291-7
Presentation Time: 9:25 AM
THE USUSHWANA COMPLEX: A TALE OF TWO ARCHEAN LIPS IN SOUTH AFRICA AND SWAZILAND, AND ITS IMPLICATIONS FOR THE MESOARCHEAN EVOLUTION OF THE SOUTH-EASTERN KAAPVAAL CRATON
GUMSLEY, Ashley P.1, OLSSON, Johan R.1, SÖDERLUND, Ulf1, DE KOCK, Michiel O.2, HOFMANN, Axel3 and KLAUSEN, Martin B.4, (1)Department of Geology, Lund University, Sölvegatan 12, Lund, 22362, Sweden, (2)Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, Johannesburg, 2006, South Africa, (3)Department of Geology, University of Johannesburg, Johannesburg, 2006, South Africa, (4)Department of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland, Stellenbosch, 7602, South Africa
The Usushwana Complex on the south-eastern Kaapvaal Craton in South Africa and Swaziland is one of the oldest known mafic intrusions on Earth in stable crust. It is remarkably pristine and well-preserved. The complex intrudes ca. 3.6-3.1 Ga Archean granitoids, gneisses and greenstone belts as well as the Mesoarchean supracrustal rocks of the Nsuze Group from the Pongola Supergroup in the same region. New high-precision U-Pb ID-TIMS baddeleyite ages of 2989 ± 1 Ma, 2990 ± 2 Ma and 2979 ± 2 Ma obtained on gabbros belonging to the Piet Retief Suite of the Usushwana Complex makes it older than previously assumed.It is a complex, long-lived magmatic feeder chamber for a major volcanic event that gave rise to the oldest known continental flood basalts on Earth, the Nsuze volcanics. Further, a SE-trending dyke in this region was dated to 2980 ± 1 Ma in this study, validating the coeval link between the Piet Retief Gabbro Suite, the Nsuze volcanics and these SE-trending dolerite dykes. Many of the other SE-trending dolerite dykes (Barberton-Badplaas Swarm) in the region have long been understood to be related to the Nsuze volcanics.
A 2869.0 ± 4.8 Ma age on a dolerite sill situated within the Mozaan Group higher up in the stratigraphy from the Nsuze Group volcanics is coeval with the ca. 2.87 Ga Hlagothi Complex and dykes further to the south on the Kaapvaal Craton. This result strengthens the presence of a younger ca. 2.87 Ga magmatic event being more widespread across the south-eastern Kaapvaal Craton than previously known [1]. The bulk of the granophyres from the Hlelo Suite of the Usushwana Complex were related to this ‘Hlagothi Event‘ as indicated from the age. They therefore should treated as a separate unit from the gabbros of the Usushwana Complex proper, and may be related to the Amsterdam volcanics. These new U-Pb baddeleyite ages have critical bearings on the tectonic, volcanic and sedimentological history of the Pongola Basin, with magmatism possibly derived from mantle plume(s) having influenced the uplift and subsidence of the basin.
[1] Gumsley, A.P., de Kock, M.O., Rajesh, H.M., Knoper, M.W., Söderlund, U. and Ernst, R.E. (2013). The Hlagothi Complex: The identification of fragments from a Mesoarchaean large igneous province on the Kaapvaal Craton. Lithos 174, 333-348.
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