A petrographic and Ar-40/Ar-39 geochronological study of the Voorspoed kimberlite, South Africa: Implications for the origin of Group II kimberlite magmatism

The Voorspoed kimberlite pipe is one of a cluster of eleven kimberlite and related rock intrusives, located north of Kroonstad in the Free State. The pipe comprises tuffisitic kimberlite plus tuffisitic kimberlite breccia and is associated with several hypabyssal sills and dykes. A large down-rafted block of Karoo lava occupies about six hectares of the pipe to a depth of at least 300 m. A recent drilling programme has enabled the recognition of two types of tuffisitic kimberlite breccia, as well as seven varieties of h ypabyssal facies kimberlite. As the Voorspoed kimberlite is located between the similar to 120 Ma Barkly West and the similar to 145 Ma Swartruggens k imberlites, accurate determination of an intrusion age is crucial to the ev aluation of the Group II age progression across southern Africa and models proposed for the origin of Group II magmatism. Ar-40/Ar-39 laser probe anal yses were carried out on six individual groundmass phlogopite grains from a hypabyssal kimberlite autolith. All grains yielded reproducible plateau ag es averaging 131.8 +/- 1.7 Ma (2 sigma). The intra- and inter-grain reprodu cibility of the current data indicates that this age represents a reliable estimate for the time of pipe emplacement. The age is also in agreement wit h recent data obtained for the nearby Lace and Besterskraal kimberlites. In combination, these results confirm the regular northeasterly age progressi on of Group II kimberlites across the Kaapvaal Craton. Postulated models to account for the distribution of Group II kimberlites have invoked hot spot tracks, subduction of oceanic crust, and rafting of the subcontinent over an oceanic lithosphere megalith. As an alternative, a model involving exten sional tectonics in relation to the disaggregation of Gondwanaland is consi dered.