The Rooi Rand is an early Jurassic sheeted dolerite dyke swarm associated with Karoo volcanism in southern Africa. The swarm is parallel to the Lebombo and represents about 40% crustal extension. It has been likened to a failed mid-ocean ridge and represents an aborted attempt of Gondwana break-up at about 183 Ma.
Detailed mapping has enabled a dyke-by-dyke dilational history to be established; in combination with petrographic and geochemical studies, 11 dykes phases have been recognised, some of which involve more than 3 magma pulses. The earliest phase represents a higher degree of partial melting (5-10%) than the next 3 phases (5%, 1-2%, 0.1%). This indicates that enough partial melt had to accumulate for the initial magma to ascend whereas, for the later magmas, successively lower degrees of partial melt were able to exploit a pre-existing pathway. The pathway opening is reflected in the increasing volumes of melt with time that correspond to the decreasing amounts of partial melt.
These first 4 phases were produced during initial lithospheric extension over an upwelling asthenosphere which was tapped by the succeeding dyke phase which is the only one to have a MORB-like REE profile. The asthenosphere ceased to rise further but its presence caused increasingly higher degrees of partial melting in the overlying mantle. The volumes of melt injected as dykes into the crust decreased as lithospheric extension slowed and may even have ceased.
Melt then gathered in sub-crustal magma chambers and underwent fractional crystallisation of a gabbroic assemblage, accompanied by a small degree of crustal contamination. Magmas from these chambers subsequently injected as the penultimate silica-rich evolved phase and the final phase of wide dykes. Heat from these chambers caused partial melting of the lower crust to yield the Lebombo rhyolites.
This research reflects part of a comprehensive project headed by Mike Watkeys at the University of Natal, Durban (UND) to unravel the history of the upheaval in southern Gondwana during the Jurassic. The research is focused on the mechanics of dyke and sill intrusion, incorporating studies from the Rooi Rand dyke swarm, and Jurassic-aged sills intruded into Karoo sediments in northern KwaZulu-Natal, and detailed geochemical and petrographic characterizations. Mike Watkeys is also involved with a comprehensive study of the structural framework of southern Africa that is indicative of Gondwana break-up. Deanna Meth completed her M.Sc. thesis about the Rooi Rand dyke swarm at UND in August 1997. Simon Kattenhorn worked on mechanics and magma flow properties of the Rooi Rand and was awarded his M.Sc. in 1994.
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