Abstract
Rubidium, Sr, U, Th and Pb concentrations and Sr and Pb isotope data are presented for a suite of Precambrian metavolcanic rocks cropping out in the lower Orange River region of southern Africa. Good agreement between total rock Rb-Sr, Th-Pb and Pb/Pb ages obtained indicates an extrusion age of 1996 ± 15Ma. The total rock U-Pb system was disturbed in recent times and ages obtained are unreliable. Parental magmas to the volcanic rocks and associated plutonic intrusives were probably derived from the upper mantle, which indicates that a major crust producing event may have taken place in the lower Orange River region during the mid-Proterozoic (2.0–1.7 Ga). Reworking of part of this older crust during a 1.2-1.0-Ga event to produce gneisses within the neighbouring Namaqua Province is supported by Sr isotopes. Initial Sr and Pb in the lavas was probably derived from a mantle source that experienced a previous history of slightly high Rb/Sr and U/Pb, relative to postulated bulk earth evolution. Enrichment in Rb/Sr for the subcratonic mantle under southern Africa was previously known, and the results of this study indicate a similar pattern for U/Pb.
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