SINGLE ZIRCON AGE FOR THE FELSIC RUBBERVALE FORMATION, MURCHISON GREENSTONE-BELT, SOUTH-AFRICA

Age data for the various greenstone belts of the northern Kaapvaal Cra ton are generally scarce and imprecise. Single zircons were dated, usi ng the vapour digestion and evaporation techniques, from a felsic meta volcanic rock of the Rubbervale Formation which constitutes an almost 4 km-thick unit within the Murchison Greenstone Belt. Both methods yie lded almost identical Pb-207/Pb-206 ages: 2965.2 +/- 1.4 Ma and 2967.5 +/- 1.3 Ma, respectively. The combined mean age of 2966.5 +/- 1.6 Ma also provides a maximum age for the associated Rooiwater Complex, whic h is locally intrusive into the Rubbervale Formation. Due to a lack of isotopic data on the mafic portion of the Murchison Belt, its regiona l setting is speculative. Suggestions are that the belt and the adjace nt gneissic terrains represent exotic crustal blocks, or that the Rooi water Complex together with the Rubbervale Formation formed in an isla nd are setting.