U-PB ZIRCON AGE FOR A TUFF IN THE CAMPBELL GROUP, GRIQUALAND WEST SEQUENCE, SOUTH-AFRICA - IMPLICATIONS FOR EARLY PROTEROZOIC ROCK ACCUMULATION RATES

An ion-microprobe U-Pb age of 2552 +/- 11 Ma has been obtained on zirc on separated from a regional banded-tuff horizon in the Nauga Formatio n (Beukes, 1980b) (upper Campbell Group, Griqualand West Sequence, Sou th Africa). This age permits time constraints to be placed on litholog ically correlated units within the adjacent Transvaal Sequence and cor relations to be made with the further removed Hamersley Group in Austr alia. Calculated rock accumulation rates of 2 to 4 m/m.y. for these st rata of mainly shale and banded iron formation suggest that sedimentat ion rates were significantly slower in the late Archean-Early Proteroz oic than is generally assumed.