DATING THE CESSATION OF KIBARAN MAGMATISM IN NATAL, SOUTH-AFRICA

Published U-Pb and Pb-Pb zircon dates from pre- to late-tectonic grani toids of the Kibaran (Grenvillian) Natal Metamorphic Province fall wit hin the range approximately 1200 to approximately 1040 Ma. A major pro blem regarding the duration and timing of magmatism in the Natal belt has been the age of the youngest intrusive events. This is important w ithin a Gondwana context, especially with respect to the nature and di stribution of the Pan-African event in this area. It is known that the youngest magmatic rocks in Natal are certain undeformed, cross-cuttin g microgranite dykes (Mbizana Microgranite) from the Margate Terrane. These dykes have previously given a Rb-Sr whole-rock date of approxima tely 950 Ma, showing them to be broadly late Kibaran in age. This pape r presents new U-Pb data from four zircon fractions from the Mbizana M icrogranite, which give a date of emplacement of 1026 +/- 3 Ma. Conseq uently, this date is considered to define the latest magmatic event in the Natal Metamorphic Province and points to the timing of the cessat ion of Kibaran magmatism in this area. The data supports recent sugges tions that there was no Pan-African magmatic activity in Natal. The ap proximately 950 Ma Rb-Sr dates previously obtained for these rocks are Te-interpreted as representing the time at which the dykes cooled thr ough the Rb-Sr biotite and K-feldspar blocking temperatures.