Dating the cessation of Kibaran magmatism in Natal, South Africa

Abstract

Published U-Pb and Pb-Pb zircon dates from pre- to late-tectonic granitoids of the Kibaran (Grenvillian) Natal Metamorphic Province fall within the range ≈1200 to ≈1040 Ma. A major problem regarding the duration and timing of magmatism in the Natal belt has been the age of the youngest intrusive events. This is important within a Gondwana context, especially with respect to the nature and distribution of the Pan-African event in this area. It is known that the youngest magmatic rocks in Natal are certain undeformed, cross-cutting microgranite dykes (Mbizana Microgranite) from the Margate Terrane. These dykes have previously given a Rb-Sr whole-rock date of ≈950 Ma, showing them to be broadly late Kibaran in age. This paper presents new U-Pb data from four zircon fractions from the Mbizana Microgranite, which give a date of emplacement of 1026 ± 3 Ma. Consequently, this date is considered to define the latest magmatic event in the Natal Metamorphic Province and points to the timing of the cessation of Kibaran magmatism in this area. The data supports recent suggestions that there was no Pan-African magmatic activity in Natal. The ≈950 Ma Rb-Sr dates previously obtained for these rocks are re-interpreted as representing the time at which the dykes cooled through the Rb-Sr biotite and K-feldspar blocking temperatures.