- © 2009 Geological Society of South Africa
The Portobello granite, which crops out along the littoral in southern Natal is an S-type, red-coloured, foliated, chlorite-biotite granite that preserves evidence of low-temperature alteration. It intruded the 1091 ± 7 Ma Munster Suite quartz monzonorites and has distinctive 1.0 to 1.5 m wide charnockitic margins adjacent to the contacts with the quartz monzonoritic country rock. SHRIMP geochronology on zircon grains from two samples, one taken from the charnockitic margins and the other from the main body of chlorite-biotite granite, produced two statistically distinct ages: 1057 ± 27 Ma for the chlorite-biotite granite and 1093 ± 7 Ma for the charnockitic margin. The age for the charnockitic margins is identical to that of the country rock and, given the previously proposed origin for the margins by way of partial assimilation of the country rock, it is interpreted that the zircons sampled from the charnockitic margins are xenocrystic. The low-precision age of 1057 ± 27 Ma for the chlorite-biotite granite was produced from milky, metamict zircon grains suggestive of low temperature alteration, which may have affected the accuracy of the age. Nevertheless, the age of 1057 ± 27 Ma for the chlorite-biotite granite is statistically similar to the intrusive ages of the Oribi Gorge Suite and to ages recorded by metamorphic overgrowths on zircon grains from the Munster Suite. The ages produced by this study fall within the range of ages thus far acquired for the Margate Terrane of the Natal Metamorphic Belt, and further corroborate the previously proposed continuation of the terrane into the Vardeklettane Terrane of Dronning Maud Land in Antarctica. A zircon core from the charnockitic margins recorded an age of 1800 ± 12 Ma, which is the oldest age thus far recorded in the Margate Terrane.