New isotope data from a neoproterozoic porphyritic garnitoid-charnockite suite from Natal, South Africa

Abstract

Abstract The tectonic setting and age of emplacement of the Oribi Gorge Suite, voluminous rapakivi-textured granites and charnockites (“megacrystic granites”) from the ∼ 1 Ga (Kibaran) Natal Metamorphic Province has been controversial. Isotopic dates of ∼ 1.0 Ga have been obtained fromfive plutons (various isotopic systems), but two of these (Fafa and Oribi Gorge plutons) have also given ∼ 0.89 Ga RbSr whole-rock isochrons. The age problem has been exacerbated by apparently equivocal field and structural relationships, in which both syn- and post-tectonic settings have been argued. New UPb isotopic analyses for zircon fractions from a sample of the Fafa pluton give a concordia data of 1029±1010Ma. Single zircon evaporation analyses of selected grains from the Oribi Gorge pluton (Bomela locality) give a data of 1092 ± 2Ma RbSr model dates of biotite separates from three plutons range between 970 and 880 Ma, generally similar to the RbSr whole-rock dates. It is significant that no Pan-African (∼ 500 Ma) dates were obtained. The zircon data strongly support a > 1.0 Ga age for the emplacement of the Oribi Gorge Suite, an age similar to both the main regional tectogenesis and syntectonic granites of the Margate Suite. The Oribi Gorge Suite can thus be regarded as late (syn)tectonic and not partly post-tectonic as previously envisaged. Consequently, the need to invoke two periods of identical A-type magmatism is removed. The younger RbSr whole-rock and biotite dates may represent the time when the various plutons or the entire terrane cooled through the relevant blocking temperatures, though this was not a simple, homogeneous process.