The petrology of the Oribi Gorge Suite: Kibaran charnokitic granitoids from southern Natal

The coarse-grained, porphyritic granitoids and charnockitoids of the Oribi Gorge Suite constitute the most voluminous of the 12 granitoid suites intrusive into the Mapumulo Metamorphic Suite, south of 300S. The Oribi Gorge Suite forms a number of batholiths and plutons which are post-tectonic with respect to the main deformation. Field relationships suggest that the rocks of the Oribi Gorge Suite were forcefully intruded and that the intrusions could be mushroom-shaped. Individual intrusions of the Oribi Gorge Suite contain varying proportions of hornblende- and biotite-bearing, pink rapakivi granite and dark-green charnockite. The apparently random distribution of granite and charnockite in any intrusion can be ascribed to local variations in pH2O, fO-2, fCO2, and fS. Rb-Sr and U-Pb data indicate that the rocks of the Oribi Gorge batholith and the Mvenyane granite were intruded approximately 1000 Ma ago. Major- and trace-element geochemistry of samples from four intrusions of the Oribi Gorge Suite show marked A-type, in addition to some I-type, granite characteristics. The granitoids are considered to have been emplaced in a within-plate setting under tensional tectonic conditions. It is suggested that the source for the rocks of the Oribi Gorge Suite was upper mantle/lower crustal material depleted by the extraction of I-type, calc-alkaline melts during an earlier compressional tectonic phase