GEOCHEMISTRY AND PLATE-TECTONIC SETTING OF MESOPROTEROZOIC VOLCANIC UNITS IN THE KONKIEP TERRANE, NAMAQUA-NATAL OROGENIC BELT, NAMIBIA

The Konkiep Terrane in Namibia is a major tectonic element within the 1.4-1.0 Ga Namaqua-Natal orogenic belt (NNOB) that extends along the southern edge of the Kalahari craton in southern Africa. In contrast to the rest of the NNOB, the terrane contains thick, surprisingly well-preserved, weakly metamorphosed volcanic successions that give insight into evolving plate-tectonic settings along this part of the craton margin. Older metamorphic rocks form the substrate to the volcanic rocks described herein, suggesting a continental margin setting. The oldest unit we have studied is the bimodal, 1.36 Ga Nagatis Formation, which contains tholeiitic basalts (SiO₂ 48-50%) and rhyolites (SiO₂ 70-78%) with A-type trace-element signatures. Basalts have nearly flat REE patterns and are inferred to have formed from decompression melting of depleted mantle in an intraplate rift setting. The 1.33 Ga bimodal Welverdiend Formation, which occurs in a different part of the terrane, ranges in composition from tholeiitic to transitional basalt and andesite (SiO₂ 49-59%) and rhyolite (SiO₂ 75-78%), with the latter rocks showing A-type features comparable to those in the Nagatis Formation. The data indicate that the Welverdiend Formation was emplaced in a similar rift-related tectonic setting to the Nagatis Formation, but trace element differences (e.g., higher Ta/Yb and Nb/Y ratios) in the Welverdiend reflect different source regions or degrees of partial melting. The extensive 1.2 Ga Barby Formation overlies the other units unconformably and contains abundant calc-alkaline to shoshonitic rocks (SiO₂ 48-59%) that are LREE enriched (La/Yb > 21.5). The unit has continental arc characteristics with pronounced Nb/Ta anomalies. Previous workers have proposed that it was emplaced in an extensional part of the arc during oblique subduction, which may explain the shoshonitic compositions. The youngest volcanic unit in the terrane, the bimodal, 1.1 Ga Guperas Formation, consists of basalts and andesites (SiO₂ 49-59%) and rhyolites (SiO₂ 72-78%) that show within-plate geochemical signatures, consistent with recent paleomagnetic and geochronological evidence showing that the unit represents part of the Umkondo LIP that was emplaced across the Kalahari craton after cessation of arc magmatism in the Konkiep terrane.