A Reassessment of Petrochemistry of ~1.2 Ga Tonalitic Rocks of the Northern Tugela Terrane, Natal Metamorphic Belt, South Africa: Further Constraints on the Evolution of This Belt

The Mesoproterozoic Natal Metamorphic Belt (NMB) constitutes the eastern sector of the Namaqua‐Natal Metamorphic Province (NNMP) of southern Africa. The NNMP borders the Archaean Kaapvaal Craton on its west, southwest and south and is modelled to have evolved in a Phanerozoic‐style island‐arc setting, although no evidence of HP/UHP metamorphism or paired metamorphic belt has been reported from this belt. This belt represents the western extremity of a continental‐scale, curvilinear, Mesoproterozoic HT/UHT metamorphism belt on the Gondwana reconstruction. The NMB (and the western NNMP) consists of a deformed meta‐supracrustal sequence younger than ~1.73 Ga, intrusive orogenic granitoids (~1.2–1.09 Ga) and late anorogenic granitoids (at ~1.03 Ga). The estimated peak metamorphism, as reported from the southern part of the NMB, was ~4–9 kbar, 600°C–890°C, suggesting a HT/UHT metamorphic condition. The new LA‐MC‐ICP‐MS (Laser Ablation Multi‐Collector Inductively Coupled Plasma Mass Spectrometry) U–Pb zircon ages of Kotongweni Tonalite and Mkondene Granodiorite from the northern NMB are ~1218 ± 26 Ma (MSWD = 1.2) and 1185 ± 12 Ma (MSWD, = 1.8), respectively. Further petrochemical studies show that these intrusive orogenic plutons are, respectively, tonalitic and granodioritic in composition, metaluminous with high Al/(Na + K) (~2.6–3.8 and ~1.8–2, respectively), calcic, magnesian and M‐type granitoid in composition with HREE content similar to that of Archaean high‐HREE TTGs forming at low pressure (< 12 kbar). The (a) common occurrence of amphibolite enclaves both within Kotongweni Tonalite and Mkondene Granodiorite; (b) the evidence of gradational contact of the former pluton with the surrounding Manyane Amphibolite country rocks; and, importantly, (c) the overlapping relation of amphibolite and intrusive orogenic plutons in incompatible trace element ratio plots, insensitive to fractionation, suggest the derivation of these plutons from the country‐rock amphibolite through in situ (or near in situ) dehydration partial melting at an estimated pressure ≤ 9 kbar and 860°C–870°C. The ε Hf values of zircon from these plutons vary between +3.9 and +8.08, supporting their derivation from dominantly mantle‐derived mafic rock sources. Relatively high ε Hf zircon value of the Kotongweni Tonalite (+6.32 to 8.08) suggests a depleted nature of its source rock. However, almost similar 143 Nd/ 144 Nd i (at 1.2 Ga) values of Kotongweni Tonalite (0.51134) and Mkondene Granodiorite (0.51131), within the range of Tugela Amphibolite (0.51124–0.51139), further confirm that the country rock amphibolite was the source for the studied granitoid pluton. It has been suggested that the orogenic plutonism of the NMB took place between ~1.2 and 1.09 Ga (Spencer et al. 2015), that is over 110 Ma, which is much higher than the duration of UHT metamorphism (≤ 30 Ma) observed in the Phanerozoic style continental arc/back arc setting. The relatively anhydrous nature of these granitoid plutons, as indicated by the interstitial occurrence of hydrous mafic silicates in these rocks, as well as their low (Sr/Y) cn (< 16), do not support an arc‐related origin for these orogenic NMB plutons. It appears that the NMB could have evolved from a riftogenic tectonic basin, intracratonic between the then‐existing Kaapvaal‐Zimbabwe and East Antarctica Cratons, through sagduction.