Metamorphic evolution and tectonic significance of Neoarchean high-pressure mafic granulites in the Central Limpopo Belt, South Africa

The Sand River area is in the northeastern part of the Central Zone (CZ) of the Limpopo Belt, and represents a key terrane to reveal early Precambrian tectonic processes in southern Africa. Based on petrology, U–Pb zircon ages, and major and trace element data of high-pressure (HP) mafic granulites from the Sand River area, the tectonic evolution of the Central Zone of the Limpopo Belt is re-evaluated here that allows revision of previous interpretations. The protoliths of the HP mafic granulites and amphibolites are sub-alkaline tholeiitic basalts similar to those formed at a mid-ocean ridge and intruded at 2.8 Ga. Based on the distinct mineral assemblages, three metamorphic stages (M1–M3) are recognized in the studied samples: (1) a peak HP granulite facies stage M1 (~1.17–1.32 GPa/~825–870°C), (2) a post-peak near-isothermal decompression stage M2 (~0.70–0.73 GPa/~835–880°C), and (3) a later, independent metamorphic stage M3 (~0.53–0.69 GPa/~793–801°C). M1 and M2 define together a clockwise P–T path. It reveals important information related to the Neoarchean collisional orogeny, and its later overprinted by an independent orogeny with subsequent exhumation and cooling. U–Pb zircon dating of two mafic granulites samples yielded protolith formation of the mafic granulites is prior to ~2.72 Ga, and the two metamorphic age peaks of ~2.72 (M1) and ~2.0 Ga (M3), which present the peak and post-peak cooling stages, respectively. The metamorphic evolution of HP mafic granulites in the Sand River area as reconstructed in this study suggests the Neoarchean collision of the Kaapvaal and Zimbabwe cratons and a discrete Paleoproterozoic orogenic overprint followed by post-orogenic exhumation and cooling.

桑德河（Sand River）区域位于林波波造山带（Limpopo Belt）中央带（Central Zone, CZ）东北部，是揭示南非早前寒武纪构造过程的关键地体。本文基于该区域高压（HP）镁铁质麻粒岩的岩石学、U-Pb锆石定年以及主量和微量元素数据，重新评估了林波波造山带中央带的构造演化历程，修正了此前的相关认识。高压镁铁质麻粒岩和角闪岩的原岩为类似洋中脊形成的亚碱性拉斑玄武岩，其侵入时代为2.8 Ga。根据独特的矿物组合，研究样品中识别出三期变质作用（M1-M3）：（1）高压麻粒岩相峰期阶段M1（约1.17~1.32 GPa/约825~870℃）；（2）峰后近等温减压阶段M2（约0.70~0.73 GPa/约835~880℃）；（3）晚期独立变质阶段M3（约0.53~0.69 GPa/约793~801℃）。M1与M2共同构成了顺时针压力-温度（P-T）轨迹。该轨迹揭示了与新太古代碰撞造山作用相关的重要信息，且该新太古代碰撞造山作用后期被一期独立的造山作用叠加，伴随随后的折返与冷却过程。对两件镁铁质麻粒岩样品的U-Pb锆石定年结果显示，麻粒岩原岩形成时代早于约2.72 Ga，且存在两个变质年龄峰值：约2.72 Ga（对应M1阶段）和约2.0 Ga（对应M3阶段），分别代表峰期变质与峰后冷却阶段。本研究重建的桑德河区域高压镁铁质麻粒岩的变质演化历程表明，卡普瓦尔克拉通与津巴布韦克拉通曾发生新太古代碰撞，随后叠加了一期独立的古元古代造山作用，之后经历了造山后折返与冷却过程。