Reworking of Archaean and Palaeoproterozoic crust in the Mozambique belt of central Tanzania as documented by SHRIMP zircon geochronology

Abstract

New SHRIMP zircon ages for high-grade rocks from the Pan-African Mozambique belt (MB) of central Tanzania document reworking of Archaean–Palaeoproterozoic crust during the formation of this Neoproterozoic collisional orogen. Several gneisses and granulites from the Great Ruaha river area yielded late Archaean emplacement ages of 2575–2680 Ma for their magmatic precursors. Core-rim relationships in some zircons revealed new zircon growth during relatively short episodes of granitoid magmatism. Metamorphic zircons, dated at 1925 Ma, provide new evidence for high-grade metamorphism during formation of the Palaeoproterozoic Usagaran mobile belt. Metamorphic rims around magmatic cores in zircons extracted from granulite-facies garnet-amphibole bearing orthogneisses yielded an age of about 640 Ma, which is interpreted to reflect the peak of Pan-African regional high-grade metamorphism. Resorbed zircons extracted from migmatitic orthogneisses show a wide range of apparently concordant results, varying between ≈500 and 700 Ma. This wide range reflects HT/HP conditions during peak metamorphism as well as subsequent strong decompression (ITC cooling path). Our data suggest that the MB in Tanzania consists largely of Archaean crust north of the Great Ruaha river and of Palaeoproterozoic crust south of this river. Both the Archaean and Palaeoproterozoic domains were strongly reworked during the formation of the MB and experienced high-grade metamorphism at ∼640 Ma. Our results imply that the Tanzania craton and Usagaran province originally extended much farther to the east and were structurally reworked, almost beyond recognition, during Pan-African ductile deformation and recrystallization.