Dating Post-Metamorphic Cooling of the Eastern Granulites in the Mozambique Belt of Northern Tanzania Using the Garnet Sm-Nd Method

Abstract

Granulites from the Usambara, Wami River and Uluguru areas in the northern part of the Mozambique Belt in Tanzania yield Sm-Nd garnet — whole rock ages of between 580 and 634 Ma with a mean of 609 ± 11 Ma (2σ). This mean age is only slightly younger than the previously published peak metamorphic age of 641 ± 2 Ma, suggesting that, contrary to some earlier arguments, garnet Sm-Nd ages can be used to closely constrain the age of peak metamorphism even in slowly cooled terranes. Using published peak metamorphic temperatures of ∼810°C and cooling rates of 1–4°C/Ma, the mean age translates into garnet closure temperatures of 690 to 780°C. The similarity in garnet ages over widely separated areas, coupled with the previously established similarity in equilibrium PT conditions, indicate that isolated complexes that form the Eastern Granulites in the Tanzanian sector of the Mozambique Belt share the same thermal history and were formed under the same geodynamic setting. A few published garnet ages of between 525 and 545 Ma indicate a younger, less pervasive event of granulite facies metamorphism in the Belt. The bimodal distribution of garnet ages supports a previously published hypothesis that the assembly of Gondwana took place in two stages. The ∼610 Ma old ages most likely date cooling from granulite facies metamorphism arising from regional crustal thickening associated with the amalgamation of India, Madagascar, parts of eastern Antarctica, the Kalahari craton, the Congo craton and the Arabian-Nubian shield (forming the IMSLEK-ANS collage). On the other hand, the 525–545 Ma ages may mark cooling from a thermal event associated with the collision of Australo-Antarctica with the IMSLEK-ANS collage.