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Young ages from old garnet in the polyphase metamorphic terrane of the Limpopo Belt, South Africa

Abstract:

The P–T–t path of high-grade metamorphic rocks is significant for understanding vertical motions and heating of the crust. In general, garnet is used to constrain P–T paths and metamorphic zircon is used to obtain t for different metamorphic stages. However, this approach may be complicated by polyphase tectonothermal events. The Central Zone of the Limpopo Belt in South Africa is a complex Precambrian metamorphic terrane that has experienced three distinct high-grade metamorphic events at 3.22 Ga (M<inf>1</inf>), 2.62 Ga (M<inf>2</inf>), and 2.02 Ga (M<inf>3</inf>), based on zircon dating. A previous investigation of a metapelite enclave in the Bulai pluton in the Central Zone showed that metamorphic zircon in the enclave recorded mainly M<inf>2</inf> granulite-facies metamorphism and was “protected” by the Bulai pluton. However, titanite and rutile U–Pb ages of the same sample record the M<inf>3</inf> event. Here we investigated garnet Sm–Nd isotopic systematics of four metapelite samples from the Evelyn enclave in the Bulai pluton. We constructed single mineral Sm–Nd isochrons for garnet using micro-samples, in order to avoid the risk of isotopic disequilibrium between different minerals or whole-rocks. Metamorphic zircon with sillimanite inclusions has yielded a granulite-facies metamorphic age of ca. 2.6 Ga. However, the Sm–Nd isotopic data for the garnet record much younger ages of ca. 2.03 Ga. These ages are consistent with the M<inf>3</inf> event in this area, but were rarely identified in previous zircon dating studies. This suggests that the M<inf>3</inf> event reached sufficiently high temperatures to totally reset the rutile and titanite U–Pb and garnet Sm–Nd systems. Moreover, the decoupling of the age of garnet formation and the date it records needs to be carefully considered when reconstructing P–T–t paths, and again highlights the usefulness of garnet in the study of polyphase metamorphic terranes.