TIMING AND NATURE OF CRUST FORMATION IN THE EAST AFRICAN OROGEN DURING THE ASSEMBLAGE OF GONDWANA: CLUES FROM ZIRCON U-PB, HF AND OXYGEN ISOTOPES

Amalgamation of the supercontinent Gondwana led to formation of the East African Orogen (EAO). In Tanzania and Kenya the EAO is a collage of high-grade metamorphic terranes with different pressure-temperature and structural histories. Published whole rock (WR) Nd and Sr and feldspar Pb isotopes indicate the presence of a widespread Archean crustal component, and a late Mesoproterozoic juvenile component restricted to several smaller areas. The juvenile material has been interpreted as part of the pre-collision, eastern active continental margin of West Gondwana. However, the nature of mantle or crustal components and timing of magmatic crystallization remain open questions.

We present the first integrated in situ oxygen, Hf and U-Pb study on zircon from East Tanzanian granulite-facies orthogneisses to assess these questions. Of six representative samples, five have Proterozoic and one has Archean basement affinity based on U-Pb ages (SIMS, this study). The samples assessed previously as juvenile and the Archean have Hf single-stage depleted mantle model ages (DMAs) closely matching the Nd WR results, and near zero (Archean) or positive values (+7 to +10) for εHf(t), indicating a juvenile nature. Samples previously interpreted to be from an area of Archean and Proterozoic mixing are different, with zircon Hf DMAs as much as 1 Byrs younger than Nd DMAs, negative εHf(t), but only one Archean Hf analysis out of n=60. Hf DMAs within samples are remarkably consistent, with SDs of only 40-150 Myrs. A sample from the easternmost, metasediment-dominated domain has a Hf DMA 0.5 Byrs younger than its Nd DMA, and a high δ¹⁸O of +10, indicating sediment assimilation. Oxygen isotopes of zircon from the other Proterozoic samples also show remarkable within-sample homogeneity, a range of δ¹⁸O of 2 units, an average within the mantle range and no systematic correlation with SiO₂ content. The Archean sample has slightly higher variability, with a lower than mantle δ¹⁸O of +4.7.

This pilot study provides additional evidence that only a specific area in the EAO of Tanzania is juvenile with late Mesoproterozoic DMA and crystallization ages of 0.65-1.02 Ga. Assimilation of older crustal components and of sedimentary material has been confirmed for two other regions. The extent of these crustal domains has to be explored in a more detailed study.