U-Pb, O and Lu-Hf isotope ratios of detrital zircon from Ghana, West-African Craton : formation of juvenile, Palaeoproterozoic crust

University essay from Lunds universitet/Geologiska institutionen

Abstract: Detrital zircon were sampled from five rivers in Ghana and they were analysed in situ for U-Th-Pb, O and Lu-Hf isotopes. Based on the analyses, their U-Pb crystallisation ages, δ18O isotope ratio, εHf and Hf-model ages were determined. This provides information about the origin and crustal evolution of the Palaeoproterozoic bedrock in Ghana. The U-Pb crystallisation ages of the zircon mainly range from 2.2-2.1 Ga, with an older zircon subset of ~2.22 Ga making up a large fraction of the Birim river sample (southeastern Ghana). These ages coincide with previously reported ages of volcanic and plutonic rocks in Ghana. Majority of the zircon have positive εHf (0.9-7.1) and relatively short residence time, indicating that they crystallised from juvenile magmas. Four zircon from the Birim river sample, that crystallised at ~2.15 Ga, do however have negative εHf and Archean model ages, suggesting substantial contribution from an Archean crust. Oxygen isotope ratios indicate that influences from reworked supracrustal rocks were minor during the initial stages of magmatic activity, with zircon having δ18O>6.5 being rare before ~2.17 Ga. A correlation is observed between younger crystallisation ages and increased proportion of zircon with elevated δ18O, which might point towards a maturing arc system. Modelling based on εHf and δ18O of the zircon indicates that the contribution from an Archean crust to the magmas was generally less than 10%. Several zircon, particularly from the Ankobra river population, have elevated δ18O and juvenile εHf, indicating rapid reworking of juvenile crust. Oxygen isotope ratios, crystallisation and Hf-model ages of detrital zircon have been used to evaluate the degree of addition of juvenile material to the crust and to model crustal growth throughout the geological record. Hf-model ages of the zircon that are reported here fall within a period when reworking of crust exceeds additions of new crust, as indicated by the detrital zircon record. This does not agree with the juvenile nature of the Birimian crust. Adding the data reported here to a dataset that was used for a previously established crustal growth model, has limited effect on the shape of the crustal growth curve, but suggests that the extent of reworking and recycling might be slightly overestimated in the Proterozoic.

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