Lead and neodymium isotopic results from metabasalts of the Haveri Formation, southern Finland: Evidence for Palaeoproterozoic enriched mantle

VAASJOKI, MATTI and HUHMA, HANNU 1999. Lead and neodymium isotopic results from metabasalts of the Haveri Formation, southern Finland: evidence for Palaeoproterozoic enriched mantle. Bulletin of the Geological Society of Finland 71, Part 1, 143-153. Tholeiitic metabasalts and coexisting sulphides have been analysed for their Pb and Nd isotopic compositions from the Proterozoic Haveri Formation, which forms the basal unit of the Tampere Schist Belt in southern Finland. Ten whole rock samples analysed for Pb isotopes form a sublinear array which yields rather uncertain age estimates in the 1900-2000 Ma range and lies on the Pb/Pb vs. Pb/Pb diagram well below the average global lead evolution curve. The initial lead isotopic composition inferred from the whole rock data and measured on chalcopyrite is the least radiogenic obtained from the Svecofennian domain, and precludes involvement of old upper crustal material in basalt genesis. This together with the geochemical composition and initial eNd(1900) of +0.5±0.6 suggest that the Haveri mafic metavolcanic rocks were not derived from convective MORB-type mantle. The source was rather a mantle, which had been enriched in LREE for a considerable time period.