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

• Main Authors: M. Vaasjoki, H. Huhma
• Format: Article
• Language: English
• Published: Geological Society of Finland 1999-06-01
• Series: Bulletin of the Geological Society of Finland
• Subjects: metavolcanic rocks; metabasalt; sulfides; absolute age; Pb/Pb; Sm/ Nd; genesis; Paleoproterozoic; Haveri; Finland
• Online Access: http://www.geologinenseura.fi/bulletin/Volume71/sgs_bt_071_1_pages_143_153.pdf

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 207Pb/204Pb vs. 206Pb/204Pb 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 εNd (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. Some chalcopyrite trace leads plot close to the whole rock array while others lie above it. This is interpreted as indicating two distinct mineralisation processes. The primary and major process involved lead which was cogenetic with the basalts, while the second mineralising fluid introduced radiogenic (high 207Pb) upper crustal lead scavenged from the adjacent sedimentary rocks. The least radiogenic leads at Haveri and in the Outokumpu ophiolite complex some 300 km NE are similar and the two occurrences can be coeval. The preservation of original mantle material at Haveri may be interpreted as suggesting that continental crust had formed in the Tampere area 1900-2000 Ma ago.