The age of mid-proterozoic phosphatic metasediments in Finland as indicated by radiometric UPb dates |
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| Authors: | Matti Vaasjoki Olli Äikäs Pentti Rehtijärvi |
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| Institution: | 1. Geological Survey of Finland, SF-02150 ESPOO 15, Finland;2. Geological Survey of Finland, P.O. Box 237, SF-70101 KUOPIO 10, Finland;3. Geological Survey of Finland, SF-02150 ESPOO 15, Finland |
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| Abstract: | Two uranium-bearing phosphatic metasedimentary occurrences at Lampinsaari, Vihanti and Temo, Nilsiä, in Finland have been dated by isotopic UPb analyses of whole rock samples to be 1876±2 Ma old. The dates reflect the time of the Svecokarelian regional metamorphism in high amphibolite facies. An analysis of a uranium thucolite from the deposit at Nuottijärvi, Paltamo, that was metamorphosed in intermediate greenschist facies gives a date of 1897±7 Ma, suggesting that the phosphatic sedimentation had occurred before that time. An upper time limit for the sedimentation is set by a previous date of 2080±45 Ma, which was obtained from a banded iron one formation. The absence of thorium in the deposits suggests a marine environment of deposition. |
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