Radiogenic whole-rock lead in Precambrian metasedimentary gneisses from south Norway: evidence of Sveconorwegian LILE mobility

Present-day whole-rock lead isotopic compositions of Precambrian metasedimentary rocks from the Sveconorwegian province of South Norway have been determined by mass spectrometry. The ²⁰⁶Pb/ ²⁰⁴Pb ratio ranges from 17.208 to 122.4 for the 39 samples analysed, most samples having ²⁰⁶Pb/²⁰⁴Pb >> 21. The samples show an imperfect fit to a regional "scatterchron' with an age of 1131 ± 30 Ma, and also some correlation of ²⁰⁸Pb/²⁰⁴Pb with ²⁰⁶Pb/²⁰⁴Pb. The metasediments must have accumulated radiogenic lead at high to very high U/Pb ratios since the Precambrian. The linear correlation of uranogenic lead isotope ratios is due to the introduction of uranium to the mestasediments during the Sveconorwegian orogeny, most probably as a result of interaction with fluids of crustal origin in amphibolite facies metamorphic conditions. The strong differentiation of the U/Pb ratio brought about by this process overshadowed any residual heterogeneity in the initial isotopic composition of lead in the rocks on a regional scale. -from Authors