Radiogenic whole-rock lead in Precambrian metasedimentary gneisses from South Norway: evidence of Sveconorwegian LILE mobility
2, 3
pp. 156-168

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 1 7.208 to 122.4 for the 39 samples analysed, most sarnples having 206Pb/204Pb :>> 21. The samples show an imperfect fit to a regional uranogenic lead correlation line ('scatterchron') with an age of 1 1 31 ± 30 Ma, and also some correlation of 208Pb/204Pb with 206Pb/204Pb. The metasediments must have aocumulated radiogenic lead at high to very high U/Pb ratios since the Precarnbrian 238U/204Pb :1> 24). 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 a bo ut by this process overshadowed an residual heterogeneity in the initial isotopic composition of lead in the rocks on a regional scale. Prior to uranium introduction, the precursors of the rocks studied had evolved in a LILE-enriched 'upper continental crust' environment for several hundred million years. If a crustal residence age of the protolith in the range 1700- 1900 Ma is assumed (in accordance with published depleted mantle neodymium isotope model ages on these and related rocks), the average 238U/204Pb ratio of the precursor would be in the range 20-24.