Trace-element characteristics and Pb isotopic evolution of metasediments and associated Proterozoic rocks from the amphibolite- to granulite-facies Bamble sector, southern Norway

A study of trace-element distribution and Rb/Sr, Sm/Nd, and Pb isotopic compositions has been performed on amphibolite- and granulite-facies metasediments, mafic granulites, dioritic gneiss, intrusive enderbitic veins and calc-alkaline tonalitic gneisses from the Proterozoic Bamble sector, southern Norway. The LILE content of the granulite-facies metapelites is similar to the average of the upper crust and post-Archaean average shales. The K/Rb ratios of the amphibolite- and granulite-facies semipelites and impure quartzites are similar, and the K/Rb ratios of the metapelites show overall similarities to the ratios of undepleted metapelites (< 300). There is evidence of incomplete Sveconorwegian correlations in the RbSr isotopic system, and a poor correlation in the SmNd system. The calculated tDM model ages for the metasediments can represent crustal residence times of 1.67 to 2.05 Ga. The present-day lead isotopic data can be reproduced by a three-stage model of Pb evolution in which (t0 = 4.57 Ga, t1 = 1.9 or 1.5 Ga, t2 = 1.1 Ga, μ1 (238U/204Pb) = 7.9, κ1 (232Th/238U) = 2.5, κ2 ≤ 4.15), where t1 and t2 are related to separation from the mantle and the Sveconorwegian orogeny, respectively. The protolith of the metasediments, the dioritic gneiss and the mafic granulites evolved in high-U/Pb environments prior to Sveconorwegian times (μ2 = 16−32, 19,and18−45, respectively). Most of these rock units experienced conditions ofμ2 > μ3 andκ2 < κ3, suggesting a large-scale fractionation in the UThPb system connected to U-mobility during the medium- to high-grade metamorphism. There are no unambiguous differences in the Pb isotopic composition between the amphibolite- and granulite-facies rocks, but the mafic granulites from the classical ‘depleted granulite’ area of Tromøy island evolved in overall lowerμ2 (238U/204Pb) environments than similar rocks from the neighbouring Hisøy island. This suggests that the similar U content of these rocks largely can be due to local pre-Sveconorwegian 238U/204Pb variations. Sveconorwegian intrusive enderbitic dykes and veins crystallized with low LILE and Nb concentrations at granulite-facies PTaCO2 conditions. Pre-Sveconorwegian granulite-facies tonalitic gneisses show a less depleted trace-element pattern and low Nb concentrations and the hornblende-free tonalitic gneisses have significantly lower U content than their hornblende-bearing counterparts. The investigated metasediments have very low contents of the less mobile elements Nb, Sr, Cu and P2O5, which probably represent characteristics of the pre-sedimentary protolith.