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

Abstract

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 Rb Sr isotopic system, and a poor correlation in the Sm Nd system. The calculated t DM 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 ( t 0 = 4.57 Ga, t 1 = 1.9 or 1.5 Ga, t2 = 1.1 Ga, μ 1 ( 238U/ 204Pb) = 7.9, κ 1 ( 232Th/ 238U) = 2.5, κ 2 ≤ 4.15), where t 1 and t 2 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 (μ 2 = 16−32, 19,and18−45, respectively). Most of these rock units experienced conditions ofμ 2 > μ 3 andκ 2 < κ 3, suggesting a large-scale fractionation in the U Th Pb 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 lowerμ 2 ( 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 P T aCO 2 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 P 2O 5, which probably represent characteristics of the pre-sedimentary protolith.