TRACE-ELEMENT CHARACTERISTICS AND PB ISOTOPIC EVOLUTION OF METASEDIMENTS AND ASSOCIATED PROTEROZOIC ROCKS FROM THE AMPHIBOLITE-FACIES TO GRANULITE-FACIES BAMBLE SECTOR, SOUTHERN NORWAY

A study of trace-element distribution and Rb/Sr, Sm/Nd, and Pb isotopi c compositions has been performed on amphibolite- and granulite-facies metasediments, mafic granulites, dioritic gneiss, intrusive enderbiti c veins and calc-alkaline tonalitic gneisses from the Proterozoic Bamb le sector, southern Norway. The LILE content of the granulite-facies m etapelites is similar to the average of the upper crust and post-Archa ean average shales. The K/Rb ratios of the amphibolite- and granulite- facies semipelites and impure quartzites are similar, and the K/Rb rat ios of the metapelites show overall similarities to the ratios of unde pleted metapelites (< 300). There is evidence of incomplete Sveconorwe gian correlations in the Rb-Sr isotopic system: and a poor correlation in the Sm-Nd system. The calculated t(DM) model ages for the metasedi ments can represent crustal residence times of 1.67 to 2.05 Ga. The pr esent-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, t(2)=1.1 G a, mu(1), (U-238/Pb-204) = 7.9, kappa(1) (Th-232/U-238) = 2.5, kappa(2 ) less than or equal to 4.15), where t(1) and t(2) are related to sepa ration from the mantle and the Sveconorwegian orogeny, respectively. T he protolith of the metasediments, the dioritic gneiss and the mafic g ranulites evolved in high-U/Pb environments prior to Sveconorwegian ti mes (mu(2) = 16-32, 19, and 18-45, respectively). Most of these rock u nits experienced conditions of mu(2) > mu(3) and kappa(2) < kappa(3), suggesting a large-scale fractionation in the U-Th-Pb system connected to U-mobility during the medium- to high-grade metamorphism. There ar e no unambiguous differences in the Pb isotopic composition between th e amphibolite- and granulite-facies rocks, but the mafic granulites fr om the classical 'depleted granulite' area of Tromoy island evolved in overall lower mu(2) (U-238/Pb-204) environments than similar rocks fr om the neighbouring Hisoy island. This suggests that the similar U con tent of these rocks largely can be due to local pre-Sveconorwegian U-2 38/Pb-204 variations. Sveconorwegian intrusive enderbitic dykes and ve ins crystallized with low LILE and Nb concentrations at granulite-faci es P-T-aCO(2) conditions. Pre-Sveconorwegian granulite-facies tonaliti c gneisses show a less depleted trace-element pattern and low Nb conce ntrations and the hornblende-free tonalitic gneisses have significantl y lower U content than their hornblende-bearing counterparts. The inve stigated metasediments have very low contents of the less mobile eleme nts Nb, Sr, Cu and P(2)O5, which probably represent characteristics of the pre-sedimentary protolith. (C) 1997 Elsevier Science B.V.