Age and origin of the Mesoproterozoic basement of the Nesodden Peninsula, SE Norway : A geochronological and isotopic study

Abstract

Abstract The Nesodden study area is part of the complex Southwest Scandinavian Domain in the Baltic Shield. To investigate its magmatic/metamorphic evolution, and to constrain magma sources, 482 in situ LA-MC-ICPMS U-Pb and Lu-Hf analyses were performed on zircon grains from granitic to tonalitic gneisses, granitoids, and a granitic pegmatite. The U-Pb data yield ages at 1.54-1.53 Ga for foliated granites and granitic gneisses, and 1.50-1.49 Ga for a second group of granitic and tonalitic rocks, whereas the pegmatite gave an age of 1.05 Ga. The present day 176Hf/177Hf ratios mainly range from 0.28191 to 0.28207; however, a smaller group (mainly zircons from the Sveconorwegian pegmatite) have higher 176Hf/177Hf ratios of 0.28208 and 0.2822. The time-corrected initial Hf isotopic composition of zircons in 6 analysed rocks has a range of 5-6 Hf–units, whereas zircons in the remaining 3 samples have a much larger range of 9-11 Hf–units. The ranges indicate that the magmas were heterogeneous, with contributions from isotopically distinct sources, including depleted mantle ( Hf = ca. +10) and Paleoproterozoic crustal rocks corresponding in age and composition to the granitoids of the Transscandinavian Igneous Belt (TIB) which have Hf = -1 to -2. Whole-rock Pb isotope compositions of the samples were determined by solution MC-ICPMS analysis. The present day 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios vary widely, from 17.282 to 29.586, from 15.487 to 16.414 and from 36.901 to 45.912, respectively. A three-stage model of Pb isotopic evolution can reproduce the present-day compositions: (1) A mantle stage ending at ca. 2.1 Ga; (2) extraction and emplacement of the crustal precursor at ca. 1.5 Ga, and; (3) anatexis and metamorphism at ca. 1.05 Ga. The data confirm previously inferred Mesoproterozoic younging to the west in the south-western part of the Baltic Shield, and support a model of westwards growth of the Shield along a long-lived active continental margin. The effect of Sveconorwegian metamorphism is reflected in discordant U-Pb data from the gneisses, and confirmed by Pb-Pb modelling. Influence of Permian magmatic activity in the Oslo Rift area at ca. 290 Ma is also reflected in the U-Pb data. Three-stage Pb-Pb modelling agrees with the crustal residence time obtained by the Hf analyses. Data from inherited zircons further indicate that the crustal component detected in the rocks does not derive exclusively from TIB equivalents, but also reflects some recycling of marginally older, calc-alkaline gneiss complexes in the region.