Provenance characteristics of Mesoproterozoic metasedimentary rocks from Telemark, South Norway: a Nd-isotope mass-balance model

Whole-rock Nd isotope data on stratigraphically constrained samples of metasedimentary rocks from the ca. 1.50 to ca. 1.12 Ga Telemark supracrustal sequence in central South Norway define a linear array with a meaningless, late Archean apparent age; this is a mixing-line defined by two distinct endmembers, characterised by 147Sm/144Nd=ca. 0.09 and ca. 0.13; and present-day 143Nd/144Nd=0.5116 and 0.5122, respectively. From published SIMS and new LAM-ICPMS U–Pb ages of detrital zircon populations, ca. 1.8 Ga, ca. 1.5 Ga and ca. 1.15 Ga source components can be identified. The initial Nd isotope composition of sediments varies with stratigraphic level and geographic position within the basins, reflecting competition between juvenile volcanic sources and Paleoproterozoic granitoids throughout the ca. 400 Ma depositional period of the sediments. Given adequate external constraints, a normal whole-rock Sm–Nd analysis of a (meta)sedimentary rock can in principle be used to determine the relative contributions of up to four provenance components. The REE concentrations in the sedimentary rocks are controlled by source characteristics and differentiation processes acting during transport and deposition, which for most units have not affected the Sm/Nd ratio of the sediment. Initial Nd isotopic compositions and model ages are inherited from the source components, and are not significantly controlled by syn- or post-depositional differentiation processes. A (semi)quantitative model of sedimentary mass balance indicates that variations in volcanogenic input are erratic in the ca. 1.4 Ga Vindeggen group and the 1.15 Ga Lifjell group (both previously assigned to the obsolete Seljord group), reflecting changes in provenance with time as well as laterally. During deposition of the 1.15 Ga and younger Oftefjell and Høydalsmo groups (parts of the obsolete Bandak group), the accumulated volcanogenic input increased with time, as a result of Sveconorwegian volcanism. The present findings support the presence of Paleoproterozoic (1.7–1.9 Ga) source terranes in central South Norway.