Crustal Evolution in the SW Part of the Baltic Shield: the Hf Isotope Evidence

The results of a laser ablation microprobe–inductively coupled by the more widely used samarium–neodymium system (e.g. Dickin, 1995). As a tracer of petrogenetic processes, plasma mass spectrometry Lu–Hf isotope study of zircons in however, Lu–Hf has one important advantage over Sm– 0·93–1·67 Ga rocks from south Norway indicate that early Nd: zircons retain a robust memory of their initial Hf Proterozoic protoliths of the Baltic Shield have present-day Hf/ isotopic compositions, because of their high Hf conHf Ζ 0·28190 [ Hf(t) = 5–6], whereas 1·52–1·60 Ga centrations, low Lu/Hf ratios and general ability to juvenile additions to the continental margin have Hf/Hf = survive metamorphic processes. Zircon has the additional 0·2820 [ Hf(t) = 12–13]. Midto late Proterozoic felsic igneous advantage of being datable by U–Pb techniques. A numrocks in the region are characterized by a range of Hf isotopic ber of benchmark papers have established the hafnium compositions suggesting mixing of material derived from Palaeoisotope signatures of the most significant reservoirs in the proterozoic crust from the Baltic Shield and/or mid-Proterozoic upper mantle and continental crust, and their evolution juvenile crust. New mantle-derived magmas were added to the crust through geological time (e.g. Patchett et al., 1981; Verat >1·48 Ga and in Sveconorwegian time. Late Sveconorwegian voort & Patchett, 1996; Blichert-Toft & Albarède, 1997; granites from the area west of the Oslo Rift have inherited zircons Vervoort & Blichert-Toft, 1999; Vervoort et al., 1999; with low Hf/Hf (<0·28180), suggesting that a pre-1·7 Ga Griffin et al., 2000). Until recently, the need for elaborate crustal source contributed to the magmas. The evolution of the laboratory techniques has prevented the widespread use continental crust in this region is thus a result of repeated interaction of hafnium isotopes in zircons as a tracer of crustal between mantle-derived magmas and midto early Proterozoic evolution. However, advances in analytical technology, crustal rocks. The results of this study confirm the presence of early which include the development of stable laser ablation Proterozoic rocks in the deep crust west of the Oslo Rift, and microprobes and multicollector, plasma-source, magnetic support tectonic models in which the protolith of the western part sector mass spectrometers, have made the in situ microof south Norway has been part of the Baltic Shield since the early analysis of hafnium isotopes in zircons a matter of routine. Proterozoic. In this paper, we use hafnium isotope data from single zircons to gain insights into the evolution of the