Metamorphic overprinting of Sm-Nd isotopic systems in volcanic rocks: the Telemark Supergroup, southern Norway

The Telemark Supergroup of southern Norway consists of a series of metamorphosed Mesoproterozoic volcanics and sediments. The volcanic pile records three metamorphic events, a pervasive regional greenschist facies event(ca. 1050 Ma, [Starmer, 1993]), a more localised early (pre-regional metamorphism) burial-type event and a late thermal metamorphism related to granite intrusion (ca. 900 Ma [Priem et al., 1973]). In the present study, major and trace element data are employed to illustrate the influence of these metamorphic events on apparent initial Nd isotopic compositions in metamorphosed volcanics. In the case of the basaltic volcanics, burial and thermal metamorphism caused substantial fractionation of the REE and alteration of Nd isotopic compositions by interaction with open system fluids, such that spurious ages might result from age calculations using the Nd isotopic data. During the regional metamorphism, element mobility was limited to the LILE (Rb, Sr, K and Ba); the REE were not fractionated and the original Nd isotopic signatures were preserved. The regional metamorphic effects on the acid volcanics are related to the degree of matrix recrystallisation, since this matrix contains a large fraction of the REE. Where groundmass recrystallisation was limited, the original Nd isotopic signatures were retained, but where the groundmass has been extensively recrystallised the REE have been fractionated resulting in a Sm-Nd isochron age [Menuge, 1985] ca. 300 Ma younger than the U-Pb zircon age [Dahlgren et al., 1990]. The results of this study emphasise the importance of distinguishing the nature of metamorphic fluid/rock reactions, the potential for REE mobility during low-grade metamorphism and the need for careful selection and geochemical screening of samples for Nd isotopic studies in metamorphosed volcanic terrains. (C) 1998 Elsevier Science B.V.