Hafnium isotope characteristics of late Palaeoproterozoic magmatic rocks from Blekinge, southeast Sweden: possible correlation of small-scale Hf and Nd isotope variations in zircon and whole rocks

Abstract

The isotopic composition of hafnium has been measured in zircons from seven magmatic rocks from the Blekinge region, southeast Sweden, previously dated by the U–Pb zircon method to 1.74–1.76 Ga, with one rock at 1.81 Ga. The results from all samples show limited variation, with a total range in initial ϵ_Hf between − 1.5 and +3.2, only slightly more than analytical uncertainty ( ± 1.5 epsilon units). However, taken together with previously published initial ϵ_Nd whole-rock values of − 0.3 to +1.0 from the same rock samples, a trend with a positive correlation between average initial ϵ_Hf (zr) for each sample and initial ϵ_Nd (WR) can be discerned. This trend could be taken to indicate mixing between a “mildly depleted” component derived from the lithospheric mantle or lower crust, and a slightly more enriched crustal component, either by mixing of separate magmas or by contamination of a mantle-derived magma with crustal material. Such mixing may have occurred both on a regional scale, as indicated by the overall trend, and locally within each intrusion and each rock sample with small-scale variations in isotope composition of the magma during crystallization, indicated by the variation in initial ϵ_Hf between the zircons in each sample. The overall results support the presence of a “mildly depleted” lithospheric mantle reservoir, both in terms of Nd and Hf isotopes, also beneath southern Fennoscandia in Proterozoic time.

Keywords::

• lutetium–hafnium
• hafnium isotopes
• granitoids
• mildly depleted mantle
• Palaeoproterozoic
• Fennoscandia
• Blekinge
• Sweden

Acknowledgements

The samples used in this study were obtained during several field trips a long time ago: sample 83113 by Eric Welin (Stockholm) in 1983, and sample 84086 together with Eric Welin and Karl-Axel Kornfält (then at SGU, Lund) the following year; sample 89068 together with Hugo Wikman (then at SGU, Lund) in 1989; and the remaining samples with Roland Gorbatschev, Svetlana Bogdanova, Andrius Rimsa (all from Lund University) and Karl-Axel Kornfält (SGU, Lund) in 2001. The map used in Fig. 1 was originally drawn by Audrius ČeČys (then at Lund University, now at Vilnius University). The manuscript benefitted from constructive comments by Anders Scherstén (Lund University) and one additional anonymous reviewer. This paper is contribution no. 44 from the Isotope Geology Laboratory of the Department of Geosciences, University of Oslo.