Cleaning up the record – revised U-Pb zircon ages and new Hf isotope data from southern Sweden

ABSTRACT

Ten samples of felsic plutonic rocks from the Eastern Segment of the Sveconorwegian Orogen in southern Sweden, previously dated by ID-TIMS on zircon, have been dated anew using SIMS spot analysis of individual zircon grains, leading to more reliable and in most cases also more precise revised magmatic crystallization ages. A gneissic monzonite within the Protogine Zone in Småland yields a revised U-Pb age of ca. 1725 Ma, four samples of orthogneiss from Skåne all yield revised ages between 1700 and 1690 Ma, while two samples of coarse-grained granitic gneiss in the same region yield ages between 1690 and 1680 Ma. These revised ages are between 15 and 250 m.y. older than previously obtained TIMS ages. Two samples of the Gumlösa-Glimåkra granite along the Protogine Zone in northern Skåne and one sample of related syenite yield ages around 1220 Ma, similar but more precise compared to the previous ages. The U-Pb zircon data have been complemented by Hf isotope analysis by LA-ICP-MS on the same grains, and previously obtained initial Sr and Nd whole-rock isotope data have been recalculated to the revised crystallization ages. The Sr isotope data scatter, while the revised initial ε_Nd values fall between +1 and +2 for the older rocks, and close to 0 for the younger intrusives along the Protogine Zone. Initial ε_Hf in magmatic undisturbed zircons shows relatively little spread within each sample, between 2 and 4 Epsilon units, disregarding a few outliers, with average values for the 1725 to 1680 Ma rocks between +3 and +5.5 and for the 1220 Ma rocks at ca. +1.5. Covariation between initial ε_Nd and initial ε_Hf in the older rocks suggests either mixing between two isotopically distinct magma sources or one magma source which was isotopically heterogeneous. The isotopic signatures of the 1220 Ma intrusive rocks along the Protogine Zone are indicative of juvenile mantle input to their magmas, rather than pure crustal melting.

KEYWORDS:

• U-Pb zircon dating
• Hf isotopes
• granitoids
• Eastern Segment
• Protogine Zone
• Sveconorwegian Orogen
• Southern Sweden

Acknowledgments

Most of the samples used in this study were obtained in the 1980:ies in cooperation with Hugo Wikman, then at the Geological Survey of Sweden (SGU) in Lund. Sample 76314 was inherited from the collection of the late professor Eric Welin at the Swedish Museum of Natural History (NRM). Paula Allart assisted with the original zircon separation. The staff of the Nordsim facility at NRM, Martin Whitehouse, Heejin Jeon and Kerstin Lindén, assisted with sample preparation, SIMS U-Th-Pb analysis and subsequent data reduction, respectively. Melanie Kielman-Schmitt from the Vegacenter facility of the NRM assisted with LA-ICP-MS analysis and data reduction of Lu-Hf. Andreas Karlsson at the Geo department of NRM helped with the BSE imaging of zircons and with mineral identification by whole-rock XRD analysis of two of the samples. Jenny Andersson at SGU contributed the digital map used in Figure 1 and Fredrik Hellström a file of the digital geochronological database of the SGU used in Appendix 5 and Figure 10. Charlotte Möller at Lund University gave useful input to an early version of the manuscript, and comments by journal reviewers Thomas Lundqvist and Ulf Bertil Andersson as well as subject editor Magnus Ripa improved the manuscript further. This study was financed by a R&D grant (no. 36-2946/2018) from the Geological Survey of Sweden to ÅJ. The Nordsim-Vegacenter facility was financed by the Swedish Research Council (VR) and the Swedish Museum of Natural History with additional support from the University of Iceland. This is Nordsim contribution no. 683, and Vegacenter publication no. 041.

Disclosure statement

No potential conflict of interest was reported by the author(s).

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Funding

This work was supported by the Sveriges Geologiska Undersökning [36-2946/2018].