U–Pb SIMS dating of some granitoids from eastern Blekinge, southern Sweden

Abstract

Zircons from seven granitoids in eastern Blekinge have been dated using secondary ion mass spectrometry. The analyzed rocks include one Småland granitoid from north of the Småland-Blekinge Deformation Zone (SBDZ), two samples of megacrystic “Filipstad-type” granite from south of that zone and one sample each of the “Småland-type” Rödeby, Almö, Tjurkö and Jämjö granites. The results yield a crystallization age of 1778 ± 5 Ma for the Småland granitoid, and crystallization ages between 1770 ± 4 and 1758 ± 5 Ma for the other granitoids, in most cases substantially older than previous TIMS ages. These data show that the “Småland-type” granitoids in eastern Blekinge are similar in age to the surrounding Tving granitoids, and the more felsic of them may represent late-stage differentiates belonging to the same magmatic suite. As the Tving granitoids show differences both in degree of deformation, in geochemistry and in age, compared to the Småland granitoids north of the SBDZ, it is suggested that these represent two separate but closely related igneous suites, which could both be included within a TIB-1 supersuite. The investigated zircons showed very limited signs of metamorphic overprinting, and no metamorphic ages could be determined. However, the combined evidence from field observations and earlier U–Pb geochronology would suggest the presence of two separate metamorphic episodes in Blekinge, one in close connection with the formation of these rocks at 1.76–1.75 Ga and one connected to the intrusion of the Karlshamn granitoid suite at around 1.45 Ga.

Keywords:

• U–Pb zircon
• SIMS dating
• Tving granitoids
• Småland granitoids
• Blekinge

Acknowledgements

The original rock samples dated in this study were collected by Karl-Axel Kornfält, or (in the case of sample 89071) in close cooperation with him. The zircon separates that had been used for conventional TIMS dating were recovered and handed over to me by Hannu Huhma at GTK (Espoo, Finland) and Per-Olof Persson at LIG (Stockholm). The staff of the Nordsim facility are thanked for their help with the analytical work: Kerstin Lindén with preparation of the zircon mount, Lev Ilyinsky with the microprobe analysis, and Martin Whitehouse with subsequent calculations of raw data. The Nordsim facility is jointly financed by the Swedish Museum of Natural History, Swedish Research Council, Norwegian Research Council, Consortium of Danish geoscience institutions, and University of Iceland. This is Nordsim contribution No. 438. Discussions with Karl-Axel Kornfält on Blekinge geology, and comments by Anders Lindh and one anonymous reviewer assisted in improving this paper.