Igneous and metamorphic geochronologic evolution of granitoids in the central Eastern Segment, southern Sweden

Abstract

The Eastern Segment abutting the Transscandinavian Igneous Belt (TIB) mostly consists of rocks with overlapping igneous ages. In the Eastern Segment west of Lake Vättern, granitoids of clear TIB affinity exhibit strong deformational fabrics. This article presents U–Pb zircon ages from 21 samples spanning the border zone between these deformed TIB rocks in the east, and more thoroughly reworked rocks in the west. Magmatic ages fall in the range 1710–1660 million years, irrespective of the degree of deformation, confirming the overlapping crystallization ages between deformed TIB rocks and orthogneisses of the Eastern Segment. A common history is further supported by leucocratic rocks of similar ages. Prolonged orogenic (magmatic) activity is suggested by continued growth of zircon at 1.66–1.60 Ga. Six of the weakly gneissic rocks show zircons with cathodoluminescence-dark patches and embayments, possibly partly replacing metamict parts of older magmatic crystals, with ²⁰⁷Pb/²⁰⁶Pb ages dominantly between 1460 and 1400 million years, whereas three of the gneisses have zircon rims with calculated ages of 1440–1430 million years. Leucosome formation took place at 1443 ± 9 and 1437 ± 6 Ma. The minimum age of SE–NW folds was determined by an undeformed 1383 ± 4 million years crosscutting aplitic dike. Sveconorwegian zircon growth was not found in any of the samples from the studied area. To our knowledge, 1.46–1.40 Ga metamorphism affecting the U–Pb zircon system has not previously been reported this far northeast in the Eastern Segment. We suggest that the E–W- to SE–NW-trending deformation fabrics in our field area were produced during the Hallandian–Danopolonian orogeny and escaped later, penetrative Sveconorwegian reworking.

Keywords:

• Transscandinavian Igneous Belt
• Eastern Segment
• Hallandian–Danopolonian orogeny
• U–Pb zircon SIMS geochronology
• Fennoscandia

Acknowledgements

This study was supported through a fund from the Faculty of Science, University of Gothenburg, and a grant (60-1159/2002) from the Geological Survey of Sweden to Sven Åke Larson. Reviews of a previous version of the manuscript by Jenny Andersson and Fernando Corfu helped to improve the text. The rapid yet careful editional handling by Journal Editor Gary Ernst is very much appreciated. Martin Whitehouse, Lev Ilyinsky, Kerstin Lindén, and Chris Kirkland at the NordSIM facility at the Swedish Natural Museum, Stockholm, are thanked for help and access during analysis sessions. The NordSIM facility is supported by the research councils in Denmark, Norway, and Sweden and the Geological Survey of Finland, together with the Swedish Museum of Natural History. This is NordSIM contribution no 272.