Timing of late- to post-tectonic Sveconorwegian granitic magmatism in South Norway.

Abstract

Dating of late- to post-tectonic Sveconorwegian granitic intrusions from South Norway by the SIMS U-Pb method on zircons and by internal Pb-Pb isochrons on rock-forming minerals indicates a major event of granitic magmatism all across southern Norway in the period 950 to 920 Ma. This magmatic event included emplacement of mantle-derived magma into the source region of granitic magmas in the lower crust east of the Mandal-Ustaoset shear zone, and formation of hybrid magmas containing crustal and mantle-derived compontents. West of the Mandal-Ustaoset shear zone, granitic magmatism started earlierm at c. 1030 Ma. A distinct group of granites, characterized by low Sr concentration and high Rb\/Sr ratio, is restricted to central Telemark, and shows evidence of involvement of a component related to the ca. 1500 Ma metarhyolite of the Telemark supracrustal sequence. Whereas on of these granites clearly belongs to the 920-950 Ma age group, two of the intrusions dated in this study (Otternes and Gunnarstul) are significantly older and may be genetically related to an earlier event of anorogenic magmatism in the region at c. 1120 to 1150 Ma.