Hornblende 40Ar/ 39Ar geochronology across terrane boundaries in the Sveconorwegian Province of S. Norway

Abstract

Hornblende incremental heating 40Ar/ 39Ar data were obtained from augen gneiss and amphibolite of the Sveconorwegian Province of S. Norway. In the Rogaland-Vest Agder and Telemark terranes, four pyroxene-rich samples, located close (≤ 10 km) to the anorthosite-charnockite Rogaland Igneous Complex, define an age group at 916 + 12/ − 14 Ma and six samples distributed in the two terranes yield another group at 871 + 8/ − 10 Ma. The first age group is close to the reported zircon UPb intrusion age of the igneous complex (931 ± 2 Ma) and the regional titanite UPb age (918 ± 2 Ma), whereas the second group overlaps reported regional mineral RbSr ages (895-853 Ma) as well as biotite KAr ages (878-853 Ma). In the first group, the comparatively dry parageneses of low- P thermal metamorphism (M2) associated with the intrusion of the igneous complex are well developed, and hornblende 40Ar/ 39Ar ages probably record a drop in temperature shortly after this phase. In other hornblende + biotite-rich samples, with presumably a higher fluid content, the hornblende ages are probably a response to hornblende-fluid interaction during a late Sveconorwegian metamorphic or hydrothermal event. A ca 220 m.y. diachronism in hornblende 40Ar/ 39Ar ages is documented between S. Telemark ( ca 870 Ma) and Bamble ( ca 1090 Ma). Differential uplift between these terranes was mostly accommodated by shearing along the Kristiansand-Porsgrunn shear zone. The final stage of extension along this zone occurred after intrusion of the Herefoss post-kinematic granite at 926 ± 8 Ma. On the contrary, the southern part of the Rogaland-Vest Agder and Telemark terranes share a common cooling evolution as mineral ages are similar on both sides of the Mandal-Ustaoset Line the tectonic zone between them. The succession within 20 m.y. of a voluminous pulse of post-tectonic magmatism at 0.93 Ga, a phase of high- T-low- P metamorphism at 0.93-0.92 Ga, and fast cooling at a regional scale ca 0.92 Ga, suggests that the southern parts of Rogaland-Vest Agder and Telemark were affected by an event of post-thickening extension collapse at that time. This event is not recorded in Bamble.