HORNBLENDE AR-40 AR-39 GEOCHRONOLOGY ACROSS TERRANE BOUNDARIES IN THESVECONORWEGIAN PROVINCE OF S. NORWAY/

Hornblende incremental heating Ar-40/Ar-39 data were obtained from aug en gneiss and amphibolite of the Sveconorwegian Province of S. Norway. In the Rogaland-Vest Agder and Telemark terranes, four pyroxene-rich samples, located close (less than or equal to 10 km) to the anorthosit e-charnockite Rogaland Igneous Complex, define an age group at 916+12/ -14 Ma and six samples distributed in the two terranes yield another g roup at 871+ 8/-10 Ma. The first age group is close to the reported zi rcon U-Pb intrusion age of the igneous complex (931+/-2 Ma) and the re gional titanite U-Pb age (918+/-2 Ma), whereas the second group overla ps reported regional mineral Rb-Sr ages (895-853 Ma) as well as biotit e K-Ar ages (878-853 Ma). In the first group, the comparatively dry pa rageneses of low-P thermal metamorphism (M2) associated with the intru sion of the igneous complex are well developed, and hornblende Ar-40/A r-39 ages probably record a drop in temperature shortly after this pha se. In other hornblende + biotite-rich samples, with presumably a high er fluid content, the hornblende ages are probably a response to hornb lende-fluid interaction during a late Sveconorwegian metamorphic or hy drothermal event. A ca 220 m.y. diachronism in hornblende Ar-40/Ar-39 ages is documented between S. Telemark (ca 870 Ma) and Bamble (ca 1090 Ma). Differential uplift between these terranes was mostly accommodat ed by shearing along the Kristiansand-Porsgrunn shear zone. The final stage of extension along this zone occurred after intrusion of the Her efoss post-kinematic granite at 926+/-8 Ma. On the contrary, the south ern part of the Rogaland-Vest Agder and Telemark terranes share a comm on cooling evolution as mineral ages are similar on both sides of the Mandal-Ustaoset Line the tectonic zone between them. The succession wi thin 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 coo ling 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. (C) 1998 Elsevier Science B.V.