Rb/Sr age determinations on late Proterozoic granitoids from the Evje area, South Norway

Abstract

Rb/Sr whole rock age determinations on different orthogneiss units, on one postkinematic granite and on young monzonitic sheets from the Evje area, Central Aust-Agder, have yielded ages ranging from 1350 to 900 m.y. The ages obtained indicate that the penultimate major regional deformation is younger than 1120 ± 31 m.y. and older than 1026 ± 57 m.y., while the last regional deformation is younger than 1026 ± 57 m.y. and probably contemporaneous with the last major period of deformation with accompanying· metamorphism dated at about 1000--900 m.y. elsewhere in South Norway. The ages suggest that a major part of the granitic basement rocks in this part of South Norway is younger than 1350 m.y. (87Sr/86Sr)0 ranges from 0.706 to 0.704 suggesting an origin from Rb poor source regions or from only slightly older crust with normal Rb/Sr ratios.