Age relationships in the proterozoic high-grade gneiss regions of southern Norway

Abstract

Abstract The Proterozoic high-grade gneiss regions of southern Norway (Bamble, Kongsberg, Rogaland and Ostfold) historically belong to the so-called Sveconorwegian (1.2—0.9 Ga) zone. This has long been correlated with the Grenville province of North America, and features prominently in Proterozoic reconstructions across the North Atlantic. Several recent radiometric studies have provided compelling evidence for an earlier high-grade gneiss-forming event at 1.6—1.5 Ga ago in parts of southern Norway. It has also been shown that at least a section of the Bamble sector suffered only a relatively low-grade thermal—deformational event during the Sveconorwegian cycle. In this paper new RbSr age determinations for metasediments and an anatectic granite from the same area confirm a pre-Grenvillian age for the high P—T event and preclude the possibility of later high-grade reworking. Previously published age determinations for all sectors are compared with the available radiometric data from the adjoining gneissic terrane of southwest Sweden, and reviewed in the light of the new data from Bamble and recent evidence that relatively low-grade events are capable of disturbing, and possibly resetting, both RbSr total-rock and UThPb zircon isotope systems. The conclusions are that the main high-grade gneiss-forming event may have been pre-Grenvillian in all sectors, and that there may have been no regional high-grade reworking during the period 1.2—0.9 Ga ago.