Geology and structure of the area west of Fyresvatn, Telemark, southern Norway. The Precambrian rocks of the Telemark area in South Central Norway no. X.

Abstract

Geological mapping has shown that the area west of Fyresvatn, Telemark, Southern Norway, is underlain by a sequence of gneisses, quartz schists, amphibolites, migmatites and epidote bearing quartzite, together with a homogenous granite pluton. A detailed structural study has revealed four periods of folding, the first three of which are represented on a macroscopic scale throughout the area: the second and the third fold episodes in particular are responsible for the present-day regional distribution of lithologies. Field relationships and petrological descriptions of the various lithologies are presented. The regional metamorphism is shown to have attained a peak within almandine-amphibolite facies during the F2 fold phase. During the same period migmatites and gneisses were developed. The late-kinematic Fyresdal granite was probably emplaced during the latest stages of F3 or possibly the F4 folding period. Biotites ages (by Rb-Sr method) of the Fyresdal granite gave an average age of 850 +- 50 m yrs. indicating the possibility of this region having taken part in 900-1000 m year orogeny. The latest event appears to have been the intrusion of diabase dykes which transect all the earlier structures; these are probably of Permian age.