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.
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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.