Amphibolites and metagabbros from the Proterozoic Telemark Suite of Setesdalsheiene, South-central Norway.

Abstract

Proterozoic supracrustal rocks of the Telemark Suite, including large thicknessof basic metavolcanites, occur as a large-scale isolated outlier within theolder gneiss-migmatitegranite complex on the mountain plateau ofSetesdalsheiene. Numerous and often large bodies of metagabbro occur mainly asconcordant lenses, but may locally show intrusive relations to thesupracrustals. Late Precambrian post-orogenic granite cross-cut all the otherrock types.Reviews are given of the lothostratigraphy and structure of the TelemarkSupracrustals as a background for a more detailed study of the petrology ofthe basic meta-igneous rocks. Petrographical investigations of these rocksshow that the amphibolites (metabasalts) are almost completely recrystallized,while the metagabbros have usually retained the outlines of primaryhypidimorphic plagioclase crystals. Mineral compositions and assemblagesindicate that the area suffered metamorphism of uppermost low-grade conditions.Chemically the rocks are metabasalts of transitional tholeiitic\/alkalinecharacter. The geological environment and the chemical data indicate that theamphibolites and metagabbros originally were formed in a within-plate(continental) environment. These findings are compared with present-daytectonic regimes and with published plate tectonic models for southern Norway.It is suggested that the amphibolites represent rift volcanism in a subsidingbasin not very far from a continental margin. The supracrustal rocks weredeformed, metamorphosed and intruded by gabbro magmas during later adjacentorogenesis which was related to subduction and calc-alkaline volcanism.