Tectonic setting of the Tronfjell Massif ; further evidence for pre-Scandian orogenesis in the Trondheim Nappe Complex, Central Norway.
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The Tronfjell Massif consists of layered mafic intrusion, containing dunites and olivine-bearing to noritic gabbros, lying within metasedimentary rocks of the Hummelfjell Group. The initially more or less flat form of the intrusion is now bowl-like, due to settling of the intrusion under its own weight. Large areas of rock were affected by upper greenschist-facies deformation during the Scandian orogeny. Rare areas of hornfels, wrapped an dcross-cut by a Scandian-age foliation, preserve an annealed tectonic fabric which, together with the inferred pre-intrusion form of structures in the metasedimentary rocks, demonstrate a pre-Scandian orogenic event. This is in agreement with the latest tectonostratigraphic model for the area, which places the Hummelfjell group within the Heidal Group of the Trondheim Nappe complex