Geology of the Tronfjell massif: placing mafic magmatism into a tectono-stratigraphic framework

Detailed mapping of a syn-orogenic intrusion and the surrounding rocks provides supporting evidence in favour of a recent re-interpretation of the stratigraphy of the Trondheim Nappe Complex (Sturt The Tron intrusion lies within rocks of the Hummelfjell group. The preservation of folds within recry- stallised hornfels close to the intrusion confirms the presence of an old tectono-metamorphic event in these rocks. This event is predicted by the corre- lation of the Hummelfjell with the Heidal series of the Otta area (Sturt 1994, B.S.Sturt pers.comm.1996). Both the intrusion and the surrounding rocks are effected by fabrics correlated, by orientation and metamorphic grade, with the main Scandian orogeny. The intrusion event is therefore placed between c.500Ma (age of post-Heldal-deformation sediments) and c.425Ma (age of main Scandian orogeny). Evidence for poly-phase deformation is seen in the country rocks, with reactiviation of pre-intrusion fabrics by post-intrusion deforma- tion. The latest foliations are associated with E-W dipping lineations, and pre-intrusion structures have similar orientation. Three phases of metamorphism are distinguished: a pre-intrusion event, represented by garnet cores; a contact metamorphic event; a late event of upper greenschist facies, similar to Scandian metamorphisn in nearby rocks. The latest event overprints all rocks including the gabbro, where zones of alteration focus deformation into discrete shear-zones. What little is preserved on the contact metamorphism suggests a narrow and low-temperature aureole, implying cool country rocks. The intrusion is a layered mafic body, sheeted along concordant margins with three distinct facies a lower contaminated layer; a transitional layer rich in dunitic rocks which yield chromite; an upper layer of coarse olivine gabbros. No roof is seen. Magmatic structures are common an

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