Svecofennian Metamorphic and Tectonic Evolution of East Central Sweden : Research report of a project entitled: Svekofennisk metamorf och tektonisk utveckling i östra mellansverige

In this report we present results from a metamorphic and structural investigation of east central Sweden, and integrate the results with age data into a model of the tectonometamorphic evolution of the region. Metamorphic grades range from medium grade in well-preserved supracrustal areas, like Hamrånge and Los, to granulite facies in intensely migmatized gneisses at Hornslandet, where pyroxene granulites and charnockites have been recorded. Thermobarometry results confirm earlier interpretations of regional low pressure-high temperature (LPM) character of the metamorphism. Results indicating medium or high pressures appear to be related to inappropriate whole-rock chemistry and in particular the CaO content in the samples. Garnets very rarely show growth zoning patterns due to the high temperature during metamorphism but imposed retrograde zoning along garnet margins is common, reflecting cooling of the crust. A PT-path obtained from a growth zoned garnet at Arlanda indicates isobaric temperature increase. Preliminary dating of monazites in pyroxene granulites yield cooling ages of 1846 and 1842 Ma respectively, which may reflect the intrusion of the Ljusdal granitoid rather than regional LPM. In the Los-Edsbyn area an age of 1803±1 Ma has been obtained on metamorphic zircon. The structural evolution comprises at least two major phases of folding and shearing, referred to two deformation episodes, D 1 and D2. Shear zones were formed during both D1 and D2. The former are folded by regional F2 folds and are characterised by complete annealing during the LPM. A preliminary age determination indicates a minimum age of 1826 Ma for D1 shear zones. The kinematics of these zones are unknown. D2 shear zones often truncate limbs of regional F2 folds and there appears to be a close relationship between folding and shearing. The kinematics of the D2 shear zones and their probable relation to F2 folds suggests that previous interpretations for the structural evolution should be revised, both considering principal stress orientation during D2 and the timing of D1 and D2. There is no systematic variation in metamorphic grade related to the existence of steep, regional shear zones. Metamorphic variations appear to reflect thermal variations in the crust possibly related to mafic underplating. Comparison of present crustal thickness and the distribution of granulites in the area and southern Finland indicate postmetamorphic modification of the crust.Svekofennisk metamorf och tektonisk utveckling i östra mellansverig