The tectonic evolution of the western part of the Svecofennian orogen, central Sweden : Insight from U/Pb and 40Ar/39Ar geochronology at Forsmark

Sammanfattning: The Forsmark area, in the western part of the Svecofennian orogen, central Sweden, is situated between two major Palaeoproterozoic tectonic domains that show contrasting histories with respect to timing of igneous activity, ductile deformation and metamorphism. At Forsmark, WNW to NW trending, ductile deformation belts anastomose around tectonic lenses with an inferred lower degree of ductile strain. Geological features common to both of the adjacent tectonic domains are found in the area, which, consequently, is of key importance for the understanding of the tectonic evolution of the Svecofennian orogen in this region. U/Pb zircon dating (6 ages), in combination with detailed field work, have revealed the existence of two calc-alkaline igneous suites at Forsmark. The older and most voluminous plutonic suite intruded at 1.89-1.87 Ga. It is affected by penetrative ductile deformation under amphibolite-facies metamorphic conditions. The younger, less voluminous and hypabyssal suite intruded at 1.86-1.85 Ga, during the waning stages of penetrative deformation and, thus, constrains the main phase of penetrative ductile amphibolite-facies deformation to between 1.87 and 1.86 Ga. Cross-cutting granite dykes, belonging to the younger suite, place an absolute minimum age for this deformational event to c. 1.85 Ga. U/Pb titanite data (4 ages) support the constraints on the penetrative deformation. However, the data also suggest that the Forsmark area has been affected by one or more tectonothermal events after the intrusion of the 1.85 Ga granite dykes. This is confirmed by 40Ar/39Ar hornblende data (16 ages), which demonstrate the existence of two age generations, 1.83-1.82 Ga and 1.81-1.80 Ga, that are suggested to represent resetting of the argon isotope system in response to retrogressive, lower amphibolite- to upper greenschist-facies deformation restricted to discrete high-strain zones within the broader deformation belts. Furthermore, the data suggest that cooling to c. 500 °C took place at around 1.85 Ga and that the area then remained at similar temperatures until the 1.81-1.80 Ga tectonothermal event, during which it was uplifted to higher crustal levels. In addition, 40Ar/39Ar muscovite (5 ages) and biotite (29 ages) data suggest that cooling to 350 °C occurred around 1.75-1.70 Ga, whereas cooling to 300 °C took place at 1.73-1.66 Ga. The estimated uplift rate was at this time c. 22 m/m.y. The Forsmark data, in combination with a compilation of available geochronological data for the time interval 1.91-1.84 Ga in central Sweden, point to the existence of at least two major tectonic cycles. Each cycle is characterised by igneous activity associated with extension, a short interval of compression (c. 10 m.y.), and migration of the tectonic activity. In this thesis, two contrasting conceptual tectonic models, which may explain the cyclic tectonic evolution of the western Svecofennian orogen in central Sweden, are discussed. The favoured model involves continuous subduction beneath a single active continental margin, combined with alternating subduction hinge retreat and advance. This model includes migration of what has been described as tectonic switching in some younger orogenic belts.