40Ar/ 39Ar hornblende geochronology from the Forsmark area in central Sweden: Constraints on late Svecofennian cooling, ductile deformation and exhumation

Abstract

At Forsmark, ca. 120 km north of Stockholm in central Sweden, ductile high-strain belts with WNW to NW trend anastomose around tectonic lenses with an inferred lower degree of ductile strain. Previous studies of ductile deformation zones with WNW to NW trend, elsewhere in the western part of the Svecofennian orogen in central Sweden, have yielded estimates for the timing of at least one phase of discrete ductile deformation that fall in the time interval 1.82–1.78 Ga. Most of these ages were determined by the U/Pb dating of titanite and, for this reason, provide no information on the thermal evolution. In this paper, we make use of 40Ar/ 39Ar hornblende geochronology to address late Svecofennian cooling, ductile deformation and exhumation. The data demonstrate the presence of three 40Ar/ 39Ar hornblende age generations. All ages have been adjusted to take account of ca. 1% systematic bias between 40Ar/ 39Ar and U/Pb ages recently reported in the literature. The oldest age, ca. 1.87 Ga, and the intermediate age generation, 1.85–1.84 Ga, are spatially restricted to the tectonic lenses. By contrast, the youngest age generation, 1.83–1.81 Ga, occurs both within the tectonic lenses and the enveloping high-strain belts. One explanation for the structurally controlled age distribution involves regional cooling beneath the closure temperature for argon isotopic mobility around or above 500 °C by 1.84 Ga, as represented in the oldest and intermediate age generations, followed by resetting of the argon isotope system in hornblende between 1.83 and 1.81 Ga, as represented in the youngest age generation. This resetting occurred in response to retrograde, lower amphibolite- to upper greenschist-facies deformation along discrete high-strain zones within the broader high-strain belts and was associated with regional exhumation. An alternative explanation involves no resetting of the ages. Instead, it is suggested that a period of slow cooling of hornblendes with slightly different closure temperatures, from ca. 1.87 to 1.82 Ga, may have caused the age variation observed within the tectonic lenses, whereas locally maintained higher temperatures, due to activity along the discrete high-strain zones, can explain the consistently younger ages in the broad, enveloping high-strain belts. In this explanation, an increase in cooling rate, in response to regional exhumation, finally closed the argon isotope system in hornblende throughout the area at 1.83–1.81 Ga. It is suggested that the regional exhumation at 1.83–1.81 Ga, which is included in both explanations, is related to far-field effects of the deformation that ended an accretionary tectonic cycle in adjacent tectonic domains.