Stratigraphies and depositional ages of Svecofennian, Palaeoproterozoic metavolcanic rocks in E. Svealand and Bergslagen, south central Sweden

Abstract

A belt of Palaeoproterozoic, dominantly felsic metavolcanic rocks occurs in south-central Sweden. The volcanic rocks comprise volcaniclastics, lavas and subvolcanic intrusions. The volcanic pile is more than 8 km thick and overlies an unexposed basement. Two new U-Pb zircon age determinations provide precise control on the age of stratigraphically and volcanologically well-constrained volcanic rocks in the belt. One age determination from a pyroclastic flow deposit at Utö in the easternmost part of the volcanic belt yielded an age of 1904±4 Ma. Stratigraphic and depositional facies analyses show that this rock marks the onset of volcanism in the area. The igneous activity was coeval with a change from deep water sedimentation to shallow water and subaerial sedimentation in a broadly continental setting. Another determination of zircons from a similar pyroclastic flow deposit at the lowest known stratigraphic level east of Hällefors in W. Bergslagen in the westernmost part of the volcanic belt gave an age of 1891±4 Ma. This age is consistent with previously published ages for metavolcanic rocks in W. Bergslagen. The age of the onset of volcanism in W. Bergslagen is not possible to determine because the lowermost sections of all known volcanic successions are truncated by intrusive rocks. Consequently, it is unknown whether volcanism also commenced around 1904 Ma in the western part of the region, or if it commenced later, just prior to 1891 Ma. The duration of volcanism also remains to be constrained.

Key Words:

• Palaeoproterozoic
• Svecofennian
• volcanic stratigraphy
• U-Pb zircon ages
• Bergslagen
• Sweden