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

A belt of Palaeoproterozoic, dominantly felsic metavolcanic rocks occurs in south-central Sweden. The volcanic rocks comprise volcaniclastics, lavas a nd subvolcanic intrusions. The volcanic pile is more than 8 km thick and ov erlies an unexposed basement. Two new U-Pb zircon age determinations provid e precise control on the age of stratigraphically and volcanologically well -constrained volcanic rocks in the belt. One age determination from a pyroc lastic flow deposit at Uta in the easternmost part of the volcanic belt yie lded 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 a ctivity was coeval with a change from deep water sedimentation to shallow w ater and subaerial sedimentation in a broadly continental setting. Another determination of zircons from a similar pyroclastic flow deposit at the low est known stratigraphic level east of Hallefors in W. Bergslagen in the wes ternmost 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. Berg slagen. The age of the onset of volcanism in W. Bergslagen is not possible to determine because the lowermost sections of all known volcanic successio ns are truncated by intrusive rocks. Consequently, it is unknown whether vo lcanism 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 al so remains to be constrained.