Geochronological and palaeomagnetic reconnaissance survey in parts of Central and Southern Sweden

Abstract

Palaeomagnetic investigations and isotopic age determinations were made on various Precambrian and Palaeozoic rocks from southern and central Sweden. (1) In Dalarna (central Sweden), Rb-Sr determinations show that the Upper Dala volcanics and the Dala “Garberg granite” have a Gothian age of 1405 ± 30 million years. K-Ar measurements on these rocks and on Jotnian basalts evidence processes of (partial) argon loss in younger time. Palaeomagnetic measurements reveal that the rocks in this region have been remagnetized in Lower Palaeozoic (probably Caledonian) time. (2) K-Ar measurements suggest that the Precambrian hyperite-dolerite dykes in the “central schistosity zone” (southern Sweden) were intruded about 1550 ± 100 million years ago. In the southern part of this zone the dykes show “over-printed” ages of 800–900 million years. The hyperite-dolerite dykes indicate a magnetic pole position at approximately 15° S and 135° W. This Precambrian pole position is thought to have an age of about 1550 + 100 million years. (3) Palaeozoic dolerite sills in Västergötland (southern Sweden) and dykes in Skåne show an Upper Palaeozoic initial remanent magnetic direction with a pole position at approximately 35° N and 175° E, which is in excellent agreement with the K-Ar age of 282 ± 5 million years (Permian/Carboniferous) measured on two sills.