Isotopic evidence on the age of the Trysil porphyries and granites in eastern Hedmark, Norway.

Abstract

Rocks from the (sub-Jotnian) acidic plutonic and volcanic basement complexes in the Trysil area, eastern Hedmark, yield a Rb-Sr isochron age of 1541+-69 million years. This agrees within the limits of error with the isochrom age of 1590+-65 million years determined for the Dala porphyries and granites in Dalarne, Sweden, which are the continuation of the acidic igneous complexes in the Trysil area. The sub-Jotnian acidic magnetism in the eastern Hedmark-Dalarna region can thus be dated at 1570+-40 million years ago, i.e. some 100 million years younger than the termination of the Svecofennian orogeny. Chemically, this magnetism is characterized by a granitic to alkali granitic and alkali syenitic composition. The Trysil area has also been affected by a tectonothermal event in Sveconorwegian time, about 925 million years ago, as evidence by the Rb-Sr and K-Ar ages of separated biotites.