Rb—Sr dating of rocks in the Värmland granite group in Sweden

Abstract

Thirteen samples of Värmland granites from south-eastern Värmland, Sweden, have been subjected to a Rb—Sr dating study. Most of the studied samples subdivide into two groups defining two separate isochrons of closely similar age but different initial ⁸⁷Sr/⁸⁶Sr ratios. Five samples from a southern sub-area of Filipstad granite around Filipstad and Karlskoga have yielded a Rb-Sr age of 1700 ± 37 Ma at an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7068 ± 0.0021. Five samples from the northern sub-area of Hagfors—Ekshärad—Uvanå, together with two samples from the vicinity of Kristinehamn, form another isochron of 1689 ± 30 Ma with an initial Sr-isotopic ratio of 0.7021 ± 0.0006. One sample could not be assigned to either isochron and is intermediate between them. The differentiation of Värmland-group granites into separate complexes and types is also brought out by field data and geochemical characteristics.