Isotopic age determinations in Bergslagen, Sweden: v. de Horssjö Granite, Filipstad area

Abstract

Rb-Sr whole rock data of 17 Horrsjö granite samples from 5 localities show a best-fit line Corresponding to an age of 1.76 ± 0.04 Ga. This result is unsatisfactory in view of the presumed age of the Horrsjö granite, which is coeval with the 1.84 ± 0.06 Ga Hyttsjö; Suite of rocks. Six samples from one locality give a best-fit line corresponding to 1 .86 ± 0.12 Ga; three samples from another locality show a best-fit line corresponding to 1.84 ± 0.44 Ga. These two localities presumably represent relic domains of Hörrsjo granite that have escaped post-emplacement, metamorphic- Rb-Sr isotopic resettings of whole-rock samples. The data show a large geological variance in excess of the analytical variance. This may be due to variation in initial 87Sr/86Sr in the samples, possibly caused by contamination of the granite magma by basic magma and/or seawater.