Rb-Sr dating and microclinization in dioritic rocks of the Haparanda group, northern Sweden

Abstract

Recent Rb-Sr data on the Haparanda plutonites, Northern Sweden, are discussed in relation to field evidence of post-intrusion microclinization. It is concluded that the plutonites did not remain closed systems after emplacement and that the whole-rock data represent mixed ages, whereas the biotite data may record the time of microclinization. The minimum age of primary crystallization is 2.25–0.10 • 10⁹ y and the maximum time since the resetting of biotite is 1.85 + 0.035 • 10⁹ y. The microcline data are anomalous. Routine statistical treatment of such Rb-Sr data does not yield the ages of primary intrusion. Criteria for the recognition of rejuvenation in the Rb-Sr data are discussed, and geological comparisons with other areas in Sweden and Finland are made.