An Rb/Sr Age of the Småland Porphyries

Abstract

Rb/Sr whole-rock radiometric dating of 14 porphyries from Småland, southeastern Sweden, yields an age of 1695 ± 20 million years (m. y.) (λ = 1.39 × 10⁻¹¹ y.⁻¹). The initial Sr⁸⁷/Sr⁸⁶ ratio is 0.7065 ± 0.0012. The relationship between the Småland porphyries may be compared with that between the Dala porphyries, with respect both to relative age and to general appearance. The Småland porphyry age compares favourable with and is well within the limits of error of the age of the sub-Jotnian Dala porphyries 1669 ± 38 m. y. (Welin 1970 a). Thus, the postorogenic sub-Jotnian igneous activity occurred, according to this age, in relatively close connection with the Svecofennian orogeny, of which the main tectonic and magmatic activity ceased approximately 1775–1800 m. y. ago. The porphyries and granites probably originated from a palingenic magma formed during the Svecofennian orogeny. Later, during the postorogenic stage, the rise of the magma is explained by the effect of buoyancy (Lundqvist 1968).