A Rb-Sr geochronological study of the older granitoids in the Åmål tectonic mega-unit, south-western Sweden

Abstract

Rocks of the tonalitic—granodioritic provisional Åmål I plutonio complex in the vicinity of Uddevalla, western Sweden, were subjected to a Rb-Sr geochronological study. The area was selected on account of the comparatively weak local manifestations of plutonic activity and metamorphism referrable to the younger, provisional, Åmål II group and later events. The sampled rocks nevertheless show a partial opening of the regional Rb-Sr isotopic system and consequently yield a poor-fit seven-point age reference line of 1690 Ma, the initial ⁸⁷Sr/⁸⁶Sr ratio being 0.7011 ± 0.0027. Feldspar-biotite whole-rock isochrons obtained from a study of two of the dated samples suggest a disturbance of the Rb-Sr isotope system at 975 ± 60 Ma. This age fits the known Rb-Sr and K-Ar ages of the late Sveconorwegian (Dalslandian) activity in the south-western Baltic shield.