Geochronology of basal gneisses and mangerite syenites of Stadlandet, west Norway

Grey gneisses from four localities yield Rb-Sr isotopic data points approaching a regression line corresponding to an age of 1760 ± 70 m.y. and an initial 87Sr/86Sr ratio of 0.702 ± 0.001 . Similar ages have been obtained from U-Pb zircon 'upper' concordia intersections. The grey gneiss precursors were of granodioritic composition and apparently did not have a long previous crustal history. Quartzites and gamet-mica schists give Rb-Sr whole rock ages of c. 1500, 1100, and 400 m.y. but are interpreted as Svecofennian sediments affected by open chemical systems during Sveconorwegian and/or Caledonian events. Microcline-rich gneisses yield Rb-Sr isotope ratios which plot below the grey gneiss isochron, while zircon U-Pb systems from one gneiss have been almost completely reset at 400 m.y. These gneisses are considered to be K-metasomatised grey gneisses tentatively associated with Caledonian processes. U-Pb zircon systems for a mangerite syenite are slightly discordant at 1520 ±10 m.y. This age is attributed to magmatic crystallisation.