The geological significance of Rb-Sr whole-rock isochrons of polymetamorphic Archaean gneisses,Fiskenaesset area,southern West Greenland

Rb-Sr whole-rock isochron ages of gneisses from the Fiskenaesset area are considerably lower (2600–2800 m.y.) than U-Pb zircon ages for the same rocks (2880–2950 m.y.). There is a significant correlation between the isochron ages and the range in Rb/Sr ratios of the samples involved. Higher ages (and lower initial⁸⁷Sr/⁸⁶Sr ratios) are obtained for sample collections with a wide range in Rb/Sr ratios. Lower ages (and higher initial ratios) are obtained for sample collections with a narrow range in Rb/Sr ratios. This relationship is explained by a model of local metamorphic Sr isotope homogenisation in restricted rock volumes. This model implies that the individual isochron ages do not date specific geological events. There is a significant inverse correlation between the isochron ages and the corresponding initial ratios. It is probable that the igneous precursors of the gneisses intruded with initial⁸⁷Sr/⁸⁶Sr ratios well below 0.701.