Isotopic constraints on the origin of AMCG-suite rocks on the Lofoten Islands, N Norway

Summary

¶Sm–Nd, Rb–Sr and Pb–Pb isotopic compositions of 34 intrusive AMCG (anorthosite–mangerite–charnockite–granite) suite rocks and spatially related ferrodiorites and gabbros from the Lofoten Islands, northern Norway, suggest that almost all Lofoten intrusive rocks can be modelled as a mixture of mainly two components: Archean lower crustal material and an about 1.8 Ga mantle-derived component. Isotopically, the gabbros and anorthosites overlap the mangeritic rocks in all three isotope systems. Hence, the isotopic data are in agreement with a model that relates the mangeritic rocks to the fractionation of crustally contaminated mantle-derived basaltic melts. Overlap in all three isotopic systems indicates that at least part of the assimilation process predates or accompanies fractionation. Whole rock geochemistry supports this model: based on major, minor, and trace element data, primitive 1.79–1.86 Ga mangerites could have formed from anorthosite residual liquids – the ferrodiorites – by fractionation processes involving Fe–Ti oxides, Fe–Mg silicates and apatite combined with contamination of the ferrodiorites with about 50 wt.% crustal anatectic melt.