The Origin of Anorthosites and Related Rocks from the Lofoten Islands, Northern Norway: II. Calculation of Parental Liquid Compositions for Anorthosites

Abstract

Clinopyroxene, orthopyroxene and plagioclase in the Proterozoic Lofoten anorthosites are inferred to be in equilibrium with ferrodioritic melts, on the basis of literature partition coefficients for these phases in equilibrium with basaltic melts. Ferrodiorites and high-Al gabbros occur as fine-grained dykes in spatial association with the ∼1.8 Ga Lofoten anorthosites. They are linked to each other by an Fe-enrichment fractionation trend typical of tholeiites, and they are characterized by Al2O3 contents of 14–18.5 wt % and XFe [= FeOtot/(FeOtot + MgO)] between 0.5 and 0.8. The association of high-Al gabbros, ferrodiorites and anorthosites is common in many Proterozoic anorthosites. We infer that the high-Al gabbros to ferrodiorites represent the parental liquids from which these Proterozoic anorthosites formed. The Lofoten rocks show fractionation trends that are similar to those of high-Al gabbros and ferrodiorites from other Proterozoic anorthosite complexes. We interpret these trends in terms of incorporation of intermediate composition plagioclase into a typical tholeiitic magma with subsequent fractionation. The first process produces positive Eu anomalies and the high Al2O3 contents of the melts, and explains the lack of negative Eu anomalies in most of the ferrodiorites from Lofoten and Laramie.