Premetamorphic lateritisation in Proterozoic metabasites of Rogaland, SW Norway

Abstract

Heterogeneous layers of granulite facies metamorphic basites of the Proterozoic supracrustal Faurefjell Formation in Rogaland, S.W. Norway, display an extreme chemical variation. Within a single layer the bulk chemical composition gradually changes from approximately basaltic in basic granulites to alumina-iron-rich in granofelses. Component-ratios and composition-volume relations indicate open-system chemical reactions mainly involving the extraction of silica. Apparent enrichment in Fe, Ti, P, Al, Zr, Ni, Co, Zn, Y, Nb, Hf and REE and variations in resulting metamorphic mineral assemblages are related to premetamorphic progressive lateritisation of a basaltic protolith. The weathering generated a continuous chemical suite from SiO2=48 wt%, Fe2O3=10 wt% and Al2O3=19 wt% in the basic granulites to 14 wt%, 40 wt% and 25 wt% in the Fe-Al granofelses. Metasomatism during diagenesis and during (very) high-grade metamorphism (1200–900 Ma) further perturbed the concentrations of relatively mobile elements Ca, Mg, K, Rb, Sr, Ba, Na and Li in the laterites without affecting the transition metal ratios. In particular, the REE did not fractionate differentially during the supracrustal and metasomatic alteration.