The Nordre Bumandsfjord ultramafic pluton, Seiland, north Norway: part 1, field relations

This pluton consists of dunite, wehrlite, and lherzolite, intruded as magmas late in the Finnmarkian phase of the Caledonian orogeny. The northern end of the pluton intrudes layered gabbros, and block stoping was the dominant mode of emplacement. The southern end displays a sheeted complex of peridotite dykes and metasediment screens. There is evidence for the fluidity of the ultramafic magmas and for high magma temperatures. Field relations and geochemistry show that the dominant primary magma was dunitic (approx Fo_72-81); other ultramafic magmas were produced by contamination of the primary magma(s) with gabbroic and sedimentary material. The formation of the primary ultramafic magma(s) was a final stage in the evolution of the very active mantle diapir responsible for the large volume of basic rocks comprising the Seiland igneous province. -G.R.