Evolution of peralkaline calcite carbonatite magma in the Fen complex, southeast Norway

Abstract

In the Fen complex, Telemark, SE Norway, peralkaline calcite carbonatite (pyroxene søvite) occurs associated with nepheline syenite, alkali pyroxenite and rocks of the melteigite-ijolite-urtite series. Nepheline line syenite and pyroxene søvite have identical liquidus mineral assemblages (sodic pyroxene + apatite, followed by nepheline + alkali feldspar and, as the last phase, calcite), and pyroxenes defining virtually identical compositional trends (Di 55Hd 25Ac 20 to Ac >90). The pyroxenite formed as a pyroxene cumulate from a magma related to ijolite, and contains a felsic interstitial mineral assemblage (alkali feldspar + nepheline + calcite + apatite) corresponding to a CO 2-rich nepheline syenite in composition. Ocelli of calcite within the nepheline syenite, and microstructural evidence for two concomitant liquids in the interstitial aggregates in the pyroxenite (nepheline syenite and peralkaline calcite carbonatite) suggest that two magma types corresponding to nepheline syenite and pyroxene søvite were present as immiscible liquids in the shallow crust during emplacement of peralkaline magmas in the Fen complex. The overlapping pyroxene trends of these rock types confirm their origin from immiscible liquids. The common parent magma was related to the ijolitic rocks of the Fen complex. The present findings question the relevance of the concept of a universal primitive carbonatite magma, regardless of its assumed composition.