Rare earth element geochemistry of the Fen alkaline complex, Norway

Determination of rare earth element (REE) abundances in rocks of the Fen complex has shown that within rocks of the first magmatic series REE abundances increase in the order urtite<ijolite<fine grained søvite <coarse grained søvite<silicocarbonatite. Low pressure differentiation of an ijolitic magma by crystallization of apatite, melanite, pyroxene and nepheline is reflected in the very variable La/Yb ratios (4–100) of the urtites and melteigites. Similar La/Yb ratios (40–60) in ijolite, tinguaite and søvite indicate that the søvites are not the result of extensive differentiation of a carbonated ijolitic magma. Within the second magmatic series REE increase in the order damtjernite<vibetoite-rauhaugite<rødberg. La/Yb ratios (30–551) are variable and reflect volatile transport of REE possibly as carbonate complexes. The effects of volatile transport, low and high pressure differentiation and liquid immiscibility on the Fen magmas are discussed and it is considered that parental magmas had relatively high La/Yb ratios (40–60). Utilizing petrological evidence from other alkaline complexes coupled with experimental studies it is considered that the parental magma was a carbonated nephelinite produced by limited (<10%) partial melting of the mantle. All the Fen rocks are placed in a petrogentic scheme in which a carbonated nephelinite magma undergoes liquid immiscibility, differentiation and volatile transport.

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