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Generation of anorthositic magma by H₂O-fluxed anatexis of silica-undersaturated gabbro: an example from the north Norwegian Caledonides

R. S. SELBEKK^*,, K. P. SKJERLIE^* and R. B. PEDERSEN

^* Department of Geology, University of Tromsø, 9037 Tromsø, Norway
Institute of Geology, University of Bergen, Allegt. 41, 5007 Bergen, Norway

Author for correspondence: runes{at}ibg.uit.no

The Skattøra migmatite complex in the north Norwegian Caledonides consists of migmatized slightly nepheline-normative metagabbros that are net-veined by numerous (up to 90%) anorthositic and leucodioritic dykes. The average chemical composition of 17 anorthosite dykes is (wt %) 58.4% SiO₂, 0.2% TiO₂, 23% Al₂O₃, 1.8% FeO^t, 0.7% MgO, 6.3% CaO, 7.8% Na₂O, 0.2% K₂O. A migmatite leucosome and a dyke have been dated by the U/Pb method on titanite to 456±4 Ma. In low melt fraction areas minor leucosomes are orientated parallel to the foliation. More intense anatexis formed stromatic to schlieric migmatites. The leucosomes are commonly connected to dykes, suggesting that melt segregated and left its source. Dyke thicknesses range from a few centimetres up to several metres. In general, early dykes are parallel to the foliation in the host rock, while the later dykes cut the foliation. Plagioclase (An_20–50) is the dominant mineral (85–100%) in the dykes and the leucosome, but 0–15% amphibole is generally present. Field relations, geochemistry and preliminary melting-experiments strongly suggest that the anorthosites originated by H₂O-fluxed anatexis of the gabbroic host rock.

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