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

The Skattora 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% FeOt, 0.7% MgO, 6.3% CaO, 7.8% Na2O, 0.2% RO. 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.