Petrogenesis of the Anorthosite Dyke Swarm of Tromsø, North Norway: Experimental Evidence for Hydrous Anatexis of an Alkaline Mafic Complex

The 456 ± 4 Ma Skattøra migmatite complex in the north Norwegian Caledonides consists of migmatitic nepheline-normative metagabbros and amphibolites that are net-veined by numerous nepheline-normative anorthositic and leucodioritic dykes. Plagioclase (An20–50) is the dominant mineral (85–100%) in the dykes and the leucosome, but amphibole is generally present in amounts up to 15%. The following observations strongly suggest formation of the anorthositic magma by anatexis of the surrounding gabbro in the presence of an H2O-bearing fluid phase: (1) the migmatites have plagioclase-rich (anorthositic) leucosomes and amphibole-rich restites; (2) crystallization of amphibole in the anorthositic and leucodioritic dykes suggests high H2O activity; (3) the presence of coarse-grained to pegmatitic dykes and miarolitic cavities indicates a fluid-rich magma; (4) hydration zones that surround many anorthosite dykes suggest that the magma probably expelled H2O-rich fluids during crystallization. Water-saturated melting experiments at 0·5–1·5 GPa and temperatures from 800 to 1000°C have been performed on a nepheline-normative gabbro to test the proposed petrogenesis of the Skattøra anorthosites. The glasses produced close to the solidus are tonalitic in composition, but they become richer in plagioclase at higher temperatures. At and below 1·0 GPa, the residues are composed of amphibole. Experiments above 1·0 GPa produced residual garnet and/or zoisite in addition to amphibole, suggesting that the anorthositic dykes in the Skattøra migmatite complex formed below 1·25 GPa. The experiments show that the high Na2O content of the anorthosite dykes can only be produced if Na is added to the charges. The glass that best fits the composition of the Skattøra dykes was produced at 1·0 GPa and 900°C with 2 wt % Na(OH) added.