Petrophysical data and opaque mineralogy of high-grade and retrogressed lithologies: implications for the interpretation of aeromagnetic anomalies in Northern Vestranden, Central Norway

A regional aeromagnetic anomaly centred around Roan, Northern Vestranden, Western Gneiss Region, Central Norway, is caused by the presence of high-grade orthogneisses and metabasites with 1–2 vol% magnetite. Granulites are surrounded, and partly covered by amphibolite-facies rocks, which consist of reworked, retrogressed granulite-facies orthogneisses, and bands of metasediments. Prograde metamorphism from amphibolite to granulite facies led to production of magnetite as a result of the breakdown of hydrous (Fe, Mg) Al-silicates. The magnetite occurs as 0.5-1 mm amoeboid grains surrounded by plagioclase and K-feldspar. This magnetite is nearly pure with only a negligible content of ulvospinel (FeTiO 4) in solid solution (