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

Abstract

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 (<l–2 mole%). The Q-value is about 0.1-0.4, which is typical of low coercivity multidomain magnetite. Early retrogression at low PH 2 O conditions is characterized by breakdown of garnet, producing fine-grained magnetite in almandine-rich varieties. This secondary magnetite is common in basic rocks where it can be the dominant type. Grains smaller than 10 μm are associated with high coercivities, and this magnetite may be the carrier of remanence in such lithologies. A probably contemporaneous development of magnetite can be seen in clinopyroxenes where it occurs as 0.1 mm long needles or rods within crystallographic planes. Clinopyroxene that crystallized at high pressure can hold an excess of Fe in the lattice, but upon decompression will release this Fe, producing Fe-Ti oxides. Late retrogression at high PH 2 O leads to reduction of the magnetite content due to replacement of magnetite by silicates, mainly hornblende and sphene (also leucoxene). Magnetite is still stable into the upper amphibolite facies. The close correlation between petrophysical data, aeromagnetic anomalies and geological units allows “mapping” of these units in the offshore areas. Metasedimentary rocks that are downfolded in the orthogneisses show low magnetic susceptibility values and can be traced as zones of lowest magnetization level.