| Summary: | This paper presents the results of a pilot study on the magnetic petrology of basement rock units in the Ross Orogen in Oates Land (Antarctica). Magnetic susceptibilities of migmatitic gneisses – the dominant lithology - are generally low (ilmenite+/-graphite-bearing rocks), with the exception of two occurrences (magnetite+/-green spinel-bearing rocks) at Harald Bay and Burnside Ridge. Similar low values are typical for most Granite Harbour Intrusives, except for the variably mylonitic Exiles Nunataks granite and the Archangel Nunataks gabbros and pyroxenites, which are among the most magnetic rock types in the area. Metamorphosed mafic and ultramafic rocks, occurring in outcrop as volumetrically minor bodies (lenses of 1 dm to 10 m size), are variably but generally about one order of magnitude more highly magnetic than country gneisses. Petrological investigations on representative samples indicate that nearly pure or low-Ti magnetite is the only ferrimagnetic phase, irrespective of lithologic type, and magnetic susceptibility values are always positively correlated with the modal amount of magnetite. The rare occurrence of magnetite in the opaque mineralogy of migmatitic gneisses indicates that appropriate chemical and fO2 conditions were only locally attained in the region, and microstructural evidence indicate that magnetite possibly formed at different metamorphic stages during the post–peak decompressional path at high T and/or the retrograde path. In variably magnetic ultramafic and mafic lenses magnetite (almost pure to low-Ti) typically occurs as very fine grained opaque inclusions in secondary hornblende and/or chlorite. This evidence indicates that a high proportion of magnetite is of secondary origin and related to amphibolite grade or lower grade metamorphic processes. Titanomagnetite and exolution features are documented in the gabbros from Archangel Nunataks, in which the decrease of magnetite content is inversely correlated with the extent of sub-solidus late-magmatic reequilibration (replacement ...
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