Oxidation During Magmatic Differentiation, Finnmarka Complex, Oslo Area, Norway: Part 1, The Opaque Oxides

Microprobe analyses of the oxide mineral assemblages from monzonitic, granodioritic, and granitic units of the Finnmarka igneous complex, Oslo area, Norway, disclose a remarkable trend of Mn enrichment in ilmenite with differentiation. Ilmenite-pyrophanite solid solutions containing up to 63 mole per cent MnTiO3 have developed under apparently unusually oxidizing conditions, a conclusion deduced in part from the fact that Fe2+ is more readily oxidized than Mn2+ and was thus preferentially extracted. Several different mineral reactions may have been involved in this process. Geothermometric applications of oxide Mn contents must be made with caution. Additional mineralogic complexities, e.g. ilmenite lamellae in magnetite, magnetite lamellae in ilmenite, and, in the granite, the breakdown of ilmenite to rutile+hematite, all provide insight into the conditions of emplacement and cooling history of the complex.