Oxidation During Magmatic Differentiation, Finnmarka Complex, Oslo Area, Norway: Part 2, The Mafic Silicates1

Electron-microprobe analyses are presented for pyroxene, amphibole,and biotite from monzonite, granodiorite, and granite at Finnmarka,Norway. Compositional trends measured in biotite, present inall three rock types, and in amphibole, present in the monzoniteand granodiorite, are markedly atypical and are interpretedas reflecting crystallization under progressively more oxidizingconditions. The average Fe/Fe + Mg for biotites from successivelymore silicic rock types changes from 0.64 0.35 0.28, and foramphiboles changes from 0.58 in the monzonite to 0.29 in thegranodiorite. Analyses of selected areas within amphibole grains in the granodioriteshow marked chemical variations, although single-crystal X-rayphotographs are sharp and do not reveal multiple phases. Onthe basis of 33 such analyses, four coupled substitutions areidentified as operative; the most unusual finding is the relationof 1 Ti cation to 4 Al^IV cations in the unit cell. Variationswithin individual amphibole grains of the granodiorite resemblechanges noted in evolution of amphibole composition from monzoniteto granodiorite and are interpreted as reflecting progressiveoxidation. Consideration of these data for the mafic silicates, data forthe opaque oxides, and the extensive formation of sphene inthe granodiorite, has allowed development of schematic reactionsand an overall picture of magmatic environment and evolutionat Finnmarka. Crystallization apparently took place at PH₂oof 1000 bars or less and a temperature of about 700 °C.The trend of oxidation during differentiation is more extremethan any heretofore reported. Amphiboles, as well as biotites,may participate in oxidation reactions and may reflect the oxidation-reductionprocesses that occurred during magmatic evolution.