Tristanites and trachytes occurring as dikes of Oki Dozen carry phenocrysts of plagioclase, biotite, clinopyroxene and iron oxide. Biotite and iron oxide are characterized by high TiO2 contents (up to 8 and 30 wt%, respectively), while clinopyroxene contains TiO2 1.5 wt% or less. Biotite phenocrysts accompanied by iron oxide specks around the crystals have higher Fe+3/(Fe+3+Fe+2) ratios and lower TiO2 contents than the apparently fresh ones. Clinopyroxene and iron oxide show positive correlations between Fe+3/(Fe+3+Fe+2) Ratios and TiO1 contents, but biotite a negative correlation.
Association of biotite and clinopyroxene phenocrysts represents low temperature crystallization of magma enriched in potash and volatile components. Crystallization of clinopyroxene from the liquid lacking in normative diopside makes the residual liquid peraluminous (corundum-normative).
Japan Association of Mineralogical Sciences