Ulreung Island, situated off the eastern coast of the Korean peninsula, is geologically char-acterized by lavas ranging in composition from alkali basalt, trachybasalt through trachyandesite to trachyte and phonolite. There is a gradational series from the alkali basalt to the phonolite in the chemistry and mineralogy. The volcanics are mildly silica undersaturated and potassic in the major chemistry.
Calcium-rich clinopyroxene is changing in composition from chromian diopside and titan augite in the alkali basalt, to ferro augite in the phonolite. Bytownite is the predominant feldspar in the alkali basalt, and labradorite and andesine are important in trachybasalt and trachy-andesite. Whereas the characteristic feldspar of the trachyte and phonolite is anorthoclase to sodic sanidine. Kaersutite and titan biotite are restricted to the phenocrysts of the trachyandesite and some trachyte.
Major and trace elements vary systematically throughout the sequence, and quantitative petrogenetic modelling suggests that compositional variations observed in the differentiated lavas can be ascribed to extensive fractional crystallization of a parental alkali basaltic magma. Over 86 per cent of the evolution from alkali basalt to phonolite occurs in the initial step from alkali basalt to trachybasalt, which represents a 42 per cent residual. The phonolite is only a 14 per cent residual from an alkali basalt parent. The mass balance models indicate that olivine, clinopyrox-ene, Fe-Ti oxide, plagioclase, kaersutite, biotite and alkali feldspar are the dominant fractionated phases. Trace element contents calculated using the Rayleigh equation show a relative good agreement in all the models.
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