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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Chromium in pyroxene generally decreases as magmatic fractionation proceeds because of its large octahedral site preference energy. Nevertheless, an increase in Cr with progressive fractionation was found in the early formed proto- and orthopyroxenes from the Bonin Island boninite and augites from the Kasayama olivine andesite. This unusual increase in Cr can be interpreted by the early removal of Cr from magma with Cr-spinel crystallization and the later reaction relations of the crystallized spinels with magma to form pyroxenes.

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Two petrographic provinces are classified in the Miocene volcanic rocks widely exposed in areas to the east of the Quaternary volcanic front of Northeast Japan. The first is that of the island arc tholeiite in the Kitakami river area, where rocks are basalts, andesites and dacites, all being of the island arc tholeiite or calc-alkali rock series. On the other hand, the second is a mixed province of the oceanic tholeiite and the island arc tholeiite series, in which occur, in association with the island arc tholeiitic rocks, basalts and andesites occasionally rich in TiO₂ and FeO^*, icelandite-like andesites and dacites similar in composition to rocks of the oceanic regions. Its area extends from Sendai southwards to the Abukuma mountains district. The boundary between the two provinces corresponds to the Matsushima-Honjo line defined by Oide and Onuma (1960).

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