초록
We have shown that the volcanic rocks from the northern part of Cheju Island can be divided into high $P_2O_5/K_2O$(HPK) and low $P_3O_5/K_2O$(LPK) groups, each with distinct geochemical characteristics(Park and Kwon, 1993a and b). This study reports mineral compositions for plagioc-lase, olivine, and clinopyroxene in order to see the dependence of mineral chemistry on the whole rock composition, and discusses equilibrium relationships between crystal and liquid. Plagioclase and olivine phenocrysts show no compositional differences for the two rock group. However, $Al^{ⅵ}/Al^{ⅳ}$ ratios of clinopyroxenes suggest that pyroxenes have fractionated at deeper level, and that the LPK group might have fractionated at higher pressure than the HPK group. These are in good agreement with our previous interpretation based on whole rock chemistry(Park and Kwon, 1993a). Although subhedral or euhedral form and homogenous composition for most plagioclase and clinopy-roxene phenocrysts suggest equilibrium relationship with liquid, the uncertainties associated with liquid, the uncertainites associated with equilibrium constant for these minerals do not allow testing equilibium relationship between mineral and liquid on the basis of chemistry. On the other hand, olivine phenocrysts in hawaiites, for which Kd is well known, show distinct nonequilibrium relationship with calculated liquid composi-tions, while those in other rock compositions are in equilibrium from those for other rocks. We report for the first time as far as we know plagioclase xenocryst and pigeonite inclusion in plagioclase, which indicates assimilation process. In conclusion, these mineralogical observations imply that mag-matic processes underneath the Cheju volcano were not simple.