The spinel lherzolite of ultramafic xenoliths are found in the alkali basalt from eastern part of the Cehju island, Korea. The xenolith is are mainly composed of olivine, orthopyroxene, clinopyroxene and spinel. Based on the chemical conmpositions of the constituent minerals, the ultramafic xenolith belong to upper mantle peridotite. Each minerals have a protogranular texture. Olivine with kink band texture partly shows undulatory extinction. Some clinopyroxenes have spongy textured rims. Brown spinels occur in the interstices between olivine and pyroxene grains. Olivine is mostly forsterits (Fo89.90). Orthopyroxene is enstatite (Wo1.3En88.4Fs10.3) with 3.87∼5.25 wt% Al₂O₃. Clinopyroxenes is diopside (Wo48.0En46.2Fs5.8) with 6.75∼5.03 wt% Al₂O₃. Spinel has the Mg value of 75.9 and its cr-number is 10.2. According to the P-T estimations for the mantle xenoliths, equilibrium temperatures of the xenoliths range from 1023 to 1038℃ and pressure is 18 kbar, Spinel lherzolith from this area, which is characterized by lower Cr-number (10.2) and homogeneous chemical compositions, supports that these ultramafic xenoliths are derived from the upper mantle.
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