Petrology of pyroxenite xenoliths enclosed in basaltic rocks from Shinsanri of Jeju Island

A variety of ultramafic xenoliths hosted in basalts is exposed in Sinsanri locality, Jeju Island. The pyroxenite xenoliths occupying about 5 vol% among these ultramafic xenoliths occur as a websteritic dyke phase with 1 similar to 2 cm in width within peridotite xenolith or as discrete xenoliths with 3 similar to 7 cm in width in the host basalt. They are olivine websterite-websterite-clinopyroxenite in modal composition. Olivine websterite and websterite show a textural continuity from protogranular to porphyroclastic texture, which is similar to peridotite xenoliths trapped in the same host basalt, whereas clinopyroxenite occur as a discrete clinopyroxene megacryst showing pegmatitic character. The major element compositions of the constituent olivine (Mg#[=100xMg/(Mg+Fe2+)]=89, NiO=0.40 similar to 0.47 wt.%), orthopyroxene (Mg#=87 similar to 90, Al2O3 =4.2 similar to 7.4 wt.%), clinopyroxene (Mg#=88 similar to 91, Al2O3 =5.8 similar to 8.9 wt.%) and spinel (Cr#[=100xCr/(Cr+Al)]=5 similar to 9, Al2O3 =58 similar to 62 wt.%) are consistent with those of the mantle-derived worldwide pyroxenite. They also show an geochemical continuity with those of peridotite xenoliths trapped in the same host basalt. The textural and major composition characteristics of the studied pyroxenite xenoliths suggest that they have been evolved from peridotite in the upper mantle by the short-time differentiation or they are the products of refertilization.