The lithology of the lower crust beneath arcs is more variable than that of the oceanic lithosphere due to variable pressure and temperature conditions during arc development. Mafic–ultramafic xenoliths in Cenozoic alkali basalts are documented near Kurose in southwest Japan, and are interpreted to have formed within the back-arc region of the Southwest Japan Arc. These rocks provide insight into the petrogenesis of the lithosphere beneath the arc. We present petrographic descriptions and geochemical data for a websterite xenolith from Kurose containing thin olivine–feldspar veins. The websterite comprises abundant green (aluminous) spinel. The major-element compositions of minerals in the websterite are approximately constant. The vein comprises plagioclase and anorthoclase, and anhedral olivine that occurs adjacent to orthopyroxene in the websterite. Clinopyroxene grains within the websterite have variable Zr and Hf concentrations of 1.9–92.6 and 0.06–8.69 μg/g, respectively. Their Zr and Hf concentrations increase considerably toward the vein, contrasting with the other trace elements (e.g., REE, Ti, Nb, Th, and U). The spatial distribution of feldspar within the vein suggests that the vein formed by a reaction between orthopyroxene in the websterite and an alkali basaltic melt. A Zr–Hf-rich melt was produced by assimilation–fractional crystallization during this reaction, resulting in the observed compositional trend in the clinopyroxene grains near the vein. The studied green spinel-rich websterite is interpreted to be a metatroctolite produced during an arc crust growth at the continental margin. The olivine–feldspar veins suggest that lower-crustal growth was terminated by back-arc spreading and that the lower crustal rocks subsequently interacted with an alkaline-rich melt.

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