Subarc magmatic and hydration processes inferred from a hornblende peridotite xenolith in spessartite from Kyoto, Japan

Hornblende peridotite xenolith in the spessartite dike in the Tamba belt, northern Kyoto city, is mainly composed of olivine with high Fo and NiO contents (Fo87−94 and 0.22-0.51 wt%). The olivine encloses chromian spinels with a high Cr/(Cr + Al) atomic ratio, 0.66-0.80, partly replaced and digested by hornblendes with a low Mg/(Mg + total Fe) atomic ratio, 0.68-0.81. Microscopic and geochemical features of the hornblende peridotite xenolith indicate a reaction between a dunitic cumulate from a high-Mg and -Cr melt and an evolved hydrous high-Mg melt to form hornblendes. Olivines are lower in Fo when in contact with hornblendes. Rare earth element (REE) patterns of the hornblendes, common to the xenolith and host spessartite, indicate that all the involved melts are similar but different in the degree of differentiation. The strong hydration of olivine in the xenolith suggests a wide distribution of hornblende-rich rocks, such as hornblendites, formed by the hydration of olivine-rich rocks, such as dunites, around the Moho beneath the arc. This may give rise to a layer with a Vp intermediate between typical lower crustal and mantle values.