Evidence for slab melt/mantle reaction: petrogenesis of Early Cretaceous and Eocene high-Mg andesites from the Kitakami Mountains, Japan

The Lower Cretaceous and Eocene HMAs show similar petrochemical characteristics to those of Cenozoic adakite (high LREE/HREE ratios and Sr contents, low Y and HREE contents) except higher Cr, Ni, and Mg contents. In addition, the Eocene HMAs and the ultramafic inclusions in the Lower Cretaceous HMAs show less radiogenic Nd-Sr isotopic characteristics than those from the adakitic granites in Kitakami, and similar to those of the Cenozoic adakite. On the other hand, the Miocene Setouchi HMAs in southwestern Japan are characterized by lower Sr/Y ratios, less fractionated REE patterns, weak negative Eu anomalies, and more radiogenic Nd-Sr isotopic characteristics than those of Cenozoic adakite. The ultramafic inclusions from Gongen have clinopyroxene characterized by high Sr concentrations and high LREE/HREE ratios. These clinopyroxenes are considered to be crystallized during interaction of slab derived adakitic melt with overlying mantle peridotite. The olivine phenocrysts of the Eocene HMA show extremely high NiO contents (maximum 0.58 wt.%), which may be an evidence for slab melt/mantle reaction. Concludingly, the petrochemical features of HMA magmas in Kitakami can be explained by reaction of slab derived adakitic melt with overlying mantle peridotite to equilibrate with mantle olivine. The difference between the chemical compositions of the Kitakami and Setouchi HMAs can be explained by the difference in compositions of initial slab melt resulted from the difference in depth of slab melting.