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

Abstract

Lower Cretaceous and Eocene igneous rocks in the Kitakami Mountains, northeast Japan, attract special interest because of the occurrence of adakitic rocks. In addition, high-Mg andesites (HMAs) were discovered from the Lower Cretaceous dike rocks and from the Eocene Jodogahama rhyolitic rocks in the Kitakami Mountains. Lower Cretaceous HMAs occur as several small dikes (less than 5 m thick, rarely up to 70 m) from Ubaishi, Gongen, Shizugawa, Numazu, and Oshika, southern Kitakami Mountains, intrude slightly older than the Lower Cretaceous plutonic rocks. Among these HMAs, the HMA dike from Gongen, characteristically contains ultramafic inclusions. Eocene HMA, a small intrusive mass (observed maximum diameter is 300 m) consisting of aphyric andesite and olivine andesite, occurs only in Matsuhashi, Iwaizumi, northern Kitakami Mountains. The Eocene HMA is associated with hornblende rhyolite belonging to the Eocene Jodogahama rhyolitic rocks. 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.