Mantle peridotite xenoliths from the Southwest Japan arc: a model for the sub-arc upper mantle structure and composition of the Western Pacific rim

Abstract

There are a number of Cenozoic ultramafic xenolith localities on the Southwest Japan arc, which enable us to deduce a petrological model of the upper mantle of the island arc or active continental margin on a regional scale. On the basis of mineral chemistry and petrology the mantle peridotites (harzburgite and lherzolite) from the Southwest Japan arc can be divided into two types, unmetasomatized and metasomatized types. The former type of peridotites, which are less predominant than the latter, have characteristics of mantle restite (olivine of Fo₈₈₋₉₁) and are not accompanied with Fe-rich black pyroxene-bearing xenoliths and related pyroxene megacrysts. They are found exclusively in isolated monogenetic volcanoes not forming volcano clusters. In contrast, the latter type of peridotites, which are always accompanied with large amounts of Fe-rich black pyroxenite xenoliths and related black pyroxene megacrysts, have less magnesian minerals (e.g., olivine of Fo₇₈₋₉₀) depending on the degree of metasomatism. They have textural and modal characteristics as mantle restites but also have chemical characteristics as metasomatites. They are mainly found in monogenetic volcanoes constituting volcano clusters.
    The Fe-rich pyroxenites and related rocks are young precipitates of possibly Tertiary ages from alkali basaltic melts, genetically related with the xenolith-carrying basalts. A large scale asthenospheric upwelling in the Miocene time may have been responsible both for the intra-plate alkali basalt magmatism, namely the formation of xenolith-bearing basalts, metasomatism and precipitation of Fe-rich pyroxenites and megacrysts, and for the Japan Sea opening. The formation of Fe-rich pyroxenites and related rocks strongly modified the pre-existing mantle-crust both in chemistry and structure, leaving a chemically and geophysically intermediate Moho transition zone. This may be one of the characteristics of the upper mantle of active continental margin or island arc in the Western Pacific.