Geochemistry of Cenozoic basalts in the Fukuoka district (northern Kyushu, Japan): implications for asthenosphere and lithospheric mantle interaction

Fukuoka volcanic field in northern Kyushu (Japan) is comprised of scattered small, monogenetic volcanoes with ages ranging from 1.1 to 4.4 Ma. A set of samples from the area, together with some from nearby localities, was collected and analyzed for major and trace element abundances and Sr, Nd and Pb isotope compositions. The basalts, unlike lavas from other nearby centers in northern Kyushu, show the highest FeO*, TiO2 and lowest SiO2 characteristics, which are interpreted to reflect high melting temperature and pressure; whereas high Sr, Sm and high-field-strength elements (HFSE) such as Zr and Nb, high light rare earth element (LREE), relatively low Ba, Rb, and broadly oceanic island basalt (OIB)-like primitive mantle normalized incompatible trace element patterns are interpreted to reflect source characteristics. In addition to lead isotopic compositions that are the most radiogenic yet analyzed from northern Kyushu and the Sea of Japan, strontium and neodymium isotopic compositions of Fukuoka lavas free from crustal contamination are among the highest and lowest, respectively (average, 0.7052 and 0.5126), in the region. The samples show the signature of enriched mantle type 2 (EM2), differing from most of the other Dupal anomaly bearing lavas reported from the Sea of Japan and elsewhere in northern Kyushu. The EM2-like characteristics and relatively low concentrations of large ionic lithophile elements (LILE), low LILE/LREE and LREE/high-field-strength elements (HFSE), and mid-ocean ridge basalt (MORB)-like Rb/Sr and Nb/Zr ratios in the Fukuoka lavas are explained by melts from an asthenospheric source that experienced previous melt extraction. Because the chemical characteristics of Fukuoka basalts are strictly, geographically localized, we suggest that, while the mantle beneath most of northern Kyushu is very much similar to that of the Sea of Japan, represented by a spectrum of depleted MORB-EM1 (Dupal-like) hybrids, the Fukuoka EM2-rich component may have been added from shallower levels.