ABSTRACT

On the basis of trace element abundances and Sr and Nd isotope ratios, Quaternary island arc low-alkali tholeiites (IAT) in Northeast Japan are divided into two types; U (undepleted)- and D (depleted)-IAT. U-IAT are characterized by high and constant abundances of Large Ion Lithophile Elements (LILE), which are 10–20 times higher than the primitive mantle value. The range of Nd isotope ratios of U-IAT is identical within errors to the Bulk Earth value. The ⁸⁷Sr/⁸⁶Sr ratios of U-IAT range from 0.7047 to 0.7057, greater than or equal to the currently accepted Bulk Earth value, and these variations can be caused by initial heterogeneity in Rb/Sr ratios of the primitive mantle. It is argued that U-IAT magmas were derived by 5–10% batch partial melting of the primitive mantle. D-IAT are characterized by convex-upward patterns with a maximum normalized value for an alkaline earth element in a primitive mantle versus ionic radii (NPR) abundance diagram. The degree of depletion in the elements with larger ionic radii is greater than those with smaller ones. These characteristics cannot be explained by magma compositional variations derived by different degrees of partial melting of a homogeneous mantle or by fractional crystallization. They can rather be explained by a variation in the composition of a previously depleted mantle source. The isotopic compositions of Sr and Nd range from the value of the Bulk Earth to that of the most depleted oceanic island basalt, and are well correlated with Nb/Y ratios. A mantle isochron of about 1 Ga is obtained for Nd and Sm isotopes. D-IAT magmas are considered to have formed in two stages consisting of depletion in LILE and Nb due to a loss of an extremely small amount of melt (<1%), about 1 Ga from a primitive mantle, and more recently followed by 5–10% partial melting episode under the conditions of formations typical of the U-IAT magmas.