The basalt-high magnesium andesite association formed by multi-stage partial melting of a heterogeneous source mantle: Evidence from Hirado-Seto, Northwest Kyushu, Southwest Japan

An association of basalts and high magnesium andesites (HMAs), erupted at 7 Ma after the opening of the Sea of Japan, exposed at Hirado-Seto in northwest Kyushu, southwest Japan. The rocks are aphyric and are characterized by enrichments in incompatible trace elements similar to those seen for oceanic island basalts, although the HMAs show a weak negative Nb anomaly. High MgO, Ni and low FeO⁎/MgO indicate that the Hirado-Seto rocks were originally primitive magmas. They do not show a positive correlation between K2O/La and SiO2, or between Pb/La and SiO2, indicating that hydrous components derived from a subducting slab did not play a significant role in the genesis of the Hirado-Seto basalt–HMA magmas. Alternatively, the normative olivine–quartz–[Jd + CaTs] compositions indicate that the Hirado-Seto basalt–HMA magmas were formed by multi-stage partial melting of the source mantle at pressures ranging from 1 to 0.5 GPa along the 1300 °C mantle adiabat, assuming anhydrous conditions. Basalt magmas separated from the source mantle at 1 GPa. HMA magmas separated at 0.5 GPa. A weak negative anomaly for Nb in HMAs can be explained by precipitation of Ti–P oxides during their ascent under high fO2 condition. Thinning of the Hirado-Seto lithosphere caused by transtensional strain during the opening of the Sea of Japan would have enabled separation of HMA magmas at unusually low pressures.