Cenozoic basaltic alkaline rocks and sub-alkaline rocks are distributed in the southern part of the Shimabara Peninsula. Most of the lavas erupted in these areas have oceanic island basalt (OIB) - like chemical compositions, in contrast to volcanic rocks produced in subduction zones. However, the chemical composition of Miocene to Pliocene (6‒4 Ma) lava distributed in the Kuchinotsu area in the southern part of the Shimabara Peninsula indicates the influence of plate subduction. We collected and analyzed basalt samples from these areas, and found that the basalts in the Kuchinotsu area are classified as sub-alkaline rocks relatively enriched in SiO₂. The basalts are depleted in incompatible elements, have negative Nb anomalies characteristic of island arcs volcanic rocks and have high LILE/HFSE ratios (e.g. Ba/Nb, Ba/Zr) and low HFSE/HFSE ratios (e.g. Nb/Y, Nb/Zr). Based on these geochemical characteristics, it is suggested that the characteristics of the Shimabara Peninsula basaltic rocks among central to northern Kyushu are due to differences in the nature of the source mantle types, their mantle melting conditions, and the addition of slab-derived components. Furthermore, the origin of the Shimabara Peninsula basaltic rocks, inferred from the trace element composition, suggests that silicate melts derived from altered oceanic crust is likely to have added to the depleted mid-ocean ridge basalt (MORB)-source mantle.

It is concluded that these basaltic rocks in the southern Shimabara Peninsula cannot be explained by resulting from the melting of a deep mantle plume similar to that at Tomie, northwestern Kyushu, and that the origin of the magma is due to the interaction between the past lithospheric mantle and the subducting Philippine Sea plate.