Partial melting controls on the northwest Kyushu basalts from Saga‐Futagoyama

Abstract The major element and compatible trace element compositions of the northwest Kyushu basalts (NWKBs) collected from Saga‐Futagoyama were analyzed to examine the petrogenesis of these basalts. Although nepheline‐normative alkaline basalts are not found in the basalts from Saga‐Futagoyama, the Saga‐Futagoyama basalts almost cover the major element variations of NWKBs. The basalts can be chemically divided into two groups: an Fe‐poor group (IPG) and an Fe‐rich group (IRG). The compositional variation of IPG basalts is essentially controlled by the partial melting of the source as suggested by the following: (i) bulk rock MgO, FeO and NiO compositions indicate that some IPG samples were equilibrated with mantle olivine; and (ii) correlations between Al2O3, CaO and MgO are consistent with those of experimental partial melts of peridotites. The inconsistent behaviors of the elements compatible with clinopyroxene (Cpx), such as V (Sc and Cu), preclude the significant role of the fractional crystallization of Cpx and spinel in IPG variation. IPG basalts have low Al and high Fe concentrations compared to the products of melting experiments involving peridotites and pyroxenites, suggesting that the IPG source would have a lithology and bulk rock composition different from those of typical peridotites and pyroxenites. IRG basalts have negative correlations between Fe2O3* and MgO, and between V and Fe2O3*/MgO, indicating that IRG basalts would have fractionated Cpx. However, the anomalously Fe‐rich feature of IRG basalts compared with NWKBs collected from other areas suggests that the role of Cpx fractionation in NWKBs is minor. Relatively low melting temperatures would have principally caused the large chemical variation of NWKBs.