Seven basaltic masses, belonging to the Ueno basaltic rocks of Pliocene to Pleistocene age, are distributed to the south of the Ontake volcano in Nagano Prefecture (the Kiso province). They are composed of (augite-) olivine basalt to basaltic andesite with 47.5-53.3 wt % SiO₂.
The UGU (Uguigawa) mass yields the least differentiated basalt (Magma I) with the lowest Fe/Mg ratio, including olivine phenocrysts of Fo89•2-90•2. The bulk chemistry suggests its primary nature of the source mantle origin.
The TKT (Takataruyama) mass yields the most differentiated basalt (Magma IV). It is characterized by high concentrations of high-field-strength (HFS) and light rare earth elements such as Ti, P, Zr and La in spite of its low SiO₂, K₂O and Rb contents. This chemical feature does not indicate its source characteristics, and is rather acquired through contamination of some highly HFS element-enriched materials.
Most of the rocks of the Kiso province constitute two distinct chemical trends in the Fe/Mg variation diagrams, except the TKT rocks. One shows only a slight increase of SiO₂ with increasing K₂O (from Magma I to Magma II). The other shows a moderate increase of SiO₂ accompanying no K₂O enrichment (from Magma I to Magma III). Magma II is derived from Magma I by crystal fractionation, and modified by “Magma IV” incorporation. Magma III is possibly derived from Magma I by crystal fractionation and assimilation of low-K acidic rocks, and also modified by “Magma IV” incorporation.

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