Lateral variations of incompatible elements are ubiquitus in the Quatemary volcanic rocks in Central Japan. Low LILE/HFSE ratio type is of the petrological characteristics of the volcanic front rocks, that is traceable from Izu-Mariana arc to the Low-K thoreiitic rocks in the volcanic front of Tohoku Honshu arc, through volcanic front of central Japan area. The rocks that belongs to the High LILE/HFSE ratio type are distributed in the back arc side (Norikura Volcanic Chain : NVC and Hakusan Volcanic Chain : HVC). In the area between the volcanic front and the back arc, rocks of a transitional chemistry are widely spreaded to form triangular shaped arc junction. Magma segregation depth inclease abruptly between the volcanic front and transition area. The upper mantle material, at magma segregation depth, may have significant compositional variation. The deplted mantle (Low-LILE/HFSE) are beneath the volcanic front, enriched one (High-LILE/HFSE) in the back arc side, and transition of the two mantle materials lies in between. Underneath the NVC, a P-wave perturbation have been detected seismologically, and considered to be mantle diapir or partially molten body (PMB) (Ando, 1986). Degree of partial melting deduced from rock chemistry is seemingly controlled by the magnitude of P-wave perturbation. The magma segregation depth of 70-60 km deep is nearly parralel to the 1250℃ geotherm, which is parralel to the earth's surface (Hayakawa, 1992). The head of possible PMB is unlikely confined by the thermal structure, it is shallower beneath Ontake and Norikura volcanoes and deeper to the north. The colser the magma root to the PMB the higher the degree of patrial melting. The variations both of magma segregation depth and degree of parrtial melting show no relationships to the subduction slab depth.

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