Temporal and spacial geochemical variation in the Neogene volcanic rocks and the dource mantle in NE Honshu arc

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The NE Honshu arc has the most completely studied Cenozoic section. In this section, there are three volcanic periods ; continental margin volcanic period, back-arc basin volcanic period, and issland arc volcanic period. The 1320ﾟC iotherm in the mantle wedge is monotonously deep to the back-arc side at the continental margin volcanic period and the island arc volcanic period. The inclination of the isotherm in the continental margin volcanic period is more gentle than in the island arc volcanic period. The magma segregation depth at the volcanic front was much deeper in the continental margin volcanic period than in the island arc volcanic period. On the other hand, the mantle temperature was the highest at the back-arc side in the back-arc basin volcanic period.The presence of mantle heterogeneity between the volcanic front side and the back-arc side was clearly shown in the process identification diagrams for the volcanic rocks of the island arc volcanic period. The two source mantles can be mineralogically heterogeneous, and vertically layred. The heterogeneity and across-arc profile of magmatic temperatures are closely related to the seismic velocity structure with the localized low-velocity regions.The Quaternary volcanism shows short-term and long-term spreading to back arc side. This eruption arrays in a temporal-spatial plot have usually commenced at the volcanic front, and propagated to the back-arc sequentially. Simultaneous magma transfer from the deeper reservoirs by tectonic pulse could be a cause of the synchronized volcanic activity. Through the Quaternary age in the southern part of the NE Honshu arc, the periodic magma eruption episodes at intervals of 50 to 100 thousand years synchronized with tectonic movements exists. The climactic eruptions seems to match the timing of the eccentricity-maximums of the Earth's orbit around the Sun. The tidal forces could work supplementary to the compressive stress from subducting plate.