Temporal and apatial variations of the Cenozoic volcanic rocks from the Notrheast Japan

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The results obtained from this research project are summarized as follows.1) The excellent petrological investigations have been made for numerous Cenozoic volcanic rocks from various localities in the Kyushu, Chugoku Hokuriku, northern Kanto, Tohoku and southwestern and northeastern Hokkaido discricts. Basaltic rocks dredged from the Okushiri Ridge, off western Hokkaido were also described prtographically. These petrological data may play an important role in revealing not only the origin of these volcanic rocks from each field but also the tectonic events around the circum Japan Sea region.2) K-Ar age determination has been carried out for many Tertiary volcanic rocks, especially those from the Northeast Japan arc. Our new data and the available rediometric age data for the Tertiary volcanic rocks provided by many previous workers have made it possible to distinguish the three different volcanic fronts in the NE Japan arc in temporal and spacial distribution.3) Based on Sr and Nd iso … More topic data for Tertiary and Quaternary basaltic rocks from the NE and SW Japan arcs, relationships are discussed between the temporal variation on magma source characteristics and the opening of the Japan Sea back-arc basin. Basaltic rocks from the back-arc side of the NE Japan arc during 35 0 Ma can be divided into two different groups with respect to initial Sr isotopic ratios, initial epsilonNd values and ages. One is characterized by higher Sri ratios (0.7040-0.7058) and lower epsilonNd values (0-+3) and the other by lower Sri ratios (0.7029-0.7040) and higher epsilonNd values (+3-+8). The former basaltic activity has taken place continuously during 30-10 Ma, whereas the latter activity occurred mainly in two periods ; i.e. ; at around 34-35 Ma and after 15 Ma. Similar evidences are also observed in basaltic rocks from the transitional zone to the back-arc side where basaltic activity characterized by higher Sri ratios took place continuously during 15-0 Ma, whereas that by lower Sri ratios occurred at around 15 Ma and after 3 Ma. These evidences suggest that the sub-continental mantle beneath the NE Japan arc may have constructed layred structure ; upper isotopically enriched (Sri=0.7040-0.7058, epsilonNd=0-+3) and lower depleted (Sri=0.7029-0.7040, epsilonNd=+3-+8) layrs. Basaltic magmas with enriched isotopic nature are considered to have been formed from the upper layr, while those with depleted isotopic one from the lower layr. Less