Sr・Nd isotopic study on basaltic rocks from the Chubu District

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(1) The Ueno and On-take basaltic rocksTen samples of the Ueno basaltic rocks (2.7-0.9 Ma) have ^<87>Sr/ ^<86>Sr-0.7055-0.7070 and epsilon _<nd>-+3.3-1.1, and show no specific secular variation in isotpic compositions. The Ueno samples are higher in ^<87>Sr/ ^<86>Sr than basaltic rocks from the On-take volcano (<0.8 Ma) which lie within the mantle array on an epsilon _<nd>-^<87>Sr/ ^<86>Sr diagram. Because (1) the Ueno and On-take basaltic rocks were formed in the same area and (2) the difference in their ^<87>Sr/ ^<86>Sr cannot be ascribed to possible contamination by the crust, some episode (arrival of the leading edge of the subducting Philippine Sea plate?) seems to have occurred at 1+alpha Ma in the upper-mantle beneath central Japan.(2) The Jizo-toge andesitic rocksThe Jizotoge andesitic rocks, formed at -2 Ma nearby the Ueno basaltic rocks, bear epsilon _<nd>-^<87>Sr/ ^<86>Sr relations similar to that of the Ueno basaltic rocks. The epsilon _<nd> and ^<87>Sr/ ^<86>Sr compositions of magmas generated at -2 Ma in this area seem to be insignificantly affected by the crustal material.(3) Volcanic rocks from the Ryohaku Mountains and Fukui PrefectureThe epsilon _<nd>-^<87>Sr/ ^<86>Sr relations of volcanic rocks from the Ryohaku Mountains (Mts. Gankyoji-yama, Dainichiga-take, Kyoga-take and Bishamon-dake) and Fukui Prefecture (Mt. Kunimi-dake and Tojimbo) are largely similar to that of the Ueno basaltic rocks, but they show a greater scatter trending towards a higher ^<87>Sr/ ^<86>Sr and lower epsilon _<nd> direction.