Temporal changes of isotopes in igneous rocks occurring in the ciucum Japan Sea regions

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The results obtained from this research project are summarized as follows.(1) On the basis of Sr and Nd isotopic data for Tertiary to Quaternary volcanic rocks from the NE Japan arc, the mantle magma sources beneath the NE Japan arc can be divided into two regions ; one is the isotopically undepleted sub-island arc upper mantle and another is the depleted asthenosphere (called the depleted source) which was upwelled to the undepleted sub-island arc upper mantle at around 15 Ma.Temporal change of HFS element (high field strength element) character two different mantle parts beneath the NE Japan arc, the undepleted and depleted sources, is investigated using four HFS elements Nb, Ti, Y and Zr in 103 basalitc samples (MgO*5%, SiO2*53%) from the trench side (including some samples from the transitional zone) and 127 ones from the back-arc side. Systematic differences are observed in HFS element composition of the older basalts relative to the younger ones. Basaltic rocks derived from the u … More ndepleted source exhibit a gradual decrease in Zr/Y ratio and a gradual increase in Ti/Zr and Zr/Nb ratios with decreasing age from -35 Ma to the present, whereas those from the depleted source show similar variations in these HFS element ratios with decreasing age from -15 Ma to the present.Plotting of basalts with minimum Zr/Y ratio among respective basaltic rocks in 2Nb-Zr/4-Y diagram proposed by Meschede (1986) suggests that the undepleted source in the mantle wedge of the NE Japan arc has gradually changed its HFS element character, from a highly incompatible HFS element-enriched OIB source type through N-MORB source type to a highly incompatible HFS element-depleted IAB source type, with the decrease in age. Gradual depletions of HFS elements of greater incompatibility in the undepleted source may be due to the successive extraction of basaltic magmas from the source since -35 Ma ago. Examination of basalts derived from the depleted source in Meschede's diagram also indicates the changing from the source depleted slightly in Zr and enriched slightly in Nb compared with N-MORB source to that depleted slightly both in Zr and Nb compared with N-MORB source during -15 Ma to the present. Variations of Ti/Zr and Zr/Nb ratios of basaltic rocks from the NE Japan arc are not compatible with the presence of residual Ti, Nb-enriched minerals such as sphene, rutile and amphibole in the source mantle.(2) The initial ^<87>Sr/^<86>Sr and ^<143>Nd/^<144>Nd ratios (Sri and Ndi ratios) were determined for the late Miocene to Pliocene volcanic rocks from twenty-six locations in the NE Japan arc and a spatial-temporal variation of these ratios was also examined using the newly obtained and published isotope data. Sri and Ndi ratios of volcanic rocks except few samples during 12-0 Ma show linear trends across the NE Japan arc from the undepletedisotopic signature in the trench side volcanics to the depleted one in the back-arc side volcanics. This can be ascribed to a across-arc isotopic heterogeneity in their source mantle material due to mixing of isotopically undepleted island arc mantle with depleted asthenospheric mantle which was up-welled to the former beneath the back-arc side during the opening of the Japan Sea. Less