|Budget Amount *help
¥7,100,000 (Direct Cost: ¥7,100,000)
Fiscal Year 1989: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 1988: ¥6,400,000 (Direct Cost: ¥6,400,000)
The Northeast Japan Arc has been divided into four volcanic zones, Aoso-Osore, Sekiryo, Moriyoshi and Chokai, from the trench to back arc side. The lateral variation of the volcanic rocks from the four volcanic zones was investigated. In the Aoso-Osore zone, low-K calc-alkaline rocks with subordinate low-K tholeiites erupted. Medium-K calc-alkaline rocks are associated with, and often follow low-K tholeiites at the volcanoes of the Sekiryo zone. The Moriyoshi zone is characterized by medium-K calc-alkaline rocks with subordinate medium-K tholeiites. The Chokai zone contains medium-K calc-alkaline rocks on the eastern side, and high-K calc-alkaline rocks on the western side. These are associated with high-alumina basalts and alkali basalts, respectively.
Instrumental photon-activation analysis has been used to determine 13 trace elements: Sc, Cr, Co, Ni, Zn, Rb, Cs, Sr, Ba, Ce, Y, Nb and Zr in these volcanics. Quaternary volcanic rocks from the Northeast Japan arc are enriched in K, Ba, Rb and Sr and depleted in Nb, Ti and Zr relative to Y and Ce. These rocks, also, show a distinct lateral variation of trace elemental abundances. Across the segment in Northeast Honshu, P_2O_5, Rb, Ce, Sr, Ba and Cs contents increase gradually with distance from the volcanic front; K_2O, Y, Zr and Nb also increase by smaller enrichment factors than the former elements. on the basis of the abundance of incompatible trace elements of each volcanic center, the degree of partial melting and the abundances of the incompatible elements of the source mantle have been calculated. The back-calculated source compositions show chemical heterogeneity within the mantle wedge.