Photon-activation analysis of volcanic rocks from Northeast Japan Arc.

日本东北弧从海沟到弧后可划分为青总、关良、盛吉和长海四个火山带。研究了四个火山岩带火山岩的横向变化。在Aoso-Osore带，低钾钙碱性岩石与次级低钾拉斑玄武岩爆发。中钾钙碱性岩石与Sekiryo带火山的低钾拉斑玄武岩相关，并经常跟随低钾拉斑玄武岩。Moriyoshi带的特征是中钾钙碱性岩石和次级中钾拉斑玄武岩。Chokai带东侧包含中钾钙碱性岩石，西侧包含高钾钙碱性岩石。用仪器光活化分析法测定了这些火山岩中Sc、Cr、Co、Ni、Zn、Rb、Cs、Sr、Ba、Ce、Y、Nb、Zr等13种微量元素。日本岛弧东北部第四纪火山岩相对于Y和Ce，富集K、Ba、Rb和Sr，亏损Nb、Ti和Zr。这些岩石，也表现出明显的微量元素丰度的横向变化。P_2O_5、Rb、Ce、Sr、Ba和Cs含量在本州东北段随远离火山前缘而逐渐增加，K_2O、Y、Zr和Nb也以较小的富集因子增加。根据各火山中心的不相容微量元素丰度，计算了源区地幔的部分熔融程度和不相容元素丰度。反算的源区成分显示了地幔楔内部的化学不均匀性。

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.