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

日本东北弧从海沟到弧后侧划分为四个火山带，即奥索-奥索尔火山带、关岛火山带、森吉火山带和周凯火山带。研究了四个火山岩带火山岩的横向变化。在奥索-奥索尔带，低钾钙碱性岩喷发，伴生低钾拉斑玄武岩。中钾钙碱性岩石与低钾拉斑玄武岩共生，并经常跟随在塞基里昂地区的火山中。盛世带以中钾钙碱性岩为主，辅之以中钾拉斑玄武岩。赤开带东侧为中钾钙碱性岩，西侧为高钾钙碱性岩。仪器光活化分析方法测定了这些火山岩中的Sc、Cr、Co、Ni、Zn、Rb、Cs、Sr、Ba、Ce、Y、Nb、Zr等13种微量元素。日本东北弧第四纪火山岩相对Y、Ce而言，富K、Ba、Rb、Sr，亏损Nb、Ti、Zr。这些岩石也显示出明显的痕量元素丰度的横向变化。在本州东北部全段，P_2O_5、Rb、Ce、Sr、Ba、Cs的含量随着距离火山前缘的增加而逐渐增加，K_2O、Y、Zr、Nb的含量增加的幅度也小于前者。根据各火山中心不相容元素的丰度，计算了源地幔不相容元素的丰度和部分熔融程度。反演源成分显示地幔楔体内部的化学不均质性。

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.