Geological and Petrological studies of the Middle Mocene volcanic rocks distributed around the volcanic front of about 15 Ma in the Northeast Japan arc

本研究项目取得的成果总结如下。1)对日本东北弧海沟侧中中新世火山岩进行了K-Ar测年。分别获得了15.1-15.4 Ma、12.6-16.0 Ma和13-15 Ma的玄武岩K-Ar年龄和托马里火山岩的K-Ar年龄。2)结合前人的研究资料和目前的研究资料，可以得出结论：13-16 Ma的火山前锋位于第四纪火山前锋以东30-50公里处。3)探讨了中中新世(形成13-16 Ma火山前缘)至晚中新世(形成0-8 Ma火山前缘)火山前缘向弧后侧转移的机制。4)根据基性斑晶组合和不相容元素丰度的差异，海沟侧中中新世火山岩带不能像第四纪奥索-奥索尔带和塞基里约带那样划分为亚带。5)在中中新世火山岩中发现了富钛玄武岩、似冰安山岩和英安岩以及化学成分原始的橄榄石玄武岩，而在第四纪火山岩中没有发现这些火山岩。6)弧后侧中中新世玄武岩的K_2O和Rb含量低于海沟侧，表明中中新世火山岩中大的亲石元素的跨弧变化与第四纪火山岩的差异明显。中中新世和第四纪火山岩岩石学特征的差异可能归因于中中新世和第四纪之间与上地幔岩浆生成有关的构造环境的不同。

The results obtained from this research project are summarized as follows. 1) The K-Ar dating was carried out on the Middle Miocene volcanic rocks from the trench side in the Northeast Japan arc. We obtained K-Ar ages of 15.1-15.4, 12.6-16.0 and 13-15 Ma for the Ryozen basalts, Inase volcanic rocks and Tomari volcanic rocks, respectively. 2) If we combine the data of previous studies with the present ones, we can conclude that the volcanic front of 13-16 Ma was estimated to lie 30-50 km east of the Quaternary volcanic front. 3) Mechanism for the shifting of volcanic front into the back-arc side from the Middle Miocene (forming 13-16 Ma volcanic front) to the Late Miocene (forming the 0-8 Ma volcanic front was discussed. 4) The Middle Miocene volcanic zone in the trench side can not be divided into subzones like the Quaternary Aoso-Osore and Sekiryo zones, based on differences in mafic phenocryst assemblages and incompatible element abundances of these volcanic rocks. 5) In the Middle Miocene volcanic rocks, TiO_2 rich basalts, icelanditelike andesites and dacites, and olivine basalts with primitive nature in chemical compositions are found although these volcanic rocks can not be observed in the Quaternary volcanic rocks. 6) The Middle Miocene basalts in the back-arc side have the K_2O and Rb contents lower than those in the trench side, indicating that the across-arc variations of large lithophile elements in the Middle Miocene volcanic rocks have striking contrast to those for the Quaternary volcanic rocks. These differences in petrologic characteristics of the Middle Miocene and Quaternary volcanic rocks may be ascribed to differences in tectonic environments in relation to the magma generation in the upper mantle between Middle Miocene and Quaternary ages.