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

本研究项目的研究结果总结如下：1)对日本东北弧海沟侧中中新世火山岩进行了K-Ar定年。寒原玄武岩、伊纳斯火山岩和托马里火山岩的K-Ar年龄分别为15.1 ~ 15.4 Ma、12.6 ~ 16.0 Ma和13 ~ 15 Ma。2)将前人研究资料与本研究资料相结合，估计13 ~ 16 Ma的火山锋位于第四纪火山锋以东30 ~ 50 km处。3)讨论了中中新世（形成13-16 Ma火山锋）至晚中新世（形成0-8 Ma火山锋）火山锋向弧后侧移动的机制。4)中间中新世火山带的战壕边不能分为个第四纪Aoso-Osore和Sekiryo区域,基于不同的镁铁质斑晶集合体和不相容元素丰度的火山岩。5)在中中新世火山岩中发现了富TiO_2的玄武岩、类冰岛安山岩和英安岩，以及化学成分具有原始性质的橄榄玄武岩，这些火山岩在第四纪火山岩中没有发现。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型亲石元素的弧跨变化与第四纪火山岩形成鲜明对比。中中新世与第四纪火山岩岩石学特征的差异可能与中中新世与第四纪上地幔岩浆生成的构造环境不同有关。

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.