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

日本东北弧约15 Ma火山前缘周围分布的中新世火山岩的地质和岩石学研究

基本信息

批准号：
03640639
负责人：
SHUTO Kenji
金额：
$ 0.96万
依托单位：
Nigata University
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1991
资助国家：
日本
起止时间：
1991 至 1992
项目状态：
已结题

项目摘要

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.

该研究项目获得的结果总结如下。 1）对日本东北弧海沟侧的中中新世火山岩进行了K-Ar测年。我们获得了 Ryozen 玄武岩、Inase 火山岩和 Tomari 火山岩的 K-Ar 年龄分别为 15.1-15.4、12.6-16.0 和 13-15 Ma。 2）如果我们将前人的研究数据与目前的数据结合起来，我们可以得出结论，13-16Ma的火山前缘估计位于第四纪火山前缘以东30-50公里处。 3) 探讨了火山锋从中中新世(形成13~16 Ma火山锋)到晚中新世(形成0~8 Ma火山锋)向弧后移动的机制。 4) 沟侧中中新世火山带不能根据基性差异划分为第四纪青苏-大山、石岭等亚带。这些火山岩的斑晶组合和不相容元素丰度。 5)中新世火山岩中发现了富TiO_2玄武岩、类冰岛安山岩、英安岩、橄榄石玄武岩等化学成分原始的火山岩,但在第四纪火山岩中看不到这些火山岩。 6)弧后侧中中新世玄武岩的K_2O和Rb含量均低于海沟侧,表明中中新世火山岩中大量亲石元素的跨弧变化与第四纪火山岩中的跨弧变化具有鲜明的对比。中中新世和第四纪火山岩岩石学特征的这些差异可能归因于中中新世和第四纪时代与上地幔岩浆生成有关的构造环境的差异。