Temporal and apatial variations of the Cenozoic volcanic rocks from the Notrheast Japan

本研究项目的研究结果总结如下：1)在九州、日本北陆、关东北部、东北以及北海道西南部和东北部等地区的不同地区，对大量新生代火山岩进行了出色的岩石学调查。从北海道西部的奥尻脊挖出的玄武岩也进行了地形学描述。这些岩石学资料不仅对揭示各油田火山岩的成因，而且对揭示环日本海地区的构造事件具有重要意义。2)对许多第三系火山岩，特别是日本东北弧的火山岩进行了K-Ar年龄测定。我们的新资料和前人已有的第三纪火山岩年龄资料，使我们能够在时间和空间分布上区分日本东北弧的三个不同的火山锋。3)利用日本东北和西南地区第三系和第四纪玄武岩的Sr和Nd等多主题资料，讨论了岩浆源特征的时间变化与日本海弧后盆地的打开之间的关系。根据初始Sr同位素比值、初始epsilonNd值和年龄，日本东北弧后侧35 ~ 0 Ma的玄武岩可分为两组。一种具有较高的Sri比率（0.7040-0.7058）和较低的epsilonNd值（0-+3），另一种具有较低的Sri比率（0.7029-0.7040）和较高的epsilonNd值（+3-+8）。前一个玄武岩活动在30 ~ 10 Ma期间连续发生，后一个玄武岩活动主要发生在两个时期；例如;约34-35毫安及15毫安以后。从过渡带到弧后侧的玄武岩中也观察到类似的证据，在15-0 Ma期间，高Sri比的玄武岩活动持续发生，而低Sri比的玄武岩活动发生在15 Ma左右和3 Ma之后。这些证据表明，日本东北弧下的次大陆地幔可能形成了层状构造；上部同位素富集层（Sri=0.7040 ~ 0.7058, epsilonNd=0 ~ +3），下部同位素富集层（Sri=0.7029 ~ 0.7040, epsilonNd=+3 ~ +8）。富同位素的玄武岩岩浆被认为是由上层形成的，而贫同位素的玄武岩岩浆则是由下层形成的。少

The results obtained from this research project are summarized as follows.1) The excellent petrological investigations have been made for numerous Cenozoic volcanic rocks from various localities in the Kyushu, Chugoku Hokuriku, northern Kanto, Tohoku and southwestern and northeastern Hokkaido discricts. Basaltic rocks dredged from the Okushiri Ridge, off western Hokkaido were also described prtographically. These petrological data may play an important role in revealing not only the origin of these volcanic rocks from each field but also the tectonic events around the circum Japan Sea region.2) K-Ar age determination has been carried out for many Tertiary volcanic rocks, especially those from the Northeast Japan arc. Our new data and the available rediometric age data for the Tertiary volcanic rocks provided by many previous workers have made it possible to distinguish the three different volcanic fronts in the NE Japan arc in temporal and spacial distribution.3) Based on Sr and Nd iso … More topic data for Tertiary and Quaternary basaltic rocks from the NE and SW Japan arcs, relationships are discussed between the temporal variation on magma source characteristics and the opening of the Japan Sea back-arc basin. Basaltic rocks from the back-arc side of the NE Japan arc during 35 0 Ma can be divided into two different groups with respect to initial Sr isotopic ratios, initial epsilonNd values and ages. One is characterized by higher Sri ratios (0.7040-0.7058) and lower epsilonNd values (0-+3) and the other by lower Sri ratios (0.7029-0.7040) and higher epsilonNd values (+3-+8). The former basaltic activity has taken place continuously during 30-10 Ma, whereas the latter activity occurred mainly in two periods ; i.e. ; at around 34-35 Ma and after 15 Ma. Similar evidences are also observed in basaltic rocks from the transitional zone to the back-arc side where basaltic activity characterized by higher Sri ratios took place continuously during 15-0 Ma, whereas that by lower Sri ratios occurred at around 15 Ma and after 3 Ma. These evidences suggest that the sub-continental mantle beneath the NE Japan arc may have constructed layred structure ; upper isotopically enriched (Sri=0.7040-0.7058, epsilonNd=0-+3) and lower depleted (Sri=0.7029-0.7040, epsilonNd=+3-+8) layrs. Basaltic magmas with enriched isotopic nature are considered to have been formed from the upper layr, while those with depleted isotopic one from the lower layr. Less