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

1)对九州、北国、关东北部、东北及北海道西南部和东北部不同地区的大量新生代火山岩进行了岩石学研究。从北海道西部的Okushiri Ridge挖出的玄武岩也被原图描述。这些岩石学资料不仅对揭示各地区火山岩的成因，而且对周边日本海地区的构造事件都可能起到重要的作用。2)对许多第三纪火山岩进行了K-Ar年龄测定，特别是日本东北弧火山岩。我们的新数据和许多前人提供的第三纪火山岩的放射性年龄数据使我们有可能在时间和空间分布上区分日本东北部弧内三个不同的火山前缘。3)基于锶和钕的iso…根据日本东北和西南弧区第三系和第四纪玄武岩的更多专题资料，讨论了岩浆源特征的时间变化与日本海弧后盆地开放的关系。35.0 Ma日本东北弧弧后一侧的玄武岩，根据初始锶同位素比值、初始epsionnd值和年龄可分为两组。一种具有较高的SRI比值(0.7040-0.7058)和较低的epsilonnd值(0-+3)，另一种具有较低的SRI比值(0.7029-0.7040)和较高的epsilonnd值(+3-+8)。前者在30~10 Ma期间持续活动，后者主要发生在34~35 Ma和15 Ma以后两个时期。从过渡带到弧后侧的玄武岩中也观察到了类似的证据，玄武岩活动的特点是在15-0 Ma期间持续发生以高SRI为特征的玄武岩活动，而在15 Ma左右和3 Ma之后以低SRI为特征的玄武岩活动。这些证据表明，日本东北弧下的次大陆地幔可能构成了层状结构：上层同位素富集层(SRI=0.7040-0.7058，epsilonNd=0-+3)，下层亏损(SRI=0.7029-0.7040，epsilonNd=+3-+8)。富含同位素的玄武岩岩浆被认为来自上层，而亏损同位素的玄武岩岩浆来自下层。较少

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