Temporal and spacial geochemical variation in the Neogene volcanic rocks and the dource mantle in NE Honshu arc

本州岛东北弧是研究最完整的新生代剖面。该剖面可划分为三个火山活动期：陆缘火山活动期、弧后盆地火山活动期和岛弧火山活动期。在陆缘火山活动期和岛弧火山活动期，地幔楔中1320 ℃的地温向弧后一侧单调加深。大陆边缘火山活动期等温线的倾斜度比岛弧火山活动期的平缓。陆缘火山期火山前缘岩浆分凝深度远大于岛弧火山期。弧后盆地火山岩期弧后一侧地幔温度最高，岛弧火山岩期火山岩过程识别图显示火山岩前缘与弧后一侧地幔不均匀性的存在。这两个源区地幔在矿物学上可能是不均匀的，并且垂直分层。岩浆温度的不均匀性和跨弧剖面特征与地震速度结构密切相关，存在局部低速区，第四纪火山活动表现出向弧后一侧的短期和长期扩展。这种喷发序列在时空上通常从火山前缘开始，依次向弧后传播。构造脉冲作用下深部储层的岩浆同时运移可能是火山活动同步化的原因之一。东北本州弧南部第四纪以来存在着与构造运动同步的5 ~ 10万年的周期性岩浆喷发幕。这种爆发的高潮似乎与地球绕太阳轨道的偏心率最大值的时间相吻合。潮汐力可以补充俯冲板块的压应力。

The NE Honshu arc has the most completely studied Cenozoic section. In this section, there are three volcanic periods ; continental margin volcanic period, back-arc basin volcanic period, and issland arc volcanic period. The 1320ﾟC iotherm in the mantle wedge is monotonously deep to the back-arc side at the continental margin volcanic period and the island arc volcanic period. The inclination of the isotherm in the continental margin volcanic period is more gentle than in the island arc volcanic period. The magma segregation depth at the volcanic front was much deeper in the continental margin volcanic period than in the island arc volcanic period. On the other hand, the mantle temperature was the highest at the back-arc side in the back-arc basin volcanic period.The presence of mantle heterogeneity between the volcanic front side and the back-arc side was clearly shown in the process identification diagrams for the volcanic rocks of the island arc volcanic period. The two source mantles can be mineralogically heterogeneous, and vertically layred. The heterogeneity and across-arc profile of magmatic temperatures are closely related to the seismic velocity structure with the localized low-velocity regions.The Quaternary volcanism shows short-term and long-term spreading to back arc side. This eruption arrays in a temporal-spatial plot have usually commenced at the volcanic front, and propagated to the back-arc sequentially. Simultaneous magma transfer from the deeper reservoirs by tectonic pulse could be a cause of the synchronized volcanic activity. Through the Quaternary age in the southern part of the NE Honshu arc, the periodic magma eruption episodes at intervals of 50 to 100 thousand years synchronized with tectonic movements exists. The climactic eruptions seems to match the timing of the eccentricity-maximums of the Earth's orbit around the Sun. The tidal forces could work supplementary to the compressive stress from subducting plate.