Geochemical study on the evolutional process of the island arc mantle, based on the analytical work on the Tertiary volcanic rocks of Northeast Japan.

本项目（05640554）旨在描述岛弧地幔的演化过程。东北日本弧起源于亚洲大陆东端的增生棱柱体。弧后张开过程形成了日本海，在日本东北部和西南部的差异旋转运动作用下，日本地区与弧后盆地形成岛弧镶嵌。日本海的形成时间是根据古地磁证据推断的，即日本东北弧在15 Ma左右逆时针旋转。在项目研究期间，我们获得了日本东北部旋转块体向北延伸至北海道大岛半岛的新资料，其规模约为500 km，并对岛弧岩浆的形成和流体的作用作了初步探讨根据对伊豆、桑义河和东北日本弧火山岩的地球化学研究，从含水相中提取了100%的碳酸盐。我们试图重新确认这些规律的第四纪火山岩收集从堪察加半岛。微量元素含量较好地解释了岛弧内三条平行火山链的分布，但Sr、Nd同位素组成均较均一，与微量元素的结果不一致。对微量元素与Sr、Nd同位素组成不一致的结果还需进一步解释，年代学工作对建立日本东北弧可靠的地质历史具有重要意义。为此，将对该地区的火山岩进行Rb-Sr同位素测年。

The present project (05640554) is designed to get a goal to describe the evolutional process of the island arc mantle. The NE Japan arc originated as accretinal prisms at the eastern end of the Asia continent. The backarc opening process had formed the Japan Sea and the Japan region developed as island arc accompaning with the back-arc basin by the differential rotational movement of NE and SW Japan. Timing of the formation of the Japan Sea is inferred by the counterclockwise rotation of NE Japan arc at around 15 Ma from paleomagnetic evidence.During the project study, we have obtained new data by which the rotational block of NE Japan extends north to Oshima Peninsula of Hokkaido and the dimension is of order of 500 km.We have already established some regularities for generation of island arc magma and the role of fluid extracted from the hydrous phases, based on geochemical study on the volcanics of the Izu, Sangihe and NE Japan arc. Wetried to reconfirm these regularitiesfor the Quaternary volcanic rocks collected from the Kamchatka. Trace element concentrations well explain the distribution of three parallel volcanic chains in island arc, but homogeneous Sr and Nd isotopic compositions are unconformable with the result of the trace elements. Further investigatoin are needed to give a reasonable interepretation about the inconsistent result between the trace elements and the isotopic compositions of Sr and Nd.Geochronologic work is very important to establish reliable geologic history of NE Japan arc. For this purpose, Rb-Sr isotopic dating will be conducted on the volcanic rocks of the terrain.