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

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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.