Genesis of high magnesium andesites from Nagasaki, Northwest Kyushu, Southwest Japan

Mashima, Hidehisa; Tajima, Toshihiko*

High magnesium andesites (HMAs) with island arc geochemical signatures are distributed in Nagasaki, Northwest Kyushu, Southwest Japan. The genesis of the Nagasaki HMAs cannot be explained by current flux melting models for HMA magmas since mantle xenoliths from NW Kyushu suggests that the NW Kyushu mantle is anhydrous. Normative Jd+CaTs-Ol-Qz compositions of the HMAs suggest that they were separated from their source mantle at 0.5 GPa. This abnormally low pressure partial melting would have been caused by the large contrast of lithospheric strength between the land area composed of the Nagasaki metamorphic rocks and the sea area where sedimentary basins have been formed. The large contrast of lithospheric strength would have concentrated mantle upwelling at the boundary between the land and the sea areas. In the genesis of the Nagasaki HMA magmas, the subduction of the Philippine Sea plate would habe played not as the source of hydrous components but as the driving force of strike-slip tectonics in NW Kyushu.