Study on the relation between sedimentation and igenous activity in the forearc area.

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We defined the tectonic origin of basaltic rock blocks within several accretionary complex of Permian to Tertiary ages in Southwest Japan based on major and minor element analysis. Basalt chemistry suggests the tectonic settings, ie. within plate basalt of hotspot origin (alkalic or tholeiite), mid-oceanic basalt (MORB)(N-or T-type), and island arc basalt. The field observations, especially the mode of occurrence between the surrounding sediments, the capped limestone or chert, degree of deformation and metamorphism are intimately related to the tectonic settings of those basalts. Basalts which are of MORB are highly deformed and metamorphosed, and capped by bedded chert, indicating the underplated oceanic crust in the deeper levels during oceanic plate subduction, while basalts which are of hotspot origin are weakly deformed and metamorphosed, and capped by reef limestone, indicating the offscraped in the shallower levels or trench collapsed seamount. The former examples are in the Northern Shimanto belt, and the latter examples are in the Akiyoshi, Mino and Konose terranes.