Origin and Emplacement Process of the Basaltic Rocks in the Accretionary Complexes and Structural Belts in Japan in View of Minor Element Analysis and Mode of Occurrence

Abstract

Basaltic rocks in several accretionary complexes in Japan were chemically analyzed. Using minor element diagrams for discrimination of their tectonic settings, it is verified that there is definitely a good relationship between the tectonic settings and modes of occurrence. Basaltic rocks in the schistose and sheared metamorphic rocks are mostly ocean floor basalts (N- or T-MORBs) which are commonly associated with bedded chert or siliceous claystone. They might have been accreted by underplating of ocean floor materials at deeper levels in the accretionary prisms during oceanic plate subduction. On the other hand basaltic rocks isolately involved within not so highly metamorphosed matrices are mostly within plate basalts of hot-spot origin commonly capped with reef of pelagic limestone. Seamount materials might be accreted either by off-scraping at shallow depths of accretionary prisms or by collapsing at the trenches from seaward. One more particular occurence is that ophiolitic materials involved in the Mineoka tectonic belt are dominantly MORBs with minor hot-spot basalts in sheared serpentinite. Oceanic plate materials associated with seamount rocks were presumed to be obducted landward during specific tectonics around the TTT-type triple junction.