ABSTRACT

The Tokoro Belt, a tectonic unit of the central axial zone of Hokkaido, is characterized by preponderant occurrence of Jurassic pillow lavas. These pillow lavas occur separately in the Eastern and the Western masses of the northern Tokoro Belt. The pillow lavas from the Eastern mass are tholeiitic basalt which is considered to have been produced in an abyssal sea floor, while, those from the Western mass are alkali basalt probably related to the formation of seamounts in Jurassic time. REE abundances of above two types of pillow lavas were studied and the results revealed pronounced contrasts between the two masses.